A Glimpse into the Past: The Recent Evolution of Globular Clusters

TL;DR

This study uncovers and analyzes distant extragalactic globular clusters around a galaxy at z=0.089, providing insights into their evolution over the past Gyr through deep imaging and spectroscopic measurements.

Contribution

It presents the discovery of the most distant globular clusters to date and examines their properties and evolution, especially comparing blue and red sub-populations.

Findings

Distant globular clusters are among the faintest detected in deep optical images.

Both blue and red globular cluster populations are present, with redder colors than local counterparts.

Color evolution of blue clusters aligns with stellar evolution predictions, red clusters suggest possible younger age or higher metallicity.

Abstract

We present the serendipitous discovery of 195 extragalactic globular clusters (GCs) in one of the deepest optical images ever obtained, a 126 orbit HST/ACS imaging study of the nearby Galactic GC NGC 6397. The distant GCs are all found surrounding a bright elliptical galaxy in the field, and are among the faintest objects detected in the image, with magnitudes 26 < F814W < 30. We measure the redshift of the parent elliptical galaxy, using GMOS on Gemini South, to be z = 0.089 (375 Mpc). This galaxy, and its associated clusters, therefore ranks as one of the most distant such systems discovered to date. The measured light from these clusters was emitted 1.2 Gyr ago (the lookback time) and therefore the optical properties hold clues for understanding the evolution of GCs over the past Gyr. We measure the color function of the bright GCs and find that both a blue and red population exist, and that the colors of each sub-population are redder than GCs in local elliptical galaxies of comparable luminosity. For the blue clusters, the observed color difference from z = 0.089 to today is only slightly larger than predictions from stellar evolution (e.g., changes in the luminosity and color of the main-sequence turnoff and the morphology of the horizontal branch). A larger color difference is found in the red clusters, possibly suggesting that they are very metal-rich and/or significantly younger than 12 Gyr.