The effect of second generation populations on the integrated colors of metal-rich globular clusters in early-type galaxies

TL;DR

This paper proposes that second-generation stars with enhanced helium in globular clusters explain their bluer colors compared to field stars, addressing observed color discrepancies in early-type galaxies.

Contribution

It introduces a new model attributing the color differences to helium-enriched second-generation stars in GCs, complementing existing mass function hypotheses.

Findings

Helium-enriched second-generation stars explain bluer GC colors.

The model accounts for strong Balmer lines and blue UV colors.

Comparison shows the model's consistency with observations.

Abstract

The mean color of globular clusters (GCs) in early-type galaxies is in general bluer than the integrated color of halo field stars in host galaxies. Metal-rich GCs often appear more associated with field stars than metal-poor GCs, yet show bluer colors than their host galaxy light. Motivated by the discovery of multiple stellar populations in Milky Way GCs, we present a new scenario in which the presence of second-generation (SG) stars in GCs is responsible for the color discrepancy between metal-rich GCs and field stars. The model assumes that the SG populations have enhanced helium abundance as evidenced by observations, and it gives a good explanation of the bluer optical colors of metal-rich GCs than field stars as well as strong Balmer lines and blue UV colors of metal-rich GCs. Ours may be complementary to the recent scenario suggesting the difference in stellar mass functions (MFs) as an origin for the GC-to-star color offset. A quantitative comparison is given between the SG and MF models.