See also:

  • Recent Results of the MPE IR/Submm Group
  • The PHIBSS2 pages for highlights from our IRAM programs on the molecular gas in distant galaxies


The Angular Momentum Distribution of z ~ 1−3 Star-forming Galaxies

Top: These panels show an example of a z ~ 1 galaxy observed in KMOS3D: HST IJH bands color map, KMOS Hα flux, velocity, velocity dispersion maps, and major axis velocity and dispersion profiles. Bottom left: The plot shows the angular momentum parameter distribution from modeling the data assuming exponential baryonic disks in NFW dark matter halos neglecting or accounting for adiabatic contraction (AC), and without AC but accounting for deviations from a pure disk profile for the galaxies. The distributions are log-normal, with a mean value ~0.037 and dispersion in logarithmic units of ~0.2 dex. Bottom right: The plot shows the angular momentum parameter as a function of stellar or gas mass surface density within the half-light radius R1/2 corrected for the redshift evolution of galaxy sizes, and of stellar mass surface density in the inner 1 kiloparsec.

The angular momentum links galaxies to their host dark matter halos and contains the imprint of their baryonic mass assembly history. We exploited the high-quality, spatially resolved Hα kinematics of a representative subset of 360 log(M/M) ~ 9.3–11.8 ~ 1–3 star-forming galaxies from our KMOS3D and SINS/zC-SINF surveys, obtained with the near-IR multiobject KMOS and AO-assisted single-object SINFONI integral field spectrographs at the Very Large Telescope. That way, we derived for the first time robustly the angular momentum distribution of massive star-forming galaxies around the peak epoch of cosmic star formation. The inferred halo scale angular momentum distribution of the galaxies is consistent with the theoretical prediction for their dark matter halos in terms of mean spin parameter 〈λ〉 ~ 0.037 and dispersion σ(log λ) ~ 0.2. Spin parameters correlate with disk size and stellar surface density but do not depend significantly on halo mass, stellar mass, or redshift. Our data support the long-standing assumption that, on average, the specific angular momentum of disks reflects that of their dark matter halos (jd = jDM). The weak correlation between λ×(jd/jDM) and stellar surface density in the inner 1 kiloparsec suggests that internal processes lead to "compaction" and dense core formation inside massive high-z disks. The analysis of our sample further yields an average stellar disk-to-dark matter mass ratio of ~ 2%, consistent with abundance matching results. Including the molecular gas, the total baryonic disk-to-dark matter mass ratio is ~ 5% for halos near 1012 M, which corresponds to 31% of the cosmologically available baryons, implying that high-redshift disks are strongly baryon-dominated.

These results can be found in Burkert et al. 2016.

The Mass Budget of Early Star-forming Galaxies from KMOS3D

Left figure, top row: The panels show the distribution of the ratio of stellar to dynamical mass (left) and of baryonic to dynamical mass (right). Here, the baryonic mass estimate accounts for the stellar content as well as molecular gas inferred from state-of-the-art scaling relations based on CO and dust observations (from Genzel et al. 2015). Left figure, bottom row: The panels show the correlation between dynamic, stellar, and baryonic masses, and indicate that when accounting for the gas, baryons strongly dominate the total enclosed mass.
Right figure: Stellar mass fractions correlate with stellar mass surface density. Qualitatively similar relations are observed for the baryonic mass fractions, also as a function of gas and total dynamical mass surface densities, and follow from the self-consistent modeling of stars, gas, and dark matter in a ΛCDM context by the Illustris cosmological hydrodynamical simulation (e.g., Vogelsberger et al. 2014; Genel et al. 2015).

We exploited our deep integral-field spectroscopic observations from KMOS3D to dynamically constrain the mass budget of 240 star-forming disks at 0.6 < < 2.6. Our sample consists of massive (≳ 109.8M) galaxies with sizes Re ≳ 2 kiloparsec. By contrasting the observed velocity and velocity dispersion profiles to dynamical models, we find that on average the stellar content contributes about 32%, and the total (stellar and gas) baryonic content amounts to about 56% of the dynamical mass budget. Nearly all disks at > 2 are strongly baryon-dominated within their half-light radius. Substantial object-to-object variations in both stellar and baryonic mass fractions are observed, correlating most strongly with measures of surface density. Our findings can be interpreted as more extended disks probing further (and more compact disks probing less far) into the dark matter halos that host them.

These results can be found in Wuyts et al. 2016, ApJ 831, 149.

Consistent Evolution of Metallicity and Metallicity Gradients from z ~ 2.7 to z ~ 0.6

These panels illustrate the mass-metallicity relationship in three redshift bins derived from the integrated [NII]/Hα line ratio, using the Pettini & Pagel (2004) calibration. The data are shown for stacked spectra of star-forming galaxies binned in stellar mass from KMOS3D (large black and blue symbols) and SINS/zC-SINF+LUCI (large orange symbols). Small gray dots indicate individual measurements from KMOS3D. Excluding galaxies with an AGN (shown with diamonds) reduces the sensitivity at high masses but otherwise results in similar relationships. The dotted line in all panels shows the z ~ 0.8 relationship by Kewley & Ellison (2008). In the middle panel, the dashed line represents the relationship derived from the FMOS survey at z ~ 1.6 by Zahid et al. (2014). In the right panel, the different dashed lines plot those derived at z ~ 2.3 from Erb et al. (2006), Steidel et al. (2014; KBSS survey), and Sanders et al. (2015; MOSDEF survey).
This plot shows the iInferred metallicity evolution for M ~ 1010 M from KMOS3D across the wide redshift interval from z ~ 2.3 to z ~ 0.9 probed consistently by our YJ-, H-, and K-band observations (large black diamonds). Other relevant measurements in the literature are plotted as circles after recalibration onto the same metallicity system of Maiolino et al. (Kewley & Ellison 2008, Zahid et al. 2014, Steidel et al. 2014, Sanders et al. 2015, Erb et al. 2006, Onodera et al. 2016, Maiolino et al. 2008). The KMOS3D match the expected evolution following the "equilibrium growth model" of Lilly et al. 2013 remarkably well, assuming the local fundamental relation between M, metallicity, and SFR of Mannucci et al. 2010 and an evolving star formation efficiency ∝ (1+z)0.34±0.15 of Genzel et al. 2015.
The inferred metallicity gradients from resolved [NII]/Hα radial profiles from KMOS3D (black symbols) are on average flat (as found in previous studies with much smaller and more biased samples) and consistent with theoretical expectations when strong feedback is involved, based on numerical simulations (colored lines; from Pilkington et al. 2012 based on Rahimi et al. 2011, Kobayashi & Nakasato 2012, Few et al. 2012, Gibson et al. 2013).

We used the [NII]λ6584/Hα ratio as probe of the gas-phase oxygen abundance in over 400 galaxies representative of the bulk of the star-forming population from our KMOS3D and SINS/zC-SINF surveys with KMOS and SINFONI at the Very Large Telescope, and LUCI sample at the Large Binocular Telescope. We constructed statistically robust mass-metallicity relationships determined consistently from the same indicator over a wide redshift range spanning z = 0.6–2.7. We found no significant dependence of the inferred metallicity on star formation rate (SFR) at fixed redshift and mass; this result, most significant for the z ~ 1 subsample, is in contrast to findings at z ~ 0 that led to the proposed "fundamental metallicity relation," whereby lower metallicities in high-z galaxies would result naturally from their elevated SFRs. With the spatially resolved KMOS3D and SINS/zC-SINF data, we derived abundance gradients in ~ 200 galaxies, tripling current literature samples. The gradients are on average flat, with only ~ 10% of them having a slope significantly offset from zero even when accounting for beam smearing. Given that most of the galaxies show no sign of interaction/merging, these results suggest efficient metal mixing mediated by strong outflows as predicted by cosmological simulations, and observed in a majority of our sample (driven by vigorous star formation and by AGN). Alternatively, shocks and ionization effects could contribute to mimic flat line ratio gradients.

These results can be found in Wuyts et al. 2016, ApJ, 827, 74.
Earlier results based on the first-year KMOS3D data, and the SINS-zC-SINF and LBT/LUCI surveys appeared in Wuyts et al. 2014, ApJ, 789, 40.


Successful Start of the KMOS3D Survey

This image illustrates the Hα velocity fields for 250 galaxies at z ~ 0.9 and z ~ 2.2 from our KMOS3D survey. The velocity fields are shown on the same angular scale, and blue to red colors correspond to blueshifted to redshifted velocities relative to the systemic velocity of each source. The galaxies are plotted within 0.1 dex of their location in the stellar mass − star formation rate (SFR) plane, where the SFRs are normalized to that of the main sequence ("MS") at log(M/M) = 10.5, and at the median z = 0.9 and 2.2 for the galaxies in the 0.7 < z < 1.1 and 1.9 < z < 2.7 intervals (observed in the YJ-and K-bands, respectively). A majority of star-forming galaxies are disks, reflected in their smooth monotonically varying velocity gradients. Several resolved disks are even uncovered among the submain sequence population at high masses.
The compilation of measurements of the disk-intrinsic velocity dispersions σ0 from KMOS3D and other near-IR IFU surveys at z > 0.7, and optical long-slit or imaging spectroscopic surveys at z ~ 0 shows an increase with redshift following σ0 ∝ 1+z. This evolution is consistent with theoretical expectations for marginally stable gas-rich disks given the observed evolution of molecular gas fractions out to z ~ 3 and the galaxies' distribution in rotation velocities.

