Star formation in a complete spectroscopic survey of galaxies

The 15R-North galaxy redshift survey is a uniform spectroscopic survey (S/N similar to 10) covering the range 3650-7400 Angstrom for 3149 galaxies wit h median redshift 0.05. The sample is 90% complete to R = 15.4. The median slit covering fraction is 24% of the galaxy, apparently sufficient to minim ize the effects of aperture bias on the EW(H alpha). Forty-nine percent of the galaxies in the survey have one or more emission lines detected at grea ter than or equal to 2 sigma. In agreement with previous surveys, the fract ion of absorption-line galaxies increases steeply with galaxy luminosity. W e use H beta, [O III], H alpha, and [N II] to discriminate between star-for ming galaxies and AGNs. At least 20% of the galaxies are star-forming, at l east 17% have AGN-like emission, and 12% have unclassifiable emission. The unclassified 12% may include a "hybrid" population of galaxies with both st ar formation and AGN activity. The AGN fraction increases steeply with lumi nosity; the fraction of star-forming galaxies decreases. We use the EW(H al pha + [N II]) to estimate the Scalo birthrate parameter, b, the ratio of th e current star formation rate to the time averaged star formation rate. The median birthrate parameter is inversely correlated with luminosity in agre ement with the conclusions based on smaller samples (Kennicutt, Tamblyn, & Congdon). Because our survey is large, we identify 33 vigorously star-formi ng galaxies with b > 3. We confirm the conclusion of Jansen, Franx, & Fabri cant that EW([O II]) must be used with caution as a measure of current star formation. Finally, we examine the way galaxies of different spectroscopic type trace the large-scale galaxy distribution. As expected the absorption -line fraction decreases and the star-forming emission-line fraction increa ses as the galaxy density decreases. The AGN fraction is insensitive to the surrounding galaxy density; the unclassified fraction declines slowly as t he density increases. For the star-forming galaxies, the EW(H alpha) increa ses very slowly as the galaxy number density decreases. Whether a galaxy fo rms stars or not is strongly correlated with the surrounding galaxy density averaged over a scale of a few Mpc. This dependence reflects, in large par t, the morphology-density relation. However, for galaxies forming stars, th e stellar birthrate parameter is remarkably insensitive to the galaxy densi ty. This conclusion suggests that the triggering of star formation occurs o n a smaller spatial scale.