The Spatially-Resolved Stellar Populations of Nearby Early-type Galaxies

Abstract

We report our investigation into the stellar populations of early-type galaxies (ETGs) in the local universe. Massive ETGs are thought to form in a manner very different to our own Milky Way, with their cores formed via a swiftly-quenched rapid starburst >10 Gyr ago and their outskirts assembled later through cumulative mergers with predominantly less-massive systems. This formation history is encoded in the properties of their stellar populations.

Various evidence indicates that in the cores of massive ETGs, the stellar initial mass function differs from that of the Milky Way, with an increased fraction of dwarf stars formed. This effect, along with signatures in the chemical abundance properties of these stars, has been linked to ETG assembly histories. We use a combination of infrared and optical spatially-resolved spectroscopy of a sample of eight nearby massive ETGs (σ_average ~ 230 km/s) to measure empirical gradients in the strengths of spectroscopic absorption features, some of which are not hitherto well-explored. Using state-of-the-art stellar population models and robust methods for statistical inference, we link these to the underlying properties of the stars in these galaxies.

We measure strong gradients in spectroscopic features linked to Na I (at 0.82μm, 1.14μm, and 2.21μm) as well as the Ca II 0.86μm triplet. We measure weak/no gradient in several features that trace the IMF, such as the FeH 0.99μm Wing-Ford band. We use models to interpret these measurements, inferring e.g. an [Fe/H] gradient of -0.16±0.05 per dex in fractional radius and an average [Na/Fe] gradient of -0.35±0.09. We find a large but radially-constant enhancement to [Mg/Fe] of ~0.4 and a much lower [Ca/Fe] enhancement of ~0.1. Finally, we find no significant IMF gradient either on average or even in individual galaxies where such gradients have been previously reported.