Isolated ellipticals and their globular cluster systems III. NGC 2271, NGC 2865, NGC 3962, NGC 4240 and IC 4889

TL;DR

This study investigates the globular cluster systems of five isolated elliptical galaxies, revealing color bimodality and low specific frequencies, challenging existing models of cluster origin and galaxy assembly.

Contribution

It provides new observational data on GCSs of isolated ellipticals, showing bimodality and low cluster frequencies, questioning the accreted origin of metal-poor clusters.

Findings

Color bimodality observed in three galaxies

GCSs are generally poor with low specific frequency

Presence of blue clusters suggests in-situ formation

Abstract

As tracers of star formation, galaxy assembly and mass distribution, globular clusters have provided important clues to our understanding of early-type galaxies. But their study has been mostly constrained to galaxy groups and clusters where early-type galaxies dominate, leaving the properties of the globular cluster systems (GCSs) of isolated ellipticals as a mostly uncharted territory. We present Gemini-South/GMOS $g'i'$ observations of five isolated elliptical galaxies: NGC 3962, NGC 2865, IC 4889, NGC 2271 and NGC 4240. Photometry of their GCSs reveals clear color bimodality in three of them, remaining inconclusive for the other two. All the studied GCSs are rather poor with a mean specific frequency $S_N\sim 1.5$, independently of the parent galaxy luminosity. Considering also previous work, it is clear that bimodality and especially the presence of a significant, even dominant, population of blue clusters occurs at even the most isolated systems, casting doubts on a possible accreted origin of metal-poor clusters as suggested by some models. Additionally, we discuss the possible existence of ultra-compact dwarfs around the isolated elliptical NGC 3962.