Download Mass and Metallicity Distribution Function of halos of simulated Disk

Mass and Metallicity Distribution Function of halos of simulated Disk galaxies
Introduction
Brook, Chris; Kawata, Daisuke; Gibson,Brad
Swinburne University, Melbourne, Australia
It is well established that cosmological N-body Smooth particle dynaamics (SPH)
simulations are able to form flattened disk structures resembling disk galaxies (e.g. Katz
1992, Steinmetz & Muller 1995, Abadi et al. 2003). However, the resulting structure are
deficient in angular momentum (e.g. Navarro, Frenk, White 1995). These simulations have
also failed to create galaxies in which most baryonic matter resides in the thin disk, as
observed in the Milky Way. Abadi et al (2003) create a stellar halo which contains over 60 %
of the total stellar mass of the system , and a thin disk which constitues only 17%, more akin
to an s0 than a late type disk galaxy. These massive halos have far higher metallicity
distributions than observed late type spiral galaxies.
This leaves open the question of what formation processes result in formation of late type
galaxies such as the Milky Way.
We examine here the halo metallicity distribution function and halo:disk mass ratio of
simulated disk galaxies, using two different feedback shemes. Using the thermal feedback
scheme, we form a galaxy which closely resembles those of previous similar studies. the
halo mass is high, and the stellar halo stars have high metallicity. Using a feedback scheme
which makes gas in star burst regions adiabatic, star formation in subclumps is regulated. A
later type spiral galaxy is realised.
code and models parameters
GCD+ (Kawata & Gibson 2003)
3d vector/parallel tree N-body/SPH code
DM and Stars
Tree N-body code
Gas Smoothed Particle Hydrodynamics (SPH)
+ Radiative Cooling
(MAPPINGSIII: Sutherland & Dopita)
+ Star Formation
SFR ∝ ρ1.5
IMF: Salpeter type
+ SNe Feedback
SNeII and SNeIa
+ Metal Enrichment
SNII (Woosley & Weaver 1994)
Intermediate
(van den Hoek & Groenewgen 1997)
SNIa (Iwamoto 1999)
+ Tracing Metals
H,He,C,N,O,Ne,Mg,Si, and Fe
Galaxy Formation Models
Semi-Cosmological (Top Hat) model
(Navarro & White 1993)
model parameters:
5x1011 M⊗
baryon fraction 0.1
solid body rotation λ=0.07
38191 dark matter particles
38191 gas/star particles
initial redshift z=40
feedback
thermal feedback model:
energy feedback from SNII and SNIa is deposited on
the nearest neighbour gas particles as thermal
energy, according to the SPH kernal (Katz 1992).
adiabatic feedback model:
the feedback region of SNII is made adiabatic.
Gas within the SPH kernal of SNII is prevented
from cooling.
SN1a energy is deposited as thermal energy.
(Gerritson 1997 , Thacker & Couchman 2000)
Final distributions
Z
Y
fig 1.a
Y
Z
fig 1.b
fig 1.a: z = 0 density plots of stars (yellow) and gas (blue) for thermal feedback model. Shown are the xy (left)
and xz planes where the z axis is the axis of rotation. A thin disk is evident in the gas distribution, but the
galaxies stellar mass is dominated by halo stars. The galaxy more closely resembles an s0 galaxy.
fig 1.b: as above for adiabatic feedback model. The galaxy is dominated by a thin stellar disk. A large
gaseous thin disk, still undergoing star formation, has also formed.
12
16
SFR (MΘ/yr))
µI / (mag/arcsec2)
thermal feedback
adiabatic feedback
10
8
6
4
fig 2
2
0
thermal feedback
adiabatic feedback
18
20
22
24
fig 3
12
10
8
6
4
2
Age/Gyr
star formation rates
26
0
0
2
4
6
8
10
12
radius (kpc)
I band surface brightness
profile
Star formation in the adiabatic feedback model is suppressed at early epochs
(fig 2), but at later times the
greater availability of gas allows increased star formation. This leads to a late type galaxy (fig 1) in the adiabatic
feedback model, with a larger scale length (fig 3).
