The H α Galaxy Survey ⋆ V . The star formation history of late - type galaxies

Aims. This study of 117 low-redshift Im and Sm galaxies investigates the star formation rates of late-type galaxies, to determine whether they are quasi-continuous or dominated by bursts with quiescent interludes. Methods. We analyse the distribution of star formation timescales (stellar masses/star formation rates) for the entire sample, and of gas depletion timescales for those galaxies with gas mass measurements. Results. We find that, on average, the late-type galaxies studied could have produced their total stellar masses by an extrapolation of their current star formation activity over a period of just under a Hubble time. This is not the case for a comparison sample of earlier-type galaxies, even those with disk-dominated morphologies and similar total stellar masses to the late-type galaxies. The earlier-type galaxies are on average forming their stars more slowly at present than the average rate over their past histories. No totally quiescent Im or Sm galaxies are found, and although some evidence of intrinsic variation in the star formation rate with time is found, this is typically less than a factor of 2 increase or decrease relative to the mean level. The Im and Sm galaxies have extensive gas reservoirs and can maintain star formation at the current rate for more than another Hubble time. The average spatial distribution of star formation in the Im galaxies, and to a lesser extent the Sm galaxies, is very similar to that of the older stellar population traced by the red light. Conclusions. Late type, bulge-free galaxies have a predominantly continuous mode of star formation, and could have assembled their stellar masses through continued star formation over a Hubble time with the currently-observed rate and spatial distribution. There is little evidence in this sample of predominantly isolated field galaxies of significant star formation through brief but intense starburst phases.