Recent Results of the MPE Infrared/Submillimeter Group

ALMA reveals the rapid evolutionary lifecycle of star-forming regions in galaxies

May 2019

Determining the physical processes controlling star formation in molecular clouds remains one of the main unsolved problems in astrophysics. New high-resolution observations of molecular clouds from ALMA and of the emission of young stars from the MPG/ESO 2.2 meter telescope for the nearby spiral galaxy NGC300 highlight the weak correlation of young stars and their parent clouds. This implies that the tight correlation between molecular gas and star formation rate observed for whole galaxies and down to kpc-scales – known as the Kennicutt-Schmidt or star formation relation – results from averaging over many star-forming relations. With a novel statistical model we have shown that the observed decorrelation of young stars and molecular clouds holds valuable information on the time evolution of star-forming regions. For NGC300, we find that molecular clouds are short-lived (10 Myr or 1 dynamical timescale) and that a short period of active star formation (1.5 Myr) during which 2-3% of the cloud's mass is turned into stars releases sufficient stellar radiation and winds to disperse the parent molecular cloud. Star formation in molecular clouds proceeds very rapidly, yet highly inefficiently. This also explains the slow consumption of the galactic molecular gas reservoir by star formation of 1 Gyr in nearby galaxies because molecular gas has to evolve through many such cycles before it is fully turned into stars. We are now applying this analysis to a large number of galaxies in the nearby universe studied by the PHANGS collaboration and to first high-resolution observations of galaxies at high redshift during the peak of cosmic star formation.

The image on the left shows that the positions of molecular clouds (blue) and emission from young stars (pink) do not coincide on small spatial scales. The two branches on the right quantify this displacement by showing that molecular clouds and young stars are only correlated when averaging over many such objects within a large part of the galaxy (1 kpc), while for individual star-forming regions (100 pc) the balance between molecular gas and young stars varies strongly with time.

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A spatially resolved quasar broad line region

November 2018

Knowing the structure, size, and dynamics of the broad line regions (BLRs) near accreting supermassive black holes would allow to constrain the inward and outward transport mechanisms and to infer the mass of the black hole. So far, however, directly measuring BLR properties was impossible because of its small physical size, on the order of hundreds of light-days (corresponding to an angular size <1mas). With GRAVITY we have now, for the first time, spatially resolved (10 micro-arcseconds or ~0.03 parsec for a distance of 550 mega-parsecs) a velocity gradient across the broad line region of the quasar 3C 273. The gradient reveals rotation perpendicular to the jet, and is consistent with line emission from a thick disc of gravitationally bound material around a black hole of 3 x 108 M. We infer a disc radius of 150 light-days (compared to 100 to 400 light-days found previously with reverberation mapping – RM). Thus, GRAVITY provides both a confirmation of RM (at least for this one object) as the main previous method to determine black hole masses in quasars and a new and highly accurate, independent method to measure such masses. In an approved VLTI Large Program, we will extend this study to a (small) sample of local AGN, and we are exploring options to expand it to larger samples and higher redshifts with potential upgrades of GRAVITY.

Our first BLR detection in 3C 273 (a), revealing a velocity gradient perpendicular to the jet direction (b, photo-center on sky vs. wavelength): a clear signature of rotation. The data are well described by kinematic models (c) of Keplerian rotation in a thick disk configuration viewed at low inclination (d, best-fitting model image on sky) and allow robust measurements of the BLR radius and black hole mass.

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Measurement of gravitational redshift in the Galactic Center

July 2018

With a distance of 26,000 light-years from earth, Sagittarius A* (SgrA*), the massive black hole in the center of our galaxy, is the closest of its kind, and with an apparent Schwarzschild radius of 53µm the largest in the sky. It is surrounded by a cluster of high-velocity stars called the S-stars, whose trajectories are governed by the gravitational field of the black hole. We used the Very Large Telescope (VLT) instruments GRAVITY and SINFONI to follow the short (16y) orbit star S2/S-02 during its recent pericenter passage in May 2018, collecting astrometric and spectroscopic data, respectively. These joint data allowed for a robust detection of the combined gravitational redshift and transverse Doppler effect on S2/S-02 for the first time.

Gravitational redshift is one of the three classical tests of general relativity. Einstein's theory predicts that due to gravitational time dilation, a light beam gets stretched to longer wavelengths by a gravitational field. The change in the wavelength of light from S2/S-02 is inconsistent with Newtonian predictions and in excellent agreement with Einstein’s theory of general relativity. On a technological level, the success of this measurement with GRAVITY/VLT opens the door to an entirely new type of laboratory to probe and test the General Theory of Relativity: the Galactic Center.

