Stellar kinematics and populations out to 1.5 effective radii in the elliptical galaxy NGC 4636

摘要

We present high quality long slit spectra along the major and minor axes out to 1.5 effective radii of the massive galaxy NGC 4636 taken by the Hobby-Eberly Telescope. Using the Fourier Correlation Quotient method,we measured the stellar line-of-sight velocity distribution along the axes. Furthermore,six Lick/IDS indices(Hβ,Mgb,Fe5015,Fe5270,Fe5335,Fe5406) are derived from the clean spectrum. By comparing the measured absorption line strengths with the predictions of Simple Stellar Population(SSP) models,we derived ages,total metallicity and α abundance profiles of the galaxy. This galaxy presents old and [α/Fe] overabundant 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's center. Interestingly,the galaxy has steep negative metallicity gradients,but the trend attens outwards. This result likely suggests that the center and outer regions of the galaxy formed through different formation processes.