Taking advantage of the new and efficient near-IR 24-IFU KMOS instrument, built by a consortium involving MPE, we began the KMOS3D survey in November 2013, an ambitious and highly successful 75-night GTO program led by a team from MPE IR/Submm, MPE OPINAS, and USM. KMOS3D is mapping the Hα+[NII]+[SII] emission of 600+ mass-selected galaxies at z ~ 0.6–2.7. The survey is carried out in well-studied extragalactic fields with extensive multi-wavelength data, including the far-IR Herschel PEP survey led by our group, and high-resolution optical/near-IR grism and imaging data from the 3D-HST/CANDELS HST Treasury programs. KMOS3D is designed to provide an unbiased census from deep integrations (~ 5 to 25 hours) of the same spectral diagnostics resolved on seeing-limited scales of 4 to 5 kiloparsec, over a wide range of galaxy parameters and 5 Gyr of cosmic time. The strategy is uniquely enabling faint-line emission mapping in individual objects and pushing IFU studies into new regimes, such as lower-mass main-sequence star-forming galaxies, and high-mass submain-sequence galaxies in the process of quenching. Altogether, KMOS3D spans two orders of magnitude in stellar mass (log [M/M] ~ 9.5–11.5) and three orders of magnitude in SFR relative to the main sequence (SFR/SFRMS ~ 0.01–10). KMOS3D now confirmed robustly our earlier results from SINS/zC-SINF on the kinematics and structure of high-z star-forming galaxies. The dynamical support of at least 70% of all z ~ 1–3 massive star-forming galaxies is dominated by ordered disk rotation, unlike what would be expected in the case of frequent (major) merging. However, the high-z disks differ significantly from nearby spiral galaxies: the measured large local random motions from Hα emission (σ0 ≳ 25 km/s) reveal turbulent ionized gas disks. The disk velocity dispersion increases with redshift as σ∝ (1+z), in line with expectations for gas-rich disks and the observed evolution in cold gas mass fractions. By targeting the same spectral diagnostics of homogeneously selected samples, observed and analyzed in the same way, KMOS3D is providing the most consistent IFU study of the evolution of resolved kinematics, star formation, and warm ISM properties of ~ 0.7–2.7 galaxies.

The KMOS3D survey design and strategy, and first-year results appeared in Wisnioski et al. 2015, ApJ, 799, 209.


Widespread AGN-driven Outflows in the Most Massive z ~ 1−2.5 Star-forming Galaxies

Top left: The stacked spectrum of the central regions of log(M/M) > 10.9 galaxies at z ~ 0.7−2.7 from our KMOS3D and SINS/zC-SINF surveys shows, at high S/N, the broad (FWHM ~ 500−2,000 km/s) emission component in Hα, [NII], and [SII] associated with AGN-driven outflows. It contrasts with the stacked spectrum of the outer disk regions of the same galaxies, which exhibits a weaker and narrower (FWHM ~ 400 km/s) broad emission component from star-formation-driven outflows. Bottom left: For five of the six log(M/M) > 10.9 SINS/zC-SINF galaxies with deep AO-assisted SINFONI observations at ~ 0.2 arcseconds resolution, the broad emission originates primarily from the center, where a stellar bulge is seen in the rest-optical light and derived stellar mass distribution. The nuclear broad emission is resolved in most cases and implies an extent of 2 to 3 kiloparsec. For BX610, the broadest outflow emission is seen around the center; strong but narrower outflow emission is also detected at the location of the bright star-forming "clump" to the southwest. For each galaxy, a pair of images is shown: the left one displays the rest-optical stellar continuum light and star formation (observed H band map from HST or extracted from the SINFONI data in red colors, HST J-band map in blue colors when available, and narrow star-formation-dominated Hα line emission from SINFONI in green colors), and the right one shows the same H-band and narrow Hα maps (in red and green, respectively) with the broad, outflow-dominated Hα+[NII] emission component overlaid in white contours. Top right: This plot shows variations in broad component velocity FWHM with galaxy stellar mass for the stacked spectra of the inner 2 to 3 kiloparsec (large circles and red-shaded area) and outer disk regions (squares and green-shaded area) of our z ~ 0.7−2.7 star-forming galaxies. Spectra were stacked per mass bin and separately for galaxies with specific star formation rate above (red circles and dark-green squares) and below (blue circles and light green squares) the main sequence. Measurements from individual nuclear spectra with best S/N are overplotted at star symbols; the three galaxies dominated by BLR emission are labeled in the plot, and excluded from the stacks. The onset of the broad nuclear emission associated with AGN-driven outflows is very sharp toward high masses, while star-formation-driven outflows with more modest velocities are detected across disks at all masses. Bottom right: The incidence of broad nuclear emission (red circles) increases sharply above log(M*/M) ~ 10.9, reaching ~2/3 at the highest masses. Although the fraction of AGN among our samples (yellow stars) or in the general galaxy population at similar redshifts in the GOODS and COSMOS extragalactic survey fields (gray- and green-shaded polygons) follows a similar trend, the frequency is < 50%. The nuclear outflow phenomenon thus has a higher duty cycle than the highly variable AGN emission.

Following the detection of powerful star-formation-driven ionized gas outflows originating throughout the disk regions and especially around intensely star-forming clumps in our z ~ 2 SINS/zC-SINF sample, new observations with SINFONI+AO and KMOS uncovered distinct high-velocity outflows in the centers of the most massive but otherwise normal star-forming galaxies. With a FWHM ~ 500−2,000 km/s seen in Hα as well as in forbidden [NII]λλ6548,6584 and [SII]λλ6716,6731 line emission, elevated [NII]/Hα ratio > 0.5, and an extent of 2−3 kiloparsec derived from five sources with high-resolution AO-assisted observations, this broad emission component is most plausibly originating from AGN-driven outflows. The frequency of these nuclear outflows rises sharply at log(M/M) ~ 10.9, reaching 2/3 above this mass. These star-forming galaxies were selected based on mass and on their location around the main sequence of star-forming galaxies, rather than by the presence of an AGN. In fact, < 50% of these galaxies are classified as hosting an AGN from classical X-ray, optical, IR, and radio indicators, suggesting the nuclear outflows have a higher duty cycle than the extremely variable AGN activity. The typical high inferred mass outflow rates (dMout/dt > SFR) and momentum deposition rates (vout×dMout/dt ~ 20×L/c), together with the presence of massive bulges, and with evidence for suppressed star formation and gravitational quenching in the inner 2 to 3 kiloparsec of half of the galaxies, make a compelling case that these nuclear winds play an important role in clearing the central regions of gas prior to quenching.

These results appeared in Förster Schreiber et al. 2014, ApJ, 787, 37 and Genzel et al. 2014, ApJ, 796, 7.

Bulge Growth and Quenching since z = 2.5

This plot shows the bulge-to-total (B/T) stellar mass ratio as a function of the total stellar mass of ~ 6,800 galaxies at 1010M and 0.5 < z < 2.5. The B/T of each galaxy was derived by fitting a disk+bulge model to the stellar mass maps obtained by modeling the spatially resolved rest-UV+optical spectral energy distributions from the multiband CANDELS HST imaging. The whole sample (black nonfilled symbols) is divided into star-forming (blue symbols) and quiescent galaxies (red symbols). The respective shaded areas mark the 50th percentile scatter of the distributions within one bin, while the error bars indicate the uncertainty on the median value. Results for the lower and upper halves of the redshift interval are marked with dotted and dashed lines, respectively. Star-forming galaxies show a clear trend of increasing B/T with increasing stellar mass, reaching B/T ~ 0.4–0.5 above 1011 M and reflecting the buildup of central stellar mass concentrations in main-sequence galaxies up to z ~ 2.5.

