2016ApJ...819...77L

We resolve a significant fraction of globular clusters (GCs) in NGC 4921, the brightest spiral galaxy in the Coma cluster. We also 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 3x10⁵ M_☉. The color distribution of the GCs in this galaxy is found to be bimodal. The turnover magnitudes of the luminosity functions of the blue (metal-poor) GCs (0.70 < (V - I) {#8804} 1.05) in the halo are estimated 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 to be 91±4 Mpc. Combining this with the Coma radial velocity, we derive a value of the Hubble constant, H₀= 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.