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 A 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 (≃3 times as many as in our earlier study) across two FOCA fields. 273 of these are galaxies, nearly all with redshifts z≃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 Hα 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≃0.4 are intense star-forming galaxies. We find that the UV flux indicates a consistently higher mean star formation rate than that implied by the Hα 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–Hα 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.