The Relationship between the High-Energy Continuum and Emission Lines in Quasars: A Low-Redshift Sample

Abstract

view Abstract Citations (52) References (60) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Relationship between the High-Energy Continuum and Emission Lines in Quasars: A Low-Redshift Sample Green, Paul J. Abstract Photoionization models dictate that many prominent quasar emission lines are sensitive to both the luminosity and shape of the quasars high- energy continuum-primarily the extreme ultraviolet (EUV) and soft X-ray continuum. Unfortunately, the EUV band is severely obscured by Galactic absorption. Using data from the adjacent UV and soft X-ray bandpasses, we initiate the first large-scale, multiline investigation of correlations between the QSO soft X-ray continuum and line emission in a sample of QSOs observed by Einstein and IUE. We present a new error analysis for objective, automated line measurements, which enables us to include the information contained in weak or undetected lines. We tabulate more than 300 UV emission-line equivalent widths from IUE spectra of 85 QSOs in the atlas of Lanzetta, Turnshek, & Sandoval, then characterize the distributions of line equivalent and velocity widths (Wlambda_ and FWHM). We then compare these line parameters to the QSO continuum spectral energy distributions from optical through soft X-ray wavelengths, using survival analysis to incorporate any nondetections for X-ray flux and/or UV emission lines. Several correlations noted in previous studies are not reproduced here. However, we illustrate that the exclusion of undetected lines from such studies may spuriously enhance apparent correlations. We find significant correlations between Wlambda_ and UV luminosity (e.g., the well-studied Baldwin effect) for Lyα, C IV, He II, and C III]. Wlambda_(C III]) and Wlambda_(He II) also show previously unreported correlations with X-ray luminosity that, for C III], appears to be primary. The line ratios C III]/Lyα and He II/Lyα both show strongest dependence on l_x_. Wlambda_(Lyα) correlates strongly with spectral slopes α_UV_ and α_OX_ (between 2500 A and 2 keV), but not with X-ray luminosity. Using these results, we argue that one simple geometrical interpretation of the Baldwin effect (BEff) as a result of a distribution of disk inclinations is not plausible. We also provide evidence that the BEff weakens or disappears when the line emission is correctly compared to the luminosity in the continuum bandpass relevant to its production. We thus support the interpretation of the BEff as a change in spectral energy distribution with luminosity, and we predict that no BEff relative to X-ray luminosity should be found for Fe II or Mg II emission lines. Extensions of our method to samples of a wider redshift/ luminosity range will be presented in a later paper, which will test these predictions. Publication: The Astrophysical Journal Pub Date: August 1996 DOI: 10.1086/177584 arXiv: arXiv:astro-ph/9603099 Bibcode: 1996ApJ...467...61G Keywords: GALAXIES: ACTIVE; GALAXIES: QUASARS: EMISSION LINES; GALAXIES: QUASARS: GENERAL; ULTRAVIOLET: GALAXIES; X-RAYS: GALAXIES; Astrophysics E-Print: 31 pages including 3 figures, AAS latex, plus 6 tables totaling 8 pages, to appear in ApJ Vol. 468, September 1, 1996 full text sources arXiv | ADS | data products NED (85) SIMBAD (10) MAST (1) INES (1)