Exploiting the deep high-resolution imaging of all five fields part of the HST CANDELS imaging survey, and accurate redshift information provided by the 3D-HST grism survey in the same areas, we investigated the relation between structure and stellar populations for a mass-selected sample of 6,764 galaxies above 1010 M, spanning the redshift range 0.5 < z < 2.5. For the first time, we fitted two-dimensional models comprising a single Sérsic fit and two-component (i.e., bulge+disk) decompositions not only to the H-band light distributions, but also to the stellar mass maps reconstructed from resolved stellar population modeling. The results confirm that the increased bulge prominence among quiescent galaxies, as reported previously based on rest-optical observations, remains in place when considering the distributions of stellar mass. Moreover, we observed an increase of the typical Sérsic index and bulge-to-total ratio (with median B/T reaching 40 to 50%) among star-forming galaxies above 1011M. Given that quenching for these most massive systems is likely to be imminent, our findings suggest that significant bulge growth precedes a departure from the star-forming main sequence. We demonstrated that the bulge mass (and ideally knowledge of the bulge and total mass) is a more reliable predictor of the star-forming vs. quiescent state of a galaxy than the total stellar mass. The same trends are predicted by the state-of-the-art, semi-analytic model by Somerville et al. Here, bulges and black holes grow hand in hand through merging and/or disk instabilities, and feedback from active galactic nuclei shuts off star formation. Further observations will be required to pin down star-formation-quenching mechanisms, but our results imply that they must be internal to the galaxies and closely associated with bulge growth.  

These results appeared in Lang et al. 2014, ApJ, 788, 11.

Evidence for Gravitational Quenching from SINS/zC-SINF

Left: This panel shows the integrated Hα line maps of 19 well-resolved star-forming galaxies from our SINS/zC-SINF AO sample with deep on-source integrations. The galaxies are plotted at their location in the stellar mass vs. star formation rate (SFR) plane, on the same angular scale and with colors scaling linearly with surface brightness; the FWHM angular resolution of these maps is ~ 0.24 arcseconds. The white solid line shows the location of the "main sequence" of z ~ 2 star-forming galaxies assuming a slope of unity, and the dashed lines indicate its 2σ scatter (±0.6 dex in log[SFR]). Many galaxies exhibit ring-like distributions in Hα, especially frequent toward more massive galaxies. In combination with the SINFONI kinematic maps and existing HST J- and H-band maps at similar resolution, the data are indicative of the presence of increasingly massive bulges and suppressed star formation in the central few kiloparsecs of the more massive galaxies.
Right: The plot illustrates derived radial profiles of the Toomre Q parameter of the galaxies, color-coded as a function of dynamical mass as follows: log(Mdyn/M
) = 10.36−10.50 in blue, 10.68−10.93 in green, 11.04−11.28 in orange, and 11.34−11.41 in red. The gray-shaded interval corresponds to the typical resolution element for the sample. All profiles exhibit an increase in Q toward their central regions, with values significantly in excess of the threshold around unity, the more so for the most massive galaxies, suggesting that their bulges may stabilize the gas against gravitational collapse in the inner few kiloparsecs.

We analyzed the radial distributions of Hα surface brightness, stellar mass surface density, and dynamical mass at ~ 2 kiloparsecs resolution in 19 z ~ 2 star-forming disks with deep AO-assisted SINFONI imaging spectroscopy from our SINS/zC-SINF survey. From the combination of the kinematic maps and the molecular gas mass surface densities inferred from the star formation rate distributions, we derived the radial profiles in Toomre Q parameter for these main-sequence star-forming galaxies, which span about two orders of magnitude in stellar mass (log[M/M] = 9.6−11.5). In more than half of these galaxies, the Hα distributions cannot be fit by a centrally peaked distribution, such as an exponential, but are better described by a ring or the combination of a ring and an exponential. At the same time, the kinematics data indicate the presence of a mass distribution more centrally concentrated than a single exponential disk component for 5 of the 19 galaxies. The resulting Q profiles are centrally peaked for all, and significantly exceed unity there for ~ 3/4 of the galaxies. The occurrence of Hα rings and of large nuclear Q values appears to be more common for the more massive star-forming galaxies. While the sample is small and biased toward larger sizes, and there remain uncertainties and caveats, the observations are consistent with the "gravitational quenching" scenario, in which cloud fragmentation and global star formation are secularly suppressed in gas-rich high-z disks from the inside out, as the central stellar mass density of the disks grows.

These results appeared in Genzel et al. 2014, ApJ, 785, 75.


The Nature of Dispersion-dominated Galaxies at High Redshift

Left: This panel shows Hα line maps of the 34 star-forming galaxies from our SINS/zC-SINF AO sample. The top two rows contain the dispersion-dominated objects (with ratio of intrinsic rotation velocity to velocity dispersion vrot0 below ~ 1) and the rest are rotation-dominated. The maps are all plotted on the same angular scale. The typical FWHM resolution of the maps presented here is 0.2 to 0.3 arcseconds (indicated with the red circle). The dispersion-dominated galaxies tend to be more compact than the rotation-dominated ones. Middle and right: These plots illustrate the dependence of vrot and σ0 on the Hα half-light radius R1/2. The measurements for the SINS/zC-SINF galaxies (blue circles) are combined with those of other z ~ 1–2.5 samples observed mostly using AO (Law et al. 2009, 2012 and Wright et al. 2009, red squares; Wisnioski et al. 2011, green circles; Swinbank et al. 2012, cyan upside-down triangles; Épinat et al. 2009, 2012 and Lemoine-Busserolle and Lamareille 2010, black triangles and crosses). The large dark grey symbols show the median values per size bins. The strong trend of velocity vs. size can be well-fit by a linear relation: log(vrot) = 0.62xlog(R1/2)+1.9. In contrast, the velocity dispersion does not appear to show a significant trend. Thus, the trend for smaller galaxies to be dispersion-dominated is in part due to the combination of a possible floor of velocity dispersion, and a linear increase of rotation velocity with size.

We analyzed the spatial distributions and kinematics of Hα, [NII], and [SII] emission in 38 star-forming galaxies from our SINS/zC-SINF survey, 34 of which were observed with SINFONI at high resolution using AO. This was supplemented by kinematic data of 43 z ~ 1–2.5 galaxies from the literature. None of these 81 galaxies is an obvious major merger. We found that the kinematic classification of high-redshift galaxies as "dispersion-dominated" or "rotation-dominated" correlates most strongly with their intrinsic sizes. Smaller galaxies are more likely "dispersion-dominated" for two main reasons: 1) The rotation velocity scales linearly with galaxy size, but intrinsic velocity dispersion does not depend on size or may even increase in smaller galaxies, and as such, their ratio is systematically lower for smaller galaxies, and 2) Beam smearing strongly decreases large-scale velocity gradients and increases observed dispersion much more for galaxies with sizes at or below the resolution. Dispersion-dominated galaxies may thus have intrinsic properties similar to the rotation-dominated ones, but are primarily more compact, have lower mass, are less metal-enriched, and may have higher gas fractions, plausibly because they represent an earlier evolutionary state. A key implication of our results is that the derived fraction of dispersion-dominated objects among massive star-forming galaxies at z ~ 1–2.5 is < 20% lower than had been inferred based largely on seeing-limited observations.

These results appeared in Newman et al. 2013, ApJ, 767, 10.

Resolved Star Formation Patterns at 0.7 < z < 1.5

This gallery presents case examples from our sample of 473 massive star-forming galaxies at 0.7 < z < 1.5 in the HST CANDELS/3D-HST extragalactic survey fields. Below the three-color composites, we show the surface brightness distribution in the ACS I (0.8 μm) and WFC3 H (1.6 μm) band, as well as Hα line emission maps extracted from HST grism spectroscopy. The physical resolution in all maps is ~ 1 kiloparsec. Blue, star-forming regions present in the I-band generally dominate the Hα emission as well. Central peaks in surface brightness (i.e., "bulges") appear more prominently in the H-band.

We analyzed the resolved stellar populations in a sample of 473 massive star-forming galaxies at 0.7 < z < 1.5, with multiwavelength broadband imaging from CANDELS and Hα surface brightness profiles at the same kiloparsec resolution from 3D-HST, two HST Treasury extragalactic surveys. Together, this unique data set sheds light on how the assembled stellar mass is distributed within galaxies, and where new stars are being formed. We found the Hα morphologies to resemble more closely those observed in the ACS I (0.8 μm) band than in the WFC3 H (1.6 μm) band, especially for the larger systems. In order to translate the Hα surface brightness profiles to maps of the star formation rate, we derived a novel prescription for Hα dust corrections, which accounts for extra extinction toward HII regions. We found the surface density of star formation to correlate with the surface density of assembled stellar mass within galaxies, akin to the so-called "main sequence" of star formation established on a galaxy-integrated level. Deviations from this relation toward lower equivalent widths are found in the inner regions of galaxies. Clumps and spiral features, on the other hand, are associated with enhanced Hα equivalent widths, bluer colors, and higher specific star formation rates than the underlying disk. Their Hα/UV luminosity ratio is lower than that of the underlying disk, suggesting that the ACS clump selection preferentially picks up those regions of elevated star formation activity that are the least obscured by dust. Our analysis emphasizes that monochromatic studies of galaxy structure can be severely limited by mass-to-light ratio variations due to dust and spatially inhomogeneous star formation histories.

These results appeared in Wuyts et al. 2013, ApJ, 779, 135.


