Merger Histories of Dark Matter Halos in CDM and Implications for the

Evolution of Milky Way-size Galaxies

Kyle StewartNPP Fellow, JPL

Astrophysics Luncheon SeminarJPL, 1-11-2010

Kyle StewartNPP Fellow, JPL

Astrophysics Luncheon SeminarJPL, 1-11-2010

Outline1. Dark Matter Halo Merger Rates

Dependence on z, m/M, MDM

Comparison to observations

2. The Problem of Disk SurvivabilityDefining the problem: too many mergers?

Gas-rich mergers: possible solution?

3. Milky Way Substructure“Stealth” galaxies

4. Preliminary & Future WorkCool halo gas in a cosmological context

3

Dark Matter Halo Merger RatesDark Matter Halo Merger RatesStewart et al. 2009a

Introduction:

1. CDM, DM dominates the matter density of the universe.

2. Galaxies reside in center of DM halos.

3. DM halos (and galaxies within them) grow hierarchically over time, by mergers of smaller halos (galaxies).

In order to understand galaxy evolution, we need to first understand dark matter merger statistics (and how mergers affect galaxies).

Stew

art et al. 2008

DM Merger Histories

• DM only, CDM, N-Body simulation.

• 80 h-1Mpc Box, 8=0.9

• 5123 particles, mp~3x108 h-1Msun

• Adaptive Refinement Tree (ART) code. 5123 cells, refined to max. of 8 levels. hpeak ~ 1.2 h-1kpc (Kravtsov et al. ‘97)

• Focus on host masses ranging from 1011-1013 h-1Msun .

(~15,000 halos at z=0, ~9,000 halos at z=2.)

• Statistics complete to 1010 h-1Msun

See, eg. Stewart et al. 2008 (galaxy size halos)Berrier, Stewart et al. 2009 (cluster size halos)

Stew

art et al. 2008

Merger Rate:

Predict: Strong evolution with redshift ~ (1+z)^2.2.

Worry: does this contradict observational evidence for flat merger fraction with redshift ? (e.g. Lotz et al. ‘08, Jogee et al. ‘08)

Stewart et al. 2009a

(Number of mergers, per halo, per Gyr,,With mass ratio larger than r = m/M)

2.22.072.054.0 )1()/1(/ zMMmMmdt

dN

Merger Fraction (in past 500 Myr*).*Sometimes used as an estimated timescale for morphological disruption.

Lotz et al. ‘08

Jogee et al. ‘08

Agrees reasonably well with observations, for 1:10 minor + major mergers.

Suggests much higher fraction at high redshift.

Stewart et al. 2009a redshift

Use number density matching to associate halos with ~0.1L* galaxies from observed luminosity function (e.g. Faber et al. 07)

Merger Fraction (in past dynamical time*).*Use halo dynamical time as a proxy for morphological dyn. time.

Lotz et al. ‘08

Jogee et al. ‘08

Agrees reasonably well with observations, for 1:3 major mergers.

Shows relatively flat redshift evolution.

Stewart et al. 2009a redshift

Use number density matching to associate halos with ~0.1L* galaxies from observed luminosity function (e.g. Faber et al. 07)

Merger Histories vs.

Disk Survival

Merger Histories vs.

Disk Survival

9

Stewart et al. 2008, 2009b

Background:

• Dark matter halos form by mergers.• Major mergers (even some minor mergers) turn disk-type

galaxies into thick, flared, more bulgy systems. (eg. Mihos & Hernquist ‘94, Kazantzidis et al. ’07, ‘08; Purcell et al. ’08b)

– And yet: majority of Milky-Way sized DM halos contain thin disk-dominated galaxies (z=0). (eg. Weinmann et al. ’06, ‘09)

• Merger rate increases with redshift.– And yet: large disk-like galaxies observed at z~2.

(eg. Förster Shreiber ‘06; Genzel et al. ‘06; Shapiro et al. ‘08.)

How is all this compatible?

Why is there a concern about disk survival?

How often do mergers occur in 1012h-1M halos?

~ 70% of halos: m > 1.0x1011~ 50% of halos:

m > 1.5x1011

~ 30% of halos: m > 2.5x1011

By strict mass cut,in last ~10 Gyr :

Stewart et al. 2008

Could a disk have formed afterwards?(Mass accretion after last major merger)

These systems probably cannot subsequently re-grow a sizeable disk (from newly accreted material from subsequent galaxy mergers)

Given a halo with mass M0 at z=0…

which we know has experienced at least 1 merger of mass > m …

What fraction of M0 was accreted AFTER the most recent merger > m?

