No sign (yet) of intergalactic globular clusters in the Local Group

TL;DR

This study used high-quality imaging to show that twelve candidate intergalactic globular clusters in the Local Group are actually distant galaxies, setting strong limits on the presence of such clusters if they resemble those in M31.

Contribution

The paper provides the first high-resolution imaging analysis of candidate IGCs, conclusively classifying them as galaxies and constraining the abundance of undetected IGCs in the Local Group.

Findings

All twelve candidates are distant galaxies.

No significant population of undetected IGCs with M_V ≤ -6 in the SDSS area.

Even a small population of 4 IGCs would likely have been detected.

Abstract

We present Gemini/GMOS imaging of twelve candidate intergalactic globular clusters (IGCs) in the Local Group, identified in a recent survey of the SDSS footprint by di Tullio Zinn & Zinn (2015). Our image quality is sufficiently high, at $\sim 0.4^{\prime\prime} - 0.7^{\prime\prime}$, that we are able to unambiguously classify all twelve targets as distant galaxies. To reinforce this conclusion we use GMOS images of globular clusters in the M31 halo, taken under very similar conditions, to show that any genuine clusters in the putative IGC sample would be straightforward to distinguish. Based on the stated sensitivity of the di Tullio Zinn & Zinn (2015) search algorithm, we conclude that there cannot be a significant number of IGCs with $M_V \le -6$ lying unseen in the SDSS area if their properties mirror those of globular clusters in the outskirts of M31 -- even a population of $4$ would have only a $\approx 1\%$ chance of non-detection.