Intermittent star-formation activities of dwarf irregular galaxies

On the basis of the propagating star-formation model, we investigated the star-formation activities of dwarf irregular galaxies (dIrrs) by considering two processes: the heating from stellar feedback and the cooling of the heated gas. After examining the timescales of the two processes, we propose that continuous global star-formation activity is difficult in dIrrs, since their small sizes make the feedback efficient and their small metallicities prevent the cooling from becoming effective. Thus, the intermittent nature of the star-formation activities of dIrrs, which is due to the small metallicity as well as the small size, is predicted. We emphasize that the size of a galaxy is an important factor concerning star-formation activity. The intermittence of the star-formation activity is also supported by the observed scatter of the UBV colors and Her equivalent widths of the dIrr sample. However, we note that efficient interstellar mixing may make the cooling time much shorter.