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.Comment: Astronomy and Astrophysics, in press, 16 pages, 8 figure
