Low-metallicity Blue Compact Dwarfs as Templates for Primordial Star Formation

Understanding how galaxies formed their first stars is a vital cosmological question, but the study of high-redshift objects, caught in the act of forming their first stars, is difficult. Here we argue that two extremely low-metallicity Blue Compact Dwarf galaxies (BCDs), I Zw 18 and SBS 0335-052, could be local templates for primordial star formation, since both lack evolved (> 1Gyr) stellar populations; but they form stars differently. The main body of I Zw18 (1/50Z⊙) comprises two star-forming complexes (SFCXs), in the NW and the SE, which are surrounded by a smooth red envelope. Our new deep near-infrared images combined with HST/WFPC2 BV I show that the red extended region is significantly contaminated by ionized gas emission (Hunt et al. 2002). Such emission reddens the observed colors, mimicking the effect of age. Indeed, an analysis of five colors (B−H , V −K, V −I, J −H , H −K) in terms of stellar populations, extinction, and gas, suggests that the oldest stars in the outskirts of I Zw18 (and in the detached C component) can be no older than 500Myr. Unlike I Zw 18, where there are no super-star clusters (SSCs), virtually all of the star formation in SBS 0335-052 (1/40Z⊙) occurs in them. The underlying extended envelope is dominated by ionized gas emission rather than old stars (Vanzi et al. 2000). There is also evidence (Thuan et al. 1999, Hunt et al. 2001) for AV = 15mag extinction, caused by 10 − 10 M⊙ of dust. Our Brα measurement suggests that more than 3/4 of the star formation in SBS 0335-052 is obscured by dust, not clearly visible even at 2μm. Taken together, these results suggest that I Zw 18 and SBS 0335-052 form stars in very different ways, in spite of their similar metallicity and lack of old stars. We propose that dust and SSCs are associated with an “active” mode of star formation, and the lack of them with a “passive” mode. Theoretical models