Determining the Galactic Halo's Emission Measure from UV and X-Ray Observations

We analyze a pair of Suzaku shadowing observations in order to determine the X-ray spectrum of the Galaxy's gaseous halo. Our data consist of an observation toward an absorbing filament in the southern Galactic hemisphere and an observation toward an unobscured region adjacent to the filament. We simultaneously fit the spectra with models having halo, local, and extragalactic components. The intrinsic intensities of the halo O VII triplet and O VIII Ly α emission lines are 9.98^+1.10 _-1.99 LU (line unit; photons cm^-2 s^-1 sr^-1) and 2.66^+0.37 _-0.30 LU, respectively. These results imply the existence of hot gas with a temperature of ~10^6.0 K to ~10^7.0 K in the Galactic halo. Meanwhile, FUSE O VI observations for the same directions and SPEAR C IV observations for a nearby direction indicate the existence of hot halo gas at temperatures of ~10^5.0 K to ~10^6.0 K. This collection of data implies that the hot gas in the Galactic halo is not isothermal, but its temperature spans a relatively wide range from ~10^5.0 K to ~10^7.0 K. We therefore construct a differential emission measure (DEM) model for the halo's hot gas, consisting of two components. In each, dEM/dlog T is assumed to follow a power-law function of the temperature and the gas is assumed to be in collisional ionizational equilibrium. The low-temperature component (LTC) of the broken power-law DEM model covers the temperature range of 10^4.80-10^6.02 K with a slope of 0.30 and the high-temperature component (HTC) covers the temperature range of 10^6.02-10^7.02 K with a slope of -2.21. We compare our observations with predictions from models for hot gas in the halo. The observed spatial distribution of gas with temperatures in the range of our HTC is smoother than that of the LTC. We thus suggest that two types of sources contribute to our broken power-law model. We find that a simple model in which hot gas accretes onto the Galactic halo and cools radiatively cannot explain both the observed UV and X-ray portions of our broken power-law model. It can, however, explain the intensity in the Suzaku bandpass if the mass infall rate is 1.35 × 10^-3 M _sun yr^-1 kpc^-2. The UV and X-ray intensities and our broken power-law model can be well explained by hot gas produced by supernova explosions or by supernova remnants supplemented by a smooth source of X-rays.