Globular clusters in the outer Galactic halo: new HST/ACS imaging of 6 globular clusters and the Galactic globular cluster age-metallicity relation

TL;DR

This study uses HST imaging to analyze the ages and metallicities of six outer halo globular clusters, revealing two distinct formation histories for inner and outer halo clusters and supporting galaxy assembly models.

Contribution

It provides new age estimates for six globular clusters using HST data, highlighting differences in formation history between inner and outer halo clusters.

Findings

Outer halo GCs are 1-2 Gyr younger than inner halo GCs with similar metallicities.

Outer halo GCs show evidence of prolonged formation, consistent with accretion from dwarf galaxies.

The GC age-metallicity relation has two distinct branches indicating different formation processes.

Abstract

Color-magnitude diagrams (CMDs) derived from Hubble Space Telescope (HST) Advanced Camera for Surveys F606W,F814W photometry of 6 globular clusters (GCs) are presented. The six GCs form two loose groupings in Galactocentric distance (Rgc): IC 4499, NGC 6426, and Ruprecht 106 at ~15-20 kpc and NGC 7006, Palomar 15, and Pyxis at ~40 kpc. The CMDs allow the ages to be estimated from the main sequence turnoff in every case. In addition, the age of Palomar 5 (Rgc ~ 18 kpc) is estimated using archival HST Wide Field Planetary Camera 2 V,I photometry. The age analysis reveals the following: IC 4499, Ruprecht 106, and Pyxis are 1-2 Gyr younger than inner halo GCs with similar metallicities; NGC 7006 and Palomar 5 are marginally younger than their inner halo counterparts; NGC 6426 and Palomar 15, the two most metal-poor GCs in the sample, are coeval with all the other metal-poor GCs within the uncertainties. Combined with our previous efforts, the current sample provides strong evidence that the Galactic GC age-metallicity relation consists of two distinct branches. One suggests a rapid chemical enrichment in the inner Galaxy while the other suggests prolonged GC formation in the outer halo. The latter is consistent with the outer halo GCs forming in dwarf galaxies and later being accreted by the Milky Way.