Stellar Populations of Dwarf Irregular Galaxies

Extract

We recently completed two-color CCD photometry of resolved stars in 11 dwarf irregular galaxies (Hopp & Schulte-Ladbeck, 1995, A&AS 111, 527), selected because of their relative isolation from massive galaxies in the Kran-Korteweg – Tammann sample (1979, AN 300, 181). The B-R color magnitude diagrams (CMD) show that all galaxies studied had star formation activity in the last ∼ 10⁸ yr. Several of them continue to form stars, the most active being UGC 5272 A (see Hopp & Schulte-Ladbeck, 1991, A&A 248, 1) while others, like DDO 210 (Hopp & Schulte-Ladbeck, 1994, ESO Conf. Workshop Proc. 49, 511), are pausing in their star formation activity. The CMDs enable us to select the brightest blue supergiants in these galaxies and to estimate their distances, D. Our values agree with the estimates based on the Virgo inflow model of Kran-Korteweg (1986, A&AS 66, 255) at the 30%-level. Prelimanary values are given in the table below.