Star formation in forming galactic disks under UV field

by RHD simulation

Hajime Susa 　Department of Physics, Konan university,

Kobe, Japan

Susa ApJ 684, 226, 2008

Ultraviolet radiation field （ >13.6eV ）

Physical processes heating ： T ～ 104 K

ionization+dissociation

background I21= 0.1 - 1 @ z= 2 – 5

I21=0.01- 0.001 @ z=0

Keep the gas in the proto-galaxies hot ( ～ 104 K).

Feedback on star formation?

Around forming gals I21 ～ 1-100 (star formation)

I21 ～ 100(AGN @10kpc)

Formation of dwarf gals.

M=1e8MsunZc ～ 5zreion ～ 7

Susa & Umemura 2004ab

M < 1e8 Msun→ SF shutdown

Aim of this work

• Assess the effects of UV on massive DM halos with Tvir > 104K. Star formation in these systems are regulated or not ?

As a first step, we investigate the stability of gas disks in massive halos under UV field, utilizing radiation hydrodynamics simulations.

RSPH code

• Parallel BH Tree• SPH• Domain decomposition : ORB• RT solver (Ray-Tracing)• Implicit solver for reactions and energy

equation• H2• On-the-Spot approximation (Case B recom.)• Multiple sources ( ～ < 10 ）• Any Spectrum

Susa 2006 (http://ads.nao.ac.jp/abs/2006PASJ...58..445S)

The model

1 2 1 2100 0 3 5-1kms kpc, kpc, .v v a a Same as Wada & Norman 2007except rotation velocity

21

100

0 05 0 1 0 3

0 1

3

300

110

H

M

I

R

T

m M

-3ini

disk

min

sph

pc

pc

or

kpc

K

=

: . , . , . ¤

¤Uniform disk with slight perturbations

radiation

radiation

Resolution

Mass resolution :

Maximally resolved density: (MJ～ Mres)

42 1 1 10N m M nei SPH . ¤

3

2

3 5 1235

4 2 2

kTn

m Gm m N

-3BH,res

p p SPH nei

cmmin

Gravity Softening: ( ～ minimal SPH softening)

1 3

313

2 4

N mmn

nei SPH

p H,res

pc

/

Bate & Burkert 1997

montage: I21=0

Disks in all modes (A 、 B 、 C) are unstable → fragmentation & star formation

21 -2H 1 3 10 cmN . 20 -2

H 6 6 10 cmN . 21 -2H 4 0 10 cmN .

Toomre Ｑ and the tmperature PDF(I21=0)

Unstable

Stable

AB

C

min@Q T T

Tmin

scQG

Tmin Tmin

diskR

montage: I21=1

C: still unstable → fragmentation & star formation

A, B : stable → star formation is strongly suppressed.

21 -2H 1 3 10 cmN . 20 -2

H 6 6 10 cmN . 21 -2H 4 0 10 cmN .

Density PDFs

No dense components

21 0I

21 1I

Distribution along z-axis

density

temperature

HI Significant self-shielding effect

Temperature PDFs0 06Mass fraction ( ) per / log .totalM M T

Ar, Br : unshielded →Hot component dominates→ large Q → stable

Ｃｒ : shielded 　→Cold component dominates→ small Q → unstable

Threshold column density for self-shielding

Numerical results : NH,sh ～ 1-4×1021cm-2

Can UV photons suppress the SF in disks hosted by massive dark halos?

Hierarchical GF

Dynamical effects of merging (shock induced SF etc.) Inhomogeneous distribution of heavy elements Radiation from forming stars in building blocks, AGN

Fresh Gas accretion Fresh Gas accretion

On going………

Analytical model of Cosmological disks (1/2)

• Dark halos parameterized by （ｚ c, M, λ ）

• Assume half of the baryons settle onto the galactic disk

• Keplar rotation & hydrostatic equilibrium in z-direction

• NH, disk > NH,sh ?• Q >1 ?

Analytical model of Cosmological disks （ 2/2 ）

H ,disk H ,shN N410 K = 1Q@

210 K = 1Q@

Unshielded& stable

Unshielded& stable

Bimordality of galaxies

Ste

llar

ag

eo

ld

　

new

Resent SF for galaxies with

Mstar < 1010Msun

(Kauffman, G. et al, MNRAS, 341, 54, 2003)

Some process to delay the SF in less massive galaxies is required.

Photoheating

Summary• We perform radiation hydrodynamics

simulations on the fragmentation of galactic disks under the ultraviolet radiation background.

• We find that ultraviolet radiation field strongly suppress the star formation in the disks in case the photoheating is not shielded enough.

• In our simulations, we find a threshold column density of the disk ( ～ 1021(I21/1)cm-2) above which the fragmentation is not suppressed.

• Presence of such critical threshold would be one of the reason for the so-called down-sizing problem in nearby galaxies. 　

Tours Symposium 2009

• Nov. 16-20 @ Tours, France• Nuclear Physics, High energy

astroparticle physics, Astrophysics• Astrophysics: “First & Second generation stars”

SOC Ferrara, Schneider, Umemura, Omukai, Tominaga, Susa