Dusty Disks and the Infrared Emission from AGN Published in Theory of Accretion Disks, 1989 DUSTY DISKS AND THE INFRARED EMISSION FROM AGN E. Sterl Phinney Theoretical Astrophysics, 130-33 Caltech Pasadena, CA 91125 U.S.A. ABSTRACT. The distortions inferred in the gaseous disks of active galaxies suggest that a significant, and possibly dominant fraction of the 1-1000 µm radiation observed from AGN must be thermal emission from gas and dust heated by the central source. We report calculations of the growth and sublimation of dust grains in the outer parts of accretion disks appropriate to AGN. The thermal state of the gas undergoes a sudden change at the radius where the dust sublimates. The outer portion of the accretion disk radiates at 0.5-5 µm; free-free emission from gas whose dust has sublimated contributes to the flux at 0.5-2 µm. If this thermal emission dominates the flux from radio-quiet quasars, it naturally explains the frequency and depth of the universal minimum in F at 1014.5 Hz. Free-free emission from the photoionized surface layers of the disk at larger radii produces a radio flux at 1011 Hz comparable to that observed in radio-quiet quasars. The far-infrared and submillimeter emission from radio-quiet quasars and Seyfert galaxies is more naturally interpreted as reradiation by dust, than as nonthermal emission from the inner accretion disk. Table of Contents INTRODUCTION DUST AND WARPS OUTSKIRTS OF THE GALAXY AND SUBMILLIMETER SPECTRUM TRANSITION DISK file:///E|/moe/HTML/Phinney/Phinney_contents.html (1 of 2) [10/14/2003 6:47:27 PM] Dusty Disks and the Infrared Emission from AGN SURFACE PHYSICS AND RADIO EMISSION SUBLIMATION AND THE TEMPERATURE GAP CONCLUSIONS REFERENCES file:///E|/moe/HTML/Phinney/Phinney_contents.html (2 of 2) [10/14/2003 6:47:27 PM] Dusty Disks and the Infrared Emission from AGN 1. INTRODUCTION It is generally accepted that emission from heated dust produces the steep far-infrared continua of Seyfert 2's and the IRAS warm galaxies. It is also generally acknowledged that the rapidly variable infrared emission in the BL Lacs and optically violently variable quasars must be produced by non-thermal processes. Both thermal and non-thermal emission evidently occurs in nature, in objects of comparable luminosities. The controversy over the nature of the emission in radio-quiet quasars is thus not over whether either emission process is physically possible or plausible. Both are, and both are probably present at a significant level in all objects. The question is simply which happens to predominate in radio- quiet quasars. In most optically selected quasars, the 1-100µ infrared luminosity comprises 10-50% of the bolometric luminosity (Sanders et al. 1989). Since the energy liberated per octave in radius in an accretion disk scales as r-1, the high relative infrared luminosity requires that the ultimate source of energy for the infrared radiation be within a few times the inner radius of the accretion disk. If the infrared radiation is emitted from a region comparable in size to that of its energy source, it must be nonthermal: blackbody limits on the source size at 1 µ and at 60 µ are respectively > 0.1 pc and > 100 pc for the most luminous sources in figure 1a and > 0.003 pc and > 10 pc for the least luminous. If the infrared radiation is thermal, and therefore emitted at large radii, it must be reprocessed energy transported to the emission radii by radiation or by mechanical means (e.g., a jet). Dust heated locally by stars may contribute to some of the infrared emission from quasars. It is unlikely, however, that this dominates in the majority of sources. For stars to produce the typical infrared luminosity of 1013 L = 0.5 M c2 yr-1 would require a star formation rate of at least 500 M yr-1 (this lower limit assumes that only massive O stars are formed; a normal IMF would require a rate approximately 10 times higher). Over the ~ 108 yr lifetime of a quasar, > 5 x 1010 M of gas would have to be processed in massive stars. Dust would form from the metals produced. So much gas and dust, pushed to high latitudes by supernova explosions, would inevitably absorb and reradiate much of the luminosity from the central source. But the optical and ultraviolet radiation from quasars, variable on timescales 10 yr (Usher 1978) must come from such a central relativistic source. Since its luminosity is comparable to the infrared luminosity, we conclude that reradiation from gas and dust heated by it would necessarily be at least comparable to anything contributed by stars. In what follows we therefore ignore the heat input from stars, except insofar as they provide a natural minimum dust temperature 25 K. Quasars and Seyfert galaxies appear to be located in galaxies amply supplied