The star formation rate density of the local universe from an HI-selected sample of galaxies

We derive observed Halpha and R-band luminosity densities of an HI-selected sample of nearby galaxies using the SINGG sample to be l'Ha = (8.6 +/- 0.9) x 1038 h 70 ergs s-1 Mpc-3 for Halpha and l'R = (4.3 +/- 0.5) x 1037 h 70 erg s-1 A-1 Mpc-3 in the R band. This R-band luminosity density is approximately 70% of that found by the Sloan Digital Sky Survey. This leads to a local star formation rate density of log( r&d2;SFR [ M⊙ yr-1 Mpc-3]) = -1.81 +0.12-0.05 (random) +/- 0.03(systematic) + log(h70) after applying a mean internal extinction correction of 0.87 mag. The gas cycling time of this sample is found to be tgas = 7.3 +0.8-1.9 Gyr, and the volume-averaged equivalent width of the SINGG galaxies is EW (Halpha) = 28.5 +5.2-2.5 A (20.0 +/- 1.6 A without internal dust correction). As with similar surveys, these results imply that r&d2;SFR (z) decreases drastically from z ∼ 1.5 to the present. A comparison of the dynamical masses of the SINGG galaxies evaluated at their optical limits with their stellar and HI masses shows significant evidence of downsizing: the most massive galaxies have a larger fraction of their mass locked up in stars compared with H I, while the opposite is true for less massive galaxies. We show that the application of the Kennicutt star formation law to a galaxy having the median orbital time at the optical limit of this sample results in a star formation rate decay with cosmic time similar to that given by the r&d2;SFR (z) evolution. This implies that the r&d2;SFR (z) evolution is primarily due to the secular evolution of galaxies, rather than interactions or mergers. This is consistent with the morphologies predominantly seen in the SINGG sample.