At most, only ~30%

Stewart et al. 2008

Summary: the problem

~70% of Milky Way-sized halos have had a > 1011 h-

1Msun merger in the past 10 Gyr. This is twice the mass of the disk!But observations show that most Milky Way size halos are disk-dominated (Weinmann et al. ‘06, ‘09)

Thus, typical Milky Way-size disks must be able to survive a 1011 h-

1Msun DM halo merger (sometimes) or we have a serious problem!

theorist

observations

Gas Rich Mergers: the Solution?• Gas rich minor mergers help form rotationally supported

gaseous disk galaxies.• Given a sufficiently high gas fraction (fgas> 50%), even

major mergers (3:1) quickly reform into a disk. (Robertson et al. ’06, Hopkins et al. ‘08)

Example: Observed disk galaxy at z~2 resembles simulated gas-rich merger remnant: Observation (Genzel et al. ’06) Simulation (Robertson & Bullock ’08)

15

Estimating Baryonic ContentEstimating Baryonic Content

1.Dark Matter Merger Trees

2.Empirical Stellar Mass—Halo Mass relation

(Conroy & Wechsler 2008)

3.Empirical Gas Mass—Stellar Mass relation

(McGaugh 2005; Erb et al. 2006)

Stewart et al. 2009b

Log Stellar Mass (Msun)

Log

(M

gas

/Mst

ar)

Step 3: Gas Masses.

Step 2: Stellar Masses.• Use number density matching to

statistically assign an average stellar mass, given DM mass (and redshift). (data from Conroy & Wechsler 2009.)

• Use observations of galaxies at z~0 (McGaugh ‘05) and z~2 (Erb et al. ‘06) to estimate Mgas vs. Mstar (for z=0-2).

Log

Ste

llar

Mas

s (M

sun)

Log Halo Mass (Msun)

z = 0

z = 2

z = 0 z = 2

Notice, since halos at high z are gas rich, total galaxy mass (cold baryons) per halo doesn’t evolve as much with z as stellar mass does…

Merger Fraction revisited: (> 1/3 mergers that hit the disk)

Squares, Xs: early-type fraction observations (Weinmann et al. 06, 09)

But what if we only look at gas rich* vs. gas poor* mergers?

* Definitions:• “Gas Poor” : both galaxies with gas fraction < 50%• “Gas Rich” : both galaxies with gas fraction > 50%

Small halos gas rich mergers

Large halos gas poor mergers

May explain disk survival?(Robertson et al. ‘06)

Stewart et al. 2009b

• Consider the DM merger rate for a 1012 M halo:

Merger rate low.

Mergers gas poor (destroys disks)

Merger rate increasing.

So is the gas rich merger fraction.

Merger rate high, but nearly ALL of them are very gas rich.

May explain assembly of massive, gas-rich disk galaxies at z~2.

(Robertson & Bullock 2008)

Section Sum-up :

Note: since relation btwn. MDM and Mstar is nontrivial, so is m/M. e.g. 1:3 DM ratio at MDM ~1011 Msun 1:20 stellar mass ratio or: 1:3 DM ratio at MDM ~1013 Msun 2:3 stellar mass ratio

Stewart 2009

An Aside: Defining “Merger Ratio”

An Aside: Defining “Merger Ratio”

21

Abundance Matching to Explore Milky Way Substructure

Abundance Matching to Explore Milky Way Substructure

Bullock, Stewart et al. 2009

MW Dwarfs: Observed Properties

Suggestive of unobserved population(s) of satellites:

• Distant and faint galaxies (somewhat expected)

•Spatially extended (higher Re, or lower total DM mass) and faint.

Obs. data from Strigari et al. ’08; Wolf et al. ’09

MW Dwarfs: Observed Properties (cont.)

Again, suggestive of unobserved population(s) of satellites:

• High velocity dispersion, faint galaxies

•Low DM mass, faint galaxies.

Obs. data from Strigari et al. ’08; Wolf et al. ’09

Use VL2 simulation + Mstar(MDM) relation:

“Sanity check” 1: does the model match the observed luminosity function?... Yes!

Also predicts many more “stealth” galaxies below luminosity & SB detection limits.

OBSERVABLE ONLY!

“Sanity check” 2: does the model fit the Strigari (2008) plot, (M300 vs L)?… Yes.

Use VL2 simulation + Mstar(MDM) relation:

Again, predicts many more “stealth” galaxies at low M300 and low Luminosity.

Flatness of Strigari plot a selection effect!

ALL MODEL DATA

26

Preliminary & Future WorkPreliminary & Future Work

• How do galaxies acquire their cool gas?– Cold flows? Cloud Fragmentation? (e.g. Keres et al.

