A large population of recently-quenched red-sequence dwarf galaxies in the outskirts of the Coma Cluster

TL;DR

This study reveals that dwarf galaxies in the outskirts of the Coma Cluster are younger and more iron-enriched than those in the core, indicating environmental effects influence galaxy evolution.

Contribution

It provides detailed spectroscopic evidence of age and metallicity gradients in dwarf galaxies across the Coma Cluster, highlighting recent quenching in the outskirts.

Findings

Outer dwarf galaxies are ~2.2 Gyr younger than inner ones.

Outer dwarfs are ~50% more iron-enriched at given luminosity.

Strong environmental dependence of stellar populations in dwarf galaxies.

Abstract

We analyse the stellar populations of 75 red-sequence dwarf galaxies in the Coma cluster, based on high signal-to-noise spectroscopy from the 6.5m MMT. The sample covers a luminosity range 3-4 magnitudes below M*, in the cluster core and in a field centred 1 deg to the south-west. We find a strong dependence of the absorption line strengths with location in the cluster. Galaxies further from the cluster centre have stronger Balmer lines than inner-field galaxies of the same luminosity. The magnesium lines are weaker at large radius, while the iron lines are not correlated with radius. Converting the line strengths into estimates of stellar age, metallicity and abundance ratios, we find the gradients are driven by variations in age (>6 sigma significance) and in the iron abundance Fe/H (~2.7 sigma significance). The light element (Mg, C, N, Ca) abundances are almost independent of radius. At radius of 0.4-1.3 degree (~0.3-1.0x the virial radius), dwarf galaxies have ages ~3.8 Gyr on average, compared to ~6 Gyr near the cluster centre. The outer dwarfs are also ~50% more iron-enriched, at given luminosity. Our results confirm earlier indications that the ages of red-sequence galaxies depend on location within clusters, and in Coma in particular. The exceptionally strong trends found here suggest that dwarf galaxies are especially susceptible to environmental "quenching", and/or that the south-west part of Coma is a particularly clear example of recent quenching in an infalling subcluster.