A Photometric Study of Star Clusters in Nearby Starburst Galaxies

Abstract

We present a photometric study of star clusters in three nearby starburst galaxies, M82, IC 10, and NGC 253. The star clusters are surveyed based on high-resolution multi-band images which cover wide-area including target galaxies. The ages of star clusters are estimated using spectral energy distribution fit method by comparing multi-band photometric data with the simple stellar population models. We find 1105 star clusters with V<23 mag in M82 based on the UBVI, YJ and H band Hubble Space Telescope images. Of those, 1070 star clusters are located in the disk region, while 35 are in the halo region. The luminosity function of the disk clusters shows a power-law distribution with a power law index alpha=-2.04+-0.03. The age distribution of the disk clusters shows that the most dominant cluster population has ages ranging from 100 Myr to 1 Gyr, with a peak at about 500 Myr. This suggests that M82 has undergone a disk-wide star formation about 500 Myr ago, probably through the interaction with M81. The brightest star clusters in the nuclear region are much brighter than those in other regions, indicating that more massive star clusters are formed in the denser environments. On the other hand, the colors of the halo clusters are similar to those of globular clusters in the Milky Way Galaxy, and their ages are estimated to be older than 1 Gyr. These are probably genuine old globular clusters in M82.

For IC 10, we find five new star clusters based on Subaru R-band images and NOAO Local Group Survey UBVRI images. All these star clusters are located far from the center of IC 10 (~650 pc to ~2 kpc), while previously known star clusters are mostly in the main body. Interestingly the distribution of these star clusters shows an asymmetrical structure elongated along the east and south-west direction. We derive UBVRI photometry of 66 star clusters including these new star clusters as well as previously known star clusters. We find that the star clusters in the halo are all older than 1 Gyr, while the star clusters in the main body region have various ages from very young (several Myr) to old (>1 Gyr). The young star clusters (<10 Myr) are mostly located in the Halpha emission regions and are concentrated on a small region (R<1 arcmin) at 2 arcmin in the south-east direction from the galaxy center, while the old star clusters (>1 Gyr) are distributed in a wider area than the disk. About ten intermediate-age star clusters (~100 Myr) are found in two groups. One is close to the location of the young star clusters and the other is at ~4 arcmin from the location of the young star clusters. The latter may be related with the past merger or tidal interaction.

For NGC 253, we survey star clusters based on the gVI band HST/ACS images that cover east-side of the disk of NGC 253. We find 762 star clusters with I<20.0 based on visual inspection, and about 88000 star clusters with 20.0 =< I < 24.0 are found from automatic selection method. The V-band luminosity functions of the star clusters show power-law distributions with power indices alpha~-3.0, those are steeper than that of young clusters in other late-type galaxies. gVI color-color diagram of the star clusters shows that most star clusters are highly reddened. The age distributions of the star clusters in the central region of NGC 253 show two dominant peaks at ~4 Myr and ~20 Myr. A mass function of the young star clusters are fitted using a Schechter function with a cut-off mass Mc = ~1.8 x 10^6 M_solar, and this value is relatively higher than those of other late-type galaxies. We suggest that the high cut-off mass is caused by the high cluster formation efficiency.