UV Properties of Galactic Globular Clusters with GALEX II. Integrated colors

TL;DR

This study provides a comprehensive catalog of UV integrated colors for 44 Galactic globular clusters, linking UV properties to stellar populations and galaxy environment, revealing new correlations with host galaxy mass.

Contribution

It presents the largest homogeneous UV color catalog for Galactic globular clusters and explores their relation to stellar populations and host galaxy characteristics.

Findings

UV colors correlate with horizontal branch morphology

Sagittarius dwarf galaxy clusters are redder in UV at same metallicity

Potential trend between cluster UV color and host galaxy mass

Abstract

We present ultraviolet (UV) integrated colors of 44 Galactic globular clusters (GGCs) observed with the Galaxy Evolution Explorer (GALEX) in both FUV and NUV bands. This data-base is the largest homogeneous catalog of UV colors ever published for stellar systems in our Galaxy. The proximity of GGCs makes it possible to resolve many individual stars even with the somewhat low spatial resolution of GALEX. This allows us to determine how the integrated UV colors are driven by hot stellar populations, primarily horizontal branch stars and their progeny. The UV colors are found to be correlated with various parameters commonly used to define the horizontal branch morphology. We also investigate how the UV colors vary with parameters like metallicity, age, helium abundance and concentration. We find for the first time that GCs associated with the Sagittarius dwarf galaxy have (FUV-V) colors systematically redder than GGCs with the same metallicity. Finally, we speculate about the presence of an interesting trend, suggesting that the UV color of GCs may be correlated with the mass of the host galaxy, in the sense that more massive galaxies possess bluer clusters.