200
50
gas (left) and stellar
rotation curves
0
-50
Vrot (km/s)
100
Vrot (km/s)
200
thermal feedback
adiabatic feedback
150
150
100
thermal feedback
adiabatic feedback
50
0
-50
-100
-150
-100
fig 4.b
-200
fig 4.a
-150
-200
-20
-15
-10
-5
0
5
10
15
-10
20
-5
0
radius (kpc)
5
10
radius (kpc)
Angular momentum imparted in the initial conditions results in gas disks with similar rotation velocities for both
simulations (fig 4.a). The increased stellar rotation of the adiabatic feedback model (fig 4.b), when coupled with
the larger scale length (fig 3), implies that less angular momentum has been lost to the dark matter halo.
0.3
The halo mass and MDF
thermal feedback
adiabatic feedback
thermal feedback
adiabatic feedback
0.29 0.15
0.57 0.06
fraction of total stellar mass
table1.
Nbin/Ntot
The fraction of total galactic stellar mass in the disk and
halo components‡ of the 2 models (table 1) shows that
the adiabatic feedback model has a far larger thin disk, 0.2
with a factor of 4¶ increase in the disk:halo stellar mass
disk
halo
ratio.
The peak of the halo metallicity distribution (fig 5)
shifted from [Fe/H]~0.1 for the thermal feedback model
(white), to [Fe/H]~ -0.7 for adiabatic feedback (green).
These halo stars are still higher metallicty than the
Milky Way halo MDF, which peaks at [Fe/H]~-1.5 (Ryan
& Norris 1991).
Much work has been done recently on the role of
accretion of subhalos in the build up of stellar halos
within a heirarchical cosmological paradigm. Further,
the adiabatic feedback was shown by Thacker &
Couchman (2000) to have a greater affect on smaller
systems. Thus, the natural place to search for the
formation processes leading to the different galaxy
properties produced by the different models was in the
subclumps of the simulation.
0.1
fig 5
0
-4
-3
-2
[Fe/H]
-1
0
1
halo metallicity distribution
‡ halo mass is determined by 2 x total mass of retrograde stars.
Bulge stars are excluded by applying a cut in binding energy to
stars within 4 kpc of the galactic centre.
‡ disk mass is determined by requiring that the distribution of
specific angular momentum as a function of specific binding energy
for the the stars follows that for the gas disk. Cuts in the z
components of velocity (|vz| < 65 km/s) and position (|z| < 1 kpc)
are also made.
¶ This is a conservative estimate: unclassified stars in the thermal
feedback model tend to be halo like, while in adiabtice model they
tend to be disk like. Also note the more massive gaseous thin disk
in the adiabatic feedback model.
fig 6.a
fig 6.b
SFR (MΘ/yr)
The disruption process for satellite 1 of the thermal feedback model. Red
particles are stars present in the satellite prior to any disruption. Gas particles
are green. Star particles formed after the disruption process begins are blue.
There are ~780 baryon particles in satellite 1, each of mass ~106 MΘ.
6 timesteps are shown, face on (xy plane, left panels) and edge on (xz
plane).
Galactic stars are not shown in the final panels. We see from fig 8 (yellow
line) that most star formation in this satellite occurs rapidly prior to disruption,
which begins around 6.7 Gyr ago (fig 6.a). The accreted stars spread
throughout the halo by z=0.Gas from the satellites fall to the central region of
the galaxy. This is where new stars (blue) are born†.
0.9
0.8
sat 1
0.7
0.6
0.5
0.4
0.3
0.2
fig 8
0.1
0
12
10
8
6
4
Age/Gyr
satellite star formation rates
2
0
fig 6.c
fig 6.d
0.9
0.8
As previous for satellite 2 in the thermal feedback model. There are
~1400 baryon particles in sat 2, each of mass ~106 MΘ..
