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Home , Seyfert galaxy

Proc. Natl. Acad. Sci. USA Vol. 75, No. 2, pp. 540-544, February 1978

Observational model of the ionized gas in Seyfert and radio- nuclei* (emission lines/plasma/extinction/rotation/turbulence) DONALD E. OSTERBROCKt School of Physics and Astronomy, University of Minnesota, Minneapolis, Minnesota 55455 Contributed by Donald E. Osterbrock, December 2,1977

ABSTRACT Equivalent widths of the total emission-line indicating the presence of significant amounts of both types of H# in Seyfert 1, Seyfert 2, and intermediate-type Seyfert gal- gas (2, 3, 8), and broad-line axies, expressed in terms of the featureless continuum, all have many radio have relatively approximately the same frequency distribution. This suggests strong narrow-line components (4). The available evidence that the energy-input mechanism to both the narrow-line, low- strongly suggests that the energy input to the narrow-line gas density gas and the broad-line, high-density gas is photoion- occurs by photoionization by radiation with an approximate ization by the featureless continuum. The reason for the power-law that extends far into the (per- weakness of the narrow emission lines in extreme Seyfert 1 haps with a cutoff at several hundred ev) that is an extension galaxies is then the absorption of most of the ionizing photons in the dense gas near the central source. The statistics of line of the observed optical featureless continuum (3, 7). The ob- widths can be fitted by a model in which the dense gas has served narrow-emission-line spectra agree qualitatively with typical rotational velocity 5000 km/sec and typical turbulent predicted spectra for such models, and the total number of velocity 2000 km/sec. A model is proposed in-which the dense ionizing photons in the extrapolated power-law spectrum is gas forms a rotating, turbulent disk with dimension -0.1 pC and approximately sufficient to balance the total number of re- height/diameter tu2/5. Seyfert 2 galaxies are objects with little combinations observed in the H I permitted-line spectrum. dense gas, and intermediate-type Seyfert galaxies are objects in which the dense gas is optically thin to ionizing radiation at The energy input to the broad-line gas is not so clear. Since least along the poles. Most radio galaxies have strong narrow only permitted line of H I, He I, and He II are observed from emission lines, suggesting that escape of radio plasma can only this region, plus Fe II lines probably excited by resonance flu- occur where some ionizing photons can also escape from the orescence, far less diagnostic information is available, and it has dense gas. Other predictions, implications, and tests of this been suggested that nonthermal particles, diffusion of thermal model are discussed. radiation, or conversion of kinetic energy into heat (9, 10) may be the source of the energy input to this region rather than In the past few years many new observational results on Seyfert photoionization. One difficulty with these alternate models is and radio galaxies have appeared in print. Most are summarized that the intensity-ratio He I X5876/H13 is approximately the in a very complete review by Weedman (1). More recently same (within a factor of two) for most Seyfert 1 galaxies, while spectrophotometric measurements have been published for the He II X4686/H,3 ratio varies strongly from one object to many Seyfert 1 galaxies by Osterbrock (2), for many Seyfert another (2). On any of the thermal-ionization models He I 2 and narrow-line radio galaxies by Koski (3), and for several X5876/Hi# would be expected to vary much more widely. On broad-line radio galaxies by Grandi and Osterbrock (4), as well the other hand, in the photoionization model the observed ratios as for NGC 4151 by Boksenberg and Penston (5). Summaries follow naturally on the assumption that most He is ionized to of the main observational results on the optical spectra of the He+ where H is ionized to H+, with relatively small and vari- nuclei of radio and Seyfert galaxies and on the physical con- able amounts of He ionized to He2+. ditions in the ionized gas in these objects have recently appeared The best evidence that photoionization is the main input (6, 7). The present