Thick Disk Formation

Chris Brook, Hugo Martel, Vincent VeilleuxUniversité 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, 2003Geisler et al 2004

Venn et al. 2004

Bensby et al `03

thick disk stars

thin disk stars

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

Simulation Results

Edvardsson 1993

+, x: simulations

Thick Disk?

Nordstrom `04

SFR

Star formation rate

Toomre metals1

Solar neighbourhood: 6 < RXY < 10 kpc |Z|< 1 kpc

[Fe/H]~-0.2

[Fe/H]~-0.6

[Fe/H]~-1.

Velocities

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.

Gal 2 ev

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.

Gal 2 ev

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.

Gal 2 ev

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.

Gal 2 ev

168 N-body “Milky Way like” dark matter halos.Traced the merger histories.

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

See also Zunter & Bullock `03

Observations:x z=0: Schwarzkopf & Dettmarr (`00)o z~1: Reshetnikov, Dettmar & Combes (`03)

Simulations: z~1□ z~0.5 z=0

Brook, Kawata, Martel Gibson,

Bailin, submitted to ApJ

#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

radius

+ thickX thin

Scale length:Thin 4.1 kpcThick 2.6 kpc

Scale height:Thin~ 0.5 kpcThick~ 1.2 kpc

Scalelengths/heights

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

Toomre metals2

Velocities

cbrook@phy.ulaval.cahttp://www.astro.phy.ulaval.ca/CHRIS/chris.html

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.

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