低金属量銀河の星形成モード (Nagoya University) L. K. Hunt (Firenze) 　　 平下 博之 (H. Hirashita)　 　 (Nagoya University) L. K. Hunt (Firenze)

Contents: Star Formation in BCDs Theory – Star Formation and Dust Conclusions

1. Star Formation in BCDs Blue Compact Dwarfs Star formation (blue) I Zw 18 (Brown et al. 2002) Blue Compact Dwarfs Star formation (blue) Small (compact) Low metallicity ⇒ early stage of evolution 300 pc BCDs are nearby “laboratories” of high-z primeval galaxies.

“Active” and “Passive” BCDs Hunt, Hirashita, et al. (2003) log (size) [pc]→ “Active”: dense and compact “Passive”: diffuse Prototypes SBS 0335–052 (1/41 Zsun) I Zw 18 (1/50 Zsun) ←Surface brightness log (density) [cm–3]

ISM Properties SBS 0335–052 I Zw 18 H2 → detected in NIR (Vanzi et al. 2000) Dust → large extinction (AV =16 mag) and large luminosity in FIR (Hunt et al. 2001; Dale et al. 2001; Takeuchi et al. 2003) SFR → large: 1.7 Msun / yr (Hunt et al. 2001) I Zw 18 H2 → not detected Dust → small extinction (AV = 0.2 mag) and not detected in FIR (Cannon et al. 2002) SFR → small: 0.04– 0.1 Msun / yr (Cannon et al. 2002; Hopkins et al. 2002) Are those properties simultaneously explained?

2. Theory – Star Formation and Dust Hirashita & Hunt (2004) Dust is supplied by Type II SNe (m* > 8 Msun). Dust per SN = 0.4 Msun (Todini & Ferrara 2001). SFR (t) ⇒ SN II rate (t) ⇒ Mdust (t) 　　　　 (IMF) e Mgas/tff (Mgas, rSF) (e = 0.1)

Dust and Metals Consistent with the data of SBS 0335–052 and I Zw 18

Luminosities IR dust emission UV ←Compact “active” ←Diffuse “passive” “Active” star-forming regions tends to be infrared luminous.

Gas State Dense and compact ⇒ rapid increase of dust optical depth ⇒ cooled and molecule rich ← characteristic of “active” Diffuse region ⇒ (converse properties) ← “passive”

3. Conclusions The properties of “active” and “passive” BCDs are explained by dust accumulation in compact and diffuse regions. Gas temperature drops fast enough (a burst of star formation possible) in “active” regions. The “active” regions tend to be rich in molecules. The “active” regions tends to be IR luminous. The dust-to-gas ratio and metallicity are consistent with observations; ASTRO-F observations of dust in metal-poor (< 1/10 Zsun) BCDs will be useful.