Paper detail

Outlying HII Regions in HI-Selected Galaxies

We present results from the first systematic search for outlying HII 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 narrow-band and continuum images for high equivalent width point sources outside the optical radius of the target galaxy (> 2 X r25 in the R-band). Follow-up longslit spectroscopy and deep GALEX images (exposure time > 1000 s) distinguish outlying HII regions from background galaxies whose strong emission lines ([OIII], Hbeta or [OII]) 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 HII 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 Halpha 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 HII 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 HII regions in this sample of gas-rich galaxies is 8 - 11% when we correct for background emission-line galaxy contamination (~75% of ELdots).