Metal-poor globular clusters ( GCs ) are our local link to the earliest epochs of star formation and galaxy building .
Studies of extragalactic GC systems using deep , high-quality imaging have revealed a small but significant slope to the color-magnitude relation for metal-poor GCs in a number of galaxies .
We present a study of the M87 GC system using deep , archival HST/ACS imaging with the F606W and F814W filters , in which we find a significant color-magnitude relation for the metal-poor GCs .
The slope of this relation in the I vs. V – I color-magnitude diagram ( \gamma _ { I } = -0.024 \pm 0.006 ) is perfectly consistent with expectations based on previously published results using data from the ACS Virgo Cluster Survey .
The relation is driven by the most luminous GCs , those with M _ { I } \lesssim - 10 , and its significance is largest when fitting metal-poor GCs brighter than M _ { I } = -7.8 , a luminosity which is \sim 1 Â mag fainter than our fitted Gaussian mean for the luminosity function ( LF ) of blue , metal-poor GCs ( \sim 0.8 Â mag fainter than the mean for all GCs ) .
These results indicate that there is a mass scale at which the correlation begins , and is consistent with a scenario where self-enrichment drives a mass-metallicity relationship .
We show that previously measured half-light radii of M87 GCs from best-fit PSF-convolved King models are consistent with the more accurate measurements in this study , and we also explain how the color-magnitude relation for metal-poor GCs is real and can not be an artifact of the photometry .
We fit Gaussian and evolved Schechter functions to the luminosity distribution of GCs across all colors , as well as divided into blue and red subpopulations , finding that the blue GCs have a brighter mean luminosity and a narrower distribution than the red GCs .
Finally , we present a catalog of astrometry and photometry for 2250 M87 GCs .