An ultraviolet-selected galaxy redshift survey - III. Multicolour imaging and non-uniform star formation histories

We present panoramic u' and optical ground-based imaging observations of a complete sample of low-redshift (0<0.4) galaxies selected in the ultraviolet (UV) at 2000 Angstrom using the balloon-borne FOCA instrument of Milliard et al. This survey is highly sensitive to newly formed massive stars and hence to actively star-forming galaxies. We use the new data to investigate further the optical, stellar population and star formation properties of this unique sample, deriving accurate galaxy types and k-corrections based on the broad-band spectral energy distributions. When combined with our earlier spectroscopic surveys, these new data allow us to compare star formation measures derived from aperture-corrected Halpha line fluxes, and UV(2000 Angstrom) and u' (3600 Angstrom) continuum fluxes on a galaxy-by-galaxy basis. As expected from our earlier studies, we find broad correlations over several decades in luminosity between the different dust-corrected star formation diagnostics, though the scatter is larger than that from observational errors, with significant offsets from trends expected according to simple models of the star formation histories (SFHs) of galaxies. Popular galaxy spectral synthesis models with varying metallicities and/or initial mass functions seem unable to explain the observed discrepancies. We investigate the star formation properties further by modelling the observed spectroscopic and photometric properties of the galaxies in our survey. We find that nearly half of the galaxies surveyed possess features that appear incompatible with simple constant or smoothly declining SFHs, favouring instead irregular or temporally varying SFHs. We demonstrate how this can reconcile the majority of our observations, enabling us to determine empirical corrections that can be used to calculate intrinsic star formation rates (as derived from Halpha luminosities) from measures based on UV (or u') continuum observations alone. We discuss the broader implications of our finding that a significant fraction of star-forming galaxies have complex SFHs, particularly in the context of recent determinations of the cosmic SFH.