Stellar Metallicity Gradients in SDSS galaxies

TL;DR

This study statistically measures stellar metallicity and abundance ratio gradients in SDSS early-type galaxies, revealing shallow gradients that are weakly dependent on stellar mass, using spectral stacking and stellar population modeling.

Contribution

It introduces a novel statistical method to infer metallicity and abundance ratio gradients from SDSS galaxy spectra without spatially resolved data.

Findings

Detected metallicity gradients in SDSS galaxies

Found shallow abundance ratio gradients

Gradients weakly depend on stellar mass

Abstract

We infer stellar metallicity and abundance ratio gradients for a sample of red galaxies in the Sloan Digital Sky Survey (SDSS) Main galaxy sample. Because this sample does not have multiple spectra at various radii in a single galaxy, we measure these gradients statistically. We separate galaxies into stellar mass bins, stack their spectra in redshift bins, and calculate the measured absorption line indices in projected annuli by differencing spectra in neighboring redshift bins. After determining the line indices, we use stellar population modeling from the EZ\_Ages software to calculate ages, metallicities, and abundance ratios within each annulus. Our data covers the central regions of these galaxies, out to slightly higher than $1 R_{e}$. We find detectable gradients in metallicity and relatively shallow gradients in abundance ratios, similar to results found for direct measurements of individual galaxies. The gradients are only weakly dependent on stellar mass, and this dependence is well-correlated with the change of $R_e$ with mass. Based on this data, we report mean equivalent widths, metallicities, and abundance ratios as a function of mass and velocity dispersion for SDSS early-type galaxies, for fixed apertures of 2.5 kpc and of 0.5 $R_e$.