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Publications of the Astronomical Society of the Pacific 93:5-19, February 1981 Publications of the Astronomical Society of the Pacific 93:5-19, February 1981 PUBLICATIONS OF THE ASTRONOMICAL SOCIETY OF THE PACIFIC Vol. 93 February 1981 No. 551 CLASSIFICATION PARAMETERS FOR THE EMISSION-LINE SPECTRA OF EXTRAGALACTIC OBJECTS J. A. BALDWIN AND M. M. PHILLIPS Cerro Tololo Inter-American Observatory,0 Casilla 603, La Serena, Chile AND ROBERTO TERLEVICH Institute of Astronomy, Madingley Road, Cambridge, England CB3 OHA Received 1980 August 21 An investigation is made of the merits of various emission-line intensity ratios for classifying the spectra of extra- galactic objects. It is shown empirically that several combinations of easily-measured lines can be used to separate objects into one of four categories according to the principal excitation mechanism: normal HII regions, planetary nebulae, ob- jects photoionized by a power-law continuum, and objects excited by shock-wave heating. A two-dimensional quantitative classification scheme is suggested. Key words: HII region—Seyfert galaxies—quasars—spectral classification I. Introduction measure of the main excitation mechanism operating on The number of extragalactic emission-line objects for the gas. Since the different types of objects described which accurate line intensities are available has grown above have characteristically different spectra, a classifi- rapidly over the past several years. Various authors have cation system based on the relative strengths of emission placed these objects into a rather confusing array of cat- lines would seem to offer a good chance of clearly dis- egories (narrow-line radio galaxies, broad-line radio gal- tinguishing between the four possibilities. There is a long axies, active galaxies, Seyfert 1, Seyfert 2, QSO, Ν galax- history of classifying planetary nebulae on the basis of ies, etc.) using classification schemes which depend more the relative intensities of their emission lines (cf. Aller on the selection criteria, morphology, or line widths than and Liller 1968). It is also well known that the emission- on the information contained in the relative intensities of line spectra of galactic Η π regions can be usefully classi- the emission lines. This has tended to obscure the factor fied according to intensity ratios such as /([O m] which links many of these objects together most strong- λ5007)/Ι(Άβ) or I{[0 π] λ3727)/7([0 m] λ5007) (Searle ly, but which clearly separates others. This factor is the 1971; Smith 1975; Alloin, Bergeron, and Pelat 1978), excitation mechanism operating on the line-emitting gas. with the range in these parameters being provided most- From detailed studies, it is known that the predominant ly by the spread in heavy-element abundances among excitation mechanism in extragalactic objects is almost the different objects. Unfortunately, power-law photo- always one of the following: (a) photoionization by O ionization or shock-wave heating are not easily distin- and Β stars, (b) photoionization by a power-law contin- guished in these one-parameter classification schemes. uum source, or (c) shock-wave heating. A fourth excita- These types of objects are instead recognized by the tion class is the planetary nebulae, which are photo- presence of lines indicating simultaneously high ioniza- ionized by stars that are in most cases very much hotter tion and low ionization. Such lines are frequently used as than normal galactic O stars. an excitation parameter in a very qualitative way, and The growing body of data on emission-line galaxies less often (cf. Heckman 1980) as a quantitative discrimi- has made increasingly obvious the need for a quan- nant. titative classification system which will give a direct In this paper we use the best available data to explore the usefulness of a number of possible parameters for set- Cerro Tololo Inter-American Observatorv is supported by the Na- ting up a more comprehensive classification system. For tional Science Foundation under contract No. AST 78-27879. maximum utility, such a classification scheme should be 5 © Astronomical Society of the Pacific · Provided by the NASA Astrophysics Data System 6 BALDWIN, PHILLIPS, AND TERLEVICH based on the most easily measured lines. Because the de- TABLE I tails of line widths, the presence of underlying stellar References for Emission-Line Intensities spectra, instrumental limitations, and other factors can Type of strongly affect which lines are accurately measurable in Obj act (s) a particular spectrum, it is desirable to be able to derive Barker 1978 PN Boksenberg and Netzer 1977 NLR-SY1 an object's classification from a variety of different sets Costero and Osterbrock 1977 NLRG Danziger 1974 H II of line strengths. We therefore attempt to tie the best of Davidson 1978 SNR Dopita 1978 HH the classification parameters together into a single two- Dufour 1975 H II Ford and Butcher 1979 SHG dimensional system which will carry with