The Star Formation Properties of Disk Galaxies: Hα Imaging of Galaxies in the Coma Supercluster

Abstract

We present integrated Hα measurements obtained from imaging observations of 98 late-type galaxies, primarily selected in the Coma supercluster. These data, combined with Hα photometry from the literature, include a magnitude selected sample of spiral (Sa to Irr) galaxies belonging to the “Great Wall ” complete up to mp = 15.4, thus composed of galaxies brighter than Mp = −18.8 (H0 = 100 km Mpc −1 s −1). The frequency distribution of the Hα E.W., determined for the first time from an optically complete sample, is approximately gaussian peaking at E.W. ∼ 25 ˚A. We find that, at the present limiting luminosity, the star formation properties of spiral+Irr galaxies members of the Coma and A1367 clusters do not differ significantly from those of the isolated ones belonging to the Great Wall. The present analysis confirms the well known increase of the current massive star formation rate (SFR) with Hubble type. Moreover perhaps a more fundamental anticorrelation exists between the SFR and the mass of disk galaxies: low-mass spirals and dwarf systems have present SFRs ∼ 50 times higher than giant spirals. This result is consistent with the idea that disk galaxies are coeval, evolve as “closed systems ” with exponentially declining SFR and that