A Deep X-ray View of the Hot Halo in the Edge-on Spiral Galaxy NGC 891
  NGC 891 is a nearby edge-on galaxy that is similar to the Milky Way and has a
hot 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 distinguish
between these models, and here we report on results that use 138 ks of archival
Chandra data and 92 ks of new XMM-Newton data to measure the temperature and
metallicity of the hot halo of the galaxy. We find good fits for a thermal
model with kT ~ 0.2 keV and Z ~ 0.1 solar, and rule out solar metallicity to
more than 99% confidence. This result suggests accretion from the intergalactic
medium as the origin for the hot halo. However, it is also possible to fit a
two-temperature thermal model with solar metallicity where kT_1 = 0.1 keV and
kT_2 = 0.25 keV. A consideration of the cooling rate and scale height prefers
the single-temperature model. We also find that the cooling rate in the hot gas
cannot explain the massive HI halo in the steady state. In addition, a galactic
fountain model cannot eject enough mass to account for the HI halo, and we
speculate that the neutral halo may be gas from a prior outflow that has since
cooled.
