Timing the formation and assembly of early-type galaxies via spatially resolved stellar populations analysis

TL;DR

This study uses spatially-resolved stellar populations to understand the formation and assembly history of early-type galaxies, revealing insights into their metallicity, age, and [Mg/Fe] gradients and their evolution through cosmic time.

Contribution

It provides a detailed analysis of stellar population gradients in elliptical galaxies and compares them with relic galaxies to infer formation processes and merger history.

Findings

Metallicity gradients steepen with galaxy velocity dispersion.

Core formation involved rapid, gas-rich star formation at high redshift.

Minor dry mergers flattened initial stellar population gradients.

Abstract

To investigate star formation and assembly processes of massive galaxies, we present here a spatially-resolved stellar populations analysis of a sample of 45 elliptical galaxies (Es) selected from the CALIFA survey. We find rather flat age and [Mg/Fe] radial gradients, weakly dependent on the effective velocity dispersion of the galaxy within half-light radius. However, our analysis shows that metallicity gradients become steeper with increasing galaxy velocity dispersion. In addition, we have homogeneously compared the stellar populations gradients of our sample of Es to a sample of nearby relic galaxies, i.e., local remnants of the high-z population of red nuggets. This comparison indicates that, first, the cores of present-day massive galaxies were likely formed in gas-rich, rapid star formation events at high redshift (z>2). This led to radial metallicity variations steeper than observed in the local Universe, and positive [Mg/Fe] gradients. Second, our analysis also suggests that a later sequence of minor dry mergers, populating the outskirts of early-type galaxies (ETGs), flattened the pristine [Mg/Fe] and metallicity gradients. Finally, we find a tight age-[Mg/Fe] relation, supporting that the duration of the star formation is the main driver of the [Mg/Fe] enhancement in massive ETGs. However, the star formation time-scale alone is not able to fully explain our [Mg/Fe] measurements. Interestingly, our results match the expected effect that a variable stellar initial mass function would have on the [Mg/Fe] ratio.