4.5. The Continuum

Today, the word "continuum" in the context of AGN might bring to mind anything from radio to gamma ray frequencies. However, in the early days of QSO studies, the term generally meant the optical continuum, extending to the ultraviolet and infrared as observations in these bands became available. Techniques of photoelectric photometry and spectrum scanning were becoming established as QSO studies began. The variability of QSOs, including 3C 48 and 3C 273 (e.g., Sandage 1963), was known and no doubt contributed to astronomers' initial hesitation to interpret QSO spectra in terms of large redshifts. In his contribution to the four discovery papers on 3C 273, Oke (1963) presented spectrophotometry showing a continuum slope L ^+0.3 in the optical, becoming redder toward the near infrared. He noted that the energy distribution did not resemble a black body, and inferred that there must be a substantial contribution of synchrotron radiation.

A key issue for continuum studies has been the relative importance of thermal and nonthermal emission processes in various wavebands. Early work tended to assume synchrotron radiation, or "nonthermal emission", in the absence of strong evidence to the contrary. The free-free and bound-free emission from the gas producing the observed emission lines was generally a small contribution. The possibility of thermal emission from very hot gas was considered for some objects such as the flat blue continuum of 3C 273 (e.g., Oke 1966). The energy distributions tend to slope up into the infrared; and for thermal emission from optically thin gas, this would would have required a rather low temperature and an excessive Balmer continuum jump. This left the possibilities of nonthermal emission or thermal emission from warm dust, presumably heated by the ultraviolet continuum.

Observational indicators of thermal or nonthermal emission include broad features in the energy distribution, variability, and polarization. For the infrared, one also has correlations with reddening, the silicate absorption and emission features, and possible angular resolution of the source (Edelson et al. 1988). For some objects, rapid optical variability implied brightness temperatures that clearly required a nonthermal emission mechanism. For example, Oke (1967) observed day-to-day changes of 0.25 and 0.1 mag for 3C 279 and 3C 446, respectively. For many objects, the energy distributions were roughly consistent with a power law of slope near ^-1.2. Power laws of similar slopes were familiar from radio galaxies and the Crab nebula, where the emission extended through the optical band. These spectra were interpreted in terms of synchrotron radiation with power-law energy distributions for the radiating, relativistic electrons. Such a power-law energy distribution was also familiar from studies of cosmic rays, and thus power laws seemed natural in the context of high energy phenomena like AGN. In addition to simple synchrotron radiation, there might be a hybrid process involving synchrotron emission in the submillimeter and far infrared, with some of these photons boosted to the optical by "inverse" Compton scattering (Shklovskii 1965). The idea of a nonthermal continuum in the optical, whose high frequency extrapolation provided the ionizing radiation for the emission-line regions, was widely held for many years. This was invoked not only for QSOs but also for Seyfert galaxies, where techniques such as polarization were used to separate the "nonthermal" and galaxy components (e.g., Visvanathan and Oke 1968).

Infrared observations were at first plagued by low sensitivity and inadequate telescope apertures. Measurements of 3C 273 in the K filter (2.2 µm), published by Johnson (1964) and Low and Johnson (1965), showed a continuum steeply rising into the infrared. Infrared radiation from NGC 1068 was observed by Pacholczyk and Wisniewski (1967), also with a flux density (F) strongly rising to the longest wavelength observed ("N" band, or 10 µm). The infrared radiation dominated the power output of this object. Becklin et al. (1973) found that much of the 10 µm emission from NGC 1068 came from a resolved source 1 arcsec (90 pc) across and concluded that most of the emission was not synchrotron emission. In contrast, variability of the 10 µm emission from 3C 273 (e.g., Rieke and Low 1972) pointed to a strong nonthermal component. Radiation from hot dust has a minimum source size implied by the black body limit on the surface brightness, and this is more stringent for longer wavelengths radiated by cooler dust. This in turn implies a minimum variability timescale as a function of wavelength. The near infrared emission of NGC 1068 was found to be strongly polarized (Knacke and Capps 1974).

Improving infrared technology, and optical instruments such as the multichannel spectrometer on the 200-inch telescope (Oke 1969), led to larger and better surveys of the AGN continuum. Oke, Neugebauer, and Becklin (1970) reported observations of 28 QSOs from 0.3 to 2.2 µm. The energy distributions were similar in radio loud and radio quiet QSOs. They found that the energy distributions could generally be described as a power law (index -0.2 to -1.6 for F ) and that they remained "sensibly unchanged" during the variations of highly variable objects. Penston et al. (1974) studied the continuum from 0.3 to 3.4 µm in 11 bright Seyfert galaxies. All turned up toward the infrared, and consideration of the month-to-month variability pointed to different sources for the infrared and optical continua. From an extensive survey of Seyfert galaxies, Rieke (1978) concluded that strong infrared emission was a "virtually universal" feature, and that the energy distributions in general did not fit a simple power law. The amounts of dust required were roughly consistent with the expected dust in the emission-line gas of the active nucleus and the surrounding interstellar medium. A consensus emerged that the infrared emission of Seyfert 2's was thermal dust emission, but the situation for Seyfert 1's was less clear (e.g., Neugebauer et al. 1976, Stein and Weedman 1976). From a survey of the optical and infrared energy distribution of QSOs, Neugebauer et al. (1979) concluded that the slope was steeper in the 1-3 µm band than in the 0.3-1 µm band, and that an apparent broad bump around 3 µm might be dust emission. Neugebauer et al. (1987) obtained energy distributions from 0.3 to 2.2 µm for the complete set of quasars in the Palomar-Green (PG) survey (Green, Schmidt, and Liebert 1986) as well as some longer wavelength observations. A majority of objects could be fit with two power laws ( - 1.4 at lower frequencies, - 0.2 at higher frequencies) plus a "3000 Å bump".

