Transcript b) How to Create Large Disks despite Major Mergers

The Formation of Realistic Galaxy Disks
Alyson Brooks
Fairchild Postdoctoral Fellow in Theoretical Astrophysics
Caltech
In collaboration with the University of Washington’s N-body Shop™
makers of quality galaxies
Outline
I. Simulating Realistic Disk Galaxies is the Necessary Starting Point for
Comparison to Observations
a) The Effect of Resolution
b) The Effect of Feedback
II. What we Learn
a) The Creation of a Bulgeless Disk Galaxy with a Dark Matter “Core”
b) How to Create Large Disks despite Major Mergers
•
•
•
•
Fully Cosmological,
Parallel,
N-body Tree Code,
+ smoothed particle hydrodynamics (SPH)
Stadel (2001)
Wadsley et al. (2004)
Galaxy Formation: Now with AMR!
Outline
I. Simulating Realistic Disk Galaxies is the Necessary Starting Point for
Comparison to Observations
a) The Effect of Resolution
b) The Effect of Feedback
II. What we Learn
a) The Creation of a Bulgeless Disk Galaxy with a Dark Matter “Core”
b) How to Create Large Disks despite Major Mergers
100,000
30,000
Rd=30% smaller
20 kpc
20 kpc
x1019 atoms cm-2
N=1,000,000
20 kpc
• Isolated (non-cosmological) galaxy simulation
• MW mass halo after 5 Gyr
Low resolution: disks heat and lose angular momentum to halo
Kaufmann et al. (2007)
The CDM Angular Momentum Problem
Log Vrot
Disks rotate too fast at a given
luminosity
Mass (dark and luminous) is too
concentrated
Disks are too small at a given
rotation speed
MI
Navarro & Steinmetz (2000)
“Zoom in” technique:
high resolution halo surrounded by low resolution region
Computationally
Efficient
Cosmic Infall and Torques correctly included
6 Mpc
1 million particles
100 Mpc
3 million particle simulation
Katz & White (1993)
baryonic
Tully-Fisher relation
(magnitudes from Sunrise)
HI W20/2
velocity widths = Vmax
Geha et al. (2006)
Brooks et al., in prep
Governato et al. (2008, 2009)
Size - Luminosity Relation
Simulated Galaxies
Observed Sample
Brooks et al., in prep.
Data from Graham & Worley, 2008
With feedback =
Disk galaxies!
As resolution
increases, Vpeak
decreases
Same mass
galaxies, but with
and without
feedback
Without feedback =
Elliptical galaxies!
Naab et al. (2007)
Mayer et al. (2008)
Outline
I. Simulating Realistic Disk Galaxies is the Necessary Starting Point for
Comparison to Observations
a) The Effect of Resolution
b) The Effect of Feedback
II. What we Learn
a) The Creation of a Bulgeless Disk Galaxy with a Dark Matter “Core”
b) How to Create Large Disks despite Major Mergers
No
feedback
Zavala et al. (2008)
Scannapieco et al. (2008)
1) “Over-cooling” leads to loss of angular momentum similar to
low resolution
Maller & Dekel (2002)
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  ?
Sub-grid physics & Blastwave Feedback Model
• 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 (SedovTaylor phase); ~20Myr (blastwave model)
• Only Free Parameters: SN & Star
Formation efficiencies
LMC HI distribution
(Stavely-Smith 2003)
Stinson et al. (2006), Governato et al. (2007)
12+log(O/H)
12+log(O/H)
The Regulation of Star Formation due to Feedback
Stellar Mass (M)
Stellar Mass (M)
Brooks et al.
Erb et al.
Tremonti et al.
Maiolino et al.
(2007)
(2006)
(2004)
(2008)
Outline
I. Simulating Realistic Disk Galaxies is the Necessary Starting Point for
Comparison to Observations
a) The Effect of Resolution
b) The Effect of Feedback
II. What we Learn
a) The Creation of a Bulgeless Disk Galaxy with a Dark Matter “Core”
b) How to Create Large Disks despite Major Mergers
Moving Forward:
“Resolving” Star Formation Regions
high resolution + high SF threshold
Density
Density
low SF threshold
X
X
Feedback becomes more efficient.
