Searching for optical counterparts to ultra-compact high velocity clouds:Possible detection of a counterpart to agc 198606

Abstract

We report initial results from a campaign to obtain optical imaging of Ultra Compact High Velocity Clouds (UCHVCs) discovered by the Arecibo Legacy Fast ALFA (ALFALFA) HI survey. UCHVCs have properties consistent with their being low-mass dwarf galaxies in the Local Volume, but do not have identified optical counterparts. We are using the WIYN 3.5 m telescope to image these objects and search for an associated stellar population. Here we present our observational strategy and method for searching for resolved stellar counterparts to the UCHVCs. We combine careful photometry, a color-magnitude filter, and spatial smoothing techniques to search for stellar overdensities in the g- and i-band images. We also run statistical tests to quantify the likelihood that detected overdensities are real and not chance superpositions of sources. We demonstrate the method by applying it to WIYN imaging of two objects: Leo P, a UCHVC discovered by ALFALFA and shown to be a star-forming dwarf galaxy in the Local Volume and AGC 198606, an ALFALFA source near in position and velocity to the Local Group dwarf galaxy Leo T. Applying the search method to the Leo P data yields an unambiguous detection (>99% confidence) of the galaxy's stellar population. Applying the method to the AGC 198606 imaging yields a possible detection (92% confidence) of an optical counterpart located similar to 2.5 arcmin from the centroid of AGC 198606's HI distribution and within the HI disk. We estimate a distance to the stellar counterpart of 373-393 kpc, an absolute magnitude M-i = -4.67 +/- 0.09, and an HI-to-stellar mass ratio of similar to 45-110.