paper represents a synthesis in the form of mechanism to the broad-line region is the fact, noticed by Searle a preliminary model that fits most of the published optical and Sargent (11), that the observed equivalent width of emission data. H(3 is more or less the same in all these objects, in the range IONIZATION 20-100 A, and that this is the range of values expected for photoionization of optically thick gas clouds by power-law The first problem is the source of energy input to the ionized spectra with the slopes observed in Seyfert 1 galaxies. gas. It exists in two different physical regimes in active nuclei, In the present paper it is shown that this conclusion is greatly the narrow-line profile (full width at half maximum t 500 strengthened by more recent spectrophotometric results. The km s'l), low-density (103 < Ne < 106 cm-3) gas that is the only observed optical continuous spectrum of an active-galaxy nu- component seen in Seyfert 2 galaxies and narrow-line radio cleus is made up of two parts, the featureless or "power-law" galaxies, and the broad-line profile (1000 km so1 < full width continuum to the active nucleus and the at half maximum < 5000 km s-'), high-density gas (Ne > 108 unique integrated- cm-3) that is the main component in Seyfert 1 galaxies and Abbreviations: pe, parsec; MO, solar mass unit; LE), solar broad-line radio galaxies. Only permitted lines are observed unit. from the broad-line regions because of collisional deexcitation * Contribution, No. 414. of the forbidden lines at these high densities. There are also t On leave from Lick Observatory, University of California, Santa intermediate-type Seyfert galaxies, with permitted-line profiles Cruz, CA 95064. 540 Downloaded by guest on September 30, 2021 Astronomy: Osterbrock Proc. Natl. Acad. Sci. USA 75 (1978) 541

Table 1. Equivalent width of H, emission in Seyfert._galaxies could be approximately measured, the mean error of fG was Galaxy fG WFC(Hfl) Galaxy WFC(Hf) approximately ±0.06. The values of fG found in this way are fG listed in Table 1, along with the resulting WFC(Hfl) calculated Mrk 10 .0.1 <80 Mrk 376 .0.1 .83 from the published Wo(Hf) for these objects (2). There may 40 0.25 125 382 0.15 24 be errors due to some or all of these Seyfert 1 galaxies having 69 0.03 53 478 .0.1 <84 different absorption-line spectra from the standard ellipticals, 79 .0.1 .150 486 .0.1 .103 or due to part or all of the K-line absorption being interstellar 106 <138 .0.1 504 0.14 77 in origin, but it seems likely that the values in Table 1 are more 110 <0.1 <168 506 0.22 138 nearly correct than the assumption fG O and WFC(Hf3) 124 0.17 77 509 .0.1 .162 Wo(H3). 141 0.12 52 541 0.14 69 Now, using the WFC(HI3) in Table 1 plus the values deter- 142 <0.1 .74 590 0.46 78 mined from Koski's (3) measurements of and we 236 0.11 85 618 0.05 90 fG Wo(Hfl), can examine the among 279 50.1 .134 NGC 3227 0.10 61 distribution of WFC(H13) the various 290 0.07 148 3516 0.14 82 types of Seyfert galaxies. This is shown in Table 2, in which are 291 0.14 22 5548 .0.04 .152 listed separately Seyfert 1, Seyfert 1.5 or intermediate-type 304 <0.1 .136 7469 .0.09 .115 profile objects, and Seyfert 2 galaxies. The Seyfert 1.5 column 335 .0.1 <97 I Zw 1 .0.02 .61 includes Mrk 6, Mrk 315, and Mrk 372 measured by Koski (3) 352 S0.1 S104 II Zw 1 .0.08 S53 plus the nine objects listed as Seyfert 1.2 or Seyfert 1.5 in table 358 0.25 72 IIZw 136 .0.03 S158 4 of my earlier paper (2). Further, As Koski suggests (3), Mrk 374 0.13 92 III Zw 2 <0.07 .156 42 is considered a Seyfert 1 galaxy in Table 1, and I Zw 81 and Mrk 378 are not considered to be Seyfert galaxies. The two objects with the largest WFC(H13) in Table 2 are Mrk 573 and galaxy continuum (containing absorption lines) due to in Mrk 270, with fFC = 0.07 and 0.02, respectively; these values or near the nucleus, and it is clear that the equivalent width of are very poorly determined and the errors in WFC(H#) are H,3 emission should be expressed in terms of the featureless correspondingly large. continuum alone to test the photoionization hypothesis. The The statistics of H,3 emission equivalent width, expressed in observed continuous spectrum can be decomposed into its two terms of the featureless continuum, are more or less the same components