Hot Gas in the Galactic Thick Disk and Halo Near the Draco Cloud

• Published in: The Astrophysical journal 2010-10, Vol.722 (1), p.302-310, Article 302
• Main Authors: Shelton, R. L, Henley, D. B, Dixon, W. V
• Format: Article
• Language: English
• Subjects: Astronomy; ASTROPHYSICS; ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; BOSONS; COOLING; EARTH PLANET; Earth, ocean, space; ELECTROMAGNETIC RADIATION; ELEMENTARY PARTICLES; ENERGY TRANSFER; Exact sciences and technology; GALAXIES; HEAT TRANSFER; IONIZING RADIATIONS; MASSLESS PARTICLES; NORTHERN HEMISPHERE; PHOTONS; PHYSICS; PLANETS; RADIATIONS; RADIATIVE COOLING; THERMAL CONDUCTION; ULTRAVIOLET RADIATION; X RADIATION
• ISSN: 0004-637X; 1538-4357
• DOI: 10.1088/0004-637X/722/1/302

This paper examines the ultraviolet and X-ray photons generated by hot gas in the Galactic thick disk or halo in the Draco region of the northern hemisphere. Our analysis uses the intensities from four ions, C IV, O VI, O VII, and O VIII, sampling temperatures of {approx}10{sup 5} to {approx}3 x 10{sup 6} K. We measured the O VI, O VII, and O VIII intensities from FUSE and XMM-Newton data and subtracted off the local contributions in order to deduce the thick disk/halo contributions. These were supplemented with published C IV intensity and O VI column density measurements. Our estimate of the thermal pressure in the O VI-rich thick disk/halo gas, p{sub th}/k = 6500{sup +2500}{sub -2600} K cm{sup -3}, suggests that the thick disk/halo is more highly pressurized than would be expected from theoretical analyses. The ratios of C IV to O VI to O VII to O VIII intensities were compared with those predicted by theoretical models. Gas which was heated to 3 x 10{sup 6} K then allowed to cool radiatively cannot produce enough C IV or O VI-generated photons per O VII or O VIII-generated photon. Producing enough C IV and O VI emission requires heating additional gas to 10{sup 5} K < T < 10{sup 6} K. However, shock heating, which provides heating across this temperature range, overproduces O VI relative to the others. Obtaining the observed mix may require a combination of several processes, including some amount of shock heating, heat conduction, and mixing, as well as radiative cooling of very hot gas.