Stellar Velocity Dispersion and Dynamical Mass of the Ultra-Diffuse Galaxy NGC 5846_UDG1 from the Keck Cosmic Web Imager

TL;DR

This study measures the stellar velocity dispersion and dynamical mass of the ultra-diffuse galaxy NGC 5846_UDG1, revealing it to have a dwarf-like stellar mass but an overly massive dark matter halo, confirming its dark matter dominance.

Contribution

The paper provides new spectroscopic measurements of stellar velocity dispersion and dynamical mass for NGC 5846_UDG1, reducing uncertainties and confirming its dark matter dominated nature.

Findings

Stellar velocity dispersion of 17 ± 2 km/s measured.

Dynamical mass indicates high dark matter contribution.

Galaxy likely pressure-supported with no rotation evidence.

Abstract

The ultra-diffuse galaxy in the NGC 5846 group (NGC 5846_UDG1) was shown to have a large number of globular cluster (GC) candidates from deep imaging as part of the VEGAS survey. Recently, Muller et al. published a velocity dispersion, based on a dozen of its GCs. Within their quoted uncertainties, the resulting dynamical mass allowed for either a dark matter free or a dark matter dominated galaxy. Here we present spectra from KCWI which reconfirms membership of the NGC 5846 group and reveals a stellar velocity dispersion for UDG1 of $\sigma_{GC}$ = 17 $\pm$ 2 km/s. Our dynamical mass, with a reduced uncertainty, indicates a very high contribution of dark matter within the effective radius. We also derive an enclosed mass from the locations and motions of the GCs using the Tracer Mass Estimator, finding a similar mass inferred from our stellar velocity dispersion. We find no evidence that the galaxy is rotating and is thus likely pressure-supported. The number of confirmed GCs, and the total number inferred for the system ($\sim$45), suggest a total halo mass of $\sim2 \times 10^{11}$ M$_{\odot}$. A cored mass profile is favoured when compared to our dynamical mass. Given its stellar mass of 1.1$\times$10$^{8}$ M$_{\odot}$, NGC 5846_UDG1 appears to be an ultra-diffuse galaxy with a dwarf-like stellar mass and an overly massive halo.