Smoother Stellar Mass Maps and the Longevity of Star-forming Clumps in High-redshift Galaxies

Left:This panel shows the fractional contribution of stars younger than 10 Myr and 100 Myr to the total emission at rest-frame far-UV to J-band wavelengths, and to the total mass present in stars. Gray shades indicate Bruzual & Charlot (2003) models with constant star formation histories that started 0.5 Gyr, 1 Gyr, and 2 Gyr prior to the epoch of observation. While the rest-frame V-band provides a better proxy for stellar mass than the UV part of the spectral energy distribution, it is still substantially biased toward the youngest generation of stars. Resolved color information is the key to constraining spatial mass-to-light ratio variations due to nonuniform star formation histories (or dust obscuration) across a galaxy, and recovering the true distribution of stellar mass. Right: These histograms compare structural parameters of concentration and galaxy (ir-)regularity as measured on light maps of different rest-frame wavelength as well as on stellar mass maps reconstructed using pixel-by-pixel stellar population modeling. In mass, galaxies are more compact and smoother than they appear in light, particularly at short wavelengths. This trend is observed over the full redshift range (0.5 < z < 2.5) considered in our analysis.

We performed a detailed analysis of the spatially resolved colors and stellar populations of a mass-complete (log(M/M) > 10) sample of 323 star-forming galaxies at 0.5 < z < 1.5, and 326 star-forming galaxies at 1.5 < z < 2.5 in the ERS and CANDELS-Deep region of the GOODS-South extragalactic field, with very deep imaging from HST. We modeled the seven-band optical ACS and near-IR WFC3 spectral energy distributions of individual bins of pixels, accounting simultaneously for the galaxy-integrated photometric constraints available over a longer wavelength range. We found evidence for redder colors, older stellar ages, and increased dust extinction in the nuclei of galaxies. Large star-forming clumps seen in star formation tracers are less prominent or even invisible on the inferred stellar mass distributions. Our results are consistent with an inside-out disk growth scenario with brief (100 to 200 Myr) episodic local enhancements in star formation superposed on the underlying disk. Alternatively, the young ages of off-center clumps may signal inward clump migration, provided this happens efficiently on the order of an orbital timescale.

These results appeared in Wuyts et al. 2012, ApJ, 753, 114.

Short-lived Star-forming Clumps in Cosmological Simulations of z ~ 2 disks: the Impact of Strong Feedback from Massive Stars

The images illustrate the impact of strong feedback from young massive stars in high-resolution cosmological numerical SPH simulations. The time sequence of gas surface density maps shows the disruption of a clump in our model (top), where t = 0 (not shown) is the formation time of the clump. To demonstrate the role of the vigorous wind, it is turned off at z = 2.03 (t = 22 Myr) and the alternative evolution of nondisruption, virialization, and migration is shown for comparison (bottom). The upper rightmost panel shows the mass of gas (solid lines) and young (< 50 Myr) stars (dashed lines) for four clumps as a function of time since their formation. The magenta lines are for the clump highlighted on the top and the black for the clump highlighted on the bottom. The jump in mass of the green lines at t ~ 60 Myr is a result of a merger between two clumps. The typical clump lifetime in the presence of winds is ~ 50 Myr, and the mass of new-formed stars is approximately 10% of the maximum clump gas mass. The mass of new-formed stars internal to the clump decreases following the decrease of the gas mass, as these stars are dispersed out of the clump when the gravitation collapse of the gas is halted by the return to stable conditions with a Toomre Q parameter > 1.

Many observed massive star-forming z ~ 2 galaxies, including the ones from our SINS/zC-SINF survey, are large disks that exhibit irregular morphologies, with luminous, kiloparsec-sized star-forming clumps. In the framework of turbulent, gas-rich, marginally unstable disks, such clumps form through fragmentation and eventually migrate toward the center of the galaxies where they coalesce to form young bulges. However, our recent findings that clumps are also launching sites of powerful gas outflows that could disrupt them rapidly (highlighted here) raise important questions about their role in the evolution of early disks. To investigate this issue, we used the largest sample to date of high-resolution cosmological smoothed particle hydrodynamics simulations that zoom-in on the formation of individual log(M/M) ~ 10.5 galaxies in log(M/M) ~ 12 dark matter halos at ~ 2. Our code includes strong stellar feedback parameterized as momentum-driven galactic winds. This model reproduces many characteristic features of this observed class of galaxies, such as their clumpy morphologies, smooth and monotonic velocity gradients, high gas fractions (~ 50%), and high specific star formation rates (~ 1 Gyr-1). In accord with other recent models, giant clumps of masses Mclump ~ 5×108–109M  form in situ via gravitational instabilities. However, the galactic winds are critical for their subsequent evolution. The giant clumps are short-lived and disrupted by wind-driven mass loss. They do not virialize or migrate to the galaxy centers. These theoretical results are in line with our recent analysis of the resolved stellar light and mass distributions of large samples of 0.5 < z < 2.5 star-forming galaxies, which revealed that bright star-forming clumps generally do not correspond to local peaks in the stellar surface density distribution of galaxies. This implies that they may be rapidly destroyed – plausibly via strong star-formation-driven feedback – unless they migrate to the center within a dynamical timescale (see highlight on "Smoother stellar mass maps").

These results appeared in Genel et al. 2012, ApJ, 745, 11.

SINS/zC-SINF Reveals the Roots of Vigorous Star-formation-driven Gas Outflows at ~ 2

This figure shows the evidence for star-formation-driven outflows from our deep, high-resolution SINFONI+AO observations of ~ 2 galaxies. Left: Broad and blueshifted wings in the Hα+[NII] line emission profiles are detected at and around the location of several bright star-forming clumps in the z = 2.19 galaxy ZC406690. Insets show the spectra extracted at the position of bright Hα emission peaks along the star-forming ring, marked in the color-composite map of the Hα (green) and rest-frame UV (red) emission. The two maps at the bottom show the distributions of the emission in the narrow component-tracing star-forming sites and of the broad underlying emission-tracing outflowing gas. These observations were the first to provide direct evidence that the origin of the ubiquitous galactic winds long observed on large > 10 kiloparsec scales around distant star-forming galaxies can be traced to extended regions within galaxies and most prominently from the actively star-forming clumps. Top right: The co-averaged spectrum of clumps in five of the SINS/zC-SINF galaxies shows that the broad emission component is also present in fainter clumps, strengthening the evidence from ZC406690, the brightest galaxy of our sample. Bottom right: By co-averaging the integrated spectra of 27 SINS/zC-SINF galaxies that do not host an AGN, we found a strong trend of increasing ionized gas outflow strength (quantifed by the ratio of the flux in the broad and narrow Hα components) with star formation rate surface density, with an apparent threshold at ~ 1 M/yr/kiloparsec2, about ten times higher than the wind breakout threshold observed in nearby starburst galaxies. The mass outflow rates inferred for the disks above this threshold are comparable to the star formation rates (and up to several times higher for bright clumps), implying that the outflows can efficiently drive large amounts of gas outside of the galaxies.

Our newest and deep SINFONI+AO observations of z ∼ 2 star-forming disks allowed us to trace the origin of powerful outflows of ionized gas in non-AGN galaxies. The outflow signature, in the form of a broad FWHM ~ 400 to 500 km/s and blueshifted Hα+[NII] emission component, had been first seen in the co-added integrated spectrum of our initial SINFONI data obtained mostly at seeing-limited resolution (highlighted here). The higher resolution and sensitivity of our new AO-assisted data revealed that the outflows are spatially extended across the galaxies over at least a few kiloparsecs, and most prominent in the immediate vicinity of giant, luminous star-forming clumps. The inferred mass outflow rates from the clumps and the disks are comparable to and even several times the star formation rates, implying that some of the clumps may lose much of their initial mass and dissolve rapidly in the disk before they can migrate to the center of the galaxy. In the galaxy with brightest clumps and highest S/N data, our analysis of line ratio diagnostics ([NII]/Hα and [SII]/Hα) together with photoionization and shock models showed that the emission around the clumps is due to a combination of photoionization from the newly formed massive stars and shocks generated in the outflowing gas component, with 5 to 30% of the emission deriving from shocks.
Among the 27 SINS/zC-SINF non-AGN galaxies observed with SINFONI+AO, we find from co-averaged spectra in bins of global galaxy properties that the inferred gas outflow strength correlates most strongly with the averaged star formation rate surface density, with an apparent threshold for powerful winds around 1 M/yr/kiloparsec2. Above this threshold, galaxies with log(M) > 10 have similar or perhaps greater wind mass-loading factors (η = dMout/SFR) and faster outflow velocities than lower-mass galaxies, suggesting that the majority of outflowing gas at ∼ 2 may derive from high-mass star-forming galaxies. The threshold at ~ 2 is an order of magnitude higher than in nearby starbursts that drive galactic-scale winds. In the framework of a simple model where the wind breakout is governed by pressure balance in the disk, the threshold for strong outflows and the mass loading derived from our observations can be explained by the higher ISM pressure in turbulent, gas-rich, and highly star-forming ~ 2 disks.

