The connection between globular cluster systems and their host galaxy and environment: a case study of the isolated elliptical NGC 821

TL;DR

This study analyzes the globular cluster systems of an isolated elliptical galaxy and a broad sample, revealing insights into their formation, efficiency, and environmental dependence, challenging some previous assumptions.

Contribution

It provides a comprehensive analysis of GC systems across various galaxy types and environments, highlighting non-universal formation efficiencies and the proportionality to halo mass.

Findings

GC numbers normalized by stellar mass increase with host mass

GC formation efficiency may not be universal

Environmental effects on GC formation are less significant than previously thought

Abstract

In an effort to probe the globular cluster (GC) system of an isolated elliptical galaxy, a comprehensive analysis of the NGC 821 GC system was performed. New imaging from the WIYN Mini-Mosaic imager, supplemented with Hubbl e Space Telescope (HST) WFPC2 images reveals a GC system similar to those found in counterpart ellipticals located in high-density environments. To put these results into the context of galaxy formation, a robustly-determined census of GC systems is presented and analysed for galaxies spanning a wide range of masses (> M_star), morphologies and environments. Results from this meta-study: (1) confirm previous findings that the number of GCs normalized by host galaxy stellar mass increases with host stellar mass. Spiral galaxies in the sample show smaller relative GC numbers than those of massive ellipticals, suggesting the GC systems of massive ellipticals were not formed from major spiral-spiral mergers; (2) indicate that GC system numbers per unit galaxy baryon mass increases with host baryon mass and that GC formation efficiency may not be universal as previously thought; (3) suggest previously reported trends with environment may be incorrect due to sample bias or the use of galaxy stellar masses to normalize GC numbers. Thus claims for environmentally dependent GC formation efficiencies should be revisited; (4) in combination with weak-lensing halo mass estimates, suggest that GCs formed in direct proportion to the halo mass; (5) are consistent with theoretical predictions whereby the local epoch of re-ionization did not vary significantly with environment or host galaxy type.