Determining The Galactic Halos Emission Measure from UV and X-ray Observations

📝 Abstract

We analyze a pair of Suzaku shadowing observations in order to determine the X-ray spectrum of the Galaxy’s gaseous halo. We simultaneously fit the spectra with models having halo, local, and extragalactic components. The intrinsic intensities of the halo OVII triplet and OVIII Lyman alpha 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. Meanwhile, FUSE OVI observations for the same directions and SPEAR CIV 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 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} Msun 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.

💡 Analysis

We analyze a pair of Suzaku shadowing observations in order to determine the X-ray spectrum of the Galaxy’s gaseous halo. We simultaneously fit the spectra with models having halo, local, and extragalactic components. The intrinsic intensities of the halo OVII triplet and OVIII Lyman alpha 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. Meanwhile, FUSE OVI observations for the same directions and SPEAR CIV 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 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} Msun 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.

📄 Content

arXiv:0906.1532v1 [astro-ph.GA] 8 Jun 2009 Determining The Galactic Halo’s Emission Measure from UV and X-ray Observations Shijun Lei, Robin L. Shelton and David B. Henley Department of Physics and Astronomy, University of Georgia, Athens, GA 30602 sjlei@physast.uga.edu ABSTRACT 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 to- ward an absorbing filament in the southern Galactic hemisphere and an observation toward an unobscured region adjacent to the filament. We simultaneously fit the spec- tra with models having halo, local, and extragalactic components. The intrinsic in- tensities of the halo O vii triplet and O viii Lyman 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 ∼106.0 K to ∼107.0 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 tempera- tures of ∼105.0 K to ∼106.0 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 ∼105.0 K to ∼107.0 K. We therefore construct a differential emission measure (DEM) model for the halo’s hot gas, consisting of two components. In each, dEM/d log 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 104.80 −106.02 K with a slope of 0.30 and the high-temperature component (HTC) covers the temper- ature range of 106.02 −107.02 K with a slope of −2.21. We compare our observations with predictions from models for hot gas in the halo. The observed spatial distribu- tion 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⊙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. Subject headings: Galaxy: general — Galaxy: halo — ISM: general — X-rays: diffuse background — X-rays: ISM — ultraviolet: ISMs – 2 – 1. Introduction Not only does hot gas (T > 105 K) reside in our galaxy’s disk, but it resides in the halo. (Here we use the X-ray astronomy convention which defines the halo as the region above the majority of the Galaxy’s H i, thus above a height of z ∼150 −200 pc given the parameterization of the H i distribution by Ferri`ere (1998a) and Dickey & Lockman (1990), although other conventions would call the lower part of this region the thick disk.) Utraviolet and X-ray observations indicate that the high-latitude sky is covered by hot gas. Absorption by Galactic O vi ions, tracers of T ∼3× 105 K gas, is seen in all of the Far Ultraviolet Spectroscopic Explorer (FUSE) halo survey spectra of sight lines that transit the halo and have high signal-to-noise data (Wakker et al. 2003). 1/4 keV X-rays, tracers of T ∼106 K gas, are also seen in all directions, but some of these X-rays are produced locally, either in the Local Bubble (LB) or in the heliosphere, and by external galaxies. After the local and extragalactic contribution are subtracted, X-rays are found to come from most, if not all, high latitude directions (Snowden et al. 1998). Such a large covering fraction does not require that the hot gas forms a smooth layer. In fact, maps of O vi column density and 1/4 keV brightness show a mottled or lumpy distribution. In comparison, maps of 3/4 keV brightness are far smoother, with the exception of the North Polar Spur/Loop I region which is bright in both 1/4 and 3/4 keV X-rays (see maps in Snowden et al. 1997). The height of the hot gas has been found from the O vi column density data. The average den- sity of O vi ions falls offexponentially with height above the plane and has scale-heights of 4.6 and 3.2 kpc for northern and southern Galactic hemispheres, respectively (Bowen et al. 2008). Although it is not possible to calculate the hot gas scale-height from observations of diffuse X-ray emission, it is possible to determine whether or not X-rays are produced beyond clouds of neutral or molecular in- terstellar gas. Such analyses, dubbed “shadowing” analysis, find 1/4 keV X-rays originating beyond clouds at heights of ∼160 pc (southern filament: Wang & Yu 1995, Shelton et al. 2007, distance from Penprase e

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