These results appeared in a series of three papers by Genzel et al. 2011, ApJ, 733, 101, Newman et al. 2012a , ApJ, 752, 111, and Newman, S.F., et al. 2012b, ApJ, 761, 43.


Galaxy structure in the Star Formation Rate – Mass Plane from ~ 2.5 until Today

This figure shows the evidence for star-formation-driven outflows from our deep, high-resolution SINFONI+AO observations of ~ 2 galaxies. Left: Broad and blueshifted wings in the Hα+[NII] line emission profiles are detected at and around the location of several bright star-forming clumps in the z = 2.19 galaxy ZC406690. Insets show the spectra extracted at the position of bright Hα emission peaks along the star-forming ring, marked in the color-composite map of the Hα (green) and rest-frame UV (red) emission. The two maps at the bottom show the distributions of the emission in the narrow component-tracing star-forming sites and of the broad underlying emission-tracing outflowing gas. These observations were the first to provide direct evidence that the origin of the ubiquitous galactic winds long observed on large > 10 kiloparsec scales around distant star-forming galaxies can be traced to extended regions within galaxies and most prominently from the actively star-forming clumps. Top right: The co-averaged spectrum of clumps in five of the SINS/zC-SINF galaxies shows that the broad emission component is also present in fainter clumps, strengthening the evidence from ZC406690, the brightest galaxy of our sample. Bottom right: By co-averaging the integrated spectra of 27 SINS/zC-SINF galaxies that do not host an AGN, we found a strong trend of increasing ionized gas outflow strength (quantifed by the ratio of the flux in the broad and narrow Hα components) with star formation rate surface density, with an apparent threshold at ~ 1 M/yr/kiloparsec2, about ten times higher than the wind breakout threshold observed in nearby starburst galaxies. The mass outflow rates inferred for the disks above this threshold are comparable to the star formation rates (and up to several times higher for bright clumps), implying that the outflows can efficiently drive large amounts of gas outside of the galaxies.

In parallel to our studies of galaxy kinematics with SINFONI, we analyzed how the structure of galaxies depends on their current star formation rate and amount of assembled stellar mass. Our sample comprised 640,000 galaxies at ~ 0.1, 130,000 galaxies at ~ 1, and 36,000 galaxies at ~ 2. Size and profile measurements for all but the ~ 0.1 galaxies were based on high-resolution HST imaging, and star formation rates were derived using a Herschel-calibrated ladder of star formation indicators. We found that a correlation between the structure and stellar population of galaxies (i.e., a "Hubble sequence") was already in place as early as ~ 2.5. At each epoch, the galaxy population can be divided into three classes that coexist over more than an order of magnitude in stellar mass, but differ in star formation activity. Most of the normal star-forming galaxies feature shallow surface brightness profiles indicative of a disk-like nature. At fixed mass, they also tend to have the largest size. More compact and cuspier morphologies are found for quiescent galaxies that already formed the bulk of their stars, and reside below the main sequence of star formation. These results imply that the processes of star formation quenching and bulge formation are closely related. It is tantalizing to speculate that the rare population of starbursting outliers above the main sequence may represent an intermediate evolutionary phase, linking the normal star-forming and quiescent populations. While their star formation is peaking, we are witnessing the rapid build-up of a central cusp that is characteristic of quiescent galaxies. Assuming all starbursting outliers will be quenched, simple duty cycle arguments assign typical timescales ~ 100 Myr for this short-lived phase.

These results appeared in Wuyts et al. 2011a, ApJ, 738, 106, and Wuyts et al. 2011b, ApJ, 742, 96.

Dynamics and Evolution of Giant Star-forming Clumps in ~ 2 disks

The images show two examples of z ~ 2 rotating disk galaxies with prominent star-forming clumps: zC406690 at z = 2.19 (left panels) and Q2346-BX482 at = 2.26 (right panels). For each galaxy, the velocity field, velocity dispersion map, and spatial distribution of star formation map from our SINFONI+AO Hα observations at a resolution of ~ 1 to 2 kiloparsecs are shown in the top row, along with the spatial distribution of the Toomre Q parameter derived from the Hα line emission and kinematic maps. The clumps correspond to minima in the Q maps, with values below unity consistent with their having formed via gravitational instabilities in a turbulent, gas-rich disk. For the first time, the quality of our data allowed us to investigate kinematic signatures of clumps from maps of the velocity residuals, i.e., subtracting the velocities from the best-fitting model disk to the observed velocity field (bottom row). Hα light and residual velocity profiles across the brightest clump in each of the galaxy shown here (arrows overplotted on the bottom row maps indicate where the plotted profiles were extracted) reveal measurable but modest velocity gradients.

New and deep SINFONI+AO observations of five ∼ 2 star-forming disks allowed us for the first time to constrain the properties of individual giant star-forming clumps to empirically test scenarios of their formation and evolution. We found that the clumps reside in disk regions where the Toomre Q parameter is below unity, consistent with their being bound and having formed from gravitational instabilities. The clumps leave a modest imprint on the gas kinematics. Velocity gradients across the clumps are 10 to 40 km/s/kiloparsec, similar to the galactic rotation gradients. Given beam smearing and clump sizes, these gradients may be consistent with significant rotational support in typical clumps. The brightest, extreme clumps may not be rotationally supported; either they are not virialized or they are predominantly pressure-supported. The velocity dispersion is elevated and fairly constant across the galaxies, and increases only weakly with star formation surface density. The large velocity dispersions may be driven by the release of gravitational energy, either at the outer disk/accreting streams interface where gas from the halo is infalling onto the disk, and/or by the clump migration within the disk.

These results appeared in Genzel et al. 2011, ApJ, 733, 101.

Pilot HST Near-IR Study: Stellar Properties of Clumps in SINS z ~ 2 Disks

This figure shows two of the SINS ~ 2 disk galaxies, with evidence suggesting radial variations in the evolutionary stage of clumps. For BX482 (left), a total of seven clumps are identified in the combined HST/NICMOS2 H-band (1.6 μm) and SINFONI+AO Hα maps. The ratio of Hα line flux to rest-optical continuum flux density from the H-band data varies monotonically with the age of a stellar population. The ratios measured for the clumps reveal a trend whereby the clump closest to the center of BX482 is the oldest. For MD41 (right), seven clumps are also identified in the HST/NICMOS2 H-band and HST/ACS I-band (0.8 μm) maps. The observed I–H colors of a stellar population correlate closely with the stellar mass to rest-frame optical light ratio, also an age indicator. The clumps in MD41 tend to be redder at smaller radii, which could be due to older ages. These radial trends provide empirical support for the scenario in which clumps formed in turbulent, gas-rich disks migrate inward, eventually coalescing and contributing to early bulge growth.

We studied the stellar properties of kiloparsec-sized clumps identified in the six galaxies observed as part of our pilot program of near-IR (1.6 μm) imaging follow-up with HST of our SINS sample (see the highlight "A Pilot HST Study"). Typically, several clumps are identified in each galaxy, individual clumps contribute a few percent of the galaxy-integrated rest-frame ~ 5,000 Å light, and the total contribution of clump light ranges from around 10 to 25%. The typical clump size and stellar mass are ~ 1 kiloparsec and ~ 10M. These values are within the ranges inferred for clumps identified in rest-UV or Hα line emission in other studies. These properties are consistent with expectations for clump formation through gravitational instabilities in gas-rich, turbulent disks (see highlights "From rings to bulges" and "Dynamics and evolution of clumps"). For two galaxies, the combination of our HST/NICMOS imaging with available SINFONI+AO Hα for one, and HST/ACS rest-UV imaging for the other, at similar kiloparsec-scale resolution reveals trends of higher Hα-equivalent width and redder rest-frame UV-optical colors at smaller galactocentric radius, in contrast to the interclump regions that exhibit little if any radial gradient. This trend can be attributed to older stellar ages of clumps nearer the galaxy center, consistent with the scenario in which massive clumps can migrate inward and contribute to form young bulges in early massive disks.

These results appeared in Förster Schreiber et al. 2011, ApJ, 739, 45.

Pilot HST Near-IR Study: Rest-frame Optical Morphologies of SINS Galaxies

These images compare, for six of our SINS galaxies, the rest-frame optical continuum emission mapped with the HST NICMOS/NIC2 camera with the Hα line maps and velocities obtained with SINFONI (left, middle, and right panels for each galaxy). The rest-optical maps have a resolution of ~ 1.5 kiloparsecs. The SINFONI maps of BX482 (top left series) were taken with AO at a similar resolution; the seeing-limited data of the other galaxies have a resolution of ~ 4 to 5 kiloparsecs. All galaxies are kinematically identified disks except BX528, for which the reversal in velocities indicates that it is a counterrotating major merger. Clearly, even the regularly rotating disks exhibit prominent clumps in their rest-optical morphologies, and are indistinguishable from the major merger when applying quantitative criteria calibrated from local galaxies. Kinematics are necessary to classify disks and mergers reliably at high redshift. For the kinematically identified disks, the HST rest-optical imaging reveals that the starlight follows exponential or even shallower disk- or ring-like profiles, similar to the Hα light, and confirms their large sizes.

