The Circum-Galactic Medium of MASsive Spirals I: Overview and a Case
  Study of NGC 5908
  The Circum-Galactic Medium of MASsive Spirals (CGM-MASS) is a project
studying the overall content, physical and chemical properties, and spatial
distributions of the multi-phase circum-galactic medium (CGM) around a small
sample of the most massive isolated spiral galaxies in the local Universe. We
introduce the sample and present a detailed case study of the XMM-Newton
observation of the hot gas halo of NGC5908. After data calibration, point
source removal, and background analysis, we find that the diffuse soft X-ray
emission of NGC5908 is significantly more extended than the stellar light in
the vertical direction. The 0.5-1.25keV radial intensity profile tracing hot
gas emission can be detected above the background out to about $2^\prime$, or
$30\rm~kpc$ from the nucleus. The unresolved soft X-ray emission can be
characterized with a $\beta$-model with a slope of $\beta\approx0.68$. The
unresolved 0.5-2keV luminosity within $1^\prime$ is
$6.8\times10^{39}\rm~ergs~s^{-1}$, but young stellar sources may contribute
significantly to it. Assuming a metallicity of 0.2solar, an upper limit
(without subtracting the very uncertain young stellar contribution) to the mass
of hot gas within this radius is $2.3\times10^9\rm~M_\odot$. The cooling radius
is $r_{\rm cool}\approx25\rm~kpc$ or $\approx0.06r_{\rm 200}$, within which the
hot gas could cool radiatively in less than 10Gyr, and the cooling of hot gas
could significantly contribute in replenishing the gas consumed in star
formation. The hot gas accounts for $\approx1.9\%$ of the baryon detected
within the cooling radius. By comparing NGC5908 to other galaxies, we find that
its X-ray luminosity per stellar mass is consistent with lower-mass
non-starburst field spiral galaxies. However, a large scatter in hot gas soft
X-ray emissivity is indicated for spiral galaxies with
$M_*\gtrsim2\times10^{11}\rm~M_\odot$.
