Globular Clusters and Spur Clusters in NGC 4921, the Brightest Spiral Galaxy in the Coma Cluster

TL;DR

This study resolves globular clusters in NGC 4921, finds bright star clusters in its arms, and uses GC luminosity functions to estimate galaxy distances and the Hubble constant, revealing galaxy transition stages.

Contribution

It provides detailed analysis of GCs and star clusters in NGC 4921, and introduces a method to estimate galaxy distances and Hubble constant from GC luminosity functions.

Findings

Bimodal color distribution of GCs in NGC 4921.

Estimated distance to Coma galaxies is 91 Mpc.

Derived Hubble constant as 77.9 km/s/Mpc.

Abstract

We resolve a significant fraction of globular clusters (GCs) in NGC 4921, the brightest spiral galaxy in Coma. Also we find a number of extended bright star clusters (star complexes) in the spur region of the arms. The latter are much brighter and bluer than those in the normal star-forming region, being as massive as 3x10^5 M_odot. The color distribution of the GCs in this galaxy is found to be bimodal. The turnover magnitudes of the luminosity functions (LF) of the blue (metal-poor) GCs (0.70<(V-I)<1.05) in the halo are estimated to be V(max) =27.11+-0.09 mag and I(max)=26.21+-0.11 mag. We obtain similar values for NGC 4923, a companion S0 galaxy, and two Coma cD galaxies (NGC 4874 and NGC 4889). The mean value for the turnover magnitudes of these four galaxies is I(max)=26.25+-0.03 mag. Adopting M_I (max) = -8.56+-0.09 mag for the metal-poor GCs, we determine the mean distance to the four Coma galaxies, 91+-4 Mpc. Combining this and the Coma radial velocity, we derive a value of the Hubble constant, H_0 = 77.9+-3.6 km/s/Mpc. We estimate the GC specific frequency of NGC 4921 to be S_N = 1.29+-0.25, close to the values for early-type galaxies. This indicates that NGC 4921 is in the transition phase to S0s.