The ACS Fornax Cluster Survey. IX. The Color-Magnitude Relation of Globular Cluster Systems

International audienceWe investigate the color-magnitude relation for globular clusters (GCs)---the so-called blue tilt---detected in the Advanced Camera for Surveys (ACS) Fornax Cluster Survey and using the combined sample of GCs from the ACS Fornax and Virgo Cluster Surveys. We find a tilt of gamma z ≡ d(g - z)/dz = -0.0257 ± 0.0050 for the full GC sample of the Fornax Cluster Survey (≈5800 GCs). This is slightly shallower than the value gamma z = -0.0459 ± 0.0048 found for the Virgo Cluster Survey GC sample (≈11,100 GCs). The slope for the merged Fornax and Virgo data sets (≈16,900 GCs) is gamma z = -0.0293 ± 0.0085, corresponding to a mass-metallicity relation of Z vprop M 0.43±0.12. We find that the blue tilt sets in at masses in excess of M ~ 2 × 105 M sun. The tilt is stronger for GCs belonging to high-mass galaxies (M * >~ 5 × 1010 M sun) than for those in low-mass galaxies (M *