Bradley C. Whitmore

The Globular Cluster System in the Inner Region of M87

We have identified 1057 globular cluster candidates in a WFPC2 image of the inner region of M87. The globular cluster luminosity function (GCLF) can be well fitted by a Gaussian profile with a mean value of m0V=23.67±0.07 mag and σ=1.39±0.06 mag. The GCLF in five radial bins is found to be statistically the same at all points, showing no clear evidence of dynamical destruction processes based on the luminosity function (LF). Similarly, there is no obvious trend between the half-light radius of the clusters and the galactocentric distance. The core radius of the globular cluster density distribution is Rc=56, considerably larger than the core of the stellar component (Rc=68). The mean color of the cluster candidates is V-I=1.09 mag, which corresponds to an average metallicity of Fe/H=-0.74 dex. The color distribution is bimodal everywhere, with a blue peak at V-I=0.95 mag and a red peak at V-I=1.20 mag. The red population is only 0.1 mag bluer than the underlying galaxy, indicating that these clusters formed late in the metal-enrichment history of the galaxy and were possibly created in a burst of star/cluster formation 3-6 Gyr after the blue population. We also find that both the red and the blue cluster distributions have a more elliptical shape (Hubble type E3.5) than the nearly spherical galaxy. The average half-light radius of the clusters is ≈2.5 pc, which is comparable to the 3 pc average effective radius of the Milky Way clusters, although the red clusters are ≈20% smaller than the blue ones.