THE OPTICAL COUNTERPART TO THE LARGE HI CLOUD IN VIRGO

We present a detailed optical imaging and spectroscopic study of the dwarf irregular galaxy located at the main peak of the large H I cloud in Virgo. The properties of this object uncovered by these new observations reemphasize it's unique and highly interesting nature. Deep images show that the optical counterpart has an extent of over 2 arcmin at low surface-brightness levels, and UBVR photometry shows that the entire galaxy is very blue: B - V = 0.10, U - B = - 0.57. Nebular abundances derived from the spectroscopic data reveal that this object is relatively unevolved chemically [log (O/H) + 12 = 7.66]. In addition, we searched for possible CO emission from the active star-forming region in the optical counterpart; no molecular emission was detected. Models which attempt to explain the observed oxygen and nitrogen abundances are presented. These, together with color evolution models, indicate that the stellar population in this galaxy is likely to be no older than roughly 1 Gyr. Between 72% and 100% of the B-band light from the low surface-brightness portions of the galaxy is being produced by stars with ages less than 40 Myr. Although the visual appearance of this object is that of a normal dwarf irregular galaxy, the extremely blue colors and immense H I cocoon suggest that it possesses a unique star-formation history.