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Alyson BrooksFairchild Postdoctoral Fellow in Theoretical Astrophysics
Caltech
In collaboration with the University of Washington’s N-body Shop™ makers of quality galaxies
The Formation of Realistic Galaxy DisksThe Formation of Realistic Galaxy Disks
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Outline
I. Simulating Realistic Disk Galaxies is the Necessary Starting Point for I. Simulating Realistic Disk Galaxies is the Necessary Starting Point for Comparison to Observations Comparison to Observations
a) The Effect of Resolution a) The Effect of Resolution b) The Effect of Feedbackb) The Effect of Feedback
II. What we Learn II. What we Learn a) The Creation of a Bulgeless Disk Galaxy with a Dark Matter “Core”a) The Creation of a Bulgeless Disk Galaxy with a Dark Matter “Core”b) How to Create Large Disks despite Major Mergersb) How to Create Large Disks despite Major Mergers
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• Fully Cosmological, • Parallel,• N-body Tree Code,• + smoothed particle hydrodynamics (SPH)
Stadel (2001)Wadsley et al. (2004)
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Galaxy Formation: Now with AMR!
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Outline
I. Simulating Realistic Disk Galaxies is the Necessary Starting Point for I. Simulating Realistic Disk Galaxies is the Necessary Starting Point for Comparison to Observations Comparison to Observations
a) The Effect of Resolution a) The Effect of Resolution b) The Effect of Feedbackb) The Effect of Feedback
II. What we Learn II. What we Learn a) The Creation of a Bulgeless Disk Galaxy with a Dark Matter “Core”a) The Creation of a Bulgeless Disk Galaxy with a Dark Matter “Core”b) How to Create Large Disks despite Major Mergersb) How to Create Large Disks despite Major Mergers
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Low resolution: disks heat and lose angular momentum to halo
Rd=30% smaller
Kaufmann et al. (2007)
20 kpc 20 kpc 20 kpc
x1019
ato
ms
cm-2
N=1,000,000 100,000 30,000
• Isolated (non-cosmological) galaxy simulation• MW mass halo after 5 Gyr
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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 concentratedL
og V
rot
MI
catastr
ophe!
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“Zoom in” technique:high resolution halo surrounded by low resolution region
3 million particle simulation1 million particles
Computationally Efficient
Cosmic Infall and Torques correctly included
6 Mpc 100 Mpc
Katz & White (1993)
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HI W20/2 velocity widths = Vmax
Geha et al. (2006)Brooks et al., in prep
Governato et al. (2008, 2009)
baryonic Tully-Fisher relation
(magnitudes from Sunrise)
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Size - Luminosity Relation
Brooks et al., in prep.Data from Graham & Worley, 2008
Simulated Galaxies
Observed Sample
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Naab et al. (2007) Mayer et al. (2008)
As resolution increases, Vpeak decreases Same mass
galaxies, but with and without feedback
With feedback = Disk galaxies!
Without feedback = Elliptical galaxies!
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Outline
I. Simulating Realistic Disk Galaxies is the Necessary Starting Point for I. Simulating Realistic Disk Galaxies is the Necessary Starting Point for Comparison to Observations Comparison to Observations
a) The Effect of Resolution a) The Effect of Resolution b) The Effect of Feedbackb) The Effect of Feedback
II. What we Learn II. What we Learn a) The Creation of a Bulgeless Disk Galaxy with a Dark Matter “Core”a) The Creation of a Bulgeless Disk Galaxy with a Dark Matter “Core”b) How to Create Large Disks despite Major Mergersb) How to Create Large Disks despite Major Mergers
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No feedback
Zavala et al. (2008)Scannapieco et al. (2008)
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1) “Over-cooling” leads to loss of angular momentum similar to low resolution
Maller & Dekel (2002)
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1) “Over-cooling” leads to loss of angular momentum similar to low resolution
2) “Over-cooling” leads to ONLY elliptical galaxies!
No feedback Thermal Disable Cooling Blastwave ?
