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

     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 m_p = 15.4, and thus composed of galaxies brighter than M_p = -18.8 (H₀ = 100 km s^-1 Mpc^-1). The frequency distribution of the Hα equivalent width, determined for the first time from an optically complete sample, is approximately Gaussian, peaking at EW ∼ 25 Å. We find that, at the present limiting luminosity, the star formation properties of spiral + Irr galaxy 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, evolving as "closed systems" with exponentially declining SFR, and that the mass of their progenitor protogalaxies is the principal parameter governing their evolution. Massive systems having high initial efficiency of collapse, or a short collapse timescale, have retained little gas to feed the present epoch of star formation. These findings support the conclusions of Gavazzi & Scodeggio, who studied the color-mass relation of a local galaxy sample, and agree with the analysis by Cowie et al., who traced the star formation history of galaxies up to z > 1.