Dr Robert Feldmann

Robert Feldmann,
Department of Astronomy,
University of California, Berkeley,
501 Campbell Hall #3411
Berkeley, CA 94720-3411 USA
E-mail: feldmann -at- berkeley.edu

How do galaxies and many of their properties evolve over cosmic history? Which processes determine their sizes, regulate their star formation rates, or shape their morphology? Nobody really knows for sure. However, compared with just 10 years ago, we are now much closer to answering those questions. This remarkable development is largely a result of two major advances. First, with the advent of large scale, digital galaxy surveys the quantitative analysis of millions of galaxies became possible -- a development that is still revolutionizing our understanding of galaxy evolution. Secondly, the continuous increase in computing power enables theoretical astrophysicists like myself to develop and evaluate increasingly sophisticated computational models. In my work I often rely on state-of-the-art hydrodynamics and gravity solvers such as the SPH-code GASOLINE or the AMR-code ART to study how galaxies evolve in a cosmological setting.
> more <

Links for the Impatient
Curriculum Vitae


PhD in Physics, Institute of Astronomy, ETH Zürich, Switzerland

Title: "The Evolution of Massive Galaxies - A numerical perspective"
06/2005 Diploma degree (equiv. M.Sc.) in Computer Science, Universität Leipzig

Title: "Planning with preferences - Solving partial satisfaction problems"
09/2004 Diploma degree (equiv. M.Sc.) in Physics, Universität Leipzig

Title: "Phase structure and photon propagator in 3D abelian lattice Higgsmodels"

Professional Experience

09/2015 - present Assistant Project Scientist at the Department of Astronomy, University of California, Berkeley
09/2012 - 08/2015
Hubble fellow at the Department of Astronomy, University of California, Berkeley
08/2010 - 08/2012
Associate Fellow of the Kavli Institute for Cosmological Physics
11/2009 - 08/2012
Theoretical Astrophysics Group at the Fermi National Accelerator Laboratory, IL, U.S.A.
2008 - 2009
Dep. Physics, ETH Zürich, Supervision of several undergraduate projects in astrophysics
2005 - 2008
Dep. Physics, ETH Zürich, Teaching assistant for several courses in physics and astrophysics


2015 SNSF Professorship, Swiss National Science Foundation
2012 Hubble Fellowship, NASA, STScI
Associate Fellowship of the Kavli Institute for Cosmological Physics
ETH Extragalactic Astrophysics Excellence Award
2004-2008 PhD fellowship from the Studienstiftung des deutschen Volkes
2001-2004 Scholarship from the Studienstiftung des deutschen Volkes


Selected Papers

D. Narayanan, M. Turk, R. Feldmann et al.
“The formation of sub-millimetre bright galaxies from gas infall over a billion years",
Nature 525, 496-499 (2015), ADS, News & Views

R. Feldmann
“The equilibrium view on dust and metals in galaxies: Galactic outflows drive low dust-to-metal ratios in dwarf galaxies",
Mon. Not. Roy. Astron. Soc. 449, 3274 (2015), ADS

D. Fiacconi, R. Feldmann, L. Mayer
“The Argo Simulation: II. The Early Build-up of the Hubble Sequence”,
Mon. Not. Roy. Astron. Soc. 446, 1939 (2015), ADS

R. Feldmann, L. Mayer
“The Argo Simulation: I. Quenching of Massive Galaxies at High Redshift as a Result of Cosmological Starvation”,
Mon. Not. Roy. Astron. Soc. 446, 1939 (2015), ADS

R. Feldmann, D. Spolyar
“Detecting Dark Matter Substructures around the Milky Way with Gaia”,
Mon. Not. Roy. Astron. Soc. 446, 1000 (2014) ADS

R. Feldmann
“Lessons from cosmic history: the case for a linear star formation - H2 relation”,
Mon. Not. Roy. Astron. Soc. 433, 1910 (2013), ADS

R. Feldmann, D. Hooper, N. Y. Gnedin
“Circum-Galactic Gas and the Isotropic Gamma Ray Background”,
Astrophys. J. 763, 21 (2013), ADS

R. Feldmann, J. Hernandez, N. Y. Gnedin
“The relation between mid-plane pressure and molecular hydrogen in galaxies: Environmental dependence”,
Astrophys. J. 761, 167 (2012), ADS

R. Feldmann, N. Y. Gnedin, A. V. Kravtsov
“The X-factor in Galaxies: II. The molecular hydrogen -- star formation relation”,
Astrophys. J. 758, 127 (2012), ADS

