Dual-component stellar assembly histories in local elliptical galaxies via MUSE

TL;DR

This study investigates the formation histories of three local elliptical galaxies using MUSE data, revealing a two-phase assembly process with an early rapid star formation phase followed by accretion-driven growth.

Contribution

The paper introduces a method to decompose elliptical galaxies into two components and analyzes their star formation histories, providing empirical evidence for the two-phase formation scenario.

Findings

Inner components formed rapidly with old, metal-rich stars.

Outer components assembled later through accretion and contain mixed stellar ages.

Results support the two-phase galaxy formation model.

Abstract

Elliptical galaxies often exhibit complex assembly histories, and are presumed to typically form through a combination of rapid, early star formation and subsequent accretion of material, often resulting from mergers with other galaxies. To investigate theories of spheroidal galaxy formation, the objective of this work is to analyse the star formation histories (SFHs) of a sample of three isolated elliptical galaxies in the local Universe observed with MUSE at $z<0.06$. With BUDDI, we decompose the integral field unit (IFU) datacubes into two components with S\'ersic profiles, which roughly correspond to the two phases of in-situ and ex-situ star formation. To constrain the mode of growth in these galaxies, we derived the mass and light-weighted stellar ages and metallicities, and created the 2D stellar population maps of each component using pPXF. We reconstructed the mass and light-weighted SFHs to constrain the contribution of different stellar populations to the mass and luminosity of the components through cosmic time. Our results show that the ellipticals in this sample have experienced an early and rapid phase of star formation either through a rapid dissipative collapse or gas-rich major mergers concentrated in the inner component, which contributes to $\sim50$% of the galaxy stellar mass. The co-dominant outer component, however, had assembled the bulk of its stellar mass shortly after the inner component did, through accretion via dry mergers and possible gas accretion. This premise is supported by our observations of the inner component being primarily composed of old and metal-rich stars. The outer component has a combination of old and intermediate-age stars, with a moderate spread in metallicities. These results are analysed through the lens of the two-phase scenario, a framework developed over the years to explain the formation histories of elliptical galaxies.