Far-Ultraviolet Imagery of the Edge-on Spiral Galaxy NGC 4631

Far-ultraviolet (FUV) imagery of the edge-on, Sc/SBd galaxy NGC 4631 reveals very strong FUV emission, resulting from active star formation, uniformly distributed along the galactic midplane. Multiband imagery, H I and H II position-velocity curves, and extinction considerations all imply that the emission is from the outer edges of the visible galaxy. The overall FUV morphology of this edge-on disk system is remarkably similar to those of the "chain galaxies" evident at high redshift, thus suggesting a similar interpretation for at least some of those distant objects. FUV, U, B, and V magnitudes, measured for 48 star-forming regions, along with corresponding Hα and Hβ measurements are used to construct diagnostic color-color diagrams. Although there are significant exceptions, most of the star-forming regions are less massive and older than 30 Doradus. Comparison with the expectations from two star formation models yields ages of 2.7 to 10 Myr for the instantaneous burst (IB) model and star formation cutoff ages of 0 to 9 Myr for the continuous star formation (CSF) model. Interpreted in terms of the IB model the photometry implies a total created mass in the 48 star-forming regions of 2.5 × 107 M☉. When viewed as resulting from constant star formation the photometry implies a star formation rate of 0.33 M☉ yr-1. These results are compared to those derived from FIR and radio observations. Corrections for FUV emission reprocessed by interstellar grains are estimated. A large ring, ~3 kpc in diameter, of 14 star-forming regions is concentrically located with an expanding H I shell toward the eastern end of the galaxy. Our observations imply that the shell may have been generated primarily by supernovae arising from 5.3 × 104 OB stars in a massive star-forming region beginning about 20 Myr ago, and that the presently observed FUV bright emission is due to second generation stars.