The Origin of Color Gradients in Early-Type Systems and Their Compactness at High-z

TL;DR

This study analyzes optical and NIR color gradients in over 5000 early-type galaxies from SDSS and UKIDSS, revealing metallicity and age variations that explain their compactness at high redshift.

Contribution

It provides the first large-scale analysis of color gradients in ETGs across multiple wavebands, linking these gradients to galaxy evolution and high-z compactness.

Findings

Color gradients are mainly due to metallicity variation.

A small positive age gradient is present in ETGs.

ETGs are more compact at high redshift, consistent with observed trends.

Abstract

In this Letter, we present mean optical+NIR color gradient estimates for 5080 early-type galaxies (ETGs) in the grizYJHK wavebands of the Sloan Digital Sky Survey (SDSS) plus UKIRT Infrared Deep Sky Survey (UKIDSS). The color gradient is estimated as the logarithmic slope of the radial color profile in ETGs. With such a large sample size, we study the variation of the mean color gradient as a function of waveband with unprecedented accuracy. We find that (i) color gradients are mainly due, on average, to a metallicity variation of about -0.4dex per decade in galaxy radius; and (ii) a small, but significant, positive age gradient is present, on average, in ETGs, with the inner stellar population being slightly younger, by ~0.1dex per radial decade, than the outer one. Also, we show that the presence of a positive mean age gradient in ETGs, as found in the present study, implies their effective radius to be smaller at high z, consistent with observations.