The SLUGGS survey: Exploring the metallicity gradients of nearby early-type galaxies to large radii

TL;DR

This study investigates metallicity gradients in 22 nearby early-type galaxies using near-infrared spectroscopy, revealing a correlation between galaxy mass and metallicity gradient steepness, and linking calcium triplet strength to the initial mass function.

Contribution

First extensive compilation of outer metallicity profiles across a wide galaxy mass range using DEIMOS spectroscopy, connecting gradients to galaxy formation processes.

Findings

Lower mass galaxies have steeper metallicity gradients.

Metallicity gradient correlates with galaxy mass, supporting in-situ and ex-situ star formation models.

Calcium triplet strength relates to the stellar initial mass function slope.

Abstract

Stellar metallicity gradients in the outer regions of galaxies are a critical tool for disentangling the contributions of in-situ and ex-situ formed stars. In the two-phase galaxy formation scenario, the initial gas collapse creates steep metallicity gradients, while the accretion of stars formed in satellites tends to flatten these gradients in the outskirts, particularly for massive galaxies. This work presents the first compilation of extended metallicity profiles over a wide range of galaxy mass. We use the DEIMOS spectrograph on the Keck telescope in multi-slit mode to obtain radial stellar metallicity profiles for 22 nearby early-type galaxies. From the calcium triplet lines in the near-infrared we measure the metallicity of the starlight up to 3 effective radii. We find a relation between the outer metallicity gradient and galaxy mass, in the sense that lower mass systems show steeper metallicity gradients than more massive galaxies. This result is consistent with a picture in which the ratio of ex-situ to in-situ formed stars is lower in less massive galaxies as a consequence of the smaller contribution by accretion. In addition, we infer a correlation between the strength of the calcium triplet feature in the near-infrared and the stellar initial mass function slope that is consistent with recent models in the literature.