Suzaku Observation of the Metallicity Distribution in the Elliptical Galaxy NGC 4636

TL;DR

This study uses Suzaku X-ray observations to measure metallicity distributions in NGC 4636, revealing radial abundance gradients, mixing of supernova products, and environmental effects on metal distribution.

Contribution

First measurement of O, Mg, Si, and Fe abundances out to 140 kpc in NGC 4636 using Suzaku, with analysis of metal distribution and supernova contribution.

Findings

Metal abundances are >1 solar in the center and decrease outward.

O/Fe ratio indicates mixing of SNe II and SNe Ia products.

Metal mass-to-light ratios are higher than in NGC 1399 and lower than galaxy clusters.

Abstract

NGC 4636, an X-ray bright elliptical galaxy, was observed for 70 ks with Suzaku. The low background and good energy resolution of the XIS enable us to estimate the foreground Galactic emission accurately and hence measure, for the first time, the O, Mg, Si and Fe abundances out to a radius of ~28 arcmin ($\simeq$ 140 kpc). These metal abundances are as high as $>$1 solar within the central 4' and decrease by ~50% towards the outer regions. Further, the O to Fe abundance ratio is about 0.60--1.0 solar in all regions analyzed, indicating that the products of both SNe II and SNe Ia have mixed and diffused to the outer regions of the galaxy. The O and Fe metal mass-to-light-ratios (MLR) of NGC 4636 are 2--3 times larger than those of NGC 1399 implying that metal distributions in NGC 4636 are less extended than those in NGC 1399, possibly due to environmental factors, such as frequency of galaxy interaction. We also found that the MLRs of NGC 4636 at 0.1 $r_{180}$ are $\sim$5 times smaller than those of clusters of galaxies, possibly consistent with the correlation between temperature and MLR of other spherically symmetric groups of galaxies. We also confirmed a resonant scattering signature in the Fe$_{XV II}}$ line in the central region, as previously reported using the XMM-Newton RGS.