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NGC 891 is a nearby edge-on galaxy that is similar to the Milky Way and has ahot X-ray emitting halo that could arise from accretion, a galactic fountain,or a combination of the two. The metallicity of the gas can help distinguishbetween these models, and here we report on results that use 138 ks of archivalChandra data and 92 ks of new XMM-Newton data to measure the temperature andmetallicity of the hot halo of the galaxy. We find good fits for a thermalmodel with kT ~ 0.2 keV and Z ~ 0.1 solar, and rule out solar metallicity tomore than 99% confidence. This result suggests accretion from the intergalacticmedium as the origin for the hot halo. However, it is also possible to fit atwo-temperature thermal model with solar metallicity where kT_1 = 0.1 keV andkT_2 = 0.25 keV. A consideration of the cooling rate and scale height prefersthe single-temperature model. We also find that the cooling rate in the hot gascannot explain the massive HI halo in the steady state. In addition, a galacticfountain model cannot eject enough mass to account for the HI halo, and wespeculate that the neutral halo may be gas from a prior outflow that has sincecooled.

A Deep X-ray View of the Hot Halo in the Edge-on Spiral Galaxy NGC 891