NGC 474 as viewed with KCWI: diagnosing a shell galaxy

TL;DR

This study uses Keck/KCWI spectroscopy to analyze the stellar populations and kinematics of NGC 474, revealing insights into its merger history and demonstrating the feasibility of spectroscopic studies of faint stellar features.

Contribution

It provides the first detailed spectroscopic analysis of the shell galaxy NGC 474, linking its stellar populations to a past merger event.

Findings

Central region has intermediate-aged stars (~8.3 Gyr) with subsolar metallicity.

Shell region contains younger stars (~4 Gyr) with solar metallicity.

Results support a merger with a ~10^10 solar mass satellite at least 2 Gyr ago.

Abstract

We present new spectra obtained using Keck/KCWI and perform kinematics and stellar population analyses of the shell galaxy NGC 474, from both the galaxy centre and a region from the outer shell. We show that both regions have similarly extended star formation histories although with different stellar population properties. The central region of NGC 474 is dominated by intermediate-aged stars (8.3 \pm 0.3 Gyr) with subsolar metallicity ([Z/H]= -0.24 \pm 0.07 dex) while the observed shell region, which hosts a substantial population of younger stars, has a mean luminosity-weighted age of 4.0 \pm 0.5 Gyr with solar metallicities ([Z/H]=-0.03 \pm 0.09 dex). Our results are consistent with a scenario in which NGC 474 experienced a major to intermediate merger with a log\((M_*/M_\odot)\sim10 \) mass satellite galaxy at least \sim 2 Gyr ago which produced its shell system. This work shows that the direct spectroscopic study of low-surface brightness stellar features, such as shells, is now feasible and opens up a new window to understanding galaxy formation and evolution.