The Circum-Galactic Medium of MASsive Spirals. I. An overview and a case study of NGC 5908

The Circum-Galactic Medium of MASsive Spirals is a project studying the overall content, physical and chemical properties, and spatial distributions of the multi-phase circumgalactic medium around a small sample of the most massive ($M_{\ast }\gtrsim 2\times {10}^{11}{M}_{\odot }$, $v_{rot}>300km{s}^{-1}$) 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 NGC 5908. After data calibration, point source removal, and background analysis, we find that the diffuse soft X-ray emission of NGC 5908 is significantly more extended than the stellar light in the vertical direction. The 0.5–1.25 keV radial intensity profile tracing hot gas emission can be detected above the background out to about $2^{\prime }$, or $30kpc$ from the nucleus. The unresolved soft X-ray emission can be characterized using a β-model with a slope of $\beta \approx 0.68$. The unresolved 0.5–2 keV luminosity within $1^{\prime }$ is $6.8\times {10}^{39}erg{s}^{-1}$, but young stellar sources may contribute significantly to it. Assuming a metallicity of 0.2 solar, an upper limit (without subtracting the very uncertain young stellar contribution) to the mass of hot gas within this radius is $2.3\times {10}^9{M}_{\odot }$. The cooling radius is $r_{cool}\approx 25kpc$ or $\approx 0.06r_{200}$, within which the hot gas could cool radiatively in less than 10 Gyr, and the cooling of hot gas could significantly contribute to replenishing the gas consumed in star formation. The hot gas accounts for $\approx 1.9\%$ of the baryons detected within the cooling radius. By comparing NGC 5908 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 $M_{\ast }\gtrsim 2\times {10}^{11}{M}_{\odot }$.