Spatial Analysis of the Hα Emission in the Local Star‐forming UCM Galaxies

We present a photometric study of the Halpha emission in the UniversidadComplutense de Madrid (UCM) Survey galaxies. This work complements ourpreviously-published spectroscopic data. We study the location of thestar-forming knots, their intensity, concentration, and the relationship ofthese properties with those of the host galaxy. We also estimate that theamount of Halpha emission that arises from the diffuse ionized gas is about15-30% of the total Halpha flux for a typical UCM galaxy. This percentage seemsto be independent of the Hubble type. Conversely, we found that an `average'UCM galaxy harbours a star formation event with 30% of its Halpha luminosityarising from a nuclear component. The implications of these results forhigher-redshift studies are discussed, including the effects of galaxy size andthe depth of the observations. A correlation between the SFR and the Balmerdecrement is observed, but such correlation breaks down for large values of theextinction. Finally, we recalculate the Halpha luminosity function and starformation rate density of the local Universe using the new imaging data. Ourresults point out that, on average, spectroscopic observations detected aboutone third of the total emission-line flux of a typical UCM galaxy. The newvalues obtained for the Halpha luminosity density and the star formation ratedensity of the local Universe are 10^(39.3+/-0.2) erg s-1 Mpc-3, andrho_SFR=0.016^(+0.007)_(-0.004) Mass_sun yr-1 Mpc-3 (H_0=50 km s-1 Mpc-1,Omega_M=1.0, Lambda=0). The corresponding values for the `concordancecosmology' (H_0=70 km s-1 Mpc-1, Omega_M=0.3, Lambda=0.7) are 10^(39.5+/-0.2)erg s-1 Mpc-3 rho_SFR=0.029^(+0.008)_(-0.005) Mass_sun yr-1 Mpc-3.