‘09, Dekel & Birnboim ‘06, Maller & Bullock ’04)

• Gas rich mergers? (Stewart et al. 2009b)

• How do we test these ideas?

Absorption systems: probes of cold halo gas

Observing Gas Around Galaxies:

QSO (Mg II)

D ~ 100 kpc(or less)

Image from Tripp & Bowen (2005)

1) Covering Fraction

2) Cloud vs. Galaxy Kinematics

Observing Gas Around Galaxies:

1)Covering Fraction2) Cloud vs. Galaxy KinematicsBut what ARE they?

Spherical halo gas?Cold Filaments? Pressure-confined gas clouds?Outflowing winds? Tidal Streams?

Mg II Cf ~20-80%

e.g. Tripp & Bowen ’05;Tinker & Chen ‘08; Kackprzak et al. '08

Observing Gas Around Galaxies:

1)Covering Fraction2) Cloud vs. Galaxy Kinematics

Kacprzak et al. ‘09

Majority (7/10) Mg II absorbers show velocities that co-rotate with galaxy

Via Lactea II (ICs)

Our Simulation:

+

GASOLINE (sph code)

Die

mand e

t al. ‘

08

Wadsl

ey e

t al. ‘

04

Some stats:WMAP3 cosmo: =0.24, =0.76, h=0.73, σ8=0.77, b=0.042mDM, mgas, mstar ~3e5, 4e5, 1e5 Msun,

Np~4 million. Sph smooth len: 332 pc. Final halo mass Mvir~2.e12 Msun

‘Blast-wave’ feedback of Stinson et al. ‘06; Haardt & Madau ‘96 UV field; NOTE: no momentum driven blow-out winds

Log HI [Msun/pc3 ]= [-8, -1] ; width 300 kpc

Log stars [Msun/pc3 ] = [-7, 1]

Results: Covering Fraction

Router ~ 50 kpc (comoving)

Ngrid ~ 1000

Rinner ~ 5 kpc (comoving)

LOS “covered” if N(HI) >1016,18,20

atoms/cm2

Results: Covering Fraction(averaged over 3 projections)

1) Cold flows: high Cf.

2) & 3) Major Mergers:“bump”drop

4) Gas-rich minor merger:gashalo. ↑Cf

5) Cloud infall / fragmentation:Keeps Cf high.

6 & 7) Choose quiescent hist:↓Cf. rapid SF.

Covering Fractio

n Depends

Strongly

on Recent Gas A

ccretion!

What about Gas and Galaxy Kinematics?

z~0.8,Image width ~60 kpc

Log HI = [-7, 1]LOS velocity: +/- 250

km/s

z~0.8,Image width ~60 kpc

Log HI = [-7, 1]LOS velocity: +/- 250

km/s

Gas and Galaxy Kinematics:

Co-rotation (as observed) is a natural

result of cosmological gas accretion &

fragmentation

Summary:Merger Rates:

• Merger rate agrees fairly well with observed “morphologically disturbed” fractions, for first order estimates of merger timescale. Merger rates depend on z, m/M, M: well fit by a “universal” function.

• “Major Mergers” (1:3 ratio) defined by DM vs. stars not trivially related.

Disk Survival:

• Milky Way-size disks must be able to survive some major* mergers (*either merger ratio > 1:3, or m > 1011 h-1M ).

• Gas rich/poor merger histories seem promising for disk survival (explains mass-morphology relation?) provided gas-rich mergers result in disks.

MW Substructure:

• We predict a population of “stealth” dwarf galaxies in the halo of the Milky Way, with SB’s too low for current techniques.

Preliminary Work—Cosmological hydrodynamic simulations:

• High CF (~50%) and cloud co-rotation both consistent with observations, from infalling gas (cold flows/mergers/clouds). No momentum winds!

• Covering fraction depends strongly on recent gas accretion.

36

Future Work:Run a suite of SPH simulations, for a systematic sampling of

“common” merger histories, from “quiescent” to “active.”

• Which properties depend strongly on merger history?

• Can “cold flows” be observed? Is high CF ubiquitous?

• What trends in CF vs. time are universal? Which aren’t?

• What do properties of the gaseous halo tell us about galaxy formation?

• Compare systematic sample of simulated galaxies to DM merger statistics. Can we infer broader statements about galaxy populations?

Side project: simulating an “invisible” major merger :

• DM mass ratio 1:3, stellar ratio 1:20. (MDM~1011 Msun)

• Is the faint galaxy distinguishable, observationally?

Thank You!