The Survey for Ionization in Neutral Gas Galaxies. II. The Star Formation Rate Density of the Local Universe

We derive observed Halpha and R band luminosity densities of an HI-selectedsample of nearby galaxies using the SINGG sample to be l_Halpha' = (9.4 +/-1.8)e38 h_70 erg s^-1 Mpc^-3 for Halpha and l_R' = (4.4 +/- 0.7)e37 h_70 ergs^-1 A^-1 Mpc^-3 in the R band. This R band luminosity density is approximately70% of that found by the Sloan Digital Sky Survey. This leads to a local starformation rate density of log(SFRD) = -1.80 +0.13/-0.07(random) +/-0.03(systematic) + log(h_70) after applying a mean internal extinctioncorrection of 0.82 magnitudes. The gas cycling time of this sample is found tobe t_gas = 7.5 +1.3/-2.1 Gyr, and the volume-averaged equivalent width of theSINGG galaxies is EW(Halpha) = 28.8 +7.2/-4.7 A (21.2 +4.2/-3.5 A withoutinternal dust correction). As with similar surveys, these results imply thatSFRD(z) decreases drastically from z ~ 1.5 to the present. A comparison of thedynamical masses of the SINGG galaxies evaluated at their optical limits withtheir stellar and HI masses shows significant evidence of downsizing: the mostmassive galaxies have a larger fraction of their mass locked up in starscompared with HI, while the opposite is true for less massive galaxies. We showthat the application of the Kennicutt star formation law to a galaxy having themedian orbital time at the optical limit of this sample results in a starformation rate decay with cosmic time similar to that given by the SFRD(z)evolution. This implies that the SFRD(z) evolution is primarily due to thesecular evolution of galaxies, rather than interactions or mergers. This isconsistent with the morphologies predominantly seen in the SINGG sample.