Population Gradients in the SDSS Galaxy Catalog. The role of merging

TL;DR

This study examines how environment influences stellar population and colour gradients in early-type galaxies, revealing that denser environments lead to shallower metallicity-driven gradients in central galaxies, likely due to dry merging processes.

Contribution

It provides new evidence linking galaxy environment to metallicity gradients and supports dry merging as a key mechanism affecting galaxy evolution in dense regions.

Findings

Shallower colour and M/L ratio gradients in denser environments for central galaxies.

Metallicity gradients primarily drive the observed environmental dependence.

No significant environmental effect on satellite galaxy gradients.

Abstract

We investigate the role of the environment on the colour and stellar population gradients in a local sample of ~3500 central and ~1150 satellite SDSS early-type galaxies (ETGs). The environment is parameterized in terms of the number of satellite galaxies, N_gal in each group. For central galaxies, we find that both optical colour and mass-to-light (M/L) ratio gradients are shallower in central galaxies residing in denser environments (higher N_gal). This trend is driven by metallicity gradients, while age gradients appear to be less dependent on the environment and to have a larger scatter. On the other hand, satellites do not show any differences in terms of the environment. The same results are found if galaxies are classified by central age, and both central and satellite galaxies have shallower gradients if they are older and steeper gradients if younger, satellites being independent of ages. In central galaxies, we show that the observed trends can be explained with the occurrence of dry mergings, which are more numerous in denser environments and producing shallower colour gradients because of more uniform metallicity distributions due to the mixing of stellar populations, while no final clues about merging occurrence can be obtained for satellites. Finally we discuss all systematics on stellar population fitting and their impact on the final results.