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The Emission Lines of Quasars and Similar Objects

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The Emission Lines of Quasars and Similar Objects The erI-iission lines of quasars and siva-filar objects Kris Davidson" Astronomy Department, Uniuersity of Minnesota, Minneapolis, MN 55455 Hagai Netzert Department of Astronomy, University of Texas, Austin, TX 78712 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 VII. Some Additional Possibilities A. Concerning the absence of broad forbidden lines 752 I. Introduction 715 B. Can densities be surprisingly high~ 753 A. Some historical remarks 715 C. Additional heating processes 753 B. Quasars, as observed optically 717 D. Nonthermal emission lines ~ 754 C. Topics not included in this review 718 E. Emission-line variability 755 D. Topics included in this review 722 VIII. Gas Motions and Dynamics; Line Profiles 756 II. Photoionized Nebulae —A Brief Overview 722 A. Gener alities 756 A. The photoionization equilibr ium: Counting B. Acceleration of gas —mainly by radiation the photons 722 pressure 757 B. Thermal equilibrium: Counting the ergs 724 C. Emission line profiles 758 C. The emergent spectrum 726 D. More remarks about certain parameters 760 D. Where to find ionic data 727 Acknowledgments 761 III. Conventional, Dustless Photoionization Models References 761 for Quasars' Spectra 728 A. Historical and pr eliminary 728 B. A specific photoionization calculation 731 C. Varying the assumed parameters 733 IV. Line Transfer 736 A. General considerations 736 I. INTRODUCTION B. Possible suppression of resonance emission A. Some historical remarks lines 738 C. The hydrogen line spectrum 739 Like other modern astronomical subjects, the study of D. The helium line spectrum 741 quasars has been greatly affected by fashion and by E. Fluor es cent process es 742 chance; Weedman (1976a) has reasonably argued that V. Dust and Quasars 743 these influences have sometimes led to misplaced at- 743 A. Correlation of visual and ultraviolet data tention and unnecessary debates. B. Dust in our Galaxy 745 C. Intergalactic dust ~ 745 The first quasars to be noticed were some radio D. Dust in quasars' galaxies 746 sources that were identified, during the early 1960s, E. Dust in quasars 746 with optically observable "stars" (see Schmidt, 1963; F. What is the intrinsic spectrum of a quasar'P 747 Oke, 1963; Greenstein and Mathews, 1963; and Bur- G. Comparing lines with continua 748 bidge and Burbidge, 1967). When it was recognized that VI. Some Additional Spectral Features 750 these were really not stars in our Galaxy, they became A. Forbidden lines 750 known as "quasistellar objects" or "QSO's" —or today, B. FeII lines 751 quasars. Their near-ultraviolet, visual, and near-in- C. The "3000 A. bump" 752 frared spectra were found to be continua plus a few emission lines, red-shifted like the spectra of very *Alfred P. Sloan Foundation Besearch Fellow, 1975-1979. distant galaxies; if the quasars' red shifts, like those ++On leave from the Dept. of Physics and Astronomy, Tel of galaxies, indicated cosmological distances, then Aviv University. their intrinsic luminosities must considerably exceed Reviews of Modern Physics, Vol. 51, No. 4, October 1979 Copyright 1979 American Physical Society Kris Davidson and Hagai Netzer: Emission lines of quasars and similar objects those of galaxies. However, some quasars were seen easily, because they are close enough to be recogniz- to fluctuate in br ightness within a few years or less; able as galaxies. But only a small fraction of galaxies this meant, according to arguments involving light- are Seyferts, and as with quasars, spectra are re- travel times, that the luminous souces must be smaller quired for identification. than a few light-years across, tiny compared with any The distances of the nearest quasars, estimated from galaxy and far too small to be resolved optically. This their red shifts, are of the order of 10' light-years; inspired a lengthy dispute about whether quasars' red while the distances of the farthest ones are comparable shifts are really cosmological (see Field, Arp, and to the size of the observable universe. Considering Bahcall, 1973). light-trave1. times, this means that we see the more But long before any quasars had been noticed, a few distant objects as they were moderately early in the galaxies, called "Seyfert galaxies, " were known to have history of the universe, not