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 sele cted in the vacuum ultraviolet (UV) at 2000 Angstrom from the FOCA balloon- borne imaging camera of Milliard et al, This work represents an extension o f the initial study by Treyer et al. Our enlarged catalogue contains 433 so urces (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 sim ilar or equal to 0-0.4. Nebular emission-line measurements are available fo r 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 th an 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 emissio n-line properties, we find no evidence for a significant number of AGN in o ur sample; most UV-selected sources to z similar or equal to 0.4 are intens e 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 Glaz ebrook et al., we interpret this discrepancy in terms of a starburst model for our UV-luminous sources. We develop a simple algorithm which explores t he 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, ther e remains a small population with extreme UV-optical colours which cannot b e understood.