Stellar kinematics and populations out to 1.5 effective radius in the elliptical galaxy NGC4636

TL;DR

This study analyzes stellar kinematics and populations in NGC4636 out to 1.5 effective radii, revealing old, alpha-enhanced stars with steep metallicity gradients, indicating different formation processes in the galaxy's core and outskirts.

Contribution

First detailed spectroscopic analysis of NGC4636's stellar populations and kinematics extending to 1.5 effective radii using high-quality long slit spectra.

Findings

Old, alpha-enhanced stellar populations in NGC4636.

Steep negative metallicity gradient that flattens outward.

Good match between observed and model-predicted colors.

Abstract

We present high quality long slit spectra along the major and minor axes out to 1.5 effective radius ($R_e$) of the massive galaxy NGC4636 taken by Hobby-Eberly Telescope (HET). Using Fourier Correlation Quotient (FCQ) method, we measured the stellar line-of-sight velocity distribution along the axes. Furthermore, six Lick/IDS indices ($H\beta,Mgb,Fe_{5015},Fe_{5270},Fe_{5335},Fe_{5406}$) are derived from the clean spectrum. By comparing the measured absorption line strengths with the predictions of Simple Stellar Populations (SSP) models, we derived ages, total metallicity and $\alpha$ abundance profiles of the galaxy. This galaxy presents old and $[\alpha/Fe]$ over abundant stellar populations. Indeed, using the SSP model, we obtained the broadband color profiles. The theoretical colors match well with the measured colors and present red sharp peaks at the galaxy center. The sharp peaks of the colors are mainly shaped by the high metallicity in the galaxy center. Interestingly, the galaxy has steep negative metallicity gradients, but trend flattens outwards. This result likly suggests that the center and outer regions of the galaxy formed through different formation process.