Our SINFONI data of SINS galaxies provide spatially-resolved maps of the ionized gas kinematics and distribution from Hα, tracing the current dynamical state and star formation activity of the galaxies. For a more complete picture, however, it is essential to also map the rest-frame optical continuum emission from the stellar populations that make up the bulk of the stellar mass and contain a record of the history of galaxies. In a pilot study, we obtained sensitive high-resolution near-IR imaging by using the NICMOS/NIC2 camera onboard HST of six z ~ 2 SINS galaxies, including five large disks and one major merger. The overall rest-frame ~ 5,000 Å of the galaxies is characterized by shallow profiles in general (Sérsic index n < 1) with a median half-light radius of R1/2 ~ 5 kiloparsecs, and no significant differences with the overall Hα surface brightness profiles. This suggests similar global distributions of the ongoing star formation and more evolved populations that dominate the rest-optical light. On smaller scales of ~ 1 kiloparsec, however, the rest-optical morphologies of the six galaxies are significantly clumpy and irregular. Commonly used quantitative morphological parameters, calibrated based on z ~ 0 galaxy samples, fail to distinguish the kinematically identified major merger from the rotating disks of our sample. Because high-redshift star-forming disks appear generally irregular with giant kiloparsec-sized clumps plausibly formed via gravitational instabilities in gas-rich disks, spatially resolved kinematics are necessary to unveil the true nature of distant galaxies.

These results appeared in Förster Schreiber et al. 2011, ApJ, 731, 65.


Mapping the Physical conditions of the Ionized Gas: Spatially Resolved Nebular Excitation and Gas Phase Abundances

The figure shows "BPT diagrams" (Baldwin et al. 1981) relating the [OIII]λ5007/Hβ and [NII]λ6584/Hα line flux ratios, which provide diagnostics for the excitation mechanism of nebular gas in galaxies. The small gray dots show the distribution of the local galaxies taken from the SDSS survey, revealing the locus of purely star-forming galaxies on the left branch with decreasing gas-phase oxygen abundances toward lower [NII]/Hα and higher [OIII]/Hβ ratios, and of galaxies with shocks and AGN dominating the gas excitation on the right branch. In the left panel, the blue data points show source-integrated measurements from our SINS galaxies (and in green and orange, star-forming galaxies taken from selected published studies for comparison). The SINS galaxies tend to populate the region between the star-forming and AGN branches, suggesting that various excitation mechanisms contribute in different proportions to the global line emission or, possibly, different physical conditions are prevailing in non-AGN actively star-forming galaxies. Detailed case studies are needed to assess those in individual galaxies, as illustrated in the middle and right panels with D3a-15504, a large star-forming disk that hosts an AGN, and zC-782941, another large disk with a small companion galaxy to the northeast. Maps of the Hα flux, [OIII]/Hβ, and [NII]/Hα (pixels with S/N < 5 are masked out) are shown at the bottom, and the distribution of ratios in individual pixels (color-coded as a function of spatial location as indicated in the top right insets) are plotted in the diagrams. For D3a-15504, the central AGN-dominated and outer star-forming disk regions separate clearly and the ratios suggest gas-phase oxygen abundances of ~ 1/3 to 1/2 solar in the outer disk. For zC-782941, both integrated and spatially-resolved line ratios are consistent with pure photoionization in HII regions, with somewhat higher abundances. Interestingly, the [NII]/Hα peaks between the main part of the galaxy and the northeast companion, possibly reflecting a different ionization parameter and/or gas fraction.

For 15 galaxies from our SINS Hα sample, we observed the [OIII]λλ4959,5007 and Hβ line emission with SINFONI, complementing our Hα and [NII]λλ6548,6584 data obtained previously. Using in particular the [OIII]λ5007/Hβ and [NII]λ6584/Hα line flux ratios in the so-called "BPT diagram" (Baldwin et al. 1981; see figure above), we investigate the excitation mechanism of the nebular gas (photoionization by hot young stars in HII regions, shocks related to galactic outflows, and/or AGN) and the gas-phase oxygen abundances. Measurements of these ratios at z ~ 2, relying on four lines redshifted in the near-IR windows with many bright telluric emission lines throughout most of this wavelength regime, are very challenging and still scarce, and have been mostly obtained from integrated spectra. Results to date show that the integrated line ratios of high-redshift galaxies tend to be offset from the locus of the low-redshift galaxy population in the "BPT diagram." This can be attributed to different physical conditions in distant star-forming galaxies, or to contributions from AGN and/or shocks. The global ratios of our SINS galaxies show such offsets in many cases. With the full spatial mapping afforded by SINFONI, we can take the next step and investigate the origin of the offsets by using spatially resolved ratio maps in individual galaxies. Examples are shown in the figure above, illustrating the power of this approach.

These results appeared in Newman et al. 2014 ApJ, 781, 21.


SINS: Largest Survey of Kinematics and Star Formation at z ~ 2

This figure shows velocity fields for 30 of the 62 galaxies of the SINS Hα sample, derived from the observed shift in wavelength of the Hα emission line across the galaxies. Blue to red colors correspond to regions of the galaxies that are approaching us and receding from us relative to the systemic or bulk velocity of each galaxy as a whole. The minimum and maximum relative velocities are labeled for each galaxy (in km/s). All sources are shown on the same angular scale; the white bars correspond to 1 arcsec, or about 8 kiloparsecs at z = 2. The galaxies are approximately sorted from left to right according to whether their kinematics are rotation-dominated or dispersion-dominated, and from top to bottom according to whether they are disk-like or merger-like as quantified by our kinemetry (Shapiro et al. 2008). Galaxies observed with the aid of adaptive optics, resolving details in the galaxies on scales as small as ~ 1 to 2 kiloparsecs, are indicated by the yellow rounded rectangles.

Upon completion of our SINFONI Guaranteed Time Observations at the ESO Very Large Telescope, we collected spatially resolved data of the ionized gas kinematics and star formation properties as traced by the Hα line emission of over 60 massive star-forming galaxies at z ~ 1.5 to 2.5. This makes SINS the largest survey of its kind to date based on near-infrared integral field spectroscopy. Our SINS Hα sample probes the z ~ 2 star-forming galaxy population over two orders of magnitude in stellar mass and star formation rates, with ranges of ~ 3×109 to 3×1011 M and ~ 10 to 800 M/yr. The ionized gas distribution and kinematics are resolved on spatial scales ranging from ~ 1.5 kiloparsecs for adaptive optics (AO) assisted observations to ~ 4 to 5 kiloparsecs for seeing-limited data. The Hα morphologies tend to be irregular and/or clumpy. About one-third of the SINS Hα sample galaxies are rotation-dominated yet turbulent disks, another third comprises compact and velocity dispersion-dominated objects, and the remaining galaxies are clear interacting/merging systems; the fraction of rotation-dominated systems increases among the more massive part of the sample. The Hα luminosities and equivalent widths suggest on average roughly twice higher dust attenuation toward the HII regions relative to the bulk of the stars, and comparable current and past-averaged star formation rates. Adopting the relation between star formation rate and gas mass surface density we presented in Bouché et al. 2007 (see the comparison of star formation properties, of different galaxy classes below), the Hα-derived star formation rates imply high fractions of gas to dynamical masses Mgas/Mdyn ~ 30% (or Mgas/[M+Mgas] ~ 45%). Combining the stellar, gas, and dynamical mass estimates, we find also high baryonic mass fractions (M+Mgas) /Mdyn ~ 60 to 80% within the central ~ 10 kiloparsecs of our SINS galaxies.

These results appeared in Förster Schreiber et al. 2009 ApJ, 706, 1364.

Stacking SINS: Broad Emission Lines revealed in High-z Star-forming Galaxies

The left panel shows the spatially integrated average spectrum of 47 galaxies observed in our SINS program; the equivalent integration time of such a spectrum is 195 hours with VLT/SINFONI. High S/N detections are obtained on five important rest-frame optical diagnostic emission lines. Fitting the Hα-[NII] region (zoomed view in right panel, with horizontal axis in velocity units) reveals excess signal above the sum of three narrow lines (green, individual components are in blue). An additional broad velocity component is required to fit the spectrum (red, individual components are in blue).

