Astronomy and Astrophysics – Astronomy
Scientific paper
Dec 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995aas...187.2405c&link_type=abstract
American Astronomical Society, 187th AAS Meeting, #24.05; Bulletin of the American Astronomical Society, Vol. 27, p.1321
Astronomy and Astrophysics
Astronomy
1
Scientific paper
Only ~ 3% of the Seyfert 1 galaxies and low-redshift QSOs observed by IUE show strong evidence for narrow absorption lines that originate near the redshift of the active galaxy. However, FOS and GHRS spectra of ~ 10 Seyfert 1 galaxies indicate that the incidence of intrinsic absorption lines is much higher than 3%, and may be closer to 50%. Only the strongest absorption lines (equivalent widths > 1 Angstroms) can be detected in IUE spectra of the brightest sources, due to the low signal-to-noise and resolution of the data. HST spectra show that the intrinsic absorption lines are typically characterized by high ionization (C IV, N V), have radial velocities in the range 0 to -1000 km sec(-1) relative to the emission lines, and are moderately broad (FWHM ~ 100 -- 300 km sec(-1) ). The origin of the absorption lines and their connection to the emission-line regions and X-ray absorption regions are under investigation. Variable UV absorption lines have previously been detected and monitored in three Seyfert 1 galaxies: NGC 4151, NGC 3516, and NGC 5548. Recent GHRS spectra have been used to detect variable C IV absorption in a fourth object: NGC 3783. These observations were used to place constraints on two types of models for the absorption-line variability: bulk motion into the line of sight, and variable ionization of existing gas in the line of sight. It is possible that the C IV absorption-line variability is related to changing ionization in the ``warm absorber'' component seen in the X-ray spectra of NGC 3783 and ~ 50% of Seyferts in general. The warm absorber is characterized by highly-ionized gas (as evidenced by O VII and O VIII absorption edges) and high temperatures (T ~ 10(5) K), and the column density of ionized hydrogen can be as high as 10(23) cm(-2) . Photoionization models indicate that the observed column densities of C IV and N V could be produced in a warm absorber. However, a direct connection between the UV and X-ray absorption regions has not yet been established.
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