with interstellar medium. The disks of gas and dust in normal galaxies exhibit warps on all scales, and would intercept and re- radiate ~ 10% of the luminosity from a central source. Several lines of evidence suggest that the warps are even more severe in Seyferts and quasars, so that an even larger fraction of the central luminosity file:///E|/moe/HTML/Phinney/Phinney1.html (1 of 2) [10/14/2003 6:47:28 PM] Dusty Disks and the Infrared Emission from AGN would be reradiated. Provided most of the reradiating material is located in an optically thick disk, the central source will never appear absorbed or reddened - either we have a clear line of sight, or the source is entirely occulted. This picture is thus consistent with the absence of reddening or absorption by broad emission line clouds in quasars. We describe below the opacities and physical state of gas in the disks at distances from 10-3 pc to 104 pc from the central source. Dust grains can form and grow in the disk within 1 pc. At much smaller radii, however, even graphite grains will sublimate. When this occurs, the gas loses its primary coolant, and heats until it reaches a new thermal equilibrium at 104 K. The superposition of emission from radii inside and outside this transition point naturally explains the minimum in L at = 1014.5 Hz observed in most quasars. The characteristic scale length of dust in galaxies (~ 2-10 kpc) naturally explains both the frequency and steepness of the drop in L at = 1014.5 at 1012 Hz. The normalization of infrared luminosity relative to the UV and X-ray luminosities of quasars is consistent with expected covering factors and space-densities. Free-free emission from the photoionized zones on the illuminated surfaces of the disk naturally provides a flat-spectrum radio flux comparable to that observed in many quasars (Antonucci & Barvainis 1988), and may contribute to the optical continuum emission. It appears therefore that the emission at wavelengths 1-1000 µm from Seyfert galaxies and quasars other than OVV's is naturally explained as thermal reradiation from the nuclear disk and the interstellar medium of the host galaxy. Although non-thermal emission from the central source may contribute in some objects at some times, a significant contribution from thermal emission seems unavoidable. file:///E|/moe/HTML/Phinney/Phinney1.html (2 of 2) [10/14/2003 6:47:28 PM] Dusty Disks and the Infrared Emission from AGN 2. DUST AND WARPS Most previous discussions of dust and gas in the host galaxy of an active nucleus have concentrated on spherically symmetrical distributions of dust (Rees et al. 1969, Bollea & Cavaliere 1976, Barvainis 1987 - the latter also considered conical sectors, or have concentrated on their effects on emission line clouds: Davidson & Netzer 1979, MacAlpine 1985 and references therein). A few have considered disk-like distributions (Begelman, McKee & Shields 1983, Begelman 1985, Shlosman & Begelman 1988), but have assumed the disks were planar and relatively thin, so that they intercepted only a small fraction ( 1%) of the luminosity of the central accretion disk. Neither of these distributions is very likely. The gas and dust most probably lie in what could crudely be described as a heavily warped (and probably clumpy) disk. Beyond about a kiloparsec radius, the orbital time t 108 (r / 3 kpc) yr is so long that gas captured orb or disturbed by a recent interaction with another galaxy will not have had time to settle into the preferred plane of the host galaxy's potential in a quasar lifetime (~ 108 yr). Since quasars seem commonly to be involved in interactions or mergers (Hutchings 1983, MacKenty & Stockton 1984), large warps and streamers are to be expected. The warped disk of NGC 5128 (Centurus A) is a famous example. Moving inwards, warps on kiloparsec scales can be produced by counter rotating bars (Vietri 1986), by continuing infall of gas (Ostriker & Binney 1989), and by Kelvin-Helmholtz instability as the disk rotates through a pressure-supported corona (Gunn 1979) Though such warps are difficult to detect in external galaxies which are not nearly edge-on, let alone in quasars, the gas at ~ 3 kpc in our own Galaxy is tilted by about 15° with respect to the plane defined at larger radii (Vietri 1986), and most well-studied Seyfert galaxies have strong kiloparsec-scale bars (Adams 1977). The same warp-inducing processes can operate on scales of parsecs. The molecular torus extending from 2-8 pc from our Galactic center is tilted by ~ 15° (in the opposite direction from the 3 kpc warp!) with respect to the plane defined by the stars. The parsec-scale dust torus in NGC 1068 (Antonucci & Miller 1985) has its axis at right angles to the kiloparsec-scale disk of gas and stars (Wilson & Ulvestad 1982), and the axes of similar tori inferred to exist in other Seyferts make random angles to the minor axes of the disks of their host galaxies (Unger et al. 1987, Haniff et al. 1988). We conclude that warps are common and substantial enough to allow the nuclear continuum to illuminate the dust and gas on scales from 1-104 parsecs. The broad-line region (BLR) will be obscured by this dust over a range of viewing angles comparable to the covering factor of the dust disk. Except in the case that the narrow line gas is cospatial with the dusty disk in a symmetrical warp, it is difficult to prevent the narrow line region from being visible from some viewing angles when the BLR is obscured. This occurs for a fraction of sky of order the covering factor of the dust disk extending beyond the NLR: typically ~ 0.05-0.1. Since molecular clouds are larger than the 0.1-1 pc scale of the BLR, the same fraction of quasars with obscured broad line regions would be expected even if the dust were in clouds at high latitudes rather than in a disk. These objects would appear as ``quasar-2's'' (by analogy to Seyfert 2's). file:///E|/moe/HTML/Phinney/Phinney2.html (1 of 3) [10/14/2003 6:47:28 PM] Dusty Disks and the Infrared Emission from AGN Such objects have been rare in optical surveys, but appear common in infrared-selected samples (Sanders et al. 1988b). In radio samples they may masquerade as narrow line radio galaxies (Scheuer 1987, Barthel 1989). We now examine the state of dust in a warped disk illuminated by the central accretion disk of a quasar, and its possible relevance to the infrared and submillimeter spectra of quasars. We postpone to section 5 a discussion of the vertical structure of an illuminated disk, and the expected optical and radio emission therefrom. The equilibrium temperature T of dust grains of characteristic radius a 20 Å at distance r from a g radiation source of luminosity density L (erg s-1 Hz-1) is determined implicitly by (1) where Q ( ) is the absorption efficiency (cross section in units of a2), at frequency . Graphite grains abs with a ~ 0.1 µm are transparent to X-rays with h > 0.4 keV and for 0.1 keV < h < 0.28 keV (K-edge of Carbon), so Q ( ) a at those energies. At wavelengths < 2 a where the grain is not transparent abs Q 1. At longer wavelengths > 2 a the grain becomes a weakly coupled antenna, Q (2 a / abs abs ) f( ) where f( ) depends on the dielectric tensor and shape of the grains (fits for various grain compositions and shapes, and discussion of the Mie scattering range can be found in Martin 1978, Draine & Lee 1984, Wright 1987, and references therein). Observations of galactic dust indicate that outside resonances f( ) 1- , with 1 2 (Whittet 1988). In quasars, most of the energy from the central source is emitted at frequencies for which Q ( ) 1, abs and reradiated at wavelengths > 2 a. Hence if we ignore heating and cooling of grains by collisions with atoms (generally weak), the temperature of directly illuminated grains is given approximately by (2) where < Q (T) > is the Planck-averaged absorption efficiency (cf. Draine 1981), < Q (T) > T . abs abs g Crudely, therefore, T [L / (r2 a)]1/(4+ ). At a given distance from the source, the smallest grains in g thermal equilibrium (~ 30 Å) will be about a factor of 2 hotter than the largest grains commonly considered (~ 0.3 µm). The heat capacity of grains with a 20 Å is so low that their temperature fluctuates, being significantly affected by the absorption of a single UV photon. Grains deeper in the dusty disk will not be exposed directly to UV radiation from the central source, but to longer-wavelength re-emission from shielding gas file:///E|/moe/HTML/Phinney/Phinney2.html (2 of 3) [10/14/2003 6:47:28 PM] Dusty Disks and the Infrared Emission from AGN and dust. This shielded dust will have a temperature nearly independent of a, and slightly lower (by a factor ~ (T / 2500 K)1/(4+ )) than that of the directly illuminated grains of temperature T . re re The emission from dust at temperature T at long wavelengths scales as 2+ T , peaks very g g sharply at h ~ (3 + )kT ( ~ 30 T -1 µm, where T = 100 T K) and declines exponentially at higher g 2 g 2 frequencies. Except for the small grains of fluctuating temperature, which can contribute high frequency emission from regions where the equilibrium temperature is low, most emission at frequency v will come from the radius where the equilibrium dust temperature T ~ h / (3 + ) k, i.e., from a radius r L1/2 - g (4+ )/2, and the flux of radiation at that frequency will (for an isotropic central source) be proportional to the fraction of the sky at the central source covered by dust at the appropriate radii. This radius-frequency scaling can cause curious effects. At frequencies