Spatial analysis of the Hα emission in the local star-forming UCM galaxies

We present a photometric study of the Halpha emission in the Universidad Complutense de Madrid (UCM) Survey galaxies. This work complements our previously published spectroscopic data. We study the location of the star-forming knots, their intensity and concentration, and the relationship of these properties with those of the host galaxy. We also estimate that the amount of Halpha emission that arises from the diffuse ionized gas is about 15% - 30% of the total Hα flux for a typical UCM galaxy. This percentage seems to be independent of the Hubble type. Conversely, we find that an "average'' UCM galaxy harbors a star formation event with 30% of its Hα luminosity arising from a nuclear component. The implications of these results for higher redshift studies are discussed, including the effects of galaxy size and the depth of the observations. A correlation between the star formation rate and the Balmer decrement is observed, but such correlation breaks down for large values of the extinction. Finally, we recalculate the Hα luminosity function and star formation rate density of the local universe using the new imaging data. Our results point out that, on average, spectroscopic observations detected about one-third of the total emission-line flux of a typical UCM galaxy. The new values obtained for the Halpha luminosity density and the star formation rate density of the local universe are 10^39.3±0.2 ergs s^-1 Mpc^-3 and ρ(SFR) = 0.016_-0.004^+0.007) M_⨀ yr-1 Mpc^-3 (H_0 = 50 km s^-1) Mpc^-1, Ω_M) = 1.0, Λ = 0). The corresponding values for the "concordance cosmology (H_0 = 70 km s^-1 Mpc^-1, Ω_M) = 0.3, Λ = 0.7) are 10^39.5±0.2 ergs s^-1 Mpc-3) and ρ_SFR = 0.029_-0.005^+0.008 M_⨀ yr^-1 Mpc^-3