THE COLOR-MAGNITUDE RELATION FOR METAL-POOR GLOBULAR CLUSTERS IN M87: CONFIRMATION FROM DEEPHST/ACS IMAGING

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 versus V-I color-magnitude diagram (γ I = −0.024 ± 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 −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, metalpoor 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 colormagnitude 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.

The inner halo of M 87: a first direct view of the red-giant population

“…In F814W there are 205 images totalling 73 800 s, while in F606W there are 49 images totalling 24 500 s. Co-adding the individual exposures in each filter yields an unusually deep pair of images at higher scale and a certain amount of gain in spatial resolution. A&A 524, A71 (2010) Composite exposures of these images have already been used for three separate studies of the population of 2000 globular clusters that lie within the area of this ACS pointing (Waters et al 2009;Peng et al 2009;Madrid et al 2009).…”

“…025 px −1 (half the native pixel size of the camera), and used the drizzle "point" (deltafunction) interpolation kernel, which is equivalent to a square kernel with a pixfrac parameter of zero. Note that in the previous globular cluster analyses of Peng et al (2009) and Madrid et al (2009) we generated combined images with the Gaussian kernel and a scale of 0. 035 px −1 .…”

The globular cluster systems of Abell 1185

“…Given the detection of distinct blue and red subpopulations, we looked for evidence of the so-called "blue tilt" in the colormagnitude relation (Fig. 9), an observed tendency for blue globular clusters to become progressively redder at brighter magnitudes (e.g., Harris et al 2006;Strader et al 2006;Peng et al 2009;Blakeslee et al 2010;Mieske et al 2010). In principle, the strength of the blue tilt, which is usually interpreted as a massmetallicity relation, could provide clues to the origin of the globular clusters in the center of A1185.…”

Massive star clusters in galaxies

“…An unexpected finding of new highprecision photometric surveys of GCSs in giant galaxies is a correlation between the mean colours of the blue-sequence GCs and their luminosities (e.g. Harris et al 2006;Mieske et al 2006;Spitler et al 2006;Strader et al 2006;Forte et al 2007;Cockcroft et al 2009;Harris 2009a,b;Peng et al 2009). Effectively, the blue GCs become progressively more heavy-element enriched at higher cluster mass.…”