Astrometric and spectroscopic measurements. Left: Position of the star (blue dots) on its trajectory around the black hole (empty circle). The depicted motion is counter clockwise from early 2017 to late 2018. Right: Velocity difference between the Newtonian and relativistic models (red curve) and residuals (circles).

GRAVITY Collaboration, 2018A&A...615L..15G

Outflow demographics and physical properties at z ~ 1 – 3

July 2018

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 facilitates the separation between the broad outflow component in Ha+[NII]+[SII] and the narrower component from star formation. Our studies show how outflows driven by SF and by 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(MÛ/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(MÛ/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. The high S/N spectra constrain for the first time 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´SFRs in warm ionized gas. The tension with theoretical work, requiring mass outflow rates SFRs to reproduce the observed galaxy mass – metallicity and galaxy mass – halo mass relationships at log(MÛ/M)<10.7, could be alleviated if substantial mass, momentum, and energy are 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.

Spatial distribution, spectral properties, and demographics of SF- and AGN-driven galactic winds (top and bottom rows).  From left to right: maps of two example galaxies from SINFONI+AO and HST (FWHM physical resolution of ~1.8 kpc), composite spectra from our KMOS3D and SINS/zC-SINF samples, and the fraction of galaxies exhibiting the broad Ha+[NII]+[SII] outflow signature as a function of galaxy stellar mass and offset in SFR relative to the main sequence relationship.

Research Paper:

Scaling relations for molecular gas in Galaxies over cosmic time

February 2018

We have developed new statistically robust scaling relations between galaxy integrated molecular gas masses, stellar masses and star formation rates (SFR) relative to that on the main sequence (δMS). Combining data from our PHIBSS and xCOLDGASS surveys with data from the literature, we  use three independent methods to determine molecular gas masses:  1) CO line fluxes, 2) Herschel far-infrared dust SEDs, and 3) ~1mm dust photometry, in a large sample of 1444 star forming galaxies and stacks between z=0 and 4.  The sample spans stellar masses from log(M*/M8)=9.0-11.8, and star formation rates relative to that on the MS, from 10-1.3 to 102.2. The most important result is that all data sets follow the same scaling trends, once we consistently account for uncertainties and apply consistent methodologies. The molecular gas depletion time tdepl, defined molecular gas mass/star formation rate, scales as (1+z)-0.6 ´ (δMS)-0.44, and is only weakly dependent on stellar mass. The ratio of molecular-to-stellar mass μgas depends on (1+z)2.5´ (δMS)0.52´ (M*)-0.36, and tracks the evolution of the specific star formation rate.

Scaling of tdepl (left) and μgas=Mmolgas/M*  (right) with redshift(bottom) and SFR relative to that on the main sequence, δMS (top). The colors show the overall distribution of our data, and the large symbols are binned averages for the different gas mass determination  methods (CO:  black circles, dust-SEDs: unfilled symbols, submm-continuum: brown filled triangles).  The dashed lines are the best  global, multi-parameter fits to the data.

Research papers:

Strongly baryon-dominated disk galaxies at the peak epoch of galaxy formation.

March 2017

The sample of six galaxies with deep SINFONI data probing their outer disk kinematics. In each pair of panels, the color images show the Halpha emission line map (left) and velocity field (right), with white contours corresponding to the rest-optical stellar continuum light distribution. The white line on the velocity fields marks the kinematic major axis along which the rotation curves are extracted.

