Matthew N. Carlson

We have analyzed HST/WFPC2 images of NGC 3597 and find ~700 compact objects surrounding the galaxy with an average (B-R)₀ ~ 0.6. We propose that the majority of these objects are young globular clusters. They have a spread in colors that is consistent with that expected for a population of young clusters with a common age and spread induced by photometric errors and reddening within NGC 3597. If these objects were similar to the Galactic globular cluster system seen at a younger age, we would predict a turnover in the luminosity function at B ~ 23. However, we find that the luminosity function for the clusters is increasing to the limits of our photometry (B ~ 27).

We present an analysis of compact star clusters in deep Hubble Space Telescope Wide Field Planetary Camera 2 images of NGC 1275. B- and R-band photometry of roughly 3000 clusters shows a bimodality in the B - R colors, suggesting that distinct old and young cluster populations are present. The small spread in the colors of the blue clusters is consistent with the hypothesis that they are a single-age population, with an inferred age of 0.1 to 1 Gyr. The luminosity function shows increasing numbers of blue clusters to the limit of our photometry, which reaches several magnitudes past the turnover predicted if the cluster population is identical to current Galactic globular clusters seen at a younger age. The blue clusters have a spatial distribution that is more centrally peaked than that of the red clusters. The individual clusters are slightly resolved, with core radii 0.75 pc if they have modified Hubble profiles. We estimate the specific frequencies of the old and young populations and discuss the uncertainties in these estimates. We find that the specific frequency of the young population in NGC 1275 is currently larger than that of the old population and will remain so as the young population evolves, even if the majority of the low-mass clusters are eventually destroyed. If the young population formed during a previous merger, this suggests that mergers can increase the specific frequency of globular clusters in a galaxy. However, the presently observed young population likely contains too few clusters to have a significant impact on the overall specific frequency as it will be observed in the future.

We present an analysis of compact star clusters in deep HST/WFPC2 images of NGC 1275. B and R band photometry of roughly 3000 clusters shows a bimodality in the B-R colors, suggesting that distinct old and young cluster populations are present. The small spread in the colors of the blue clusters is consistent with the hypothesis that they are a single age population, with an inferred age of 0.1 to 1 Gyr. The luminosity function shows increasing numbers of blue clusters to the limit of our photometry, which reaches several magnitudes past the turnover predicted if the cluster population were identical to current Galactic globulars seen at a younger age. The blue clusters have a spatial distribution which is more centrally peaked than that of the red clusters. The individual clusters are slightly resolved, with core radii <~ 0.75 pc if they have modified Hubble profiles. We estimate the specific frequencies of the old and young populations and discuss the uncertainties in these estimates. We find that the specific frequency of the young population in NGC 1275 is currently larger than that of the old population and will remain so as the young population evolves, even if the majority of the low mass clusters are eventually destroyed. If the young population formed during a previous merger, this suggests that mergers can increase the specific frequency of globulars in a galaxy. However, the presently observed young population likely contains too few clusters to have a significant impact on the overall specific frequency as it will be observed in the future.