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Abstract

Isolated neutral hydrogen (HI) clouds, or "ultra-compact high-velocity clouds" (UCHVCs), form a small subset of detections by the Arecibo Legacy Fast ALFA (ALFALFA) blind extragalactic survey. These systems, if located within ~1 Mpc, would populate the lowest-mass end of the HI mass function. Subsequent optical imaging has revealed that some of these UCHVCs harbor associated (thought sparse) stellar populations, suggesting that they may be some of the most extreme galaxies known in the Local Volume, with properties akin to ultra-faint dwarf galaxies but with significant neutral gas reservoirs. In this work, we investigate the neutral hydrogen properties of six UCHVC candidate galaxies using deep spectral line imaging from the Karl G. Jansky Very Large Array (VLA). We produce images and discuss the morphological and kinematic properties of six sources: AGC 198606, AGC 215417, AGC 219656, AGC 249525, AGC 258237, and AGC 268069. We also apply the modeling code 3D-Barolo to our deep HI images in order to derive rotation curves and constrain inclination angles for our two most promising galaxy candidates: AGC 198606 and AGC 249525. Successful modeling allows us to determine the dynamical masses of these objects and thus to consider them in the context of various fundamental scaling relations defined by more massive galaxies.

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