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Highly ionized gas in the Galactic halo

1994; IOP Publishing; Volume: 427; Linguagem: Inglês

10.1086/174185

1538-4357

J. Michael Shull, Jonathan D. Slavin,

Ionosphere and magnetosphere dynamics

view Abstract Citations (81) References (33) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Highly Ionized Gas in the Galactic Halo Shull, J. Michael ; Slavin, Jonathan D. Abstract We reexamine the values of electron density ne and gas pressure P/k in the interstellar medium (ISM) of the Galactic halo, as inferred from C IV emission and absorption lines and using current C IV atomic data. In a homogeneous model with 4.7 less than or equal to log T less than or equal to 5.3, the data are consistent with 0.01 less than or equal to ne less than or equal to 0.02/cu cm and 2200 less than or equal to P/k less than or equal to 3700/cu cm K, a factor of 2-3 higher than advocated by Martin & Bowyer (1990) and comparable to the thermal pressure in the disk. If some of the C IV absorption arises from nonemitting, photoionized gas, then the inferred density and pressure will increase accordingly. The volume filling factor for homogeneous models ranges from 0.5% to 5%. Because of the constraints arising from filling factor and radiated power, most of the C IV must arise from gas near the peak of the cooling curve, at log t less than or equal to 5.6. We relate both emission-line and absorption-line observations to recent models in which turbulent mixing layers and isobarically cooling supernova remnants (SNRs) provide significant amounts of halo gas at approximately 105.3 K and process 20-40 solar mass/yr with a power of approximately 1041 ergs/sec. Since the observed C IV and N V absorption scale heights have been reported to differ, at 4.9 kpc and 1.6 kpc, respectively, we examine inhomogeneous models with different exponential scale heights of T, P, and SN energy input. The ISM may change its character with distance above the Galactic plane, as superbubbles and mixing layers dominate over isolated SNRs as the source of the C IV. For appropiate scale heights, the midplane pressure is twice the homogeneous values quoted above. The O IV lambda 1034 diffuse emission line, which can be used as a temperature diagnostic of the hot gas, is predicted to be comparable in strength to that of C IV lambda 1549 (approximately 6000 photons/cu cm/s/sr. The ions C IV, N V, and O VI are predicted to show a decreasing trend in vertical extent, with the O IV scale height perhaps as low as approximately 1 kpc. Publication: The Astrophysical Journal Pub Date: June 1994 DOI: 10.1086/174185 Bibcode: 1994ApJ...427..784S Keywords: Absorption Spectra; Electron Density (Concentration); Emission Spectra; Galactic Halos; Gas Temperature; High Temperature Gases; Interstellar Gas; Ionized Gases; Ultraviolet Astronomy; Gas Pressure; Hubble Space Telescope; Iue; Mixing Layers (Fluids); Supernova Remnants; Turbulent Boundary Layer; Ultraviolet Absorption; Astrophysics; GALAXY: HALO; ISM: ABUNDANCES; ISM: GENERAL full text sources ADS | data products SIMBAD (1)