(more outflows per unit mass of stars formed)
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
Resolving Star Forming Complexes
The effect of
altering the SF
density
threshold
The effect of
altering
resolution
Governato et al., 2009, Nature, accepted
arXiv:0911.2237
“Observed” Rotation Curve
Governato et al., 2009, Nature, accepted
arXiv:0911.2237
Mag/arsec2
“Observed” Surface Brightness Profile
18
20
Diffuse Star Formation
22
24
0
1
2
3
4
Radius (kpc)
5
6
7
Mag/arsec2
22
24
“Resolved” Star Formation
26
28
0
1
2
Radius (kpc)
3
4
Governato et al., 2009, Nature, accepted
arXiv:0911.2237
Gas removalRemove
from the galaxy
Outflows
Lowcenter
Angular Momentum Gas
Hot gas perpendicular to disk plane: 100km/sec
Cold Gas in shells = 30 km/sec
Accreted material
J
Outflows
Time
At high z
outflows
remove
low angular
momentum
gas at a rate
2-6 times
SFR(t)
See also: van den Bosch (2002),
Maller & Dekel (2002), Bullock et al. (2001)
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
Dark Matter with a Central Core
DM Density
Diffuse SF: weak outflows
5 million particles
SF in high
density
regions:
strong
outflows
2 million particles
Clumpy Gas
transfers orbital
energy to DM via
dynamical friction
DM expands as gas
is rapidly removed
Log Radius
(kpc)
e.g., Mashchenko et al. (2007, 2008); El-Zant et al. (2004);
Navarro et al. (1996); Mo & Mao (2004); Tonini et al. (2006)
Outline
I. Simulating Realistic Disk Galaxies is the Necessary Starting Point for
Comparison to Observations
a) The Effect of Resolution
b) The Effect of Feedback
II. What we Learn
a) The Creation of a Bulgeless Disk Galaxy with a Dark Matter “Core”
b) How to Create Large Disks despite Major 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
Mergers or Smooth Gas Accretion?
CDM  mergers
Mergers destroy or
thicken disks
Or do they?
e.g., Toth & Ostriker 1992, Kazantzidis
et al. 2007, Bullock et al. 2008, Purcell
et al. 2008
Baugh et al. 1996, Steinmetz & Navarro
2002, Robertson et al. 2006, Hopkins et
al. 2008, Robertson & Bullock 2008
Therefore, disk galaxies must grow
rather quiescently
Not if the disks are gas rich (fgas > 50%)
How Do Galaxies Get Their Gas?
Cold Disk
Infalling Gas
Dark Matter Halo + Hot Gas
Dark Matter Halo + Hot Gas
e.g., Peebles (1969), Rees & Ostriker (1977), Silk (1977), Binney (1977),
White & Rees (1978), Fall & Efstathiou (1980), Somerville & Primack (1999)
Standard
•
Not all gas is shock heated!
•
Fraction of shocked gas is a strong
function of galaxy mass
Case 1
•
Cold flow gas accretion due to both:
1.
2.
This is already in
the SAMs.
Mass threshold for stable
shock
Even after shock develops, there can
be cold gas accretion at high z in
dense filaments
This is not.
Keres et al. (2005), Dekel & Birnboim (2006),
Ocvirk et al. (2008), Agertz et al. (2009), Dekel et al. (2009)
Case 2
Gas Accretion Rates at the Virial Radius
3.4x1010 M
1.1x1012 M
1.3x1011 M
3.3x1012 M
Brooks et al. (2009)
Gas Accretion Rates
Disk Star Formation Rates
3.4x1010 M
1.3x1011 M
1.1x1012 M
3.3x1012 M
Brooks et al. (2009)
Historic Problem :
Disk Growth After z=1,
dramatic change in scale lengths
since z=1
3.4x1010 M
1.3x1011 M
z=1
Massive Disks at z=1:
1.1x1012 M
3.3x1012 M
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)
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
Bulge SFR (M/yr)
Develop a gas
reservoir, yet fgas
never > 25%
Brooks et al. (2009), Governato et al. (2009)
5
10
Age of Universe (Gyr)
The Role of Feedback
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)
Bulge SFR (M/yr)
SFR increases
by ~2-3x in
mergers
5
10
Age of Universe (Gyr)
Disk Regrowth
Young stellar disk, formed
after last major merger
(z < 0.8);
30% of z=0 disk mass (but
dominates the light)
~30 % due to cold gas
accreted prior to lmm;
~35% due to cold gas
accreted after lmm;
~30% due to hot gas
accretion
Brightness not to scale!
Old stellar disk, formed
prior to last major merger
(z > 0.8);
70% of z=0 stellar disk
mass
Disk Regrowth
B/Di = 1.1
B/DM* = 1.2
M*disk = 2.07x1010 M
B/Di = 0.49
B/DM* = 0.87
M*disk = 3.24x1010 M
Sunrise: Jonsson (2006)
www.ucolick.org/~patrik/sunrise/
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