A Neighboring Dwarf Irregular Galaxy Hidden by the Milky Way

We have obtained VLA and optical follow-up observations of the low-velocity HI source HIZSS 3 discovered by Henning et al. (2000) and Rivers (2000) in asurvey for nearby galaxies hidden by the disk of the Milky Way. Its radiocharacteristics are consistent with this being a nearby (~1.8 Mpc) low-massdwarf irregular galaxy (dIm). Our optical imaging failed to reveal a resolvedstellar population, but did detect an extended H-alpha emission region. Thelocation of the H-alpha source is coincident with a partially-resolved H Icloud in the 21-cm map. Spectroscopy confirms that the H$\alpha$ source has asimilar radial velocity to that of the H I emission at this location, and thuswe have identified an optical counterpart. The H$\alpha$ emission (100 pc indiameter and with a luminosity of $1.4\times10^{38}$ ergs s$^{-1}$) ischaracteristic of a single H II region containing a modest population of OBstars. The galaxy's radial velocity and distance from the solar apex suggeststhat it is not a Local Group member, although a more accurate distance isneeded to be certain. The properties of HIZSS 3 are comparable to those of GR8, a nearby dIm with a modest amount of current star formation. Furtherobservations are needed to characterize its stellar population, determine thechemical abundances, and obtain a more reliable distance estimate.Comment: Scheduled for the November 2003 issue of the Astronomical Journa