PROBING THE GALACTIC DISK AND HALO .1. THE NGC-3783 SIGHT LINE

We report a study of Galactic disk and halo absorption toward the Seyfert galaxy NGC 3783 which has Galactic coordinates l = 287.46 and b = +22.95. The data were obtained with the Goddard High Resolution Spectrograph operating at medium resolution with the Large Science Aperture, which produces a line spread function having a sharp core (FWHM approximately 20 km s-1) and broad wings extending to +/-70 km s-1. Ion species detected in absorption near zero LSR velocity include C IV and N V for high ions, and C I, Mg II, Si II, and S II for low ions. Absorption from a high-velocity cloud (HVC) at a velocity of +240 km s-1 along the sight line is also detected in the ion species of S II, Si II, and probably C I. This is the first reported case where S II and C I absorption has been detected in a HVC. The S II lines are especially useful since metal abundance estimates based on S are largely unaffected by dust grains. The study is aided by the availability of 21 cm emission data. The main results are the following: 1. The integrated high ion column densities are log N(N V) = 13.55 +/- 0.12 and log N(C IV) = 14.30 +/- 0.03. These values are consistent with the high ions being produced in the gas of a galactic fountain cooling from temperatures exceeding 2 x 10(5) K. 2. The C IV absorption toward NGC 3783 exhibits an asymmetric extension to large positive velocity. The C IV profile is roughly modeled by a corotating absorbing gas with an exponential scale height of 3.5 kpc. This scale height estimate is similar to the 4.4 kpc value found for the direction of the Large Magellanic Clouds and also agrees with the C IV scale heights determined from the N\sin b\ versus Absolute value of z analysis technique. 3. For the intermediate velocity gas at +62 km s-1 toward NGC 3783 we estimate an approximate S abundance of 0.5 + 0.2 times the solar value. The abundance measurement may be influenced by differences in sampling geometry as it relies on a H I column density based on 21 cm data obtained with a large radio beam. The S abundance is consistent with that expected from the known metallicity gradient in the Galactic disk if the intermediate velocity gas is a corotating high-z extension of an outer spiral arm of the Galaxy at a Galactocentric distance of approximately 14 kpc. The implied z distance of the gas is approximately 5 kpc. 4. For the HVC at +240 km s-1, the unsaturated S II lines result in a S abundance of about 0.15 +/- 0.05 times the solar value. The estimate may be affected by differences in sampling geometry between the UV and radio data that were used to derive the H I column density. Based on the velocity and the S abundance of the HVC, we conclude that the HVC is most likely associated with gas being stripped away by the Galaxy from an extragalactic object, such as the Magellanic Clouds or some other member of the Local Group. Such a phenomenon may have significant implications for the study of quasar absorption line systems.