SPIDER VII - Revealing the Stellar Population Content of Massive Early-type Galaxies out to 8Re

TL;DR

This study analyzes the radial stellar population trends in massive early-type galaxies out to 8 times their effective radius, revealing metallicity decreases outward and outer regions are older, supporting minor mergers in galaxy growth.

Contribution

First comprehensive optical and NIR color analysis of ETGs out to 8Re, constraining age and metallicity gradients and their environmental dependence.

Findings

Metallicity decreases outward with steeper slopes in outer regions.

Outer regions of ETGs are older than cores, especially in massive galaxies.

Age gradients are more significant in high-density environments.

Abstract

Radial trends of stellar populations in galaxies provide a valuable tool to understand the mechanisms of galaxy growth. In this paper, we present the first comprehensive analysis of optical-optical and optical-NIR colours, as a function of galaxy mass, out to the halo region (8Re) of early-type galaxies (ETGs). We select a sample of 674 massive ETGs (M*>3x10^10MSun) from the SDSS-based SPIDER survey. By comparing with a large range of population synthesis models, we derive robust constraints on the radial trends in age and metallicity. Metallicity is unambiguously found to decrease outwards, with a measurable steepening of the slope in the outer regions (Re<R<8Re). The gradients in stellar age are found to be more sensitive to the models used, but in general, the outer regions of ETGs feature older populations compared to the cores. This trend is strongest for the most massive galaxies in our sample (M*>10^11MSun). Furthermore, when segregating with respect to large scale environment, the age gradient is more significant in ETGs residing in higher density regions. These results shed light on the processes leading from the formation of the central core to the growth of the stellar envelope of massive galaxies. The fact that the populations in the outer regions are older and more metal-poor than in the core suggests a process whereby the envelope of massive galaxies is made up of accreted small satellites (i.e. minor mergers) whose stars were born during the first stages of galaxy formation.