Evolutionary Studies of the Low Metallicity Dwarf Irregular Galaxy UGCA 20
van Zee, L., Haynes, M.P., Salzer, J.J., & Broeils, A.H. 1996, AJ, 112, 129.
Abstract
Optical imaging, spectroscopic, and HI synthesis imaging observations
of the irregular dwarf galaxy UGCA 20 were undertaken to constrain the 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 HI emission result in a revised estimate forM_H/L_B of 2.8 M_sun/L_sun, a factor of 3 higher than typical
for dwarf galaxies. UGCA 20 is under luminous for its HI mass; its absolute
blue magnitude is -14.9 at an assumed distance of 12.8 Mpc. UGCA 20 has an
oxygen abundance of only 1/17th 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 population.
Based on a maximum--disk dark matter model, approximately 90% of the mass in
UGCA 20 is dark. Furthermore, the dynamical time scale of UGCA 20 is
quite short, slightly less than 1 Gyr, so it is unlikely 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 key to its star formation history is
that the majority of the HI gas appears to be below the nominal threshold required
for star formation. The minimal sites of active star formation are spatially
correlated with peaks in the HI column density. Only in those regions does the HI
column density exceed the threshold density.
Thus, in UGCA 20, star formation appears to have been occurring at a constant, but
suppressed rate for at least the last 4 Gyr. UGCA 20 has
an irregular HI distribution which is possibly indicative of
two components embedded in a common HI envelope.
ADS abstract