long after the first galaxies luminous, compact nuclei, with emission lines in their formed. Although we do not know much about quasars' spectra. Often mentioned in connection with quasars lifetimes, it is probable that the early ones are now ex- during the 1960s, Seyfert galaxies assumed new im- tinct. Quasars span such a wide range of intrinsic portance when some of their central brightnesses were luminosities that some of the more distant ones appear seen to fluctuate like quasars (see references cited by as bright as some of the nearest. The nuclei of Seyfert Weedman, 1976a, 1977). Today it seems probable that galaxies are intrinsically less luminous than most quasars are like a certain class of Seyfert nuclei, but quasars but more luminous than most ordinary more luminous; they are probably at the centers of galaxies. galaxies (which are detectable in some cases —see Many theorists have "explained" the radio, infrared, Morton, Williams, and Green, 1978; Wehinger and Wy- visual, ultraviolet, x-ray, and y-ray continua of quas- ckoff, 1978; Hawkins, 1978; and earlier references ars by invoking nonthermal processes such as synchro- cited by these authors, as well as comments by Bahcall, tron radiation (magnetobremsstrahlung) and the inverse 1971), a,nd their emission-line red shifts do indeed in- Compton effect, while the basic energy sources are dicate cosmological distances. suspected to be gravitational fields of black holes (see Although it was their radio emission that drew atten- Sec. I.C below). But the details of these explanations tion to the first-known quasars, today we know that are unsatisfying, because many parameters are both most quasars are not prominent radio sources (Sandage important and unobservable. There must be supplies and Luyten, 1969; Lynds and Wills, 1972; Osmer and of energetic electrons, accelerated by mechanisms that Smith, 1977a; Murdoch and Crawford, 1977; Sramek are imperfectly understood (!), in magnetic fields whose and Weedman, 1978). Of the nearly 1000 presently geometries are model dependent; one must guess known quasars, several hundred were first noticed as whether the electron-energy spectra are correlated with radio objects, but only a minority of these are strong magnetic field strengths, and how the electrons' veloci- radio sources. Among those which were first noticed ties might be correlated in direction with magnetic at optical wavelengths, only a small fraction —10% to field lines; the relativistic electrons may lose most of 30/~ —can be detected with present-day radio telescopes. their energies to radiation, or they may escape; ener- Even in a "radio-bright'* quasar, the visual-wavelength getic protons and other massive particles or i.ons may radiation usually carries significantly more energy than or may not contain as much total energy as the elec- the radio emission. But this is not to say that these trons; magnetic fields may be quasistatic, or there objects are predominantly visual-wavelength emitters; may be strong low-frequency electromagnetic waves; their infrared, ultraviolet, x-ray, and perhaps even and so on. Even though the theoretical situation is so y-ray fluxes are also important. complex, it produces few observable quantities: usually At visual wavelengths the most conspicuous quasar only a total Qux, a few tentative statements about the is a "12th magnitude" object, i.e., it seems about shape of the emitted continuum, and perhaps statements 1/300th as bright as a faint naked-eye star; most of about linear or circular polarization; and these results the other known quasars are of the 15th to 20th mag- may not follow uniquely from a.ny one particular model. nitude, 10 to 1000 times fainter. Being so faint, quas- Thus, being dissatisfied with a largely theoretical ars that fail to draw attention as radio emitters are nonthermal continuum approach, we think that emission difficult to identify, because on most photographs they lines are presently more useful for learning about look just like the equally faint foreground stars, which quasars. The emission-line spectra have several good are multitudinous. Sometimes colors have been useful attributes. First, there are a number of individual (most quasars are "bluer, " i.e., tend to emit at shorter lines whose relative intensities can be measured, giv- wavelengths, than most stars), but generally the emis- ing a fair number of well-defined observables.
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