An ultraviolet-selected galaxy redshift survey - II. The physical nature of star formation in an enlarged sample

We present further spectroscopic observations for a sample of galaxies selected in the vacuum ultraviolet (UV) at 2000 Angstrom from the FOCA balloon-borne imaging camera of Milliard et al, This work represents an extension of the initial study by Treyer et al. Our enlarged catalogue contains 433 sources (similar or equal to 3 times as many as in our earlier study) across two FOCA fields. 273 of these are galaxies, nearly all with redshifts z similar or equal to 0-0.4. Nebular emission-line measurements are available for 216 galaxies, allowing us to address issues of excitation, reddening and metallicity. The UV and H alpha luminosity functions strengthen our earlier assertions that the local volume-averaged star formation rate is higher than indicated from earlier surveys. Moreover, internally within our sample, we do not find a steep rise in the UV luminosity density with redshift over 0 < z < 0.4. Our data are more consistent with a modest evolutionary trend, as suggested by recent redshift survey results. investigating the emission-line properties, we find no evidence for a significant number of AGN in our sample; most UV-selected sources to z similar or equal to 0.4 are intense star-forming galaxies, We find that the UV flux indicates a consistently higher mean star formation late than that implied by the H alpha luminosity for typical constant or declining star formation histories. Following Glazebrook et al., we interpret this discrepancy in terms of a starburst model for our UV-luminous sources. We develop a simple algorithm which explores the scatter in the UV flux-H alpha relation in the context of various burst scenarios. Whilst we can explain most of our observations in this way, there remains a small population with extreme UV-optical colours which cannot be understood.