Nicholas John Smith, 2022
Title: A Systematic, Wide-Field Search for Stars Associated with ALFALFA Ultra-Compact High Velocity Clouds
Abstract: The Arecibo Legacy Fast ALFA (ALFALFA) survey was a blind neutral hydrogen (HI) survey that used the Arecibo radio telescope to identify 31,500 sources distributed over 7,000 square degrees of sky. Among these objects are roughly 100 Ultra Compact High-Velocity Clouds (UCHVCs), which have HI masses indicating they may reside in low-mass dark matter halos and velocities that suggest they may be located within or near the Local Group. Crucially, the UCHVCs don't have clear optical counterparts and are therefore ideal locations to search for stars associated with undiscovered dwarf galaxies.
This dissertation is a continuation of an optical survey of UCHVCs begun by our group. We observed 26 UCHVCs with the WIYN 3.5-m telescope, adding to the sample of 23 presented in Janesh et al. (2019). In addition to analyzing more UCHVCs, I corrected an error in the original star filtering method and added steps to the detection pipeline to improve the data quality and detection accuracy. I also implemented cross-matching of our source lists with Gaia and SDSS to remove possible contaminants from our star catalogs.
From the sample of UCHVCs we analyzed, we identified six dwarf galaxy candidates with absolute V-band magnitudes -2 to -8 and distances between 270 and 870 kpc. We compare the estimated optical properties of our most convincing candidate with predictions from theoretical simulations and a recent census of the Local Group and find them broadly consistent. We also investigate the possibility that one of the UCHVCs is a gas cloud associated with the Galactic globular cluster Pal 3. We identify two stellar over-densities with modest significance at similar distances to Pal 3 and near the HI source, but note that they would imply a much larger gas mass than one would expect to be associated with a globular cluster.