by adopting a standard integrated-galaxy spectrum for Seyfert 1, 1.5, and 2 galaxies, with a broad distribution and calculating or measuring how much it must be diluted by ranging from 20 to 160 A, and a median of the order of 100 A. a featureless continuum to give the observed absorption-line Given that photoionization followed by recombination is very strengths. Such a decomposition has been carried out by Koski probably the mechanism responsible for the narrow-line (3) for all the objects he observed, using chiefly the strengths emission, which makes up the entire H# profile in Seyfert 2 of the G band, Mg I b, Na I D, Ca II K, and a broad blend near galaxies, a good fraction of the profile in Seyfert 1.5 galaxies, X6230 to estimate fc, the relative strength of the galaxy con- and only a small part of the profile in Seyfert 1 galaxies, this tinuum to the entire continuum (at Hi3). From fC and Wo (Hfi), table strongly suggests that photoionization is responsible for the observed equivalent width of emission HB (in the rest frame the emission in the entire profile, broad and narrow components of the observed galaxy) measured by Koski, WFC, the equiva- together. The calculated WFC(Hf3) resulting from photoion- lent width of HB in terms of the featureless continuum, can be ization by a power-law source in an optically thick, dust-free immediately calculated as WFC(H1) = fFcWo (HO) = (1 - gas cloud ranges from 30 to 300 A for power-law indices fG)Wo(H13). In a typical Seyfert 2 galaxy fG t 0.7 and fFC ranging from -1.5 to -0.5 (3); these calculated equivalent 0.3, although fFc ranges from 0.02 to 0.57 in different Seyfert widths are decreased somewhat by dust that can absorb some 2 galaxies, being of course only poorly determined in the former of the ionizing photons or by a high-frequency cutoff. On the case. other hand, collisional excitation and self-absorption from the In contrast to Seyfert 2 galaxies, in typical Seyfert 1 galaxies the featureless continuum is much stronger than the inte- grated-stellar continuum. To a good first approximation Table 2. Statistics of HB equivalent width in as WFC(H/B) Wo(Hfl) these objects, Searle and Sargent Seyfert realized. In the present paper a better approximation is pro- vided for the 36 Seyfert 1 galaxies for which Lick spectropho- WFc(HO) 1 1.5 2 tometric measurements were previously published (2). The 0-19 3 equivalent width of the Ca II K line was measured absorption 20-39 3 1 4 on the directly tracings of each of these objects, and from the 40-59 1 2 1 adopted integrated-galaxy equivalent width 12.2 A (the mean 60-79 9 2 value determined from the two absorption-line elliptical gal- 80-99 5 2 axies NGC 6482 and NGC 6702) the fractional contribution of 100-119 1 1 2 the galaxy spectrum was determined as fG = Wo(K)/12.2. In 120-139 4 2 3 a few of the Seyfert 1 galaxies it was also possible to measure 140-159 5 2 the depression d(b) at the deepest part of the Mg I X5170 b 160-179 feature, expressed as a fraction of the continuum to the long- 180-199 1 wavelength side of this absorption. Again, by comparison with 200-249 1 the two standard spectra, an independent value 250-299 fG = d(b)/0.37 was found. In most of the Seyfert 1 galaxies, 300-349 however, it is impossible to see the Mg I absorption because of 350-399 1 strong Fe II emission features in this spectral region. For the 400-449 1 six Seyfert 1 galaxies for which both Ca II and Mg I absorption Median 90 90 110 Downloaded by guest on September 30, 2021 542 Astronomy: Osterbrock Proc. Nati. Acad. Sci. USA 75 (1978)

Table 3. Statistics of [O III] X5007 equivalent width dense, broad-line gas observed in Seyfert 1 and broad-line radio galaxies is then that it forms a turbulent, rotating disk just out- Seyfert side and around the of the black hole. The con- WFC(X5007) 1 1.5 2 cept of rotational velocity as the origin of much of the observed line broadening in Seyfert 1 galaxies dates back to the pi- 0-19 11 1 oneering paper of Woltjer (13); a specific detailed model based 20-39 8 2 2 40-59 5 3 on this concept has recently been published by Shields (14). In 60-79 1 2 the present paper the aim is to describe the areas of agreement 80-99 3 1 1 between observational results and this general type of model 100-119 2 1 and to specify its