The diverse X-ray properties of four truly isolated elliptical galaxies:
  NGC 2954, NGC 6172, NGC 7052, and NGC 7785
  We investigate the X-ray properties of four isolated elliptical galaxies,
selected from the Updated Zwicky Catalog according to strict isolation
criteria. Isolated galaxies are not influenced by the group/cluster
environment, and their X-ray emission can be studied independently of the often
overwhelming contribution of the hot intergalactic medium. They are therefore
suited to studying the X-ray characteristics relative to their intrinsic
properties. We analyzed our own XMM-Newton and archival Chandra data in detail
for three objects, and derived, when possible, the spatial and spectral
characteristics of each source. An upper limit for the fourth one was obtained
from archival ASCA data. We compared their characteristics with those of other
23 isolated objects for which X-ray and optical data are available in the
literature. We explored possible theoretical explanations to interpret our
results. In spite of our attempt to select very homogeneous objects, both in
terms of optical properties and environmental characteristics, we find a wide
range in X-ray luminosities and LX/LB ratios for the four objects: two of them
show a hot gaseous halo, whereas no gas is detected in the other two, to a
factor >10 in luminosity. In fact, we find a large spread in the LX/LB for all
galaxies considered, suggesting that the presence of hot gas is not easily
related to the optical luminosity or to the mass, even in isolated systems.
Younger objects tend to be less luminous in X-rays than older systems. However,
it appears that older objects could span a wide range in luminosities.
