EVOLUTIONARY STUDIES OF THE LOW METALLICITY DWARF IRREGULAR GALAXY UGCA-20

Optical imaging, spectroscopic, and H I synthesis imaging observations of the irregular dwarf galaxy UGCA 20 were undertaken to constrain th e star formation history and potential for future star formation. The striking features of UGCA 20 are its observed low metal abundance and high ratio of M(H)/L(B). New observations of the optical and H I emiss ion result in a revised estimate for M(H)/L(B) of 2.8 M(.)/L(.), a fac tor of 3 higher than typical for dwarf galaxies. UGCA 20 is under lumi nous for its H I mass; its absolute blue magnitude is -14.9 at an assu med distance of 12.8 Mpc. UGCA 20 has an oxygen abundance of only 1/17 th of solar; however, the observed colors, U-B=-0.20+/-0.03, B-V=0.36/-0.01, and V-R=0.50+/-0.01, indicate the presence of an old stellar p opulation. Based on a maximum-disk dark matter model, approximately 90 % of the mass in UGCA 20 is dark. Furthermore, the dynamical time scal e of UGCA 20 is quite short, slightly less than 1 Gyr, so it is unlike ly that UGCA 20 has been caught in the process of collapse. Rather, it appears that the evolution of the system has been ''hung up.'' One ke y to its star formation history is that the majority of the H I gas ap pears to be below the nominal threshold required for star formation. T he minimal sites of active star formation are spatially con-elated wit h peaks in the H I column density. Only in those regions does the H I column density exceed the threshold density. Thus, in UGCA 20, star fo rmation appears to have been occurring at a constant, but suppressed r ate for at least the last 4 Gyr. UGCA 20 has an irregular H I distribu tion which is possibly indicative of two components embedded in a comm on H I envelope. (C) 1996 American Astronomical Society.