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

Abstract

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 H2 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 H2. 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 $\la 50$ pc) and dense (with gas number density $\ga 500$ 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 H2 formation over a period of several Myr. This explains the high infrared luminosity, high dust temperature, and large H2 content of active BCDs. Moreover, the gas in "active" galaxies cools ($\la 300$ 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 $\ga 100$ pc and gas number density $\la 100$ cm$^{-3}$). We finally discuss primordial star formation in high-redshift galaxies in the context of the "active" and "passive" star formation scenario.