0.6
SFR (MΘ/yr)
Star formation is rapid between 11 and 8 Gyrs ago (fig 8 blue line).
By 5.7 Gyrs ago, accretion is well underway (fig 6.c & d) and star
formation has ceased within the dwarf. The accreted stars spread
throughout the halo by z=0, although a slight flattening may be
detected in fig 6.d . Gas from the satellite falls to the central region
and also the disk region of the galaxy. This is where new stars
(blue) are born†.
†a few new stars are born in the centre of the sattelite after disruption
has commenced, and these are seen to end up in the galactic halo.
sat 2
0.7
0.5
0.4
0.3
0.2
fig 8
0.1
0
12
10
8
6
Age/Gyr
satellite star formation rates
4
2
0
fig 7.a
fig 7.b
0.9
0.8
As previous for satellite 3 of the adiabatic feedback model. There are ~1000
Stars formation in the subclump is regulated (fig 8, purple line) prior to disruption.
Gas remains less densely concentrated than stars and is prefferentilly stripped.
The stripped gas accretes smoothly to the disk region. Stars from the satellite are
accreted into the halo of the galaxy by z=0. Pre-enriched gas stripped from the
satellites feeds the thin disk, where new stars (blue) are born over the past ~7.8
Gyrs.
0.6
SFR (MΘ/yr)
baryon particles in this satellite, each of mass
sat 3
0.7
~106 MΘ..
0.5
0.4
0.3
0.2
fig 8
0.1
0
12
10
8
6
4
Age/Gyr
satellite star formation rates
2
0
fig 7.c
fig 7.d
0.9
0.8
Again star formation is regulated in the subclump prior to accretion
(fig 8 green line). The stars which end up in the halo are the oldest
stars formed from the baryons in this subclump, as well as the most
metal poor. With pre-enriched gas stripped from the satellites feeding
the thin disk, the g-dwarf problem does not eventuate.
0.6
SFR (MΘ/yr)
As previous for satellite 4 of the adiabatic feedback model. There are
~1000 baryon particles in satellite 4, each of mass ~106 MΘ..
sat 4
0.7
0.5
0.4
0.3
0.2
fig 8
0.1
0
12
10
8
6
4
Age/Gyr
satellite star formation rates
2
0
Discussion
In the thermal feedback model, the gas cools to the centre of subhalos, rapidly forming stars (fig 8)
and dwarf galaxies with steep central density profiles. The rapid star formation cycles also result in
metallicities rapidly approaching ~solar levels. The satellites subsequently take a few Gyrs to
disrupt (fig 6), and the largest fraction of the baryons from these dwarfs end up in the halo of the
final galaxy. This is due to the rapid star formation meaning a high fraction of these baryons are
turned to stars prior to accretion. Star particles subsequently act dynamically like dark matter
particles, and are accreted into the halo. The halo thus becomes very massive, and very metal rich.
With adiabatic feedback, gas is slowed from cooling in star burst regions.
Star formation is regulated in dwarf galaxies and lower fractions of the initial baryonic content of
these dwarfs are converted to stars. The dwarfs thus have lower metallicty stars and shallower
stellar and gas central density profiles. The gas remains less centrally concentrated than the stars,
and is thus prefferentially stripped from the dwarfs. This pre-enriched gas accretes smoothly into
the forming galaxy, feeding the thin disk, and avoiding the g-dwarf problem. The stars which formed
in these dwarf galaxies are accreted preferentially onto the halo of the galaxy. As a smaller fraction
of baryons are turned into star particles in the dwarf galaxies of the adiabiatic feedback model, the
halo of the main galaxy is less massive, and more metal poor. Also, even if they are accreted into
the galaxy later than the gas, the stars from these dwarfs are older than the stars which
subsequently form in situ in the thin disk from gas accreted from the dwarfs: halo stars are old and
metal poor, disk stars are new and metal rich.
The result is a disk galaxy which is a far better realisation of the Milky Way or M31, or other typical
late type spirals.