The Coevolution of Galaxies and Dark Matter Halos

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The Co-evolution of Galaxies and Dark Matter Halos
Charlie Conroy (Princeton University) with Andrey
Kravtsov, Risa Wechsler, Martin White,
Shirley Ho.
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Outline

• What we learn from
• Observed clustering of galaxies
• Observed evolution in the stellar mass function
  and the intracluster light (ICL)
• Observed multiplicity function of LRGs in groups
  and clusters

• The Big Picture
• What does LCDM plus observations of galaxies
  tell us about the relation between galaxies and
  dark matter?
• Uncertanties in cosmology ltlt Uncertainties in
  galaxy formation/evolution

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The Clustering of Galaxies and Halos from z4
to z0
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Galaxy Clustering I

• Luminosity dependent clustering
• ??is a power-law, but why??

brighter
fainter
Davis et al 88
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Galaxy Clustering II

• Galaxy clustering is a function of luminosity,
  confirmed in both SDSS and 2dF surveys (among
  others)

More clustered
Definition of galaxy bias
b2 ?gal???dm
Brighter galaxies
Note bias measured at a particular scale
Zehavi et al. 05
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Galaxy Clustering III

• Clustering at z1

• Clustering at z4

Coil et al 06
More clustered
Brighter galaxies
Brighter galaxies
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Dark Matter Clustering
?CDM Simulation

• Galaxy clustering is a power-law and evolves
  only weakly with redshift
• DM clustering is not a power law, and is a
  strong function of redshift
• How do we reconcile this with observed galaxy
  clustering?

Colin et al 99
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Halo Clustering
Colin et al 99

• The clustering of dark matter halos is similar to
  the clustering of galaxies (i.e. power-law, slow
  function of redshift)
• (How) do galaxies correspond to dark matter halos?

The Big Question
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The Model
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Simulation Details

• ?CDM Cosmology ?m0.3, ??0.7, ??0.9, h0.7
• ART N-body code (Klypin Kravtsov)
• Box Sizes 80 120 Mpc/h. Npart 5123
• Particle Mass 3.2 x 108 1.1 x 109 Msun/h
  hpeak1-2 kpc/h
• Halos identified using BDM algorithm

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Distinct halos vs. Subhalos
Distinct halos
Subhalos their centers are within the virial
radii of larger parent halos
Note subhalos used to be distinct halos!
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Merger Trees
(for a distinct halo)
Time
Wechsler et al 02
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Halo Evolution
Distinct Halo Evolution
Subhalo Evolution
Accretion epoch
Constant increase
increase
decrease
Vmax, mass
Vmax, mass
Time
Time
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Connecting Halos to Galaxies I

• Find a relation between galaxy luminosity and
  halo Vmax, the maximum of the circular velocity
  function GM(ltr)/r1/2, or, equivalently, Mvir.
• Why?
• Tully-Fisher Faber-Jackson relations
  demonstrate a strong correlation between galaxy
  velocity and luminosity.
• Strong theoretical expectations that galaxy
  luminosity will correlate with halo mass (e.g.
  White Rees 78)
• brighter galaxies live in more massive halos

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Which Vmax Correlates with Luminosity?

• For distinct halos, we use Vmax measured at z
  zobs.
• For subhalos we use Vmax at the epoch of
  accretion

Accretion epoch
Why? Vmax at accretion should more accurately
reflect the build-up of stellar mass, and hence
luminosity
Vmax, mass
Time
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Connecting Halos to Galaxies II
ngal(gtL) nhalo(gtVmax)
r-band luminosity
Vmax
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ResultsComparing Galaxy Clustering to Halo
Clustering
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z 0
SDSS data

• Data red points
• Halos blue lines
• DM dotted lines

Notice the bump
Projected correlation function
Data Zehavi 05
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Vmax at accretion vs. Vmax today

• In order to match observed clustering at z0, we
  must use accretion epoch Vmax for subhalos
• Using accretion epoch effectively increases the
  fraction of galaxies that are satellites

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z 1
DEEP2 data

• Data red points
• Halos blue lines
• DM dotted lines

Data Coil 06
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z 4
Subaru data
Notice strong break on small scales
Notice strong linear bias b5
Data Ouchi 05
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Are We Missing Satellites?

• We have assumed that a satellite galaxy is
  destroyed when the subhalo is destroyed
• Are there satellite galaxies which have no
  counterparts in (our) simulations??
• No.
• Significant fraction (gt20) of missing subhalos
  ruled out observationally, for the mass ranges we
  probe
• In other words, subhalos in our simulations do
  not experience significant overmerging.

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What About ?8?

• The 2-pt auto-correlation of halos in this model
  does not depend on ?8
• Large scale clustering of halos decreases, but
  Nsat increases for lower ?8

Mrlt-21 dashed Mrlt-20.5 solid
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Implications

• ?gal is a power-law because ?halo is a power-law
• deviations from a power-law at high z and high
  luminosity are due to the clustering of halos
  (incl subhalos).
• High-res dissipationless N-body simulations can
  completely describe explain the dependence of
  galaxy clustering on luminosity, scale, and
  redshift with a simple assumption regarding the
  relation between galaxy luminosity and Vmax
• Understanding luminosity scale dependent
  clustering is separable.

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Build-up of stellar mass and the ICL since z1
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Evolution in the Stellar Mass Function

• Observations indicate mild/no evolution in the
  stellar MF since z1 at the massive end
• Evolution in the LF also consistent with passive
  evolution at the bright end since z1.

