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 o
f 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.
Ultraviolet, optical and near infrared images of the nearby (D ~ 5.5 Mpc) SBm galaxy NGC 1311, obtained with the Hubble Space Telescope, reveal a small population of 13 candidate star clusters. We identify candidate star clusters based on a combinati
on of their luminosity, extent and spectral energy distribution. The masses of the cluster candidates range from ~1000 up to ~100000 Solar masses, and show a strong positive trend of larger mass with increasing with cluster age. Such a trend follows from the fading and dissolution of old, low-mass clusters, and the lack of any young super star clusters of the sort often formed in strong starbursts. The cluster age distribution is consistent with a bursting mode of cluster formation, with active episodes of age ~10 Myr, ~100 Myr and ~1 Gyr. The ranges of age and mass we probe are consistent with those of the star clusters found in quiescent Local Group dwarf galaxies.