R. Feldmann, N. Y. Gnedin, A. V. Kravtsov
“The X-factor in Galaxies: I. Dependence on Environment and Scale”,
Astrophys. J. 747, 124 (2012), ADS

R. Feldmann
, C. M. Carollo, L. Mayer
“The Hubble Sequence in Groups: The Birth of the Early-Type Galaxies”,

  Astrophys. J. 736, 88 (2011),

R. Feldmann, N. Y. Gnedin, A. V. Kravtsov
“How Universal is the SFR - H2 Relation?”
Astrophys. J 732, 115 (2011),

R. Feldmann, N. Y. Gnedin
“On the time variability of the star formation efficiency”
Astrophys. J Letters 727, 12 (2011)

R. Feldmann, C. M. Carollo, L. Mayer et al.
“The Evolution of Central Group Galaxies in Hydrodynamical Simulations”
Astrophys. J. 709, 218 (2010)

R. Feldmann, L. Mayer, & C. M. Carollo,
“Tidal Debris in Elliptical Galaxies as Tracers of Mergers with Disks”
Astrophys .J. 684, 1062 (2008)

R. Feldmann, C. M. Carollo, C. Porciani et al.
“The Zurich Extragalactic Bayesian Redshift Analyzer (ZEBRA) and its first application: COSMOS”
Mon. Not. Roy. Astron. Soc. 372, 564 (2006)

Link to the full list of ADS published papers

Refereed Publications outside Physics

R. Feldmann, G. Brewka, S. Wenzel
“Planning with Prioritized Goals”
In Proceedings of the 10th International Conference on Knowledge Representation and Reasoning (KR-06),
503-514. Menlo Park, CA, AAAI Press, (2006), abstract&fulltext
, pdf

Student Projects

I have supervised a number of undergraduate and high school student projects over the years. Feel free to drop me an e-mail if you are interested in pursuing a project related to the numerical study of galaxy evolution.

Research Highlights

The Formation of submillimeter bright galaxies >arXiv:1509.06377<

Submillimeter-luminous galaxies at high-redshift are the most luminous, heavily star-forming galaxies in the Universe, and are characterized by prodigious emission in the far-infrared at 850 microns (S850 > 5 mJy). They reside in halos ~ 10^13 Msun, have low gas fractions compared to main sequence disks at a comparable redshift, trace complex environments, and are not easily observable at optical wavelengths. Their physical origin remains unclear. Simulations have been able to form galaxies with the requisite luminosities, but have otherwise been unable to simultaneously match the stellar masses, star formation rates, gas fractions and environments. We report on a cosmological hydrodynamic galaxy formation simulation that is able to form a submillimeter galaxy which simultaneously satisfies the broad range of observed physical constraints. We find that groups of galaxies residing in massive dark matter halos have rising star formation histories that peak at collective rates ~ 500-1000 Msun/yr at z=2-3, by which time the interstellar medium is sufficiently enriched with metals that the region may be observed as a submillimeter-selected system. The intense star formation rates are fueled in part by a reservoir gas supply enabled by stellar feedback at earlier times, not through major mergers. With a duty cycle of nearly a gigayear, our simulations show that the submillimeter-luminous phase of high-z galaxies is a drawn out one that is associated with significant mass buildup in early Universe proto-clusters, and that many submillimeter-luminous galaxies are actually composed of numerous unresolved components (for which there is some observational evidence).

Snapshot, taken from a supercomputer simulation, depicting the distribution of gas and light in a small region around a submillimeter-luminous galaxy (SMG): it contains a bright central galaxy (white) that is accreting gas along a filamentary structure (pink), a large spiral galaxy (left of centre), and numerous smaller galaxies that contribute to the total luminosity of the SMG. Ambient gas (blue-green), much of which was expelled by the galaxies at earlier epochs, gravitates towards the center of the proto-SMG. This fuels the prodigious star-formation activity of the system, which is unlike anything seen in the present-day Universe. [ from the News & Views article by R. Dave ]