Using a high S/N spectrum created by combining data from 47 SINS galaxies, we detect a broad emission component underneath the narrow Hα and [NII] lines. This feature is found in galaxies with and without a known active nucleus. It exists preferentially in the more massive and more rapidly star-forming galaxies, which tend to be older and larger. The two possible explanations for such a feature are starburst-driven galactic winds and active supermassive black holes. If galactic winds are responsible for the broad emission, the luminosity and velocity of the emission line imply gas outflow rates comparable to the star formation rate (= 72 M/yr for those 47 SINS galaxies). On the other hand, if the central disk of accreting gas associated with active black holes is powering the broad feature, we can use the dynamics of this gas (and therefore of the broad emission line) to probe the mass of the associated black hole. In this scenario, we find a black hole that is a factor of ten less massive than in local galaxy bulges of similar mass, implying that bulges are assembled first and observed already at ~ 2 (see the SINS "From rings to bulges" result below), with the black hole being somewhat delayed in its formation.

These results appeared in Shapiro et al. 2009 ApJ, 701, 95.

First Determination of the Stellar Mass Tully-Fisher Relation at z ~ 2

The left panel shows the spatially integrated average spectrum of 47 galaxies observed in our SINS program; the equivalent integration time of such a spectrum is 195 hours with VLT/SINFONI. High S/N detections are obtained on five important rest-frame optical diagnostic emission lines. Fitting the Hα-[NII] region (zoomed view in right panel, with horizontal axis in velocity units) reveals excess signal above the sum of three narrow lines (green, individual components are in blue). An additional broad velocity component is required to fit the spectrum (red, individual components are in blue).

We have modeled the dynamics of 18 star-forming galaxies at ~ 2 using the Hα line emission as observed with SINFONI. The galaxies were selected from the larger SINS "Hα sample," based on the prominence of ordered rotational motions with respect to more complex merger-induced kinematics. The quality of the data allowed us to carefully select systems with kinematics dominated by rotation, and to model the gas dynamics across the entire galaxies, using suitable exponential disk models. We obtained a good correlation between the dynamical mass Mdyn and the stellar mass M*, finding that large gas mass fractions (Mgas M) are required to explain the difference between the two quantities. We used the derived maximum rotational velocity vmax from the modeling together with the stellar mass to construct the stellar mass Tully-Fisher relation at ~ 2 for the first time. The tight Tully-Fisher relation connects the luminosity (or stellar mass) and maximum rotational velocity of disk galaxies, and was discovered for spirals in the nearby universe by Tully & Fisher (1977). It is a key property for understanding the structure and evolution of these galaxies, as it directly links the luminosity (or mass) of the stars in disk galaxies to the angular momentum of the dark matter halos in which they reside. The relation obtained at high redshift shows a slope similar to what is observed at lower redshift, but we detected an evolution of the zero point, with galaxies at z ~ 2 rotating faster than those in the local universe at a given stellar mass. This result is consistent with the predictions of some of the latest N-body/hydrodynamical simulations of disk formation and evolution, which invoke gas accretion onto the forming disk via "cold flows" associated with filaments in the dark matter cosmic web. This scenario is in agreement with other dynamical evidence obtained as part of our SINS survey, where relatively smooth but rapid gas accretion from the parent dark matter halo of galaxies is required to reproduce the observed properties of a significant fraction of the ~ 2 massive star-forming galaxies.

These results appeared in Cresci et al. 2009 ApJ, 697, 115.


Millennium Simulation and Observations: The Role of Secular Evolution at High Redshift

This plot shows the distribution (shaded contours) of z ~ 2 halos in the dark matter accretion rate (left y-axis) vs. halo mass plane. Major merger fractions are displayed (red contours), increasing as the specific dark matter accretion rate grows. Associated star formation rates (right y-axis) assume an effective star formation efficiency of 1 (between baryonic matter, taken to follow the assembly of dark matter, and star formation rate). SINS galaxies are indicated by blue symbols, grouped based on our disk/merger classification applying kinemetry. The star formation rates for submillimeter galaxies (SMGs) are shown as horizontal lines in the upper part of the plot (their halo masses cannot be observationally well-constrained). When a high effective star formation efficiency is assumed, the host halos of SINS galaxies lie in the region where most halos of their mass are expected to be concentrated. Furthermore, the expected mass accretion rates are sufficient to account for the observed star formation rates, and the predicted major merger fraction is small ( 0.5), consistent with observations. The halo mass of SMGs is inferred from this analysis to be typically 1012.5 M.

We have used the Millennium Simulation to show that in a Lambda-CDM universe, even dark matter halos not undergoing major mergers have mass accretion rates that are plausibly sufficient to account for the high star formation rates observed in z ~ 2 disk galaxies as studied in our SINS survey. However, the fraction of major mergers in the Millennium Simulation is sufficient to account for the number counts of submillimeter galaxies (SMGs), in support of observational evidence that these are frequently major mergers (e.g., from their dynamical properties). When following the fate of these two populations in the Millennium Simulation to z = 0, we find that subsequent mergers are not frequent enough to convert all ~ 2 turbulent disks into elliptical galaxies at z = 0. Similarly, mergers cannot transform the compact SMGs/red sequence galaxies at z ~ 2 into present-day massive cluster ellipticals. We argue therefore that secular and internal dynamical processes must play an important role in the evolution of a significant fraction of z ~ 2 rest-UV/optical- and submillimeter-selected galaxy populations.

These results appeared in Genel et al. 2008 ApJ, 688, 789.

From Rings to Bulges: Evidence for Rapid Secular Evolution at z ~ 2

These images (left panels) show the central mass concentration as a function of evolutionary stage for five z ~ 2 non-AGN disk galaxies from SINS (right panel). The central mass concentration is taken as the ratio of the dynamical masses within a radius of 3and 10 kiloparsecs, derived from detailed modeling of the gas kinematics traced by the Hα line emission. The [NII]/Hα emission line ratio is a measure of the gas-phase chemical abundance, related to the evolutionary stage of galaxies. The properties of these disks suggest evolution driven by efficient secular processes in a globally unstable fragmenting disk, where massive clumps rapidly migrate inward due to dynamical friction, and coalesce to form a young bulge. The trend between central mass concentration and [NII]/Hα is consistent with such a scenario. The maps at the top left show the Hα line flux and velocity field from our adaptive-optics-assisted SINFONI data and the rest-frame 5,000 Å continuum emission from HST/NICMOS H-band imaging of BX482 (all at a resolution of 0.18 arcseconds or 1.5 kiloparsecs), which appears to be in early stages of this "clump-driven" dynamical evolution sequence. Despite irregular morphology, the Hα kinematics show compelling signatures of disk rotation on large scales, along with small-scale perturbations likely induced by the massive clumps. The maps at the bottom left show again the Hα line flux and velocity field as well as the K-band continuum (rest-frame 6,600 Å) from SINFONI taken under excellent seeing conditions (0.5arcseconds or 4 kiloparsecs) of D3a-6004, which lies at the more evolved end of the sequence, with morphological and dynamical evidence for a nascent central bulge component.

In a detailed study of several of our best-resolved SINS galaxies, we found evidence for rapid secular/internal dynamical evolution taking place in massive early disks at z ~ 2 based on the morphologies and kinematics of the Hα line emission. Our Laser Guide Star AO and good-seeing data show the presence of turbulent rotating star-forming outer rings/disks, plus central bulge/inner disk components, whose mass fractions relative to the total dynamical mass appear to scale with the [NII]/Hα line flux ratio (an indicator of the nebular gas chemical abundances) and the star formation age, both related to the global stellar evolutionary stage of galaxies. We propose that the buildup of the central disks and bulges of massive galaxies at z ~ 2 can be driven by the early secular evolution of gas-rich disks in formation. High-redshift disks exhibit large random motions. This turbulence may in part be stirred up by the release of gravitational energy in the rapid "cold" accretion flows along the filaments of the dark matter cosmic web. As a result, dynamical friction and viscous processes occur on a timescale of less than one billion years, at least an order of magnitude faster than in present-day disk galaxies. Early secular evolution thus drives gas and stars into the central regions and can build up exponential disks and massive bulges, even without violent and dissipative major mergers. Secular evolution along with increased efficiency of star formation at high surface densities may also help to account for the short timescales of the stellar buildup observed in massive galaxies at z ~ 2.

These results appeared in Genzel et al. 2008, ApJ, 687, 59.

Kinemetry at High-z: Confirmation of a Majority of Rotating Disks among SINS Galaxies

This diagnostic diagram is used to distinguish between regularly rotating "disk-like" galaxies and systems undergoing major mergers (right). Our templates for each group, shown in blue and red respectively, have been analyzed as if they were observed at redshift z ~ 2 with the VLT/SINFONI setup used for the SINS survey observations. The total kinematic asymmetry is defined as the sum in quadrature of the measured asymmetry in the velocity and velocity dispersion fields, and the probability distribution functions of this parameter for nonmerging and merging systems (inset) is used to identify the boundary between unperturbed and merging systems (black line). Performing this analysis on SINS galaxies (black points), we find that the majority (8/11) of our best-resolved systems are disk-like. A visual analysis of the velocity fields of SINS galaxies (shown at the left) confirms the efficacy of our classification. The centers of stellar continuum emission in each system is indicated with a black cross, and the maxima and minima of the Hα velocities are shown (in km/s).

