The Hα galaxy survey -: IV.: Star formation in the local universe

Aims. We present an analysis of the star formation properties of field galaxies within the local volume out to a recession velocity limit of 3000 km s(-1). Methods. A parent sample of 863 star-forming galaxies is used to calculate a B-band luminosity function. This is then populated with star formation information from a subsample of 327 galaxies, for which we have H alpha imaging, firstly by calibrating a relationship between galaxy B-band luminosity and star formation rate, and secondly by a Monte Carlo simulation of a representative sample of galaxies, in which star formation information is randomly sampled from the observed subset. Results. The total star formation rate density of the local Universe is found to be between 0.016 and 0.023 M-circle dot yr(-1) Mpc(-3) with the uncertainties dominated by the internal extinction correction used in converting measured Ha fluxes to star formation rates. If our internally derived B-band luminosity function is replaced by one from the Sloan Digital Sky Survey blue sequence, the star formation rate densities are similar to 60% of the above values. We also calculate the contribution to the total star formation rate density from galaxies of different luminosities and Hubble T-types. The largest contribution comes from bright galaxies with M-B similar to -20 mag, and the total contribution from galaxies fainter than M-B = -15.5 mag is less than 10%. Almost 60% of the star formation rate density comes from galaxies of types Sb, Sbc or Sc; 9% from galaxies earlier than Sb and 33% from galaxies later than Sc. Finally, 75-80% of the total star formation in the local Universe is shown to be occurring in disk regions, defined as being > 1 kpc from the centres of galaxies. Conclusions. The star formation rate density estimates found here are consistent with values from the recent literature using a range of different star formation indicators. Even though they are numerous, dwarf galaxies contribute little to the star formation in the local Universe, and the bulk of the star formation takes place in L* spirals.