The stellar populations of massive galaxies in the local Universe

TL;DR

This review summarizes recent findings on the stellar populations of massive, early-type galaxies, highlighting their complex evolution, the role of environment, and the importance of spatially-resolved studies in understanding galaxy formation.

Contribution

It provides a comprehensive overview of recent observational and theoretical advances, emphasizing the non-universality of the IMF and the significance of spatially-resolved stellar population analyses.

Findings

Presence of young stars and cold gas in early-type galaxies.

Evidence supporting a non-universal IMF.

Outer envelopes of galaxies contain clues about accretion versus in situ star formation.

Abstract

I present a brief review of the stellar population properties of massive galaxies, focusing on early-type galaxies in particular, with emphasis on recent results from the ATLAS3D Survey. I discuss the occurrence of young stellar ages, cold gas, and ongoing star formation in early-type galaxies, the presence of which gives important clues to the evolutionary path of these galaxies. Consideration of empirical star formation histories gives a meaningful picture of galaxy stellar population properties, and allows accurate comparison of mass estimates from populations and dynamics. This has recently provided strong evidence of a non-universal IMF, as supported by other recent evidences. Spatially-resolved studies of stellar populations are also crucial to connect distinct components within galaxies to spatial structures seen in other wavelengths or parameters. Stellar populations in the faint outer envelopes of early-type galaxies are a formidable frontier for observers, but promise to put constraints on the ratio of accreted stellar mass versus that formed 'in situ' - a key feature of recent galaxy formation models. Galaxy environment appears to play a key role in controlling the stellar population properties of low mass galaxies. Simulations remind us, however, that current day galaxies are the product of a complex assembly and environment history, which gives rise to the trends we see. This has strong implications for our interpretation of environmental trends.