properties and parameters to the extent this 120-139 can be done from observational data. 140-159 The statistics of line widths do not agree with the hypothesis 160-179 that rotation alone is the sole source of broadening (2), essen- 180-199 2 tially because there are no Seyfert 1 galaxies with near-zero line 200-249 1 widths. However, the combination of turbulence, with a root- 250-299 1 mean-square velocity component of about 2000 km/s, and rotation, with an equatorial velocity of about 5000 km/s, gives 1000-1999 2 4 an expected distribution that agrees well with the distribution 2000-2999 1 of observed widths. (Note that Mrk 42 with an H I full width 3000-3999 2 at zero intensity 4500 km/s should be added to the Seyfert 1 4000-4999 1 statistics.) Several Seyfert 1 galaxies have asymmetric line Median 30 70 275 profiles, all with wings extending farther to the long-wavelength side of the peak than to the short wavelength side. Attributing this asymmetry to extinction by dust mixed with the ionized level 2s level of HO both tend to increase WFC(HB). Thus, the gas, these profiles suggest that there is some contraction or flow observed Hf3 emission-line equivalent widths are in approxi- into the central source, but there is no evidence for expansion. mate agreement with the idea that photoionization is respon- Presumably, the ratio of turbulent to rotational velocity, 0.4, sible for the energy input to the broad- and the narrow-line is approximately the ratio of height to diameter of the rotating gas. disk of dense ionized gas. Observed variations of the broad If photoionization were responsible for the narrow-line emission lines in some Seyfert 1 galaxies suggest a diameter component but not for the broad-line component, then the about 0.1 pC, and an active nucleus with this diameter, L(Hfl) distribution of equivalent widths of the narrow H#3 emission = 109 Lo and Ne = 109 cm-3 would have a filling factor 0.1, lines alone would be expected to be approximately the same in all apparently quite reasonable values. Also the average rota- all three types of Seyfert galaxies. Although Koski has measured tional velocity 2500 km/s at an average radius 0.025 pc requires equivalent widths for the narrow Hf3 lines in Seyfert 2 galaxies a central black hole of mass 4 X 107 MO, about the value cor- and in three Seyfert 1.5 galaxies, similar data are not yet responding to the observed luminosity (10). available for most of the Seyfert 1 galaxies. Until these have Only ionizing photons not absorbed by the dense, turbulent been measured, the best procedure is to study the distribution disk rotating about the central source will get through to ionize of the equivalent widths of [O III] X5007 because its intensity the lower-density gas in its much greater volume, with diameter > ratio to the narrow-line component of HO3 is more or less con- 102 pc. The dense disk will have the greatest optical depth stant at X5007/H,3n ; 10 in Seyfert 2 galaxies and the nar- in the equatorial directions and the smallest optical depth in the row-line regions of Seyfert 1 galaxies (3, 6). Therefore polar directions. Therefore, Seyfert galaxies with relatively WFC(X5007), the equivalent width of [O III] X5007 in terms of massive disks, or relatively thick disks, will be extreme Seyfert the featureless continuum (actually at X4861 rather than at 1 galaxies, with only small amounts of narrow-line gas ionized X5007, but the difference is small), was calculated for each along their polar axes, while objects with smaller amounts of Seyfert galaxy from the relation WFC(X5007) = [I(X5007)/ dense gas along their polar directions will be optically thin down I(Hfl)IWFC(H#3), and Table 3 was constructed to study their to larger colatitudes and hence will have larger proportions of distribution. It can be seen that the three distributions are ionized low-density gas. The example shown in Fig. 1 is a case quantitatively quite different. It is clear from this table that the in which the central dense disk is optically thin to colatitude 45°, photoionization of the narrow-line gas is inhibited strongly in so that the narrow-line gas is ionized in two cones, each with Seyfert 1 galaxies, and to a lesser extent in Seyfert 1.5 galaxies, vertex angle 90°. This object would be an