?M-4 0.2dex
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Evolution in the Halo Mass Function

• Strong evolution in the Halo MF from z1 to z0
  at the massive end
• Growth of halo does not track growth of central
  galaxy at zlt1 in massive halos
• But halos accrete most of their mass in 1/10
  Mhalo size clumps

Log(Mvir)
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Observations of the ICL
Gonzalez et al. 2005

• BCG surface brightness profiles in excess of
  deVaucouleurs at large scales
• Best fit by a 2-compenent deV profile, rather
  than a generalized Sersic profile
• Associate 2nd deV profile with ICL

Surface brighness (mag/arsec2)
deV
Semi-major axis (kpc)
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Modeling Stellar Mass Build-up

• Use the observed z1 galaxy stellar MF to connect
  stellar mass to halo mass at z1
• Follow the build-up of stellar mass with time
  using halo merger trees.
• Ignore star-formation and other dissipative
  physics
• Appropriate for the most massive galaxies where
  zform,starsgt2
• Therefore a lower bound to stellar mass build-up

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Fate(s) of Satellite Galaxies

• The evolution of satellite galaxies is tracked
  along with its dark matter subhalo until the
  subhalo dissolves. When the subhalo dissolves we
  have a decision to make
• Keep the satellite galaxy
  KeepSat
• Put the satellites stars into the BCG Sat2Cen
• Put the satellites stars into the ICL
  Sat2ICL
• Equally split between 2) and 3)
  Sat2CenICL

model name
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Evolution in the Galaxy Stellar MF

• Sample the observational uncertainties
• Model Sat2Cen ruled out by observed evolution in
  stellar MF.
• Other models OK.

Sat2Cen
Observationally allowed range
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BCG Luminosity - Mass Relation

• Mstar / LK 0.72
• Model Sat2Cen ruled out (again).
• Model Sat2CenICL marginally ruled out.
• Implies that lt50 of satellites from disrupted
  subhalos deposit their stars onto the central BCG

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The Intracluster Light
Gonzalez et al. 2005

• Model Sat2ICL (red points) reproduces observed
  total BCGICL luminosities.
• Model KeepSat (blue points) dramatically fails
  this test.
• We assumed that ICL is built-up at zlt1 by major
  mergers, tidal stripping not important.
• Validated by hydro-sims.

• Model Sat2ICL (red points) reproduces observed
  ICL light fraction better than model Sat2CenICL
  (blue points).
• Depends on modeling of observed surface
  brightness profile and defn of ICL.

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Implications

• Model Sat2ICL is the only model that matches an
  array of observations
• In massive halos (gt1013.5 Msun), satellite
  galaxies dissolve when their associated subhalo
  dissolves, and the satellite stars are dumped
  primarily into the ICL.
• This explains the apparent contradiction between
  the lack of evolution in the stellar MF and the
  strong evolution in the halo MF.
• This model predicts strong evolution in the total
  (BCGICL) light since z1 (very hard to observe
  this at z1!)

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Implications for Star-formation

• Match z0 stellar MF to the z0 halo MF in the
  usual way
• Compare z0 true stellar mass to the z0
  stellar mass predicted by our dissipationless
  models
• The difference should reflect the amount of
  star-formation since z1
• Galaxies in halos above 1013.5 Msun have had
  little star-formation
• At lower masses, fraction of stars formed since
  z1 decrease with increasing halo mass.

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Evolution in Mstar-Mvir Relation I
Evolution in galaxy stellar MF measured out to
z4 by Fontana et al. 2006
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Evolution in Mstar-Mvir Relation II

• Match stellar MF to halo MF at various epochs.
• As Universe evolves, the peak conversion
  efficiency evolves to lower halo masses
  (downsizing)
• At low halo masses stellar mass and halo mass
  increase with time, whereas at higher halo masses
  only the halo mass increases with time.
• Simple model matches observations, and favors
  ?80.75 (dashed line).

Fraction of baryons stars
Log(Mstar)
Log(Mvir)
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The LRG Multiplicity Function
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Observed LRG Multiplicity Function
Shirley Ho, et al. in prep

• 43 Clusters identified at 0.2ltzlt0.5 in
  Rosat/Chandra x-ray data that overlap SDSS
  footprint.
• Cluster masses determined from x-ray
  observations
• Mvir gt 1014 Msun
• LRGs identified in SDSS with photo-zs (dz0.03)

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Modeling the LRG Multiplicity Fcn
?t 3.2 Gyr
?t 1.6 Gyr
?t time for LRG to merge once accreted
Shape and normalization are important
constraints
?t 4.3 Gyr
?t 5.9 Gyr
Data MLRGgt6E12 MLRGgt1E13
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Conclusions

• By utilizing the observed number density of
  galaxies and LCDM simulations we can learn a
  great deal about the relation between galaxies
  and halos and the evolution of this relation with
  time.
• Observations which are thought to evolve
  dissipationlessly with time are particularly
  attractive because they are easy to model and yet
  much can be learned.
• The ICL is built up by merging satellites at zlt1.
  LRG multiplicity function provides information
  about merger/DF timescales.

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What about scatter?

• We expect scatter between Mvir and Mstar, what
  effect does this have?

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The Importance of Scatter I
z 0

• Scatter strongly affects the clustering of bright
  galaxies, but does not affect fainter galaxies
• Use this sensitivity to constrain the amount of
  scatter for bright galaxies
• Work in progress

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The Importance of Scatter II
Tasitsiomi et al. 04

• Scatter needed to match the observed galaxy-mass
  cross correlation function for bright galaxies
• Note these plots were made using the current
  Vmax for subhalos. We have not yet investigated
  scatter using accretion epoch Vmax for subhalos

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The Importance of Scatter III

• bla

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