A single population of red globular clusters around the massive compact galaxy NGC 1277

TL;DR

This study reveals that the relic galaxy NGC 1277 hosts a unimodal, red globular cluster system, indicating minimal mass accretion since its initial formation, contrasting with typical massive galaxies.

Contribution

It provides direct observational evidence that some massive galaxies can remain largely unchanged after their initial formation, challenging existing galaxy evolution models.

Findings

NGC 1277's globular clusters are unimodal and red.

The galaxy has likely experienced less than 10% stellar mass accretion.

Blue, metal-poor clusters are primarily accreted populations.

Abstract

Massive galaxies are thought to form in two phases: an initial, early collapse of gas and giant burst of central star formation, followed by the later accretion of material that builds up their stellar and dark matter haloes. The globular cluster systems of such galaxies are believed to form in a similar manner. The initial central burst forms metal-rich (red) clusters, while more metal-poor (blue) clusters are brought in by the later accretion of less massive satellites. This formation process is thought to lead the creation of the multimodal optical colour distributions seen in the globular cluster systems of massive galaxies. Here we report HST/ACS observations of the massive relic galaxy NGC 1277 and its globular clusters, a nearby unevolved example of a high redshift "red nugget". The g-z cluster colour distribution shows that the globular cluster system of the galaxy is unimodal and uniquely red. This is in strong contrast to normal galaxies of similar and larger stellar mass, whose cluster systems always exhibit (and are generally dominated by) blue clusters. We argue that the globular cluster system of NGC 1277 indicates that the galaxy has undergone little (if any) mass accretion after its initial collapse and use analytic merger trees to show that the total stellar mass accretion is likely less than ~ 10 %. These results confirm that NGC 1277 is a genuine relic galaxy and show that the blue, metal-poor globular clusters constitute an accreted population in present day massive galaxies.