Intermittent Star-Formation Activities of Dwarf Irregular Galaxies

On the basis of the propagating star-formation model, we investigated thestar-formation activities of dwarf irregular galaxies (dIrrs) by consideringtwo processes: the heating from stellar feedback and the cooling of the heatedgas. After examining the timescales of the two processes, we propose thatcontinuous global star-formation activity is difficult in dIrrs, since theirsmall sizes make the feedback efficient and their small metallicities preventthe cooling from becoming effective. Thus, the intermittent nature of thestar-formation activities of dIrrs, which is due to the small metallicity aswell as the small size, is predicted. We emphasize that the size of a galaxy isan important factor concerning star-formation activity. The intermittence ofthe star-formation activity is also supported by the observed scatter of theUBV colors and H$\alpha$ equivalent widths of the dIrr sample. However, we notethat efficient interstellar mixing may make the cooling time much shorter.Comment: 6 pages LaTeX, to appear in PAS