We have developed a set of quantitative kinematic criteria, based on templates from observations of nearby galaxies and from simulations, which enable us to differentiate between systems with and without recent major mergers in the SINS sample. Applying these criteria to our highest-quality data, we find that ~ 3/4 of the resolved systems (with a half-light radius larger than 4 kiloparsecs) display no dynamical evidence of having had a recent major merger. This quantitatively confirms earlier results from our survey, which provided qualitative evidence that there is a significant population of rapidly-star-forming systems (with star formation rates ~ 100 M/yr) in regularly rotating, unperturbed configurations. Our detailed analysis of the kinematics showed that indeed the high star formation rates in these z ~ 2 systems are not driven by major mergers. Instead, these young (typical stellar ages of ~ 500 million years) but rapidly evolving galaxies must have formed via smoother accretion processes, such as gas inflow along cold filamentary streams, or rapid series of minor mergers.

These results appeared in Shapiro et al. 2008 ApJ, 682, 231.


First Comparison of the Dynamical and Star Formation Properties of Different Galaxy Classes at z ~ 1.4–3.4

The plots illustrate dynamical and star formation properties of galaxy samples at z ~ 1.4–3.4: rest-frame UV and optically selected star-forming galaxies from the SINS survey (blue and red points, respectively), and bright submillimeter-selected galaxies (SMGs) observed as part of a program to map the CO molecular gas line emission with millimeter interferometry (black points). The left panel shows the location of these samples in the size vs. rotation velocity diagram, along with nearby spiral galaxies revealing the local velocity-size relation (gray crosses, from Courteau 1997). The right panel shows the surface density of star formation rate and gas mass for the high-redshift galaxies compared to that for nearby galaxies, including normal as well as active starburst and (ultra)luminous infrared galaxies (gray plus symbols and crosses, from Kennicutt 1998). For SMGs, the gas masses are inferred directly from the CO line fluxes (adopting the so-called "ULIRGs CO-H2 conversion factor"). For the SINS galaxies, they are derived from the dynamical masses from Hα kinematics assuming the average gas mass fraction of 40% found for the SMGs.

Combining the results from our SINS survey with SINFONI at the VLT with those from a study carried out with the IRAM/Plateau de Bure millimeter interferometer, we made the first comparison of the dynamical and star formation properties of different classes of galaxies at redshift z ~ 1.4–3.4. Both surveys provide spatially-resolved information on the dynamics and distribution of gas closely related to star formation activity. The sample from SINS included 16 rest-frame UV and 16 rest-frame optically selected objects, probing the bulk of actively star-forming galaxies at the high-mass end. The Hα emission line originating from HII regions was the tracer of gas kinematics and star formation. The millimeter interferometric observations targeted the CO line emission tracing molecular material from which stars form. This sample consisted of 13 bright submillimeter-selected galaxies (SMGs) at similar redshifts, which represent the most luminous and most intensely star-forming systems in the early universe. These data were taken as part of a long-term IRAM program involving several members of our team. The SMG sample is highly complementary to the SINS sample, as it probes a more extreme regime of star formation in systems that are also often so severely obscured by very large amounts of dust that they are difficult to observe at shorter wavelengths.

The main results from this first comparison are the following:
(i) We find that rest-frame UV and optically bright (K < 20 mag) z ~ 2 star-forming galaxies are dynamically similar, and follow the same velocity-size relation as spiral galaxies in the nearby universe (left panel of the figure above). In contrast, the bright SMGs (S850μm > mJy) have significantly larger velocity widths and are much more compact, implying higher central matter densities by nearly an order of magnitude and lower angular momenta than for the SINS galaxies. Together with the spatially resolved CO line mapping obtained for several of them showing strongly perturbed kinematics on scales of ~ 1–2 kiloparsecs, these results suggest that dissipative gas-rich major mergers are more frequent among the bright SMG population compared to more "normal" star-forming galaxies at high redshift.
(ii) Due to their small sizes and high densities, SMGs lie at the high end of a "Schmidt-Kennicutt" relation between matter or gas surface density and star formation rate surface density. The best-fit relation implies that the star formation rate per unit area scales as the surface gas density to a power ~ 1.7, suggesting that a "universal" Schmidt-Kennicutt law holds out to z ~ 2.5) (see rightmost panels of the figure above).

These results appeared in Bouché al. 2007, ApJ, 671, 303 (see also Tacconi et al. 2006, ApJ, 640, 228; Tacconi et al. 2008, ApJ, 680, 246).


Detailed Anatomy of a Young Massive Star-forming Disk at z = 2.38

This figure shows the hydrogen recombination line emission of Hα of the massive star-forming galaxy BzK-15504 11 billion light-years away (redshift z = 2.38). The observations were carried out with SINFONI in adaptive-optics mode, resulting in an angular resolution of ~ 0.15 arcseconds, or a mere 1.2 kiloparsecs (4,000 light-years; indicated by the gray filled circle) at the redshift of BzK-15504. The top left panel is a color-composite map of the integrated Hα line emission, showing from blue to red the ionized gas that is blueshifted to redshifted relative to the systemic velocity of the galaxy. The other panels are channel maps displaying the spatial distribution of the Hα emitting gas moving at different velocities (given in km/s) relative to the systemic velocity.

Our finely resolved SINFONI data of BzK-15504 reveal a large galaxy about 16 kiloparsecs (53,000 light-years) across, with several prominent bright knots corresponding to luminous sites of active star formation. It appears to be a disk rotating with a maximum speed of 230 km/s, implying a large dynamical mass of ~ 1011 M. The details of the kinematics further suggest that gas is being channeled via radial flows (outlined by the dotted line) towards a growing central bulge, and indicate the presence of a broad and high velocity component (bottom right panel) likely due to an outflow from the active galactic nucleus (AGN) powered by a massive accreting black hole. The high surface density of gas (~ 350 M/pc2), the high rate of star formation (~ 150 M/yr), and the moderately young stellar ages (~ 500 million years) suggest rapid assembly, fragmentation, and conversion to stars of an initially gas-rich protodisk. Surprisingly, there are no obvious signs of a recent major merger event, which would have led to the rapid mass assembly and triggered the intense star formation activity. This may suggest that BzK-15504 assembled its mass via smoother infall such as in the "cold flow" accretion mechanism, or through a series of minor mergers. BzK-15504 could later evolve into a massive elliptical galaxy.

These results appeared in Genzel et al. 2006, Nature, 442, 786.

Dynamical Evidence for Large Massive Rotating Disks at ~ 2

These images show the spatial distribution (top row) and motions (bottom row) of the Hα line emitting gas in six large star-forming galaxies at cosmological redshifts z ~ 2. The observations were carried out with SINFONI in seeing-limited mode, under typical seeing conditions of ~ 0.5 to 0.6 arcseconds, corresponding to a spatial resolution of 4 to 5 kiloparsecs at the redshift of the sources. The maximum velocities, relative to the systemic velocity, are given in km/s for each source.

During the first year of the SINS survey, our SINFONI observations revealed many large star-forming galaxies with irregular and clumpy morphologies in Hα line emission but smooth and regular velocity fields. For the majority of the larger systems, the ionized gas kinematics exhibit monotonic variations across the galaxy with the steepest gradient along the geometric/kinematic major axis (e.g., the four left-most galaxies in the figure above) and in three of them, the velocity profile flattens at large radii. These features are expected signatures of ordered rotation in a disk-like structure, and provided key dynamical evidence for the existence of large massive rotating disks at ~ 2. The case of BzK-15504, observed with adaptive optics but otherwise similar in its overall properties, offers an unparalleled view into one such system, with three to four times finer detail. Interestingly, Q1623-BX663 is probably more consistent with an advanced merger or disturbed spiral hosting an AGN responsible for the high velocity dispersion measured at the location of the dominant off-center Hα peak. For Q1623-BX528, the reversal in velocities along the major axis could be indicative of a counterrotating merger.
The discovery of so many massive rotating disks among our SINS sample was surprising. In view of the higher rate of major mergers at high redshift, we had expected most of the larger systems to exhibit more complex gas motions. From a more detailed analysis of the best resolved cases, it appears that their disks are quite turbulent, probably fairly gas-rich, and likely unstable to global star formation and fragmentation. As some simulations of the evolution of gas-rich galactic disks suggest, the star-forming clumps could later sink toward the gravitational center by dynamical friction to form a central bulge on a timescale of order 1 billion years. This could provide a mechanism whereby some of the young disks uncovered in the SINS survey evolve into elliptical galaxies or disk galaxies with a dominant massive bulge, as those observed in the present-day universe.

These results appeared in Förster Schreiber et al. 2006, ApJ, 645, 1062.

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