How are galaxies influenced by their

environment?

rachel somervilleSTScI

Predictions & insights from hierarchical models

with thanks toEric Bell the COMBO/GEMS teamRisa WechslerAndrey KravtsovSandy Faber

what are observations telling us?

old wisdom: familiar correlations between galaxy observables

color-morphologycolor-magnitudeluminosity-metallicity

new wisdom: correlation between intrinsic/physical properties , e.g.

stellar mass and star formation ratestellar mass & metallicity

new wisdom: many fundamental galaxy properties have bimodal distributions, divided by a critical stellar mass

Baldry et al. 2003co

lor

luminosity

SDSS: color and magnitude

Blanton et al. 2003

stellar mass and age…~s

tell

ar a

ge

Kauffmann et al. 2003, 2004

SDSS

stellar mass

stellar mass and relative star formation rate

rela

tive

sta

r fo

rmat

ion

rate

stellar mass

Brinchmann et al. 2004

what imposes these relationships on galaxies

(internal/external)?the old wisdom:

morphology-density relation: early type fraction increases with densityButcher-Oemler effect: early/blue fraction decreases with cosmic time

the new wisdom:(Hogg et al., Blanton et al., Balogh et al., Kauffmann et al.):

structural properties have weak dependence on environment spectro-photometric properties have a stronger dependence on environment -- critical density?

morphology-density as a function of redshift

Smith et al. 2004

low density

high density

frac

tion

of

earl

y ty

pes

projected density lookback time

structure and density

Kauffmann et al. 2004

increasing density

age and relative star formation

and local density

increasing N

Kauffmann et al. 2004

increasing N

Hogg et al. 2003

luminosity has a strongdependenceon local density

color has a weakerdependence on local density

Balogh et al. 2004

fraction of red galaxiesincreases with density

but the mean color of the red and bluedistributions changes little with density

colo

r

magnitude

Blakeslee et al. 2003 (ACS GTO team)

RDCS1252 z=1.24

the color magnituderelation is in place at z~1 and evolution is consistent with passive

red dots:early typeblue dots:late type

rest

U-V

col

or

rest V magnitude (luminosity)

age =8.4Gyr

age = 5.5 Gyr

Bell et al. 2003

…in the field aswell as clusters(COMBO-17/GEMS)

time

•do hierarchical models predictthis behaviour?•can they give us any insight intowhat is going on?

Wechsler et al.

cluster halo

‘Milky Way’ halo

Kauffmann et al. 1999VIRGO/GIF simulationssee also Benson et al. 2001;

Springel et al. 2001

hierarchical simulationsshow a clear correlationbetween color/morphologyand density, in qualitativeagreement with observations

dependence of mean color and morphological fraction on halo

mass

Diaferio et al. 2001

color fraction of bulge/disk galaxies

log halo mass

primordial power spectrum

star formationstellar feedbackchemical enrichment

collisional heatingradiative cooling

merger tree

stellar populationsdust absorption & emission

galaxy galaxy observablesobservables

inflation

specific model ingredients

€

SFR =mcold

τ *

€

τ* = τ *0tdyn

Vc

200km /s

⎛

⎝ ⎜

⎞

⎠ ⎟α *

€

˙ m rh = SFR ×εSN0 Vc

200km /s

⎛

⎝ ⎜

⎞

⎠ ⎟α rh reheated gas

ejected if Vc>150 km/s

•major mergers (>4:1) trigger bursts of star formation•Bruzual & Charlot 2003 multi-metallicity stellarpopulation models

SDSS & 2MASS luminosity functions

n um

ber

p er

u nit

vol

ume

magnitude

u-band g-band r-band

i-band z-band K-band

(observed LF from Bell et al.

luminosity functions by morphology

disk dominated bulge dominated

gas fraction

log stellar massSDSS Bell et al.

gas fraction distributionsincreasing stellar mass -->

color-magnitude relation

r-band magnitude

color histogramsbright-->

faint

g-r color

color of a passivelyevolving burst formedat z=5

u-r

g-r Z=2xsolarZ=solar

Global star formation history

stellar massassemblyhistory

rss et al. 2004

new observationalestimates fromCOMBO-17 andGOODS

stellar mass assembly history

rss et al. 2004

estimates fromGlazebrook et al. (GDDS)Rudnick et al. (FIRES)Dickinson et al. (HDFN)Fontana et al. (K20)

red: n>2blue: n<2

red: B/T>0.5blue: B/T<0.5

color-magnitude and morphology at high redshift

KAB<22

13.55.83.21.00.50.1

rss et al. 2004 GOODS ApJLGOODS

missing EROs

status: low redshift

hierarchical models can be made to reproduce global luminosity/stellar mass distributions at low redshift but don’t produce enough luminous red galaxiescolor magnitude relation has correct slope (well, sort of) but distributions do not match data and are not bimodal

status: high redshift

hierarchical models produce enough massive galaxies to z~2but, do not produce enough red galaxiesthe mean stellar ages of the massive galaxies are old enough -- color problem is caused by ‘frosting’ of young stars

what makes red galaxies red?

need a process that quenches star formation in the most massive galaxies without drastically altering the mass assembly/star formation history

environment: ram pressure or tidal stripping, harassment?internal: SN or AGN driven wind? global instability?

neighbor counts (R=2 Mpc)

L=114 Mpc mp=3x108 M_sun r_force=1.5 kpc

number of neighbors in 2 Mpc spheres

cyan: 0-1

blue: 2-3

green: 4-6

rust: 7-11

red: >11

number of neighbors vs. halo mass

log(1+N)

log halo mass

color-magnitude by density

g-r

r-band mag

Kauffmann et al. 2004

N=0-1 N>17

increasing density-->decreasing lum

ino si ty - ->

N=0-1 N=2-3 N=4-6 N=7-11 N>11

-22.5

-21.5

-20.5

-19.5

-18.5

Balogh et al. 2004

increasing density-->

decreasing luminosity-->

u-r

relative SFR vs. mass

log stellar mass

log(

SF

R/s

tell

ar m

ass)

star formation/age as function of local density

increasing N

Kauffmann et al. 2004

increasing N

where does this leave us?

where do galaxies become red?simulations: massive halosreal universe: all environments (though more often in dense ones)

how do galaxies become red?simulations: whole distribution shiftsreal universe: galaxies ‘hop’ from one distribution to the other

why do galaxies become red?simulations: strangulationreal universe: ???

internal processes

all gas driven out of galaxy after a merger by SN or AGN winds?

has almost no effect on colors because of continuous infall of fresh gas

star formation ‘turned off’ when a bulge (BH?) has grown beyond a critical mass?

g-r

colo

r

r-band magnitude

star formation shutoff when m_bulge> 2 x 1010 M_sun

better!

color distributions

bright

faint

g-r color

bimodal!