Ultra-Compact Dwarfs in the Fossil Group NGC 1132

TL;DR

This study identifies and analyzes ultra-compact dwarfs and star clusters around the galaxy NGC 1132, revealing their properties, potential origins, and extending the understanding of UCDs beyond galaxy clusters.

Contribution

It provides the first detailed analysis of UCDs in a fossil group environment, showing their diversity and possible formation scenarios.

Findings

Identified 11 UCDs and 39 star cluster candidates around NGC 1132.

Discovered two types of UCDs with different properties and origins.

Extended the mass-metallicity relation of globular clusters to higher luminosities.

Abstract

Eleven Ultra-Compact Dwarf (UCD) and 39 extended star cluster candidates are found to be associated with the galaxy NGC 1132. This giant elliptical galaxy is the remnant of a fossil group. UCD and extended star cluster candidates are identified through the analysis of their structural parameters, colors, spatial distribution, and luminosity using deep Hubble Space Telescope observations in two filters: the F475W (Sloan g) and F850LP (Sloan z). The median effective radius of these UCDs is r_h=13.0 pc. Two types of UCDs are identified in the vicinity of NGC 1132, one type shares the same color and luminosity of the brightest globular clusters and traces the onset of the mass-size relation. The second kind of UCD is represented by the brightest UCD candidate, a M32-type object, with an effective radius of r_h=77.1 pc, located at ~6.6 kpc from the nucleus of NGC 1132. This UCD candidate is likely the remaining nucleus of a minor merger with the host galaxy. With the exception of a particularly blue UCD candidate, UCDs are found to extend the mass-metallicity relation found in globular clusters to higher luminosities. The results of this work support the growing body of evidence showing that UCDs are not circumscribed to galaxy clusters as previously thought. UCDs are likely to be a common occurrence in all environments. The milder tidal field of a fossil group, when compared to a galaxy cluster, allows UCDs and extended star clusters to survive up to present time at small galactocentric distances.