The Formation and Evolution of Massive Galaxies
  >arXiv:1404.3212< >updated version<
Observations show a prevalence of high redshift galaxies with large stellar masses and predominantly passive stellar populations. A variety of processes have been suggested that could reduce the star formation in such galaxies to observed levels, including quasar mode feedback, virial shock heating, or galactic winds driven by stellar feedback. However, the main quenching mechanisms have yet to be identified. We have studied the origin of star formation quenching using Argo, a cosmological, hydrodynamical zoom-in simulation that follows the evolution of a massive galaxy at z≥2. Argo adopts the same sub-grid recipes of the Eris simulations, which have been shown to form realistic disk galaxies, and, in one version, adopts also a mass and spatial resolution identical to Eris. The resulting galaxy has properties consistent with those of observed, massive (M_* ~ 1e11 M_sun) galaxies at z~2 and with abundance matching predictions. Our models do not include AGN feedback indicating that supermassive black holes likely play a subordinate role in determining masses and sizes of massive galaxies at high z. The specific star formation rate (sSFR) of the simulated galaxy matches the observed M_* - sSFR relation at early times. This period of smooth stellar mass growth comes to a sudden halt at z=3.5 when the sSFR drops by almost an order of magnitude within a few hundred Myr. The suppression is initiated by a leveling off and a subsequent reduction of the cool gas accretion rate onto the galaxy, and not by feedback processes. This "cosmological starvation" occurs as the parent dark matter halo switches from a fast collapsing mode to a slow accretion mode. Additional mechanisms, such as perhaps radio mode feedback from an AGN, are needed to quench any residual star formation of the galaxy and to maintain a low sSFR until the present time.

The galaxy evolves off the star forming sequence at z~3.5
Evolution of the specific star formation rate (SFR / unit stellar mass) of the simulated massive galaxy over cosmic time. The galaxies lies on the star forming sequence at z>3.5, but falls below the sequence at z~3.5, i.e., it starts its transit from the star forming to the quiescent galaxy population. The drop of the specific star formation rate is a consequence of cosmic starvation, i.e., a consequence of the reduced matter accretion rate onto the halo hosting the galaxy at z<3.5.

Detecting dark matter substructures with Gaia  >arXiv:1310.2243< >updated version<

Cold Dark Matter (CDM) theory, a pillar of modern cosmology and astrophysics, predicts the existence of a large number of starless dark matter halos surrounding the Milky Way (MW). However, clear observational evidence of these "dark" substructures remains elusive. We propose a detection method of orbiting substructure that relies on the small velocity changes imposed on the stars in the MW disk. Using high-resolution numerical simulations we estimated that the new space telescope Gaia should detect the kinematic signatures of a few starless substructures provided the CDM paradigm holds. Such a measurement will provide unprecedented constraints on the primordial matter power spectrum at low-mass scales and offer a new handle onto the particle physics properties of dark matter.

Kinematic signature of the substructure
Kinematic signature of a low mass substructure passing vertically through the disk of the MW. Each panel shows a velocity map of the face-on stellar disk of the MW model at a different time (see legend). The galactic center (white cross) is at X = Y = 0. Panels A through E show the change in vertical velocity caused by the gravitational pull of the substructure in 500 x 500 pc^2 bins. Upward (downward) motions are shown in red (blue) colors. The blue (white) circle in each panel indicates the projected center of mass of the substructure when it is above (below) the MW disk plane.
We show the position of the substructure in a frame co-rotating with the mean tangential velocity of stars at 8 kpc from the galactic center. The MW–substructure interaction results in well-localized maxima and/or minima of the vertical velocity of disk stars, visible in panels A–D.

On The Isotropic Gamma Ray Background  >arXiv:1205.0249<

Interactions of cosmic rays with the interstellar gas and radiation fields of the Milky Way provide the majority of the gamma rays observed by the Fermi Gamma Ray Space Telescope. In addition to the gas which is densely concentrated along the Galactic Disk, hydrodynamical simulations and observational evidence favor the presence of a halo of hot (T~10^6 K) ionized hydrogen, extending with non-negligible densities out to the virial radius of the Milky Way. We show that cosmic ray collisions with this circum-galactic gas should be expected to provide a significant flux of gamma rays, on the order of 10% of the observed isotopic gamma ray background at energies above 1 GeV. In addition, gamma rays originating from the extended halos of other galaxies along a given line-of-sight should contribute to this background at a similar level.

The gas density profile - simulations vs observations
The density profile of ionized hydrogen in a simulated Milky-Way halo and comparison with observations. The total matter density (solid blue line) is well approximated by an NFW profile, except in the central region of the halo (< 20 kpc), where cooling and adiabatic contraction lead to a density enhancement. The density profile of ionized hydrogen that our simulation predicts (solid red line) is in good agreement with observations (symbols) and can be fit with a beta-profile (dotted red line). Cosmic rays that originate in the disk of the Milky Way, but diffuse outward, interact with this extended halo of hydrogen and produce a diffuse and highly isotropic background of gamma-rays.  

