The PHIBSS2 pages for highlights from our IRAM programs on the molecular gas in distant galaxies
Evidence for Cored Dark Matter Distributions in Galaxies at z~1–2
Kinematics are a powerful tool to understand galaxies’ structures and mass composition, as they directly trace a galaxy’s entire mass. This is one of the only ways to probe components that do not emit light, such as the dark matter halo. To decompose a galaxy’s total dynamics into the contributions from the bulge, disk, and halo, it is necessary to have deep observations so the kinematics can be traced to large galactic radii.
However, distant galaxies at the epoch of peak cosmic star formation (“cosmic noon”, z ~ 1–3) are faint, so obtaining such deep data requires large amounts of observing time even on the largest, most sensitive telescopes currently in operation. Initial studies focusing on a handful of individual galaxies (Genzel et al. 2017, Übler et al. 2018) and on stacked galaxy profiles (Lang et al. 2017) provided useful first constraints, but analyses of larger samples of individual galaxy kinematics are necessary to examine widespread trends.
We have compiled a sample of 100 star-forming galaxies at high redshift (z ~ 1–2) with deep, spatially resolved observations from multiple sources, including the KMOS3D, SINS/zC-SINF, and NOEMA3D surveys as well as additional observations with LBT/LUCI. Using these deep data, we are able to perform such mass modeling to detangle the galaxy and dark matter halo profiles (Genzel et al. 2020, Price et al. 2021).
One of the key findings is that galaxies at cosmic noon (z ~ 1–2) are baryon-dominated on galaxy scales, with higher-mass (or higher-mass surface density) galaxies having lower dark matter fractions. Some of these galaxies have such low fDM(Re) that the profiles of their dark matter halos are likely to be cored, in contrast to predictions for more “peaked” halo profiles (i.e., the NFW profile).
Compared to today’s galaxies (z = 0), the RC100 star-forming disk galaxies at z ~ 1 have similar fDM(Re) – Mbaryon values to the star-forming, disky galaxies, while the RC100 galaxies at z ~ 2 are more like today’s quiescent, elliptical galaxies. These z ~ 2 RC100 galaxies probably represent the progenitor population of today’s elliptical galaxies, with the transformation to quiescence possibly happening not long after the observed epoch, given the overlap with quiescent galaxies seen at z = 1.7 (grey region).
With the ongoing NOEMA3D survey, work will continue to expand this kinematic modeling to more galaxies. Furthermore, using existing data and upcoming observations with VLT/ERIS, we will begin to explore signatures of mass transport and outflows, to better understand bulge growth and other physical processes regulating galaxy growth.
Outflow Demographics and Physical Properties at z ~ 1–3
Exploiting our full KMOS3D and SINS/zC-SINF surveys of near-IR IFU spectroscopy of z ~ 1–3 galaxies, we obtained the most complete census to date of galactic-scale ionized gas outflows at the peak epoch of cosmic star formation and AGN activity. The sample of ~ 600 primarily mass-selected galaxies spans wide ranges in stellar mass and star formation rate; the selection by mass, rather than by properties biased toward star formation (SF) or AGN activity, makes it ideally suited for a population-averaged characterization of winds as relevant to galaxy evolution. Compared to slit spectra, the IFU data greatly facilitate the separation between the broad outflow component in Hα+[NII]+[SII] and the narrower component from SF. Our studies show how outflows driven by SF and AGN are spatially, spectrally, and demographically distinct. SF-driven winds, launched near bright star-forming clumps across disks, have typical speeds of ~ 450 km/s below the hosts’ escape velocity except at log(M⁎/M☉) < 10.3; the prevalence of these winds depends on SF properties, not mass. AGN-driven winds originate from the nuclear regions, are ubiquitous in log(M⁎/M☉) > 10.7 galaxies hosting a massive bulge but rare at lower masses, irrespective of SF activity; with velocities of ~ 1,500 km/s, they can escape the galaxies. For the first time, the high S/N spectra constrain the density in high-z SF-driven winds from the broad [SII] doublet ratio, yielding ne ~ 400 cm-3; for AGN-driven winds, a higher ne ~ 1,000 cm-3 is inferred. These densities are a factor of several higher than previously assumed values, and lead to correspondingly more modest mass outflow rates ~ 0.1–0.4 x SFRs in warm ionized gas. The tension with theoretical work, requiring mass outflow rates ≳ SFRs to reproduce the observed relationships between galaxy mass and metallicity as well as galaxy mass and halo mass at log(M⁎/M☉) < 10.7, could be alleviated if substantial mass, momentum, and energy were ejected in hotter and/or colder phases than the ~ 104 K ionized gas probed by our data. The fast, high-duty-cycle AGN-driven winds at high masses carry significant energy (~ 1% that of the AGN), which may contribute to heat halo gas and help prevent further gas infall. Our results are consistent with recent EAGLE and Illustris/TNG numerical simulations, which suggest that such a mechanism, acting also at the modest luminosities and Eddington ratios of the majority of the KMOS3D and SINS/zC-SINF AGN, may be more effective at widespread and long-term quenching than ejective "QSO mode" feedback in rare, high-luminosity, high-Eddington-ratio AGN.
The Angular Momentum Distribution of z ~ 1−3 Star-Forming Galaxies
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 z ~ 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 1012M☉, which corresponds to 31% of the cosmologically available baryons, implying that high-redshift disks are strongly baryon-dominated.
The Mass Budget of Early Star-Forming Galaxies from KMOS3D
We exploited our deep integral-field spectroscopic observations from KMOS3D to dynamically constrain the mass budget of 240 star-forming disks at 0.6 < z < 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 z > 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.
Consistent Evolution of Metallicity and Metallicity Gradients from z ~ 2.7 to z ~ 0.6
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
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 σ0 ∝ (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 z ~ 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
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.
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 1010M⊙, 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.
Evidence for Gravitational Quenching from SINS/zC-SINF
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.
The Nature of Dispersion-Dominated Galaxies at High Redshift
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.
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.
Smoother Stellar Mass Maps and the Longevity of Star-Forming Clumps in High-Redshift Galaxies
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.
Short-Lived Star-Forming Clumps in Cosmological Simulations of z ~ 2 disks: the Impact of Strong Feedback from Massive Stars
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 z ~ 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").
SINS/zC-SINF Reveals the Roots of Vigorous Star-Formation-Driven Gas Outflows at z ~ 2
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 z ∼ 2 may derive from high-mass star-forming galaxies. The threshold at z ~ 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 z ~ 2 disks.
Galaxy Structure in the Star Formation Rate – Mass Plane from z ~ 2.5 until Today
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 z ~ 0.1, 130,000 galaxies at z ~ 1, and 36,000 galaxies at z ~ 2. Size and profile measurements for all but the z ~ 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 z ~ 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.
Dynamics and Evolution of Giant Star-Forming Clumps in z ~ 2 disks
New and deep SINFONI+AO observations of five z ∼ 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.
Pilot HST Near-IR Study: Stellar Properties of Clumps in SINS z ~ 2 Disks
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 ~ 109 M☉. 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.