  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. 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. 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.