New observations of rotating galaxies at z~2, the peak epoch of galaxy formation, show that these massive star-forming galaxies are strongly dominated by baryonic mass, with dark matter playing a much smaller role in comparable regions of their outer disks than in typical present-day spiral galaxies. This result was obtained from observations of unprecedented sensitivity with the SINFONI and KMOS near-IR integral field spectrometers, mapping the 2D kinematics of ionized gas through the Halpha line emission of six galaxies out to 2-3 times their half-light radius. On these scales, dark matter starts to dominate the total mass budget in z~0 late-type galaxies, causing their rotation curves to stay mostly flat or to slightly increase with radius.  In contrast, the six individual massive z~2 galaxies in our study exhibit declining rotation curves, which can be explained by the combination of two factors: baryons dominate more strongly than dark matter on galactic scales, and the elevated gas turbulence characteristic of z~2 disks provides a significant amount of the dynamical support -- thereby leading to a decrease in the rotation velocity.   Such a falloff with radius is further seen in the average outer rotation curve derived through a novel stacking technique of ~100 typical massive z~1-2.5 disk galaxies observed with KMOS and SINFONI, suggesting that it is a common feature among the star-forming population at these epochs. Two additional studies of the resolved inner disk kinematics (out to 1-1.5 half-light radius or the velocity turnover radius) of 240 star-forming disks support the results based on the outer rotation curves.  Detailed dynamical modeling shows that while the stars+gas account on average for ~56% of the total enclosed mass at z~1-2.5, the baryon fraction reaches ~90% at the higher redshifts, and is largest in disks with highest central baryonic mass surface density.  The evolution of the zero-point stellar and baryonic Tully-Fisher relationships based on the same data is further compatible with the increase in gas and baryon mass fractions with redshift, with a lesser role of dark matter in the central disk regions. The low galactic-scale dark matter fractions found in massive high-redshift galaxies in these studies are comparable to those of present-day massive early-type galaxies, their likely descendants.  The high gas fractions in z~2 star-forming galaxies could help to dissipate angular momentum, efficiently driving gas inwards and leading to the strong dominance of baryons early on.

(Left) The rotation curves of the six individual galaxies, and of the stack of ~100 other star-forming disks are plotted in radial and velocity units normalized to the values at which the maximum velocity is reached. The outer rotation curve falloff is seen in both individual sources and appears to be common among z~1-2.5 star-forming galaxies based on the stack. (Right) The individual and stacked rotation curves are combined together (grey circles and shaded area corresponding to the uncertainties) and compared to the average rotation curve of z=0 massive (log(M*/Msun)=11) star-forming galaxies and to those of the Milky Way and M31 taken from the literature, as well as to the predictions for an infinitely thin Freeman disk without dark matter, and for a thick disk with ratio of rotation velocity to velocity dispersion v_rot/sigma_0 = 5 characteristic of high-redshift disks. Both the effects of pressure support from the elevated gas turbulence and little dark matter on disk scales contribute to the observed falloff of the rotation curves of distant star-forming disks.

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KMOS3D survey of spatially-resolved gas kinematics, star formation, and ISM properties sheds new light on the physics of galaxy evolution

June 2016

Our comprehensive and highly successful multi-year KMOS3D survey takes advantage of the efficient multiplexing of the new near-IR 24-IFU KMOS instrument at the Very Large Telescope to spatially resolve the ionized gas kinematics, star formation, outflows, excitation, and metallicities of a large and homogeneous sample of z ~ 0.7 − 2.7 mass-selected galaxies. KMOS3D is a 75-night Garanteed Time Program begun in November 2013 and led jointly by the MPE IR/Submm group and MPE/OPINAS+USM. After 2.5 years and 52 nights of productive observing campaigns, the sample has now reached 564 galaxies, with an overall Hα detection fraction of 80% (and as high as 90% for main-sequence star-forming galaxies). The survey is carried out in well-studied extragalactic fields, benefitting from extensive multi-wavelength data that include the far-IR Herschel PEP survey and high-resolution near-IR/optical grism and imaging data from the 3D-HST/CANDELS HST Treasury programs. KMOS3D is designed to provide an unbiased census from deep integrations (~ 5h − 25h) of the Hα+[NII]+[SII] line emission resolved on seeing-limited scales of 4 − 5 kpc, over a wide range of galaxy parameters and 5 billion years of cosmic time. The strategy is uniquely enabling faint line emission mapping in individual objects and pushing near-IR IFU studies into new regimes such as lower mass star-forming galaxies, and high-mass sub-main sequence galaxies in the process of quenching. Our results, with highlights shown in the Figure, now 1) robustly confirm the earlier findings from our SINS/zC-SINF near-IR IFU survey with SINFONI on the majority of disks among high-z star-forming galaxies and their elevated gas turbulence compared to present-day spirals, and on the ubiquity and origin of powerful AGN- and star formation-driven gas outflows, 2) provide new constraints on the angular momenta, baryonic mass fractions, outer disk structure, mass-metallicity-star formation relation, and gas-phase O/H abundance gradients, and 3) shed new light on dense core formation and star formation quenching in high-mass galaxies.

