Outlying HII Regions in HI-Selected Galaxies

We present results from the first systematic search for outlying H II regions, as part of a sample of 96 emission-line point sources (referred to as ELdots-emission-line dots) derived from the NOAO Survey for Ionization in Neutral Gas Galaxies (SINGG). Our automated ELdot finder searches SINGG narrowband and continuum images for high equivalent width point sources outside the optical radius of the target galaxy (>2 x r(25) in the R band). Follow-up long-slit spectroscopy and deep Galaxy Evolution Explorer (GALEX) images (exposure time > 1000 s) distinguish outlying H II regions from background galaxies whose strong emission lines ([O III], H beta, or [O II]) have been redshifted into the SINGG bandpass. We find that these deep GALEX images can serve as a substitute for spectroscopic follow-up because outlying H II regions separate cleanly from background galaxies in color-color space. We identify seven SINGG systems with outlying massive star formation that span a large range in Ha luminosities corresponding to a few O stars in the most nearby cases, and unresolved dwarf satellite companion galaxies in the most distant cases. Six of these seven systems feature galaxies with nearby companions or interacting galaxies. Furthermore, our results indicate that some outlying H II regions are linked to the extended-UV disks discovered by GALEX, representing emission from the most massive O stars among a more abundant population of lower-mass (or older) star clusters. The overall frequency of outlying H II regions in this sample of gas-rich galaxies is 8%-11% when we correct for background emission-line galaxy contamination (similar to 75% of ELdots).