The role of dust in active and passive low-metallicity star formation

We investigate the role of dust in star formation activity of extremely metal-poor blue compact dwarf galaxies (BCDs). Observations suggest that star formation in BCDs occurs in two different regimes: active and passive. The active BCDs host super star clusters (SSCs), and are characterised by compact size, rich H-2 content, large dust optical depth, and high dust temperature; the passive BCDs are more diffuse with cooler dust, and lack SSCs and large amounts of H-2. By treating physical processes concerning formation of stars and dust, we are able to simultaneously reproduce all the above properties of both modes of star formation (active and passive). We find that the difference between the two regimes can be understood through the variation of the compactness of the star-forming region: an active mode emerges if the region is compact (with radius less than or similar to50 pc) and dense (with gas number density greater than or similar to500 cm(-3)). The dust, supplied from Type II supernovae in a compact star-forming region, effectively reprocesses the heating photons into the infrared and induces a rapid H-2 formation over a period of several Myr. This explains the high infrared luminosity, high dust temperature, and large H-2 content of active BCDs. Moreover, the gas in active galaxies cools (less than or similar to300 K) on a few dynamical timescales, producing a run-away star formation episode because of the favourable (cool) conditions. The mild extinction and relatively low molecular content of passive BCDs can also be explained by the same model if we assume a diffuse region (with radius greater than or similar to100 pc and gas number density less than or similar to 100 cm(-3)). We finally discuss primordial star formation in high-redshift galaxies in the context of the active and passive star formation scenario.