intermediate-type in comparison with Seyfert 2 galaxies. The most natural hy- Seyfert galaxy, with approximately equal strengths of narrow pothesis is that this inhibition occurs by the absorption of some and broad-line components of Hj3. Pure Seyfert 2 galaxies of the ionizing photons in the broad-line gas and that photo- would be objects in which the dense disk is nonexistent or op- ionization is the mechanism for its ionization as well as for the tically thin even in the equatorial plane, so that most of the ionization of the narrow-line gas. ionizing photons are absorbed in the low-density gas. On this model the highest stages of ionization (such as He2+), if present, would be expected close to the central source, where GEOMETRY the ionizing flux is high. Thus, He II X4686 should have a From the theoretical point of view, the evidence summarized broader profile than the H I lines because the rotational ve- by Rees (10) suggests that physically the central source is locities are higher close to the central black hole. This prediction probably a rotating black hole with mass of order 108-109 Mo. is hard to verify because in most Seyfert 1 galaxies Fe II X4570 Approximate calculations indicate that such a source can radiate emission is strong and partly overlaps the Hell X4686 profile. a continuous spectrum extending to high energies (10, 12), as However, Mrk 10, Mrk 69, Mrk 509, Mrk 618, and NGC 3516 required by the photoionization model. A natural model for the are all cases in which X4570 is relatively weak compared with Downloaded by guest on September 30, 2021 Astronomy: Osterbrock Proc. Natl. Acad. Sci. USA 75 (1978) 543 in having relatively weak Fe II emission (2). The excitation r Fe+10 mechanism of this emission is almost certainly resonance flu- orescence (15, 16), which means that Seyfert 1 galaxies must have considerable amounts of Fe+, a low stage of ionization, ie++ relatively close to the source of ultraviolet continuum radiation. Hence, the Seyfert 1 galaxies must have, on the average, rela- tively high-density gas in their broad-line regions close to the nucleus. Again, perhaps this relatively high-density gas inhibits 3LG+ the escape of radio plasma from the active nucleus and thus prevents formation of a radio source. In this connection, it may be noted that Seyfert 2 galaxies are, on the average, consider- ably stronger radio sources than Seyfert 1 galaxies, as first re- -BLG0 ported by Sramek and Tovmassian (17). The fact that especially high-density gas close to the nucleus is required to account for strong emission in Fe II may explain the correlation between Fe II emission and emission noticed by Allen (18), 10 2 PC supposing that the infrared comes from dust heated by radiation I-102 p from the central source. by Grandi (19) show that the highest ioniza- FIG. 1. Model, not to scale, showing dense (BLG) and low-density Measurements (NLG) gas components in cylindrical and spherical distributions, tion line observed from the narrow-line region, [Fe XI] X7892, respectively, about a central source. Ionized regions are indicated by is often slightly broader than and slightly blue-shifted with cross hatching; highly ionized regions by double cross hatching. Flow respect to the other narrow lines. Similar but smaller effects out from nucleus in low-density gas decreases outward. have been noted for [0 III] and [Ne III] with respect to other lower ionization lines (3, 16). The most straightforward inter- pretation is that in the narrow-line gas there is an outward flow X4686, and in all of them the He II profile appears broader than from the nucleus, with velocity decreasing outward, so that the HfB. In both Mrk 509 (6) and Mrk 618 (Fig. 2), the main im- highest ionization lines, emitted closest to the central source, pression is that although the full width at zero intensity of X4686 come from regions of highest flow velocity, and that dust mixed and H/i are comparable, the full width at half maximum of with the gas reduces the contribution to the observed profile X4686 is much larger than that of Hfl. In other words, more of of the emission from the far side of the active nucleus. the He2+ is at high velocities than of the H+, although their The observations also show a very strong correlation between limits of