Astronomy and Astrophysics – Astronomy
Scientific paper
Dec 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993aas...18311405s&link_type=abstract
American Astronomical Society, 183rd AAS Meeting, #114.05; Bulletin of the American Astronomical Society, Vol. 25, p.1462
Astronomy and Astrophysics
Astronomy
Scientific paper
We re-examine the values of electron density n_e 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 <= log T <= 5.3, the data are consistent with 0.01 <= n_e <= 0.02 cm(-3) and 2200 <= P/k <= 3700 cm(-3) K, a factor of 2 -- 3 higher than advocated by Martin & Bowyer and comparable to the thermal pressure in the disk. 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 <= 5.6. We relate both emission-line and absorption-line observations to recent models in which turbulent mixing layers and isobarically cooling supernova remnants provide significant amounts of halo gas at ~ 10(5.3) K and process 20 -- 40 Msun yr(-1) with a power of ~ 10(41) ergs s(-1) . 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 appropriate scale heights, the midplane pressure is twice the homogeneous values quoted above. The O VI lambda1034 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 lambda1549 ( ~ 6000 photons cm(-2) s(-1) sr(-1) ). The ions C IV, N V, and O VI are predicted to show a decreasing trend in vertical extent, with the O VI scale height perhaps as low as ~ 1 kpc.
Shull Michael J.
Slavin Jonathan D.
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