The physical properties of star forming galaxies in the low redshift universe

We present a comprehensive study of the physical properties of ∼ 10 galaxies with measurable star formation in the SDSS. By comparing physical information extracted from the emission lines with continuum properties, we build up a picture of the nature of star-forming galaxies at z < 0.2. We develop a method for aperture correction using resolved imaging and show that our method takes out essentially all aperture bias in the star formation rate (SFR) estimates, allowing an accurate estimate of the total SFRs in galaxies. We determine the SFR density to be 1.948 −0.01(rand.) +0.11 −0.41(sys.) h7010 −2 M⊙/yr/Mpc at z = 0.1 (for a Kroupa IMF) and we study the distribution of star formation as a function of various physical parameters. The majority of the star formation in the low redshift universe takes place in moderately massive galaxies (10–10M⊙), typically in high surface brightness disk galaxies. Roughly 15% of all star formation takes place in galaxies that show some sign of an active nucleus. About 20% occurs in starburst galaxies. By focusing on the star formation rate per unit mass we show that the present to past-average star formation rate, the Scalo b-parameter; is almost constant over almost three orders of magnitude in mass, declining only at M∗ > 10M⊙. The volume averaged b parameter is 0.400 −0.007(rand.) +0.024 −0.084 (sys.)h 70 . We use this value constrain the star formation history of the universe. For the concordance cosmology the present day universe is forming stars at at least 1/3 of its past average rate. For an exponentially declining cosmic star formation history this corresponds to a time-scale of 7 −1.5Gyr. In agreement with other work we find a correlation between b and morphological type, as well as a tight correlation between the 4000Å break (D4000) and b. We discuss how D4000 can be used to estimate b parameters for high redshift galaxies.