The halo globular clusters of the giant elliptical galaxy Messier 87

Abstract

New photographic UBR photometry of about 1700 stellar images surrounding the Virgo giant elliptical M87 is presented. Over the measured radial range (1.5 to 9.0 arcmin), the M87 clusters become systematically bluer (more metal-poor) with increasing galactocentric distance. The color gradient in the cluster system has the same slope as that for the M87 integrated halo light, but the clusters are bluer by Delta(U-R) = 0.5 at all radii. The cluster galaxies are not correlated with color, and the cluster luminosity function that is derived here is similar to that observed previously by Hanes and appears similar in all radial regions. It is suggested from the results that galaxies were not likely to have been formed by the accumulation of masses of globular cluster size or by mergers of separate smaller galaxies.