  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 Iron abundance to be ~0.5 solar. 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 [+34.2,-35.8] solar at 50 kpc,
42.7 [+14.6,-21.2] solar at 5 effective radii, 1 sigma errors). We discuss this
disparity and consider possible mechanisms by which galaxies might reduce their
dark matter content.
