Deep HST Observations of Star Clusters in NGC 1275

TL;DR

This study analyzes star clusters in NGC 1275 using HST data, revealing distinct old and young populations with different spatial distributions and luminosity functions, and discusses implications for galaxy merger history.

Contribution

It provides the first detailed analysis of star cluster populations in NGC 1275, identifying bimodal color distributions and spatial differences, and discusses their implications for galaxy evolution.

Findings

Bimodal color distribution indicating old and young clusters.

Young clusters are more centrally concentrated.

The young population's specific frequency exceeds that of the old.

Abstract

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.