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低金属量銀河の星形成モード. 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. I Zw 18 (Brown et al. 2002). Blue Compact Dwarfs Star formation ( blue )
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低金属量銀河の星形成モード 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 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) “Active”: dense and compact “Passive”: diffuse log (size) [pc]→ Prototypes SBS 0335–052 (1/41 Zsun) I Zw 18 (1/50 Zsun) ←Surface brightness log (density) [cm–3]
ISM Properties SBS 0335–052 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?
SFR (t) ⇒ SN II rate (t) ⇒ Mdust (t) (IMF) eMgas/tff (Mgas, rSF) (e = 0.1) 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).
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.
