A Search for Intergalactic Globular Clusters in the Local Group

TL;DR

This study conducted a comprehensive search for intergalactic globular clusters in the Local Group using SDSS data, identifying a few candidates and suggesting such clusters are rare or faint.

Contribution

First large-scale search for intergalactic globular clusters in the Local Group using multi-wavelength photometry and visual confirmation methods.

Findings

Identified 17 new GC candidates, 5 confirmed as GCs.

Most candidates are not associated with known galaxies.

The Local Group likely lacks a large population of luminous intergalactic GCs.

Abstract

The whole Sloan Digital Sky Survey (SDSS, 14,555 deg^2 has been searched for intergalactic globular clusters (IGCs) in the Local Group (LG). Using optical, infrared, and ultraviolet photometric selection criteria and photometric redshifts, the 2.1x10^8 of objects in the SDSS Galaxy Catalogue were reduced to only 183,791 brighter than r_o = 19 that might be GCs. Visual examination of their SDSS images recovered 84 percent of the confirmed GCs in M31 and M33 and yielded 17 new GC candidates, 5 of them of high confidence, which we could confirm as GCs in MegaPrime images from the Canada, France, Hawaii Telescope. These 5 GCs are within M31's halo, but the other 12 candidates are not close to LG galaxies or galaxies within 3 Mpc of the LG. Even though this search covers only one-third of the sky and some GCs could have been missed, it suggests that the LG does not contain a large population of IGCs more luminous than Mv ~ -6. In the direction of the M81 Group, the search yielded five candidate GCs, probable members of that group.