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The Assembly of Massive Galaxies in Cosmological Hydro Simulations

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Title: The Assembly of Massive Galaxies in Cosmological Hydro Simulations


1
The Assembly of Massive Galaxies in Cosmological
Hydro Simulations
  • Romeel Davé, Univ. of Arizona
  • With Lars Hernquist (CfA), Dušan Kereš Neal
    Katz (UMass), Volker Springel (Garching) and
    David Weinberg (Ohio State)

2
Introduction
  • While the growth of dark matter is now reasonably
    well-determined, the growth of stellar mass
    remains poorly understood.
  • Near-IR surveys can directly measure the stellar
    mass growth in galaxies since moderate redshifts,
    with fewer/different systematics than star
    formation rate indicators.
  • How do current simulations of galaxy formation
    compare against such surveys?

3
Near-Infrared Galaxy Surveys
  • - Proliferation of NIR surveys K20, FIRES, SDF.
    Also improved optical surveys constrain zlt1
    stellar mass growth SDSS, 2dF, DEEP2, Combo-17.
    Soon Spitzer...
  • - General result Massive galaxies are in place
    at least as early as z2, indicating an early
    epoch of stellar mass growth in the Universe.
  • - Consistent with what Ive seen qualitatively in
    simulations, but...

The existence of such systems when the Universe
was only one-quarter of its present age, shows
that the build-up of massive early-type galaxies
was much faster in the early Universe than has
been expected from theoretical simulations. -
Cimatti et al 2004, Nature
Cimatti 2004
4
NIR Surveys vs. Semi-Analytic Models
  • Semi-analytic models (lt2003) predict too little
    stellar mass at high-z, and too much evolution to
    z0.
  • - 2004 SAMs do better, but Nagamine et al. hydro
    sims predicted correct growth in 2001.

Glazebrook et al 2004
5
Gadget2 Hydro Simulations
  • - G6 model
  • 2x4863 particles mbary1.38x108M?,
    mdm8.97x108M?.
  • 100 Mpc/h box size
  • 2 kpc/h resolution (equiv Plummer)
  • LCDM W0.3,H070,s80.9, Wb0.04
  • - D5,Q5 models (2x3243 particles)
  • 33.75,10 Mpc/h box sizes
  • mbary1.8x107M?,, 4.6x105M?.
  • 1.5, 0.4 kpc/h resolutions (comoving)
  • Galaxy spectra found by integrating SF histories
    of individual galaxies using BC03 models,
    assuming solar metallicity, Salpeter IMF,
    E(B-V)0.1 (at all redshifts).
  • Entropy-conservative SPH Tree-PM (Springel
    Hernquist)
  • HHe cooling, Jn, star formation according to
    Kennicut Law.
  • Multi-phase subgrid ISM model, based on McKee
    Ostriker 1977.
  • Supernova feedback
  • Thermal, added to hot ISM phase.
  • Wind, that randomly expels gas from star
    forming regions.
  • Wind speed (constant) chosen to roughly reproduce
    observed mass density in stars today reduces
    stellar mass by factor of 2.5.
  • - Galaxies identified using SKID gt64mbary is a
    resolved galaxy.

6
Global Star Formation History Hopkins 2004
  • - G6 run underpredicts SFR at zlt2, and
    overpredicts(?) at zgt3 Early epoch of global
    star formation.
  • z0 baryon fraction in stars 6.3.
  • Low due to limited resolution, since it
    underestimates early star formation Dashed line
    shows Q5.
  • Dotted line shows 50 of baryons in stars at
    z2.1 (age3 Gyr).

7
Global Luminosity Density Evolution
  • Rudnick et al (2004) SDSS (z0), Combo-17 (zlt1),
    FIRES (1ltzlt3).
  • Select Kslt23.2 and LVgt1.4x1010L?,v.
  • Rest-frame V, B, and U luminosities broadly match
    _at_ z0, and show increase out to z3, by the
    following factors
  • U-band, G6 5.5, Observed 4.91.0.
  • B-band, G6 3.8, Observed 2.90.6.
  • V-band, G6 2.6, Observed 1.90.4.
  • At z3, G6 shows an overabundance of stellar mass
    (V), while SFR (U) matches again, early epoch of
    SF.
  • Good agreement with rate of global luminosity
    decrease with time.

