The Young Stellar Population of the Nearby Late-Type Galaxy NGC 1311
  We have extracted PSF-fitted stellar photometry from near-ultraviolet,
optical and near-infrared images, obtained with the Hubble Space Telescope, of
the nearby (D ~ 5.5 Mpc) SBm galaxy NGC 1311. The ultraviolet and optical data
reveal a population of hot main sequence stars with ages of 2-10 Myr. We also
find populations of blue supergiants with ages between 10 and 40 Myr and red
supergiants with ages between 10 and 100 Myr. Our near-infrared data shows
evidence of star formation going back ~1 Gyr, in agreement with previous work.
Fits to isochrones indicate a metallicity of Z ~ 0.004. The ratio of blue to
red supergiants is consistent with this metallicity. This indicates that NGC
1311 follows the well-known luminosity-metallicity relation for late-type dwarf
galaxies. About half of the hot main sequence stars and blue supergiants are
found in two regions in the inner part of NGC 1311. These two regions are each
about 200 pc across, and thus have crossing times roughly equal to the 10 Myr
age we find for the dominant young population. The Luminosity Functions of the
supergiants indicate a slowly rising star formation rate (of 0.001 Solar masses
per year) from ~100 Myr ago until ~15 Myr ago, followed by a strong enhancement
(to 0.01 Solar Masses per year) at ~10 Myr ago. We see no compelling evidence
for gaps in the star-forming history of NGC 1311 over the last 100 Myr, and,
with lower significance, none over the last Gyr. This argues against a bursting
mode, and in favor of a gasping or breathing mode for the recent star-formation
history.