reasonable Fosbury et al. 1978 SHG Hawley 1978 H II precision a reasonable amount of information about the Heckman 1980 SHG Koski 1978 SY2, NLRG conditions in the ionized gas. We confine our attention Miller 1974 SNR Miller 1978 SNR to emission-line regions which have strong forbidden Neugebauer et al. 1976 DXH 11 0'Conne11 et al. 1978 DXH II lines. We hope that the resulting classification system, Osterbrock and Costero 1973 SNR Osterbrock and Dufour 1973 SNR although only a modest extension of the existing practice Osterbrock and Koski 1976 NLR-SY1 Osterbrock and Miller 1975 NLRG in the field, will allow better intercomparison of the dif- Peimbert and Costero 1969 H II 2 Peimbert and Torres-Peimbert 1971 PN 2 ferent types of emission-line objects, and the easier rec- Peimbert and Torres-Peimbert 1974 H II 2 Peimbert et al. 1978 H II 2, 3 ognition of the most unusual of these objects. Searle and Sargent 1972 DXH II Smith 1975 H 11 II. The Data Base Key. - H II = galactic H II region, DXH 11 detached extragalactic Observational experience and published model calcu- H II region, PN = planetary nebula, SNR= supernova remnant, HH = Herbig lations both suggest that the following lines might be Haro object, SHG = shock-heated galaxy, SY2 = Seyfert 2 galaxy, NLRG = useful for discriminating between different excitation narrow-line radio galaxy, NLR-SY1 = narrow-1ine region of a Seyfert 1 mechanisms: [Ne ν] λ3426, [Ο ιι] λ3727. He π λ4686, Ηβ galaxy. λ4861, [Ο πι] λ5007, [Ο ι] λ6300, Ηα λ6563, and [Ν ιι] NOTLS: - (1) VV3, M1-1, VV8, NGC 4361, M3-27, and HB 12 not included. λ6584. Other potentially useful lines such as [Ne m] (2) Only objects with measured [0 I] λ6300 emission were included. λ3869? [Ο m] λ4363, and Ηγ λ4340 have been avoided (3) S 298 not included. because of measurement difficulties arising from blend- ing with other emission and absorption lines. shock-heating again strongly points to photoionization A set of reasonably accurate data on the lines of inter- by a flat spectrum. Although the best published models est was compiled from the literature, using as much as of this sort (Shields and Oke 1975; Boksenberg and Net- possible papers that contain large sets of internally con- zer 1977) fail badly at predicting the strength of the sistent data. Table I lists the sources used. All line [Ο π] λ3727 emission line, recent models of planetary strengths are based on photoelectric measurements. In nebulae (Aller et al. 1980) suggest that the problem was cases where the spectrum of an underlying stellar popu- due to an underestimate of the 0+ charge exchange cross lation could affect the measurements of the emission-line section rather than to any misinterpretation of the prin- strengths, we made a particular effort to use only the cipal excitation mechanism. As further examples of pow- higher-resolution (5 Â-10 Â) data obtained with the new er-law photoionization, we also include in this group the generation of spectrometers. best-measured narrow-line regions of Seyfert 1 galaxies. We subdivided the objects in this data base into four The best examples of galaxies with shock-heated gas groups according to their principal excitation mecha- are NGC 1052 (Koski and Osterbrock 1976; Fosbury et nism. There is little ambiguity in classifying objects as ei- al. 1978), M87 (Ford and Butcher 1979), and the so- ther planetary nebulae or normal Η π regions (from here called "LINERS" studied by Heckman (1980); all show on we will use the term "Η π region" to mean gas io- the characteristic of a relatively large [O m] nized by normal galactic O and Β stars). λ4363/λ5007 intensity ratio, combined with strong lines For our prototype power-law photoionization cases of [Ο i] and [Ο π] and relatively weak [Ο m] lines. We we have lumped together Seyfert 2 galaxies and narrow- take as our shock-heated prototypes those objects from line radio galaxies, since Koski (1978) has shown that the this group which have also been satisfactorily fitted to relative emission-line intensities of these two groups of detailed shock-heating models. objects are indistinguishable. Koski also found that the Using these criteria, we assembled line strengths for 80 "average" Seyfert 2 spectrum is approximately matched Η π regions, 23 plànetary nebulae, and 38 extragalactic by a combination of the emission-line spectra of a typi- objects presumed to be excited either by power-law radi- cal planetary nebula and the Crab nebula, implying that ation or shock-wave heating. the gas is photoionized by a relatively flat spectrum of The general goal is to find combinations of intensity photons. Comparison of this average spectrum to models ratios which clearly separate, both from each other and of nebulae heated by power-law photoionization and by from normal Η π regions, those gas clouds excited by © Astronomical Society of the Pacific · Provided by the NASA Astrophysics Data System CLASSIFICATION OF EMISSION-LINE SPECTRA 7 shock-heating and power-law photoionization.
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