8
Global Color Evolution
  • - Universe is bluer in the past.
  • Better agreement in B-V than U-B U-B colors
    slightly too red (0.1 mag) at high-z.
  • Both colors slightly too blue at z0? (but
    sensitive to dust).
  • D5 colors virtually identical to G6 colors not
    sensitive to numerical resolution (unlike global
    luminosity).

9
Rest-frame K-band Luminosity Function Evolution
K20
  • K20 Survey (Cimatti et al 2003) 52?,
    spectroscopic survey to Kvega20 on VLT.
  • Excellent agreement at z0.5 1 (up to
    resolution limit of G6).
  • Slight overprediction of LF at z1.5
    overabundance of stellar mass showing up already
    at z1.5? (cf cosmic variance)
  • Remarakable agreement overall Not only is
    global stellar mass growth in G6 correct, but
    stellar mass is ending up in the correct galaxies
    (i.e. the massive ones).

10
Rest-frame K-band Luminosity Function Evolution
Subaru
  • Subaru Deep Field Survey (Kashikawa et al 2003)
    3.74?, spectroscopic survey to Kvega23.7 with
    Subaru. Points show their LF Schechter function
    fits, over the luminosity range where they have
    data.
  • G6 underpredicts LF at z1 (cosmic variance?),
    but matches at z2,3.
  • D5 run (dashed line), shows good overlaps with G6
    at bright end, but possibly too many faint
    galaxies.
  • Bright galaxies match well out to z3, but still
    possibly a faint-end excess despite the strong
    feedback in G6.

11
Rest-frame B-band Luminosity Function Evolution
  • G6 (solid line) shows more rapid rest-frame
    B-band evolution from z1?3 than observations.
  • D5 run shows poorer agreement than in K-band
    case, with both shape and amplitude mismatches,
    and a prominent faint galaxy excess.
  • Overall conclusion Bright end better predicted
    than faint end, and galaxy stellar masses better
    predicted than SFRs as a function of luminosity.

12
Evolution of Color-Magnitude Relation
  • Combo-17 Red sequence persists to z1, with mild
    evolution (Bell et al).
  • While mild evolution is OK, G6 show no red
    sequence the bright-end tail gets bluer.
  • More significantly Simulations show no evidence
    for the observed color gap between early
    late-type galaxies.

13
Birthrates of Simulated Galaxies
  • Birthrate tHubblexSFR/M
  • Birthrate of intermediate low mass galaxies is
    constant, but birthrate of highest mass galaxies
    drops with time thats good.
  • While generally smaller galaxies have higher
    birthrates, the most massive galaxies also show
    elevated birthrates not so good.
  • This trend weakens with time but does not
    disappear or reverse bad.
  • Reveral of this trend, i.e. a truncation of the
    small yet persistent star formation in massive
    galaxies, is required to obtain a color gap.
  • Likely candidate AGN feedback.

14
Conclusions
  • - The Good Simulations broadly reproduce the
    observed early growth of stellar mass, as well as
    the global luminosity and color evolution of
    large galaxies. The bright-end K-band luminosity
    function is also in good agreement with estimates
    out to z3.
  • - The Bad Simulations tend to overproduce
    stellar mass at high-z. Also, the B-band LF
    evolution and shape do not agree well, signalling
    that star formation is occurring in the wrong
    galaxies.
  • - The Ugly Simulations do not produce a color
    gap between old, massive, red galaxies and
    later-type galaxies. The putative red sequence
    looks wrong in slope and amplitude, possibly due
    to incorrect/missing feedback physics.
  • - The Bottom Line We now mostly understand how
    gas gets into galaxies to form stars, and can
    reproduce observations of the growth of stellar
    mass. However, feedback remains poorly
    understood, and the simple model implemented here
    still results in classic disagreements with data.

