The survey for ionization in neutral gas galaxies. II. The star formation rate density of the local universe

We derive observed Hα and R-band luminosity densities of an H I-selected sample of nearby galaxies using the SINGG sample to be l _Hα′ = (9.4 ± 1.8) × 10³⁷ h ₇₀ ergs s^-1 Å^-1 Mpc^-3 for Hα and l_R′ = 4.4 ± 0.7) × 10³⁷ h₇₀ ergs s^-1Å^-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(ρ̇_SER [M_⊙ yr^-1 Mpc ^-3]) = -1.80_0.07^-0.13 (random) ± 0.03(systematic) + log (h₇₀) after applying a mean internal extinction correction of 0.82 mag. The gas cycling time of this sample is found to be t_gas = 7.5_-2.1^+1.3 Gyr, and the volume-averaged equivalent width of the SINGG galaxies is EW(Hα) = 28.8_-4.7^+7.2 Å (21.2_3.5^+4.2 Å without internal dust correction). As with similar surveys, these results imply that ρ̇_SFRSFR(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 H I 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 ρ̇_SFR(Z) evolution. This implies that the ρ̇_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.