Galactic diffuse hot gas
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Galactic diffuse hot gas of temperature ~10^6 K that constitutes most of the interstellar space remains the least well known component of the interstellar medium. We present in this dissertation two studies of the Galactic diffuse hot gas using shadowing observation and joint analysis methods. We first analyze a pair of Suzaku shadowing observations in order to determine the X-ray spectrum of the Galaxy's gaseous halo. Our X-ray measurements, together with the FUSE OVI and SPEAR CIV observations for the same (or nearby) direction, indicate the existence of hot halo gas at temperatures of ~10^5 K to ~10^7 K. We construct a broken power-law differential emission measure model for the hot halo gas. We find that a simple model in which hot gas accretes onto the Galactic halo and cools radioactively cannot explain both the observed UV and X-ray portions of our broken power-law model. However, the UV and X-ray intensities and our broken power-law model can be well explained by hot gas produced by supernova explosions supplemented by a smooth source of X-rays. We also construct a sample of 19 directions with both OVII emission intensity and absorption equivalent width measurements made from XMM-Newton archival data for the study of the Galactic diffuse hot gas. Both the OVII emission and absorption strengths are significantly enhanced toward the inner region of the Galaxy, where the Galactic center soft X-ray enhancement (GCSXE) is seen in the ROSAT 3/4 keV map. We find a tight correlation between the OVII absorption equivalent width and the OVII emission intensity at the 97.9% confidence level for these 19 directions, strongly suggesting that the OVII emission and absorption are largely co-spatial. Our joint analyses of the OVII emission and absorption show that the hot gas on the directions off the GCSXE are in good agreement with a thick disk model. While for the hot gas associated with the GCSXE, our results support its Galactic center/bulge origin. We also propose in this dissertation a future study of the hot gas associated with the GCSXE using differential/joint analyses.