Extended Hot Halos Around Isolated Galaxies Observed in the ROSAT
  All-Sky Survey
  We place general constraints on the luminosity and mass of hot X-ray emitting
gas residing in extended "hot halos" around nearby massive galaxies. We examine
stacked images of 2165 galaxies from the 2MASS Very Isolated Galaxy Catalog
(2MVIG), as well as subsets of this sample based on galaxy morphology and
K-band luminosity. We detect X-ray emission at high confidence (ranging up to
nearly 10\sigma) for each subsample of galaxies. The average L_X within 50 kpc
is 1.0\pm0.1 (statistical) \pm0.2 (systematic) x10^40 erg/s, although the
early-type galaxies are more than twice as luminous as the late-type galaxies.
Using a spatial analysis, we also find evidence for extended emission around
five out of seven subsamples (the full sample, the luminous galaxies,
early-type galaxies, luminous late-type galaxies, and luminous early-type
galaxies) at 92.7%, 99.3%, 89.3%, 98.7%, and 92.1% confidence, respectively.
Several additional lines of evidence also support this conclusion and suggest
that about 1/2 of the total emission is extended, and about 1/3 of the extended
emission comes from hot gas. For the sample of luminous galaxies, which has the
strongest evidence for extended emission, the average hot gas mass is 4x10^9
Msun within 50 kpc and the implied accretion rate is 0.4 Msun/yr.