velocity are comparable. the presence of a relatively strong featureless-continuum source Although the quantitative discussions in Tables 2 and 3 in- and the presence of appreciable broad-line, dense gas. As Table clude only Seyfert galaxies, for which the best observational 1 shows, in a typical Seyfert 1 galaxy fFC > 0.9, while in a data are available, this model presumably also applies to radio typical Seyfert 2 galaxy fFc t 0.3 (3). The Seyfert 1 nuclei must galaxies. Most radio galaxies with emission lines are narrow-line be more luminous; perhaps they all have close to radio galaxies; those that are broad-line radio galaxies have or even above the (10, 12) and the origin relatively strong [O111] and other forbidden lines and are more of the turbulence and the continued presence of dense gas near similar to intermediate-type Seyfert galaxies than to Seyfert 1 the nucleus is connected with L > LEdd. galaxies (4). This suggests that the escape of radio-emitting One remaining problem is why all Seyfert 2 galaxies are not plasma from the nucleus to the distant double source is corre- radio sources. There must be another parameter in addition to lated with the escape of ionizing photons from the nucleus to column density of gas in the polar direction that determines the narrow-line gas region. Probably the broad-line radio gal- whether or not an active nucleus can become at least a weak, axies have, on the average, less mass per unit area in the polar single radio source. As has been emphasized before (7), most direction than typical Seyfert 1 galaxies. broad-line radio galaxies are N galaxies and most narrow-line Broad-line radio galaxies also differ from Seyfert 1 galaxies radio galaxies are cD, D, or E galaxies, while most Seyfert galaxies are, insofar as they can be classified by form, S galaxies (20-22). The basic difference between elliptical and spiral

1.0 galaxies is probably connected with the ratio of disordered .MVRK 618 [O III] (turbulent) to ordered (rotational) velocities per unit mass, and [01I1 Fe H this must be related to the present conditions of the gas in the 3727 4570 [Ne I1111H6 H7y He Fe II Fe nucleus. 3869 4686 5190 5320 HelI 3889 I am very grateful to the Hill Family Foundation for support of this research under a Professorship at the University of Minnesota, to the University of California for support under a sabbatical leave, and to the National Science Foundation for support under Research Grant AST-76-18440.

0.0 4000 5000 1. Weedman, D. W. (1977) Annu. Rev. Astron. Astrophys. 15, x 69-95. FIG. 2. Measured spectrum ofSeyfert 1 galaxy Mrk 618 in relative 2. Osterbrock, D. E. (1977) Astrophys. J. 215,733-745. energy units per unit wavelength interval against wavelength. Note 3. Koski, A. T. (1976) Dissertation (University of California, Santa the broad He II X4686 profile. Cruz, CA). Downloaded by guest on September 30, 2021 544 Astronomy: Osterbrock Proc. Nati. Acad. Sci. USA 75 (1978)

4. Grandi, S. A. & Osterbrock, D. E. (1978) Astrophys. J. 220, in 13. Woltjer, L. (1959) Astrophys. J. 130, 38-44. press. 14. Shields, G. A. (1977) Astrophys. Lett. 18, 119-123. 5. Boksenberg, A. & Penston, M. V. (1976) Mon. Not. R. Astron. 15. Wampler, E. J. & Oke, J. B. (1967) Astrophys. J. 148, 695- Soc. 177, 127P-131P. 704. 6. Osterbrock, D. E. (1978) Phys. Scr. 17, in press. 16. Phillips, M. M. (1977) Dissertation (University of California, Santa 7. Osterbrock, D. E. (1978) Phys. Scr. 17, in press. Cruz, CA). 8. Osterbrock, D. E. & Koski, A. T. (1976) Mon. Not. R. Astron. Soc. 17. Sramek, R. A. & Tovmassian, H. M. (1975) Astrophys. J. 196, 176, 61P-66P. 339-345. 9. Ptak, R. & Stoner, R. E. (1975) Astrophys. J. 200,558-566. 18. Allen, D. A. (1976) Astrophys. J. 207,367-375. 10. Rees, M. J. (1977) Ann. N.Y. Acad. Sci. 302,613-636. 19. Grandi, S. A. (1978) Astrophys. J. 221, in press. 11. Searle, L. & Sargent, W. L. W. (1968) Astrophys. J. 153, S. (1975) Astrophys. J. 198, L1-L2. 1003-1006. 20. van den Bergh, 21. Adams, T. F. (1977) Astrophys. J. Supp. Ser. 33, 19-34. I. D. & Thorne, K. S. (1973) in Black Holes, eds. De 12. Novikov, Soc. Witt, C. & DeWitt, B. S. (Gordon & Breach, New York), pp. 22. Wehinger, P. A. & Wyckoff, S. (1977) Mon. Not. R. Astron. 343-450. 181,211-231. Downloaded by guest on September 30, 2021