THE COLOR-MAGNITUDE RELATION FOR METAL-POOR GLOBULAR CLUSTERS IN M87: CONFIRMATION FROM DEEPHST/ACS IMAGING
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abstract
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 ($\gamma_I=-0.024\pm0.006$) 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 $M_I<-10$, and its significance is largest when fitting metal-poor GCs
brighter than $M_I=-7.8$, a luminosity which is ~1 mag fainter than our fitted
Gaussian mean for the luminosity function (LF) of blue, metal-poor GCs (~0.8
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 cannot 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.
