Finding gas-rich dwarf galaxies betrayed by their ultraviolet emission

TL;DR

This study uses ultraviolet imaging to identify potential gas-rich dwarf galaxies from HI cloud catalogs, revealing that nearly half of the compact HI clouds have UV counterparts similar to known dwarf galaxies, aiding future spectroscopic confirmation.

Contribution

It demonstrates that UV emission is an effective method to identify candidate dwarf galaxies among HI clouds, especially velocity outliers, and estimates the potential abundance of such galaxies across the sky.

Findings

48% of compact HI clouds have UV counterparts similar to dwarf galaxies.

UV emission effectively identifies promising dwarf galaxy candidates for follow-up.

Potentially hundreds of new tiny galaxies may exist across the sky based on HI and UV data.

Abstract

We present ultraviolet (UV) follow-up of a sample of potential dwarf galaxy candidates selected for their neutral hydrogen (HI) properties, taking advantage of the low UV background seen by the GALEX satellite and its large and publicly available imaging footprint. The HI clouds, which are drawn from published GALFA-HI and ALFALFA HI survey compact cloud catalogs, are selected to be galaxy candidates based on their spatial compactness and non-association with known high-velocity cloud complexes or Galactic HI emission. Based on a comparison of their UV characteristics to those of known dwarf galaxies, half (48%) of the compact HI clouds have at least one potential stellar counterpart with UV properties similar to those of nearby dwarf galaxies. If galaxies, the star formation rates, HI masses, and star formation efficiencies of these systems follow the trends seen for much larger galaxies. The presence of UV emission is an efficient method to identify the best targets for spectroscopic follow-up, which is necessary to prove that the stars are associated with the compact HI. Further, searches of this nature help to refine the salient HI properties of likely dwarfs (even beyond the Local Group). In particular, HI compact clouds considered to be velocity outliers relative to their neighbor HI clouds have the most significant detection rate of single, appropriate UV counterparts. Correcting for the sky coverage of the two all-Arecibo sky surveys yielding the compact HI clouds, these results may imply the presence of potentially hundreds of new tiny galaxies across the entire sky.