where is large (e.g., 1 < < 7 µm, where 1.7 - Whittet 1988), the temperature is nearly independent of radius, enhancing the probability of having a large warp at an appropriate radius, and hence a large sky covering factor and a large ~ 3 µm flux (see figure 2). This may be the cause of the ``3-5 µm bumps'' commonly observed in AGN (Edelson & Malkan 1986). file:///E|/moe/HTML/Phinney/Phinney2.html (3 of 3) [10/14/2003 6:47:28 PM] Dusty Disks and the Infrared Emission from AGN 3. OUTSKIRTS OF THE GALAXY AND SUBMILLIMETER SPECTRUM At low frequencies is also large ( -> 2 when 0-frequency isotropic conductivity dominates the dielectric tensor), so that the temperature changes only slowly with radius at the outskirts of the galaxy (~ 3 - 30 kpc) where the molecular gas distribution of galaxies rapidly becomes very patchy. The characteristic temperature at the outskirts of a typical galaxy would be ~ 30 - 50 K (see figure 1). Figure 1. Temperature of directly illuminated 0.1 µm graphite grains as a function of distance from a central UV source of luminosity L = 1046 erg s-1. Planck-averaged UV absorption efficiencies used are from Draine & Lee 1984. For sources of other luminosities and grains of other sizes, the temperature in the flatter portions of the curve (T > 300 K, T < 60 K) scales roughly as (L / a)1/6, and in the steeper UV portion (60 K < T < 300 K) as (L / a)1/4. For a galaxy disk UV with a smooth logarithmic warp, dust at the given temperatures contributes predominately to the flux at wavelengths marked on the right. A dust layer wil absorb UV flux incident at an angle to the normal of the layer if its column density > 10-2 cos g cm-2. With a Galactic gas-to-dust ratio spread over a disk of radius r this column kpc corresponds to a mass of gas M 2 x 108 cos r2 M . The gas masses in nearby AGN are inferred H kpc file:///E|/moe/HTML/Phinney/Phinney3.html (1 of 3) [10/14/2003 6:47:29 PM] Dusty Disks and the Infrared Emission from AGN from observations of CO (which of course depletion relates more directly to the dust than to M !) to be H of order 108-1010 M (Sanders, Scoville, & Soifer 1988a), and even in a young galaxy such as might surround a high redshift quasar it is unlikely that there would be more than ~ 1011 M of processed gas. Consequently the disk will become optically thin beyond a few kpc (it could become thin at a smaller radius if most of the dust is clumped into clouds with >> 10-2 cos g cm-2, and such clouds at larger radii could preserve dust in neutral cores, but their covering factor would necessarily be very small). Since the covering factor of dusty material at 20 K (~ 100 kpc) is expected to be very small, the spectrum of dust reradiation will be characterized by a rather well-defined minimum temperature, and should roll over sharply at wavelengths 200 µm with F 2+ at longer wavelengths (as for similar reasons it is observed to do in starburst galaxies and Galactic H II regions - cf. Telesco & Harper 1980). The precise form of the spectrum at 60 µm < < 200 µm will vary depending on the covering factor at large radius, which could be enhanced by the presence of companion galaxies (whose starlight maintains a minimum dust temperature ~ 20 K!), tidal tails, and the like. As we discuss in section 5, at frequencies < 1011 Hz, free-free emission from photoionized gas at the illuminated face of the disk will dominate the spectrum. Figure 2 shows the spectrum of continuum reradiation from gas and dust in an exponential disk with a logarithmic warp (d(covering factor) / d ln r = const). To illustrate how material at large radii can affect the far infrared and submillimeter spectrum, we show the effect of adding reradiation from a 2 x 20 kpc slab of dust extending from 10 to 30 kpc (which could represent a companion galaxy or a tidal tail). file:///E|/moe/HTML/Phinney/Phinney3.html (2 of 3) [10/14/2003 6:47:29 PM] Dusty Disks and the Infrared Emission from AGN Figure 2. Spectrum of reradiation from dust and photoionized gas in a warped disk surrounding a quasar with L = 1046 erg s-1. The solid line UV represents a disk containing 0.1 µm graphite dust having C = d(covering factor) / dln r = 0.1 exp(-r / 10kpc). Note the ``5 µm bump.'' The dot-dashed line shows the result of adding 0.03 to C for 10 < r < 30 kpc - representing a tidal tail or companion galaxy. The dashed line shows the spectrum of reradiation from (very large) grains assumed to radiate as black bodies, with the same covering factor distribution as for the solid line. The dotted line shows the contribution of free-free emission from the photoionized zones above the dust. There is no freedom in its normalization relative to the dust spectra. file:///E|/moe/HTML/Phinney/Phinney3.html (3 of 3) [10/14/2003 6:47:29 PM]