SPIDER X - Environmental effects in central and satellite early-type galaxies through the stellar fossil record

TL;DR

This study investigates how environment influences the star formation history of early-type galaxies, revealing that velocity dispersion is the main driver, with environment effects mainly affecting central galaxies in groups.

Contribution

It provides a detailed analysis of environmental effects on ETGs using high-quality spectra, highlighting the role of halo mass and galaxy central/satellite status in stellar population properties.

Findings

Velocity dispersion is the primary driver of stellar population properties.

No environmental dependence for satellite ETGs except at low sigma.

Central ETGs in groups are younger and have different chemical properties than isolated centrals.

Abstract

A detailed analysis of how environment affects the star formation history of early-type galaxies (ETGs) is undertaken via high signal to noise ratio stacked spectra obtained from a sample of 20,977 ETGs (morphologically selected) from the SDSS-based SPIDER survey. Two major parameters are considered for the study: the central velocity dispersion (sigma), which relates to local drivers of star formation, and the mass of the host halo, which relates to environment-related effects. In addition, we separate the sample between centrals (the most massive galaxy in a halo) and satellites. We derive trends of age, metallicity, and [alpha/Fe] enhancement, with sigma. We confirm that the major driver of stellar population properties in ETGs is velocity dispersion, with a second-order effect associated to the central/satellite nature of the galaxy. No environmental dependence is detected for satellite ETGs, except at low sigma - where satellites in groups or in the outskirts of clusters tend to be younger than those in the central regions of clusters. In contrast, the trends for centrals show a significant dependence on halo mass. Central ETGs in groups (i.e. with a halo mass >10^12.5 M_Sun) have younger ages, lower [alpha/Fe], and higher internal reddening, than "isolated" systems (i.e. centrals residing in low-mass, <10^12.5 M_Sun, halos). Our findings imply that central ETGs in groups formed their stellar component over longer time scales than "isolated" centrals, mainly because of gas-rich interactions with their companion galaxies.