Transcript Slide 1

Thick Disk Formation
Chris Brook, Hugo Martel, Vincent Veilleux
Université Laval
Brad Gibson
Swinburne University, Melbourne, Australia
Daisuke Kawata
Carnegie Observatory
Introduction
• -review our knowledge of thick disks
• -show that our simulated gals have thin disk,
thick disk, & stellar halo components
• -thick disk formation in our simulated galaxies
• -merger history of Milky Way sized gals
• -properties of our simulated thick disks
• -relationship to halo formation
• -subsequent thin disk growth
Milky Way Thick Disk
• Gilmore & Reid (`83)- star counts
•
•
•
•
•
•
•
•
scale height~ 0.6-1 kpc (e.g. Phelps et al `99)
~5% of the mass of the thin disk
lags thin disk by ~ 40 km/s
dynamically hot
old ~10 Gyrs (e.g. Gilmore & Wyse `95)
-1<[Fe/H]<-0.2 (peak~-0.6)
no vertical metallicity gradient
distinct chemical abundance patterns
thin disk
thick disk
halo stars
□ dwarf spheroidal stars
Shetrone et al 2001, 2003
Geisler et al 2004
Venn et al. 2004
thick disk stars
thin disk stars
Bensby et al `03
Milky Way Thick Disk
• Gilmore & Reid (`83)- star counts
• large scale height~ 0.6-1 kpc (e.g. Phelps et al `99)
• ~5% of the mass of the thin disk
• lags thin disk by~40 km/s
• dynamically hot
• old stars ~10 Gyrs (e.g. Gilmore & Wyse `95)
• -1<[Fe/H]<-0.2 (peak~-0.6)
• no vertical metallicity gradient
• distinct chemical abundance patterns
Kinematics, metal abundances and ages support
the hypothesis it is a distinct component
Extra-Galactic Thick Disks
photometric observations
(Dalcanton & Bernstein `02)
• thick disks common in disk galaxies
• thick disk stars relatively old and metal rich
resolved stellar populations
(Mould `05; Davidge `05; Tikhonov `05;
Seth, Dalcanton & de Jong `05)
• more evidence that thick disks are common
• thick disk stars old, relatively metal rich
• lack of vertical colour gradient
• counter-rotating thick disk? (Yoachim & Dalcanton `05)
GCD+: Details of the Code
parallel chemo-dynamical galaxy evolution code
• tree N-body -DM & stars
• SPH -gas
• radiative cooling -metallicity (Sutherland & Dopita)
• SFR ~ ρ1.5
• supernovae feedback Ia (Kobayashi et al. 2000) & II
• metal enrichment:
H, He, C, N, O, Ne, Mg, Si, Fe
SNII (Woosley & Weaver 1994)
Intermediate (van den Hoek & Groenewgen 1997)
SNIa (Iwamoto 1999)
Simulation Results
Thick Disk?
Edvardsson 1993
Nordstrom `04
+, x: simulations
Star formation rate
Solar neighbourhood: 6 < RXY < 10 kpc |Z|< 1 kpc
[Fe/H]~-0.2
[Fe/H]~-0.6
[Fe/H]~-1.
Abrupt increase in velocity dispersion, as well as
period of rapid star formation, coincide with period
of chaotic merging of gas rich building blocks,
during which a central galaxy forms.
Abrupt increase in velocity dispersion, as well as
period of rapid star formation, coincide with period
of chaotic merging of gas rich building blocks,
during which a central galaxy forms.
Abrupt increase in velocity dispersion, as well as
period of rapid star formation, coincide with period
of chaotic merging of gas rich building blocks,
during which a central galaxy forms.
Abrupt increase in velocity dispersion, as well as
period of rapid star formation, coincide with period
of chaotic merging of gas rich building blocks,
during which a central galaxy forms.
168 N-body “Milky Way like” dark matter halos.
Traced the merger histories.
See also Zunter & Bullock `03
Number of merging building blocks vs lookback time.
•>1010 Msun.
•Contribute >4% mass of the halo (Quinn et al 1993)
•Be merged by the next timestep
Simulations:
 z~1
□ z~0.5
 z=0
Brook, Kawata, Martel Gibson,
Bailin, submitted to ApJ
Observations:
x z=0: Schwarzkopf & Dettmarr (`00)
o z~1: Reshetnikov, Dettmar & Combes (`03)
#1 T. Nykytyuk
Enhance star formation
and infall of pre-enriched
gas satisfy nicely the
criteria set by Kim Venn.
Halo: Robertson et al. `05
+ thick
X thin
radius
Scale length:
Thin 4.1 kpc
Thick 2.6 kpc
Scale height:
Thin~ 0.5 kpc
Thick~ 1.2 kpc
Formation scenarios Gilmore et al. (1989):
1) a slow, pressure supported collapse (Larson 1976);
2) Enhanced kinematic diffusion of the thin disk
stellar orbits (Norris 1987);
3) a rapid dissipational violent dynamical heating of
the early thin disk (Quinn et al. 1993, Jones &
Wyse 1983);
4) direct accretion of thick disk material (Statler 1988)
5) collapse triggered by high metallicity (Wyse &
Gilmore 1988).
-information of the metallicity, ages, and chemical
abundances of thick disk stars can be compared to
the predictions that the various scenarios make.
scenario 2 well supported by Galactic observations
(Quillen & Garret 2001; Wyse 2000; Gilmore et al.
2002; Freeman & Bland-Hawthorn 2002; Feltzing
et al. 2003).
scenario 3 also has contemporary support from
observations and simulations (Abadi et al. `03,
Helmi et al. `05, Yoachim & Dalcanton `05, but see
poster #60 Brooks & Governato, metallicity?)
Thick disk formation during the high redshift
epoch of multiple mergers of gas rich building
blocks is consistent with observations of the Milky
Way and extra-galactic thick disks.
Thick disk and thin disk material are spatially
well separated at high redshift
Support hierarchical models
Decoupled cores, counter-rotating disks
Two accretion events
Conclusions
Thick Disk: chemical abundance evidence suggest
seperate formation from thin disk
(although ongoing research req’d)
Early heating of thin disk most accepted model
(e.g. Freeman & Bland-Hawthorn 2000)
Recent observations suggest that old, metal rich thick
disks are prevalent (perhaps even ubiquitous) in disk
galaxies.
Our work suggests thick disk formed through chaotic
merging of gas rich “building blocks” at high redshift.
[email protected]
http://www.astro.phy.ulaval.ca/CHRIS/chris.html
Disk
• -overcooling
• -angular momentum
• -disk size
• -feedback from AGN, supernovae, solar
winds…
• -multi-phase gas
• -different recipes: thermal, kinematic, adiabatic
feedback, subgrid physics
• -resolution issues persist
• -regulate star formation in earliest forming
halos
SFRs
Reddy et al 2003