Alyson BrooksFairchild Postdoctoral Fellow in Theoretical Astrophysics

Caltech

In collaboration with the University of Washington’s N-body Shop™ makers of quality galaxies

The Interplay Between Gas Accretion and Feedbackin the Formation of Galactic Disks

How Do Galaxies Get Their Gas?

Dark Matter Halo + Hot Gas Dark Matter Halo + Hot Gas

Cold Disk

Infalling Cold Gas

• Not all gas is shock heated!

• Fraction of shocked gas is a strong function of galaxy mass

• Cold flow gas accretion due to both:

1. Mass threshold for stable shock

2. Even after shock develops, there can be cold gas accretion at high z in dense filaments

Case 1

Standard

Case 2

This is already in the SAMs.

This is not.

Gas Accretion Rates at the Virial Radius

3.4x1010 M 1.3x1011 M

1.1x1012 M 3.3x1012 M

Gas Accretion Rates

Disk Star Formation Rates

3.4x1010 M 1.3x1011 M

1.1x1012 M 3.3x1012 M

1.3x1011 M

z = 1

3.4x1010 M

1.1x1012 M 3.3x1012 M

Massive Disks at z=1: Vogt et al. (1996); Roche et al. (1998); Lilly et al. (1998); Simard et al. (1999);

Labbe et al. (2003); Ravindranath et al. (2004); Ferguson et al. (2004);

Trujillo & Aguerri (2004); Barden et al. (2005); Sargent et al. (2007);

Melbourne et al. (2007); Kanwar et al. (2008); Forster-Shreiber et al. (2006);

Shapiro et al. (2008); Genzel et al. (2008);

Stark et al (2008); Wright et al. (2008); Law et al. (2009)

Historic Problem : Disk Growth After z=1,dramatic change in scale lengths since z=1

Mergers or Smooth Gas Accretion?

Mergers destroy or thicken disks

e.g., Toth & Ostriker 1992, Kazantzidis et al. 2007, Bullock et al. 2008, Purcell et al. 2008

Therefore, disk galaxies must grow rather quiescently

NO GAS

NO GAS

CDM mergers

Mergers or Smooth Gas Accretion?

CDM mergers

Mergers destroy or thicken disks

e.g., Toth & Ostriker 1992, Kazantzidis et al. 2007, Bullock et al. 2008, Purcell et al. 2008

Therefore, disk galaxies must grow rather quiescently

Baugh et al. 1996, Steinmetz & Navarro 2002, Robertson et al. 2006, Hopkins et al. 2008, Robertson & Bullock 2008

Not if the disks are gas rich (fgas > 50%)

Or do they?

NO GAS

NO GAS

NO GAS ACCRETION

NO GAS ACCRETI

ON

The Formation of a Milky Way-Mass Galaxy to z=0

30 kpc on a side

Green = gas

Blue/Red = age/metallicity weighted stars

The Role of Cold Flows

Disk growth prior to z=1 due to cold flows

Brooks et al. (2009), Governato et al. (2009) Keres et al. (2008), Ocvirk et al. (2008), Agertz et al. (2009), Dekel et al. (2009), Bournaud & Elmegreen (2009)

The Role of Feedback

Bul

ge S

FR (M

/yr)

Develop a gas reservoir, yet fgas never > 25%

Age of Universe (Gyr)5 10Brooks et al. (2009), Governato et al. (2009)

The Role of Feedback

Bul

ge S

FR (M

/yr)

SFR increases by ~2-3x in mergersonly

Age of Universe (Gyr)5 10

Jogee et al. (2008), Stewart et al. (2008), di Matteo et al. (2008), Hopkins et al. (2008), Cox et al. (2008), Daddi et al. (2007), Bell et al. (2005), Bergvall et al. (2003), Georgakakis et al. (2000)

Disk Regrowth

~30 % due to cold gas accreted prior to lmm; ~35% due to cold gas accreted after lmm; ~30% due to hot gas accretion

Young stellar disk, formed after last major merger (z < 0.8); 30% of z=0 disk mass (but dominates the light)

Old stellar disk, formed prior to last major merger (z > 0.8); 70% of z=0 stellar disk mass

Brightness not to scale!

Sunrise: Jonsson (2006)www.ucolick.org/~patrik/sunrise/

Disk Regrowth

B/Di = 1.1B/DM* = 1.2M*disk = 2.07x1010 M

B/Di = 0.49B/DM* = 0.87M*disk = 3.24x1010 M

Conclusions

• New disk grows due to a) subsequent gas accretion, b) remaining gas from existing reservoir, c) cooling of hot gas from the galaxy halo

• Do not require fgas > 50% to rebuild disks, due to subsequent gas accretion and cooling from hot halo

• Although mergers are expected to be common in CDM, this is not at odds with the existence of disks.

• Cold gas accretion leads to the building of disks at higher z than predicted by standard models

• Feedback regulates SFR in galaxies, building a gas reservoir and limiting gas consumption in mergers

The Effects of Feedback

Not all feedback is created equal

High density threshold high efficiency.

The CDM Angular Momentum Problem

Navarro & Steinmetz (2000)

Disks are too small at a given rotation speed

Disks rotate too fast at a given luminosity

Mass (dark and luminous) is too concentratedLo

g V

rot

MI

catas

troph

e!

The Effects of Feedback:

The Tully-Fisher RelationThe Effects of Feedback:

HI W20/2 velocity widths = Vmax

Governato et al. (2009), Geha et al. (2006)

baryonic Tully-Fisher relation

(magnitudes from Sunrise)

AAGGEE

ofof

SSTTEELLLLAARR

PPOOPP

More Massive Galaxies

Have older Stellar Populations

MacArthur, Courteau & Bell (2004)

“Downsizing”The Effects of Feedback:

Brooks et al. (2007)Tremonti et al. (2004)

Erb et al. (2006)

Stellar Mass (M)

12+l

og(O

/H)

Stellar Mass (M)12

+log

(O/H

)

The Mass-Metallicity RelationshipThe Effects of Feedback:

Image by C. Brook, using Sunrise, courtesy P. Jonsson.

Feedback and resolution combine to yield simulated galaxies that match observed galaxy relations both at high z and at z=0

(Only some of these galaxies are simulated. Can you tell which?)

Steps to Rapid Disk Regrowth

1) Cold gas accretion leads to the building of disks at higher z than predicted by standard models

2) Feedback regulates SFR in galaxies, building a gas reservoir and limiting gas consumption in mergers

3) Existing disk is thickened in merger events4) New disk grows due to a) subsequent gas

accretion, b) remaining gas from existing reservoir, c) cooling of hot gas from the galaxy halo

Gas Accretion at the Virial Radius

Low mass High mass

3.3x10121.1x1012 1.3x1011 3.4x1010

Galaxy Mass (M)

Hot vs Cold

“clumpy” accretion gas (from other halos)

unshocked, smoothly accreted gas

shocked, smoothly accreted gas

Disk Growth

1.1x1012 M 3.3x1012 M

Heating gas shifts SF to z < 1, as models predict

Models also predict dramatic change in scale lengths since z=1

Historic Problem : Disk Growth After z=1

hot

dense

Hot vs Cold

Disk Growth

3.4x1010 M

1.3x1011 M

1.1x1012 M3.3x1012 M

Mo et al. (1998); Mao et al. (1998); van den Bosch (1998); Somerville et al. (2008)

Historic Problem : Disk Growth After z=1

Hot vs Cold

No shock yet

Shock exists

Unshocked gas in filaments

Definitions exclude “clumpy” accretion gas: gas that ever belonged to another galaxy halo