Keck Cosmic Web Imager (KCWI) Spectra of Globular Clusters and Ultra Compact Dwarfs in the Halo of M87

TL;DR

This study uses KCWI spectra to analyze the stellar populations of globular clusters, ultra compact dwarfs, and halo light in M87, revealing dual populations and supporting the idea that UCDs are remnant galaxy nuclei.

Contribution

First consistent stellar population analysis across GCs, UCDs, and halo light in M87 at similar radii, providing new insights into their origins and properties.

Findings

Dual stellar populations in M87 halo light.

UCDs share similar populations with some GCs, suggesting a common origin.

Evidence supporting UCDs as remnant galaxy nuclei.

Abstract

Using the Keck Cosmic Web Imager we obtain spectra of several globular clusters (GCs), ultra compact dwarfs (UCDs) and the inner halo starlight of M87, at a similar projected galactocentric radius of $\sim$5 kpc. This enables us, for the first time, to apply the same stellar population analysis to the GCs, UCDs and starlight consistently to derive ages, metallicities and alpha-element abundances in M87. We find evidence for a dual stellar population in the M87 halo light, i.e an $\sim$80\% component by mass which is old and metal-rich and a $\sim$20\% component which is old but metal-poor. Two red GCs share similar stellar populations to the halo light suggesting they may have formed contemporaneously with the dominant halo component. Three UCDs, and one blue GC, have similar stellar populations, with younger mean ages, lower metallicities and near solar alpha-element abundances. Combined with literature data, our findings are consistent with the scenario that UCDs are the remnant nucleus of a stripped galaxy. We further investigate the discrepancy in the literature for M87's kinematics at large radii, favouring a declining velocity dispersion profile. This work has highlighted the need for more self-consistent studies of galaxy halos.