15
Analytic Analysis of Shock Stability
  • - Birnboim Dekel (2003) Shocks near Virial
    radius are unstable for Mhalo lt few x
    1011M?.
  • - Virial shock is not formed until this (L-ish)
    halo has accreted the bulk of its mass.

16
SFR vs. Environment The Hot Cold View
  • - SF peaks at group densities, mainly due to
    increase in hot mode, then drops dramatically
    within clusters.
  • - Drop is evident out to several Rvir, consistent
    with SDSS observations (Gomez et al 2003).

17
Merging vs. Smooth Accretion
  • - Merging Gas/star was in galaxy at some
    previous step, and is now in larger galaxy.
  • - Smooth accretion Gas was never part of any
    (resolved) galaxy before entering current galaxy
    and forming stars.
  • - Galaxies obtain most of their mass by smooth
    accretion, not merging.
  • - This is not a numerical resolution issue
    Sub-resolution merging contributes little (lt20
    overall at zlt2).
  • - Globally, SFR follows smooth accretion rate,
    not merger rate.

18
Cumulative Contribution of Hot vs. Cold
  • Note that even hot mode does not come in exactly
    at Tvir, but only within a factor of a few of
    Tvir.

19
Phase Diagram of Accretion
  • - Cold and hot mode distinguished by Tmax, the
    maximum temperature a gas particle reaches before
    it forms stars. Division at T2x105K.

20
Global Accretion Rate in Hot Cold Modes
  • - Differential smooth accretion rate shows two
    distinct modes.
  • - Cold mode dominates at high-z, when galaxy
    formation is most vigorous, and becomes
    comparable to hot mode from z2?0.
  • - Dividing temperature roughly 2.5x105K, but can
    also divide based on individual halos' Tvir.
  • - At z0, 70 of accreted baryons never reached
    their halos' Tvir.
  • - At z3, this number is 95, and 70 never came
    within an order of magnitude of Tvir.

21
Example Accretion at z5 in a 3x1011M?
halo, shown to Rvir.
Hot particles in green
Cold particles in green
22
Physics of Stellar Mass Growth in Simulations
  • - Two modes of gas accretion onto galaxies (Keres
    et al 2004)
  • - Hot Mode Gas shock heats at halos virial
    radius up to Tvir, cools slowly onto disk. The
    canonical White Rees (1978) scenario.
  • - Cold Mode Gas radiates its potential energy
    away in line emission at TltltTvir, and never
    approaches virial temperature.
  • - Punch line Cold mode dominates during the
    epoch of galaxy assembly, especially in smaller
    halos and at earlier times.

23
Global Accretion in Cold and Hot Modes
  • - Cold accretion (defined by T2x105K cutoff)
    dominates at zgt2.
  • - When defined as T/Tvirlt1, cold accretion is
    even more dominant.

24
Accretion Geometry Plays a Role
  • Cold accretion is generally more filamentary.
  • Histogram of radius vector dot products shows
    peak in cold mode accretion at cosine1.
  • This enhances cooling rate by increasing the
    density relative to spherical accretion, and
    further destabilizing the virial shock.

25
Accretion Rates vs. Halo Mass
  • - Cold accretion dominates for Mhalo lt 3x1011M?,
    virtually independent of redshift.
  • - Halo mass as driving parameter suggests that
    accretion shock physics plays key role.
  • - In fact, this dividing halo mass is
    analytically predictable.

26
Infall Velocity
  • Velocity of gas within virial radius shows
    gradual acceleration down to disk.
  • Not free fall ? No strong X-ray emission at disk.

27
Lya Cooling Radiation
  • Cold mode energy release detectable as Lya
    emission.
  • At z3, 70 of energy is emitted at 104K, with
    much of it in the Lya line.
  • Luminosity structure are very broadly
    comparable to Steidels Lya blob at z3.

28
SFR vs. Environment
  • Gomez et al SFR begins to shut off well outside
    Rvir, at S 1 gal/Mpc2.
  • Simulations show identical behavior.
  • Driven by drop in hot mode accretion rate.
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