低金属量環境でのダストを
触媒とした水素分子形成
平下 博之 (台湾中央研究院)
内容
水素分子とダストの重要性
DLAの水素分子・ダストの観測
水素分子から探るガスの物理状態
星形成(分子形成)に対するダスト
の効果
5. まとめ
1.
2.
3.
4.
1. 水素分子とダストの重要性
水素分子 (H2)
• 宇宙の中で最も豊富な分子
• 星形成領域に付随（分子雲）
ダスト
• ダスト表面でH2形成
• 紫外線を吸収し、遠赤外域で再輻射
• 星間ガスの加熱・冷却過程
H2 形成・破壊に関する物理
水素分子 (H2)
自己遮蔽
光解離
分子形成
UV
Dust
shielding
Damped Lya Clouds (DLAs)
QSO
Damped Lya cloud
Lya absorption
• High H I column density (> 2×1020 cm–2)
Reservoir of a large amount of H I
⇒ progenitors of nearby large galaxies?
• Unique objects at high z for detailed study ISM by
using various species.
2. DLAの水素分子・ダストの観測
水素分子の吸収線
Ledoux et al. (2002)
ダストの存在
Depletion (太陽組成比に対する「欠乏」)
太陽組成比
Ledoux et al. (2002)
Dust and H2 in DLAs
Ledoux, Petitjean, & Srianand (2003)
H2の割合に
厳しい上限
metal depletion
log (molecular fraction)
log (molecular fraction)
相関あり
分散大
log (dust/gas)
3. 水素分子から探るガスの物理状態
H2が検出されているDLA (z ~ 2–3)の解析
J = 0, 1
J = 4, 5
Dust-to-gas ratio +
H2 fraction
励起温度T ~ ガスの温度
H2 formation rate 密度 n
||
H2 destruction rate
UV field
30 < n < 300 cm–3
30 < T < 300 K
3 < UV/UV(Galactic) < 30 “cold phase”
Hirashita & Ferrara (2005)
H2 の形成と破壊
H2 formation on dust
4×10–17(D/0.01) S (Tgas, Tdust) cm3 s–1
Hollenbach & McKee (1979)
H2 destruction (光解離)
self-shielding effect included
s–1
Abel et al. (1998)
平衡H2 fractionが分かる
Equilibrium Molecular Fraction
log (molecular fraction)
(n
[cm–3],
T [K])
NH = 1021 cm–2
UV = G0 (=Galactic)
(33, 300)
(10, 1000)
(3.3, 3000)
log (dust/gas)
self-shielding
◆: Ledoux et al. (2003)
H2 fraction
データの入る確率
95%
50%
30 < n < 300 cm–2
30 < T < 300 K
3 < UV/UV(Galactic) < 30
“Cold phase”
dust-to-gas ratio
物理状態の判別
log (H2 fraction)
High density
and low UV
Low density
and high UV
“cold phase”
30 < n < 300 cm–3
3 < c < 30
30 < T < 1000 K
log (dust-to-gas ratio)
H2 forms
in gas phase.
星形成率
3 < UV/UV(Galactic) < 30
星形成率面密度 = 0.005 – 0.05 Msun/yr/kpc2
典型的半径 = 3 kpc
(e.g. Kulkarni et al. 2000)
SFR = 0.1 – 1 Msun/yr
星形成率は通常の渦巻き銀河や
矮小銀河に類似
DLAの模擬観測
◆Numerical calculation (2D, vcir
Hirashita et al. (2003)
= 100 km/s, zform = 3)
Code: Wada & Norman (2001)
Density
1 kpc
Temperature
H2の分布
log (molecular fraction)
平衡となるH2量
(1) ダスト上での形成
(2) UVによる破壊
50 pc
(1) = (2)
i21 = 0.1, D = 0.1Dsun
H2は非常に非一様に分布し、小さな領域に局在する
観測シミュレーション
数値計算された銀河上で任意に視線を選び「観測」
log (molecular fraction)
相関
 log (D/Dsun) ~ –1.5で
molecular fraction急増
(←self-shielding)
 molecular fractionの
分散大
×: Ledoux et al. (2003)
◆: our simulation
log (dust-to-gas ratio)
4. 星形成(分子形成)に対するダストの
効果
Hirashita & Ferrara (2002); Hirashita & Hunt (2004)
We concentrate on young (t < 1 Gyr) galaxies.
(1) Dust is supplied by Type II SNe (m* > 8 Msun).
(2) Dust per SN = 0.4 Msun (Todini & Ferrara 2001).
(3) Galaxies are treated as one zone.
SFR (t) ⇒ SN II rate (t) ⇒ Mdust (t)
(Salpeter IMF)
Governed by free-fall time
Nearby BCDs as Laboratories
Hirashita et al. (2002); Hirashita & Hunt (2004, 2006)
Nearby blue compact dwarf galaxies (BCDs)
(low-metallicity and star-forming)
Vanzi et al. (2000)
300 pc
D = 53 Mpc
SBS 0335–052 (Zsun/41) is
genuinely young (< 5 Myr).
A “laboratory” for
high-z primeval galaxies.
Evolution of Dust Mass and
FIR Luminosity
Dust is concentrated
⇒ large t
Vanzi et al. (2000)
Vanzi et al. (2000)
Gas State
Dense and compact ⇒ rapid increase of dust optical depth
⇒ cooled and molecule rich
Diffuse region ⇒ (converse properties)
ISM Properties of 2 BCDs
Compact
Diffuse
SBS 0335–052 (1/41Zsun) I Zw 18 (1/50Zsun)
H2 → detected in NIR (Vanzi H2 → not detected
et al. 2000)
Dust → small extinction
Dust → large extinction (AV (AV = 0.2 mag) and not
=16 mag) and large
detected in FIR (Cannon et al.
luminosity in FIR (Hunt et al. 2002)
2001; Dale et al. 2001; Takeuchi SFR → small: 0.04– 0.1
et al. 2003)
Msun / yr (Cannon et al. 2002;
SFR → large: 1.7 Msun / yr
Hopkins et al. 2002)
(Hunt et al. 2001)
Those properties simultaneously explained!
5. Summary
(1) Our simulations of H2 distribution reproduce
a. Overall correlation between dust/gas ratio and H2
fraction
b. Clumpy H2 rich regions (⇒ lack of H2 detection)
c. Effect of self-shielding (⇒ large variation of H2
fraction)
(2) The physical state in DLAs indicates
a. The cold phase suggested by H2 detected objects
covers all the data in the likely range.
b. The upper limit data are consistent also with the
warm phase.
c. DLAs are objects with SFR ~ 0.1 – 1 Msun/yr.