NE VIII lambda 774 and time variable associated absorption in the QSO UM 675

Details NE VIII lambda 774 and time variable associated absorption in the QSO UM 675 NE VIII lambda 774 and time variable associated absorption in the QSO UM 675

Apr 1995

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995apj...443..606h&link_type=abstract

Astrophysical Journal, Part 1 (ISSN 0004-637X), vol. 443, no. 2, p. 606-616

Astronomy and Astrophysics

Astronomy

49

Carbon, Hydrogen, Interstellar Gas, Light (Visible Radiation), Neon, Nitrogen, Oxygen, Quasars, Ultraviolet Astronomy, Variability, Absorption Spectra, Astronomical Models, Astronomical Observatories, Gas Density, Metallicity, Photoionization, Spectrographs

Scientific paper

We discuss measurements of Ne VIII lambda 774 absorption and the time variability of other lines in the za approximately equal ze absorption system of the ze = 2.15 QSO UM 675 (0150-203). The C IV lambda 1549 and N V 1240 doublets at za = 2.1340 (shifted approximately 1500 km/s from ze strengthened by a factor of approximately 3 between observations by Sargent, Boksenberg and Steidel (1981 November) and our earliest measurements (1990 November and December). We have no information on changes in other za approximately equal ze absorption lines. Continued monitoring since 1990 November shows no clear changes in any of the absorptions between approximately 1100 and 1640 A rest. The short timescale of the variability (less than or approximately equal to 2.9 yr rest) strongly suggests that the clouds are dense, compact, close to the QSO, and photoionized by the QSO continuum. If the line variability is caused by changes in the ionization, the timescale requires densities greater than approximately 4000/cu cm. Photoionization calculations place the absorbing clouds within approximately 200 pc of the continuum source. The full range of line ionizations (from Ne VIII lambda 774 to C III lambda 977) in optically thin gas (no Lyman limit) implies that the absorbing regions span a factor of more than approximately 10 in distance or approximately 100 in density. Across these regions, the total hydrogen (H I + H II) column ranges from a few times 1018/sq cm in the low-ionization gas to approximately 1020/sq cm where the Ne VIII doublet forms. The metallicity is roughly solar or higher, with nitrogen possibly more enhanced by factors of a few. The clouds might contribute significant line emission if they nearly envelop the QSO. The presence of highly ionized Ne VIII lambda 774 absorption near the QSO supports recent studies that link za approximately equal to ze systems with X-ray 'wamr absorbers. We show that the Ne VIII absorbing gas would itself produce measurable warm absorption -- characterized by bound-free O VII or O VIII edegs near 0.8 keV -- if the column densities were NH greater than or approximately equal to 1021/sq cm (for solar abundances).

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