The CO-H2 conversion factor   >arXiv:1112.1732< >arXiv:1204.3910<

Characterizing the conversion factor between CO emission and column density of molecular hydrogen, X_CO, is crucial in studying the gaseous content of galaxies, its evolution, and relation to star formation. In most cases the conversion factor is assumed to be close to that of giant molecular clouds in the Milky Way, except possibly in mergers and star-bursting galaxies. However, there are physical grounds to expect that it should also depend on the gas metallicity, surface density, and strength of the interstellar radiation field. We study the dependence of X_CO on such gas properties and on spatial resolution using a model that is based on a combination of results of sub-parsec scale magnetohydrodynamic simulations and on the gas distribution from self-consistent cosmological simulations of galaxy formation. We show that neglecting these dependencies can strongly bias the inferred distribution of column densities of molecular clouds to have a narrower and offset range compared to the true distribution. We further show that observations of molecular gas based on CO emission can result in a biased slope of the star formation -- gas relation. Specifically, our model predicts that this relation appears to steepens at high surface densities as a result of the gas surface density dependence of X_CO. Finally, we show that on sub-kpc scales most of the scatter of the star formation -- gas relation is a consequence of discreteness effects of the star formation process. In contrast, we expect variations of X_CO to be responsible for most of the scatter measured on super-kpc scales.

Relation between star formation rate and molecular gas The relation between the surface density of the star formation rate and the molecular hydrogen surface density. The relation used in the simulation is perfectly linear (dashed black line). However, the relation inferred from CO data (blue solid curve) deviates from the actual relation at high surface densities, similar to what observations indicate (grey dashed line).

Time variability in the star formation efficiency?   >arXiv:1009.5674<

A star formation efficiency per free fall time that evolves over the life time of giant molecular clouds (GMCs) may have important implications for models of supersonic turbulence in molecular clouds or for the relation between star formation rate and H2 surface density. In the paper we discuss observational data that could be interpreted as evidence of such a time variability. In particular, we investigate a recent claim based on measurements of H2 and stellar masses in individual GMCs. We show that this claim depends crucially on the assumption that H2 masses do not evolve over the life times of GMCs. We exemplify our findings with a simple toy model that uses a constant star formation efficiency and, yet, is able to explain the observational data.

Estimator of the star formation efficiency as function of molecular mass of the GMC
The estimator ηGMC of the star formation efficiency vs. mass of the GMC. Empty Squares and triangles are  observational data. Overplotted are solid and dot-dashed lines that refer to the predictions of the our model. Filled circles and filled stars indicate when the age of the modeled GMC is half its total life time or when the cloud is 1 Myr away from the end of its life, respectively. The diagonal dashed line indicates a slope of -0.75, which is approximatively the slope predicted by our model. The observed data is consistent with this slope. The horizontal line denotes the value of the actual (time-independent) star formation efficiency of the GMC that entered into our model.

Galaxies in Groups - Satellites   >arXiv:1008.3386<

Many physical processes may be responsible for making elliptical morphologies and quenching star formation, but the mainly responsible mechanisms, and the epochs and timescales at/on which they operate have not been yet identified. In this paper we use a simulation of the formation of a group of galaxies with sufficient resolution to track the evolution of gas and stars of about a dozen galaxy group members over cosmic history. Ellipticals form, as suspected, through galaxy mergers. In contrast with what has often been speculated, however, these mergers occur at z>1, before the merging progenitors enter the virial radius of the group and before the group is fully assembled. Quenching of star formation in the still star-forming elliptical galaxies lags behind their morphological transformation, but, once started, is taking less than a billion years to complete.

Assembly histories since z = 2 of the z~0 group satellites
Assembly histories since z = 2 of the z ∼ 0 group satellites. Red solid (blue dashed) lines show the stellar mass of z ∼ 0 elliptical (disk) satellites as function of time (bottom axis) and redshift (top axis). Filled (empty) symbols indicate the time when the progenitor of a z ∼ 0 gas-poor (gas-rich) satellite enters the virial radius of the group. Sudden increases in stellar mass are caused by merger events, often before infall into the group, while close pericentric passages that lead to tidal stripping reveal themselves through a step-wise stellar mass loss occurring after their entrance in the group. The gas-poor disk satellites do not grow significant stellar mass after they enter the group, due to quenching of their star formation.

