Color-Magnitude Diagrams of Merged Globular Clusters: Metallicity Effects

Abstract

Mergers of globular clusters (GCs) once associated with dwarf spheroidal (dSph) galaxies have recently been suggested as an explanation for the bimodal horizontal branches (HBs) of some Galactic GCs, most notably NGC 1851, NGC 2808, and NGC 6229. Through analysis of the available color-magnitude diagrams for the GCs in the Fornax and Sagittarius dSph satellites of the Milky Way, as well as their metallicity distributions, we argue that the merger of two GCs would most likely produce a bimodal distribution in red giant branch (RGB) colors, or at least a significant broadening of the RGB, due to the expected difference in metallicity between the two merging globulars. No GC with a bimodal RGB is currently known, and the tightness of the RGB sequences in the above bimodal-HB GCs implies that a merger origin for their HB bimodality is unlikely.