A long standing problem in cosmology is the mismatch between the number of low mass dark matter halos predicted by simulations and the number of low mass galaxies observed in the Local Group. We recently presented a set of isolated ultra-compact high velocity clouds (UCHVCs) identified within the dataset of the Arecibo Legacy Fast ALFA (ALFALFA) HI line survey that are consistent with representing low mass gas-bearing dark matter halos within the Local Group (Adams+ 2013). At distances of ~1 Mpc, the UCHVCs have HI masses of ~10^5 Msun and indicative dynamical masses of ~10^7 Msun. The HI diameters of the UCHVCs range from 4' to 20', or 1 to 6 kpc at a distance of 1 Mpc. We have selected the most compact and isolated UCHVCs with the highest average column densities as representing the best galaxy candidates. These systems have been observed with the Westerbork Synthesis Radio Telescope (WSRT) to enable higher spatial resolution studies of the HI distribution. From these data, the sources break into two clear categories. Two of the sources maintain a smooth HI morphology at higher resolution, show a velocity gradient and have the highest peak column densities of the sample, indicating they are good candidates to represent gas in dark matter halos. In fact, one of these sources, AGC 198606, has a tentative stellar counterpart detection (Janesh+ 2015). Nine of the sources break into clumps at higher angular resolution, show no ordered velocity motion, and have significantly lower peak column densities, indicating they are likely Galactic halo HI clouds. One source straddles the two categories with a relatively smooth HI morphology and some evidence for ordered velocity motion while having a lower peak column density. These observations show that higher resolution HI data is a good way to address the galaxy hypothesis for isolated HI clouds, and future HI surveys with phased-array feeds on interferometers, such as Apertif, will be able to directly detect and discriminate among these sources.
|Status||Published - 2016|