The emission lines of quasars and similar objects

Abstract

Much of our information about quasars is derived from their emission-line spectra. The analysis of such spectra has become an intricate subject which differs considerably from traditional, low-density nebular astrophysics. This review is intended to explain our present understanding of the situation, including some aspects of galactic nuclei whose luminosities are more modest than quasars. Quasars line-emitting regions are probably photoionized (even if supplementary heating processes also occur). So fax, models have been constructed which include ionization and thermal equilibria, the transfer of resonance-line and related photons, and the likely effects of absorption and scattering by dust grains. From comparisons between emission-line intensities produced in these models and observed quasars spectra, it appears that certain densities and pressures and size scales occur in or around quasars. The relative abundances of elements are not very far from solar values, although it is suspected that heavy elements -carbon, nitrogen, and oxygen in particular -are moderately "overabundant" in quasars. The emission-line intensities also provide indirect information about quasars ultraviolet and soft-x-ray continua; there are hints that photons with energies between 20 and 300 eV -which are not directly observable -may even represent the peak of the luminous output of a typical quasar. Finally, some gas-dynamical questions, while extremely important, are very difficult to answer, because of a lack of observables. CONTENTS 715

Fractal Quasar Clouds

“…Of the set of solutions obtained, the mean covering fraction is 0.50^0.02, and the mean geometric factor is L \ 3.16^0.10. Of particular interest is the solution at log (v) \ [3.90 because this solution has a BLR cloud element column density (cm~2)] \ 23.0 corresponding to column denlog [N BLR sities inferred in the BLR (Davidson & Netzer 1979). The solution is marked by a star in the graphs of Figure 4 and is summarized numerically, in greater detail, in Table 1.…”

“…Many ideas have been presented, including magnetically conÐned blobs (Rees 1987), winds above an accretion disk (Blandford & Payne 1982), continuum radiation driven winds (Mathews 1986), the disk itself (Collin-Sou †rin 1987), tails behind ablating stars (Mathews 1983 ;Netzer & Alexander 1994), or Ðlaments similar to those in the Crab Nebula (Davidson & Netzer 1979). No ab initio theory for the origin of these clouds is now possible, so a semiempirical approach is taken, often arguing by analogy with geometries encountered elsewhere in nature.…”

“…(2) The column density of individual BLR cloud elements should be of the order of cm~2 (Davidson & N BLR D 1023Netzer 1979. (3) Since the observed EW of the Lya line is about 10% of that produced for full coverage of the continuum source, the ensemble should have a covering fraction (the fraction of the sky covered by BLR clouds, as seen by the continuum source) of about 10% (Peterson 1997).…”

Classification of active galaxies and the prospect of a unified phenomenology

“…It has been believed for some time that the broad-line spectra of Seyferts and quasars require photoionization with a fairly tightly defined ionization parameter (U ~ 0.01) and density (N e ~ 10 9 cm -2 ). This belief arose from comparison of observed emission-line ratios with single-cloud photoionization models (e.g., Davidson and Netzer 1979;Kwan and Krolik 1981). These models were a considerable advance, but have important remaining weaknesses, such as the discrepancy between implied and observed (i.e., deduced via variability) sizes of the BLR, the failure to predict profile differences between lines, the lack of a predicted asymmetry in Lymanalpha, and the discrepancy between the continuum shape required for model fits and that actually observed (see review by Ferland and Shields (1985) and the discussion in Netzer (1987)).…”

Size and disk-like shape of the broad-line region of ESO 399-IG20

“…This has been demonstrated by Bentz et al (2009), who presented the most recent compilation of host-subtracted A&A 552, A1 (2013) AGN luminosities for several reverberation-mapped Seyfert 1 galaxies obtained through host-galaxy modeling of highresolution Hubble Space Telescope (HST) images. Compared to the previous slope of α = 0.7 they determined an improved slope of α = 0.519 0.063 −0.066 close to α = 0.5 expected from photoionization models of the BLR (Davidson & Netzer 1979). A different approach to estimate the host galaxy contribution is the flux variation gradient method (FVG, Choloniewski 1981;Winkler 1997).…”