Astronomy and Astrophysics – Astronomy
Scientific paper
Jul 1996
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996aj....112..129v&link_type=abstract
Astronomical Journal v.112, p.129
Astronomy and Astrophysics
Astronomy
31
Galaxies: Irregular, Galaxies: Individual: Ugca 20, Galaxies: Evolution
Scientific paper
Optical imaging, spectroscopic, and H I 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 H I emission result in a revised estimate for M_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 H I 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 H I 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 H I column density. Only in those regions does the H I 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 H I distribution which is possibly indicative of two components embedded in a common HI envelope.
Broeils Adrick H.
Haynes Martha P.
Salzer John Joseph
van Zee Liese
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