The isolated elliptical NGC 4555 observed with Chandra

Abstract

We present analysis of a Chandra observation of the elliptical galaxy NGC 4555. The galaxy lies in a very low density environment, either isolated from all galaxies of similar mass or on the outskirts of a group. Despite this, NGC 4555 has a large gaseous halo, extending to ≃60 kpc. We find the mean gas temperature to be ≃0.95 keV and the Fe abundance to be ≃0.5 Z ⊙. We model the surface brightness, temperature and abundance distribution of the halo and use these results to estimate parameters such as the entropy and cooling time of the gas, and the total gravitational mass of the galaxy. In contrast to recent results showing that moderate luminosity ellipticals contain relatively small quantities of dark matter, our results show that NGC 4555 has a massive dark halo and large mass-to-light ratio (56.8 -35.8+34.2 M⊙/LB⊙ at 50 kpc, 42.7-21.2+14.6 at 5re, 1σ errors). We discuss this disparity and consider possible mechanisms by which galaxies might reduce their dark matter content.