NASA/IPAC EXTRAGALACTIC DATABASE
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For refcode 1997AJ....113.2158S:
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1997AJ....113.2158S ABSORPTION BY HIGHLY IONIZED INTERSTELLAR GAS ALONG EXTRAGALACT[C AND GALACTIC SIGHT LINES BLAIR D. SAVAGE Department of Astronomy, University of Wisconsin, 475 N. Charter Street, Madison, Wisconsin 53706-1582, Electronic mail: savage@astro.wisc.edu KENNETH R. SEMBACH Center for Space Research, 6-216, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 Electronic mail: sembach@sundoggie.pha.jhu.edu LIMIN LO Department of Astronomy. 105-24, California Institute of Technology, Pasadena, California 91125, Electronic mail: ll@troyte.caltech.edu Received 1997 January 14; revised 1997 March 18 ABSTRACT We present intermediate resolution (FWHM ~ 10-20 km s^-1^) Goddard High Resolution Spectrograph observations of Galactic C IV and N V absorption along the sight lines to Fairall 9, Mrk 509, PKS 2155-304, and NGC 5548. These observations also provide measures of SiII and S II along the sight lines. Recent measurements of Si IV absorption for Mrk 509 and archival C IV data for NGC 3516 and NGC 4151 increase the high ion information available for complete paths through the Galactic halo. We combine these data with earlier GHRS and IUE measurements to study the distribution and properties of highly ionized gas in the Milky Way. Exponential scale heights determined from an N sin |b| vs |z| analysis are h(Si IV) = 5.1+/-0.7 kpc, h(C IV) = 4.4+/-0.6 kpc, and h(N V)=3.9+/-1.4 kpc. A kinematical analysis of the Milky Way C IV profiles toward 6 extragalactic objects yields <h (C IV))=4.5 +/- 1.4 kpc, which is in essential agreement with the N(C IV)sin|b| vs |z| result. The extension of the highly ionized gas away from the Galactic plane exceeds that of the H I gas measured for the same sight lines by about a factor of 10. Values of N(C IV)/N(N V) and N(Si IV)/N(N V) for objects with 2 < |z| <5 kpc are approximately a factor of 2 larger than for objects with smaller and larger |z|. The enhancement in these ratios at intermediate |z| distances is not described well by simply assuming the species have different exponential scale heights; more complex z distributions are implied by the observations and the nature of the ionizing processes may be changing with |z|. While some of the ionization may be due to photoionization from the extragalactic EUV background, models for the expected behavior of the photoionized halo gas predict an increase in N( C IV)/N( Si IV) with |z| which is not observed. The unusual ionization conditions of the observed gas may be associated with the non-equilibrium processes occurring in Galactic supershells and chimneys. The C IV turbulent velocity of ~60 km s^-1^, which includes the effects of inflow and outflow, falls short by a factor of approximately 2.8 in being able to support the highly ionized gas at its observed scale height of 4.4 kpc by turbulent pressure alone. The additional support required must be provided by other sources, such as the Galactic magnetic field or cosmic ray pressure. The observed line profiles suggest that highly ionized disk and halo gases are kinematically coupled and that the highly ionized gases corotate to |z| ~ 5 kpc for sight lines with Galactocentric distances exceeding 5 kpc. A clear trend for increasing velocity dispersion with increasing ionization level in the gas is seen. In high latitude directions, a net inflow of highly ionized gas is found along 7 sight lines, while outflow is found for 2. The magnitude of these high ionization |z| motions averages ~20 km s^-1^, or twice that of the neutral gas.
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