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 \sim 0.2 keV and Z \sim 0.1 Z _ { \odot } , 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 } \sim 0.1 keV and kT _ { 2 } \sim 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 can not explain the massive H i halo in the steady state .
In addition , a galactic fountain model can not eject enough mass to account for the H i halo , and we speculate that the neutral halo may be gas from a prior outflow that has since cooled .