[a] The KMOS3D survey spans a wide range of galaxy mass and star formation rate (SFR) at 0.7 < z < 2.7, overlapping with and substantially extending the ranges covered by our previous SINFONI “SINS/zC-SINF” survey. [b, c] With the first 2.5-year results for 564 galaxies (80% detection fraction) from KMOS3D, the prevalence of rotating disks among star-forming galaxies (≳ 70%) and the significant increase at higher redshift in gas velocity dispersion (∝ 1+z) are now firmly established. About 2/3 of the detected massive sub-main sequence galaxies have unexpectedly strong Hα emission given their UV and IR luminosities — possibly signaling rejuvenation — and are often extended rotating disks. [d] The distribution of inferred halo-scale angular momenta of the galaxies is consistent with the theoretical predictions for their dark matter halos in terms of mean spin parameter (〈λ〉 ~ 0.037) and dispersion (σ(log λ) ~ 0.2), suggesting conservation of the net angular momentum of baryons as they settle onto the disks. [e] The dynamical masses from KMOS kinematics, together with the stellar and gas mass distributions derived from HST imaging and our molecular gas mass – specific SFR scaling relations (see below), imply that high-z disks are on average strongly baryon-dominated, with baryon-to-total mass fractions of ~ 65% within their half-light radius and baryon-to-dark matter fractions of ~ 5% within their dark halo virial radius. [f] The inferred gas-phase metallicity gradients from 200 star-forming galaxies, tripling existing samples, are typically flat, in line with cosmological simulations involving strong feedback and the observed ubiquitous powerful outflows discovered in our KMOS3D and SINS/zC-SINF surveys (see below).

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ALMA reveals planetary construction sites

December 2015

Using the Atacama Large Millimeter/submillimeter Array (ALMA), the clearest indications yet have been found that planets with masses several times that of Jupiter have recently formed in the discs around young stars. New images of transitional discs show that there are significant amounts of gas within the dust gaps, and that the gas also possessed a gap, up to three times smaller than that of the dust. This can only be explained by the scenario in which newly formed massive planets have cleared the gas as they travelled around their orbits, but trapped the dust particles further out. The data rule out other scenarios for the observed set of discs.

Evidence for widespread AGN-driven outflows in the most massive z~1-2 star-forming galaxies

November 2014

Adaptive optics assisted near-infrared integral field spectroscopy shows broad emission line components (white contours) superposed on the central region of massive z~2 star forming galaxies. The broad components extend over 2-3kpc and most likely indicate AGN-driven outflows.

Incidence of AGN in massive z~1-2 star-forming galaxies. All indicators suggest a rise with stellar mass, but the incidence of AGN-driven outflows (blue and red filled circles) is larger than traditional X-ray/optical/IR evidence for AGN in the same galaxies (stars) or similar evidence in large samples in popular deep fields (shaded bands).

Very deep laser guide star assisted adaptive optics observations have allowed us to detect ubiquitous powerful nuclear outflows in massive (1011 MSun) z ∼ 2 star-forming galaxies, which are plausibly driven by an active galactic nucleus (AGN). The spectra in their central regions exhibit a broad component in Hα and forbidden [N II] and [S II] line emission, with typical velocity FWHM ∼ 1500 km s-1, high [NII]/Hα ratio ≈ 0.6, and intrinsic extent of 2–3 kpc. At larger radii, weaker and less wide broad components suggest star formation driven outflows. The high inferred nuclear mass outflow rates and frequent occurrence suggest that the nuclear outflows efficiently expel gas out of the centers of the galaxies with high duty cycles and may thus contribute to the process of star formation quenching in massive galaxies. Extending to a large sample observed without adaptive optics, we find that the incidence of the most massive galaxies with broad nuclear components is at least as large as that of AGNs identified by X-ray, optical, infrared, or radio indicators, as expected for rapidly varying AGN whose outflows are visible with larger duty cycle than X-ray or optical continuum. The mass loading of the nuclear outflows is near unity. Our findings provide compelling evidence for powerful, high-duty cycle, AGN-driven outflows near the Schechter mass, and acting across the peak of cosmic galaxy formation.