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• Star Formation: reproduces the Kennicutt-Schmidt Law; each star particle a SSP with Kroupa IMF
• Energy from SNII deposited into the ISM as thermal energy based on McKee & Ostriker (1977)
• Radiative cooling disabled to describe adiabatic expansion phase of SNe (Sedov-Taylor phase); ~20Myr (blastwave model)
• Only Free Parameters: SN & Star Formation efficiencies
Sub-grid physics & Blastwave Feedback Model
Stinson et al. (2006), Governato et al. (2007)
LMC HI distribution LMC HI distribution (Stavely-Smith 2003)(Stavely-Smith 2003)
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Brooks et al. (2007)Erb et al. (2006)
Tremonti et al. (2004)Maiolino et al. (2008)
Stellar Mass (M)
12+
log(
O/H
)
Stellar Mass (M)12
+lo
g(O
/H)
The Regulation of Star Formation due to Feedback
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Outline
I. Simulating Realistic Disk Galaxies is the Necessary Starting Point for I. Simulating Realistic Disk Galaxies is the Necessary Starting Point for Comparison to Observations Comparison to Observations
a) The Effect of Resolution a) The Effect of Resolution b) The Effect of Feedbackb) The Effect of Feedback
II. What we Learn II. What we Learn a) The Creation of a Bulgeless Disk Galaxy with a Dark Matter “Core”a) The Creation of a Bulgeless Disk Galaxy with a Dark Matter “Core”b) How to Create Large Disks despite Major Mergersb) How to Create Large Disks despite Major Mergers
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Moving Forward: “Resolving” Star Formation Regions
Den
sity
X
low SF threshold
Den
sity
X
high resolution + high SF threshold
Feedback becomes more efficient.(more outflows per unit mass of stars formed)
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The Formation of a Bulgeless Dwarf Galaxy
Mvir = 2x1010 M
M* = 1.2x108 M
15 kpc on a side
Green = gas
Blue/Red = age/metallicity weighted stars
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The effect of altering the SF density threshold
Resolving Star Forming Complexes
The effect of altering resolution
Governato et al., 2009, Nature, accepted
arXiv:0911.2237
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“Observed” Rotation Curve
Governato et al., 2009, Nature, accepted
arXiv:0911.2237
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Diffuse Star Formation
“Observed” Surface Brightness Profile
“Resolved” Star Formation
Radius (kpc)0 1 2 3 4 5 6 7
18
20
22
24
Mag
/ars
ec2
0 1 2 3 4 Radius (kpc)
22
24
26
28Mag
/ars
ec2
Governato et al., 2009, Nature, accepted
arXiv:0911.2237
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At high zoutflowsremove
low angularmomentum gas at a rate
2-6 timesSFR(t)
Time
J
Accreted material
Outflows
Gas removal from the galaxy center
Hot gas perpendicular to disk plane: 100km/sec
Cold Gas in shells = 30 km/sec
Outflows Remove Low Angular Momentum Gas
See also: van den Bosch (2002), Maller & Dekel (2002), Bullock et al. (2001)
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Angular Momentum of Stellar Disk vs DM halo
van den Bosch et al. (2001)
The Angular Momentum Distribution of baryons must
be altered to match observed galaxies
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Dark Matter with a Central Core
Diffuse SF: weak outflows
5 million particles
2 million particlesSF in high density regions:strong
outflows
DM
D
en
sity
Log Radius (kpc)
Clumpy Gas transfers orbital energy to DM via dynamical friction
DM expands as gas is rapidly removed
e.g., Mashchenko et al. (2007, 2008); El-Zant et al. (2004); Navarro et al. (1996); Mo & Mao (2004);
Tonini et al. (2006)
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Outline
I. Simulating Realistic Disk Galaxies is the Necessary Starting Point for I. Simulating Realistic Disk Galaxies is the Necessary Starting Point for Comparison to Observations Comparison to Observations
a) The Effect of Resolution a) The Effect of Resolution b) The Effect of Feedbackb) The Effect of Feedback
II. What we Learn II. What we Learn a) The Creation of a Bulgeless Disk Galaxy with a Dark Matter “Core”a) The Creation of a Bulgeless Disk Galaxy with a Dark Matter “Core”b) How to Create Large Disks despite Major Mergersb) How to Create Large Disks despite Major Mergers
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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
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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
ACCRETIO
N
NO GAS
ACCRETIO
N
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How Do Galaxies Get Their Gas?
Dark Matter Halo + Hot Gas Dark Matter Halo + Hot Gas
Cold Disk
Infalling Gas
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• 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.
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Gas Accretion Rates at the Virial Radius
3.4x1010 M 1.3x1011 M
1.1x1012 M 3.3x1012 M
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Gas Accretion Rates
Disk Star Formation Rates
3.4x1010 M 1.3x1011 M
1.1x1012 M 3.3x1012 M
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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
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The Formation of a Milky Way-Mass Galaxy to z=0
30 kpc on a side
Green = gas
Blue/Red = age/metallicity weighted stars
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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)
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The Role of Feedback
Bulg
e SF
R (M
/yr)
Develop a gas reservoir, yet fgas never > 25%
Age of Universe (Gyr)5 10Brooks et al. (2009), Governato et al.
(2009)
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The Role of Feedback
Bulg
e SF
R (M
/yr)
SFR increases by ~2-3x in mergers
Age of Universe (Gyr)5 10
Heiderman et al. (2009), 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)
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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!
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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
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Conclusions
• Simulations are improving! (due to resolution and feedback)
• Bulgeless galaxies with shallow DM cores are compatible with a CDM cosmology
• Strong gas outflows can selectively remove low angular momentum gas (but force resolution < 100pc is required)
• Although mergers are expected to be common in CDM, this is not at odds with the existence of disks
• Filamentary 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