Galaxies in Groups - Centrals    >arxiv:0906.3022<

In this paper we trace the evolution of central galaxies in three ~10^13 M_sun galaxy groups simulated at high resolution in cosmological hydrodynamical simulations. The evolution in the group potential leads, at z=0, to central galaxies that are massive, gas-poor early-type systems supported by stellar velocity dispersion resembling either elliptical or S0 galaxies. Their central stellar densities stay approximately constant from z~1.5 down to z=0. Instead, the galaxies grow inside-out, by acquiring a stellar envelope outside the innermost ~2 kpc. Consequently the density within the effective radius decreases by up to two orders of magnitude. Both major and minor mergers contribute to most of the mass accreted outside the effective radius and thus drive the evolution of the half-mass radii. Our simulations demonstrate that, in galaxy groups, the interplay between halo mass assembly, galaxy merging and gas accretion has a substantial influence on the star formation histories and z=0 morphologies of central galaxies.

Simulated galaxy groups at different redshifts
Simulated galaxy groups at different redshifts. The size of each image is 1.2 Mpc × 1.2 Mpc. The four columns correspond to z = 2.5, z = 1.5, z = 0.6 and z = 0. Each row corresponds to a different galaxy group. Color coded are dark matter in blue (from 3.6 to 1460 M⊙ / pc^2), cold gas (here the gas with T < 2.5 × 10^5 K) in green, stellar matter in yellow (both from 0.9 to 365 M⊙ / pc^2), and hot gas (here the gas with T ≥ 2.5 × 10^5 K) as red surface mass isocontours (3 contour lines at 1, 4.5 and 20 M⊙ / pc^2). The white circle shows the virial radius of the group at the each redshift.

Tidal Debris around Massive Ellipticals  

We use a set of high-resolution N-body simulations of binary galaxy mergers to show that the morphologies of the tidal features that are seen around a large fraction of nearby, massive ellipticals in the field cannot be reproduced by equal-mass dissipationless mergers; rather, they are well explained by the accretion of disk-dominated galaxies. The minor cold-accretion events that explain the presence, brightness, and structural and color properties of the tidal debris cause only a modest mass and luminosity increase in the accreting massive elliptical. These results, coupled with the relative statistical frequencies of disk- and bulge-dominated galaxies in the field, suggest that massive ellipticals assemble most of their mass well before their tidal debris forms through the accretion of relatively little, kinematically cold material rather than in very recent, dissipationless major mergers.

Morphological signatures of mergers
Morphological signatures of mergers. (a) Observed image of an elliptical galaxy that shows tidal tails at large galactocentric distances; (b-d) Mock images of: a simulated 1:4 merger between an elliptical galaxy and a disk+bulge system 600 Myrs after the merger;  a simulated 1:10 merger between an elliptical galaxy and a disk 500 Myrs after the merger; a simulated 1:4 merger between elliptical galaxies 380 Myrs after the merger. The tidal features originating in mergers between ellipticals and  those involving a disk-dominated companion are strikingly different, independent of the mass ratio and orbit characteristics. While all mergers can lead to shells and diffuse features, only the mergers involving disks show strong tidal arms and loops, similar to the observed features around bright ellipticals.

Accurate Photometric Redshifts

Observational studies of galaxy evolution are increasingly dependent on accurate photometric redshifts. As part of my PhD program I designed and implemented ZEBRA, the Zurich Extragalactic Bayesian Redshift Analyzer. ZEBRA is a publicly available software package that allows to compute photometric redshifts with outstanding accuracy and has been successfully used by many research groups. You can find more information about ZEBRA on its dedicated website.
Simple Maximum-Likelihood estimate of redshifts
ZEBRA's Bayesian estimate of redshifts with template optimization
A simple Maximum-Likelihood estimate of redshifts: (top) spectroscopic vs. photometric redshifts; (bottom) the error in the redshift estimation as function of spectroscopic redshift for different template types; The standard deviation of dz/1+z  is ~ 0.043 and  the outlier fraction ~ 2.1 %. Systematic biases at z~1 are visible. ZEBRA's Bayesian estimate of redshifts with template optimization. Top and bottom panels as in a). Both the standard deviation of dz/1+z ~ 0.027 and the outlier fraction ~ 0.8 % are improved. Systematic biases are minimized.

External Collaborators

C. M. Carollo (ETH Zürich)
C. A. Faucher-Giguère (Northwestern University)
N. Y. Gnedin (Fermilab)
O. Hahn (Stanford U)
A. Kravtsov (U Chicago)
L. Mayer (U Zürich)
R. Teyssier (U Zürich)

Image & Movie Gallery

Image and movie gallery  Click on image to enter the movie gallery!

Oct 9, 2014