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Molecular gas, extinction, star formation and kinematics in the z=1.5 star forming galaxy EGS130111661

In the top row, the molecular gas is shown as the backdrop image in color and the contours display the stellar light (from left to right: H band map, stellar mass, extinction corrected star formation rate). In the bottom row (center image), the backdrop shows the extinction and the white contours show the CO integrated flux map -- demonstrating how well the absorption seen in the rest-frame optical coincide with the CO emission. The high-resolution HST H-band maps show clumps of star formation, typical for galaxies at the peak of the cosmic star formation rate (see results from the high-redshift galaxy survey SINS) 

Comparison of CO (IRAM/PdB) and H-alpha (LBT/LUCI) data at the same resolution of FWHM 0.75". There is a remarkable agreement between the kinematics as derived from the molecular (traced by the sub-mm CO line) and ionized gas (traced by the H-alpha line) that shows that both tracers can be used equally well to study rotation curve and kinematics in high-redshift galaxies.

As a follow-up to the high-redshift molecular gas survey PHIBBS (Tacconi et al. 2013; see news item below), a detailed study was performed of one of the most massive galaxies in this survey. For the study of the galaxy named EGS13011166, CO 3-2 line observations from the IRAM Plateau de Bure millimeter interferometer were combined with Large Binocular Telescope (LBT) LUCI observations of the H-alpha line in this galaxy, at matched spatial resolutions of 0.75 arcseconds. The galaxy was scanned perpendicular to the slit with LUCI to obtain spatially resolved spectra both along and perpendicular to the slit. Additionally, Hubble Space Telescope (HST) V-I-J-H band maps were used. Together these data allow to derive the stellar surface density and star formation rate, molecular gas surface density, optical extinction and gas kinematics.

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New observations on the Galactic Center gas cloud "G2"

September 12, 2012

New observations on the gas cloud "G2" falling towards the Galactic Center confirm its highly elliptical orbit, but with updated orbital parameters. With the new data, the cloud is now expected to come even closer -- the updated pericenter distance is 2200 Schwarzschild radii -- to the super-massive black hole at the center of our galaxy. While its origin is still unclear, its apocenter is near the inner edge of the disk of young stars. This supports speculations that the cloud, which has a mass of only about three earth masses, originated as a wind of one of these stars.

Data were recorded with the integral-field spectrograph SINFONI which produces data cubes containing a spectrum for each pixel. From this cube, data were extracted along a curved slit that matches the trajectory of the gas cloud and from a fit to the gas emission lines in the spectra, the velocity of the cloud was deduced for each position along the slit. The resulting plot shows at which velocity (x axis) the gas is moving depending on its position on the orbit (y axis) and color-coded for the epoch of observation (2008: red, 2011: blue, 2012: green). Apart from the cloud itself, a slow moving tail with velocities < 1000 km/s is seen. While the observations clearly show that the gas cloud is being disrupted, up to now the evolution of the cloud can be fully explained with a simple test particle simulation without any hydrodynamical effects.

In the course of this year, more observations of the cloud are planned, of course, and not only in the infrared but campaigns have been started by many groups to observe this accretion event in the whole electromagnetic spectrum. A wiki page has been set up to collect all information on "G2".

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Galactic Black Hole disrupts Gas Cloud

December 14, 2011

This composite image shows the positions of the gas cloud in 2002, 2007, and 2011 marked in colour. The cross indicates the position of the black hole in the galactic centre.
Image: MPE

Over the next few years, astronomers will be able to observe first-hand how the super massive black hole at the centre of our Milky Way is being fed: an international team of astronomers led by the Max Planck Institute for Extraterrestrial Physics has found a gas cloud that is falling towards the black hole in the galactic centre. While some distortion due to the huge gravitational pull of the black hole can already be seen, the gas cloud will be completely disrupted and ultimately swallowed by the black hole, resulting in largely increased X-ray emission. The observations and analysis are described in a Nature paper, published online on 14 December 2011.

For more information see
    MPE Press Release.

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Caught in the act: Herschel detects gigantic storms sweeping entire galaxies clean

May 09, 2011

This illustration shows an Ultra-Luminous InfraRed Galaxy (ULIRG) that exhibits massive outflows of molecular gas.
Image: MPE

With observations from the PACS instrument on board the ESA Herschel space observatory, an international team of scientists led by the Max Planck Institute for Extraterrestrial Physics have found gigantic storms of molecular gas gusting in the centres of many galaxies. Some of these massive outflows reach velocities of more than 1000 kilometres per second, i.e. thousands of times faster than in terrestrial hurricanes. The observations show that the more active galaxies contain stronger winds, which can blow away the entire gas reservoir in a galaxy, thereby inhibiting both further star formation and the growth of the central black hole. This finding is the first conclusive evidence for the importance of galactic winds in the evolution of galaxies.

For more information see
    MPE Press Release.

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