Measurements at shorter and longer wavelengths were facilitated by the International Ultraviolet Explorer (IUE) and the Infrared Astronomical Satellite (IRAS), launched in 1978 and 1983, respectively. Combining such measurements with ground based data, Edelson and Malkan (1986) studied the spectral energy distribution of AGN over the wavelength range 0.1-100 µm. The 3-5 µm "bump" was present in most Seyferts and QSOs, involving up to 40 percent of the luminosity between 2.5 and 10 µm. All Sy 1 galaxies without large reddening appeared to require a hot thermal component, identified with the increasingly popular concept of emission from an accretion disk. Edelson and Malkan (1987) used IRAS observations to study the variability of AGN in the far infrared. The high polarization objects varied up to a factor 2 in a few months, but no variations greater than 15 percent were observed for "normal" quasars or Seyfert galaxies. The former group was consistent with a class of objects known as "blazars" that are dominated at all wavelengths by a variable, polarized nonthermal continuum. Blazars were found to be highly variable at all wavelengths, but most AGN appeared to be systematically less variable in the far infrared than at higher frequencies. This supported the idea of thermal emission from dust in the infrared. This was further supported by observations at submillimeter wavelengths that showed a very steep decline in flux longward of the infrared peak at around 100 µm. For example, an upper limit on the flux from NGC 4151 at 438 µm (Edelson et al. 1988) was so far below the measured flux at 155 µm as to require a slope steeper than ^+2.5, the steepest that can be obtained from a self-absorbed synchrotron source without special geometries. Dust emission could explain a steeper slope because of the decreasing efficiency of emission toward longer wavelengths.

Sanders et al. (1989) presented measurements of 109 QSOs from 0.3 nm to 6 cm (10¹⁰ - 10¹⁸ Hz). The gross shape of the energy distributions was quite similar for most objects, excepting the flat spectrum radio loud objects such as 3C 273. This typical energy distribution could be fit by a hot accretion disk at shorter wavelengths and heated dust at longer wavelengths. Warping of the disk at larger radii was invoked to give the needed amount of reprocessed radiation as a function of radius. As noted by Rees et al. (1969) and others, the rather steep slope in the infrared, giving rise to an apparent minimum in the flux around 1 µm, could be explained naturally by the fact that grains evaporate if heated to temperatures above about 1500 K. Sanders et al. saw "no convincing evidence for energetically significant nonthermal radiation" in the wavelength range 3 nm to 300 µm in the continua of radio quiet and steep-spectrum radio-loud quasars. This paper marked the culmination of a gradual shift of sentiment from nonthermal to thermal explanations for the continuum of non-blazar AGN.

The blazar family comprised "BL Lac objects" and "Optically Violent Variable" (OVV) QSOs. BL Lac objects, named after the prototype object earlier listed in catalogs of variable stars, had a nonthermal continuum but little or no line emission. OVVs have the emission lines of QSOs. These objects all show a continuum that is fairly well described as a power law extending from X-ray to infrared frequencies. They typically show rapid (sometimes day-to-day) variability and strong, variable polarization. The continuum in blazars is largely attributed to nonthermal processes (synchrotron emission and inverse Compton scattering). 3C 273 seems to be a borderline OVV (Impey, Malkan, and Tapia 1989). The need for relativistic motions, described above, arises in connection with this class of objects. A comprehensive study of the energy distributions of blazars from 10⁸ to 10¹⁸ Hz was given by Impey and Neugebauer (1988). Bolometric luminosities ranged from 10⁹ to 10¹⁴ L, dominated by the 1 to 100 µm band. There was evidence for a thermal infrared component in many of the less luminous objects, and an ultraviolet continuum bump associated with the presence of emission lines. When gamma rays are observed from AGN (e.g., Swanenburg et al. 1978), they appear to be associated with the beamed nonthermal continuum. The relationship of blazars to "normal" AGN is a key question in the effort to unify the diverse appearance of AGN.

IRAS revealed a large population of galaxies whose luminosity was strongly dominated by the far infrared (Soifer, Houck, and Neugebauer 1987). (Rieke [1972] had found early indications of a class of ultraluminous infrared galaxies.) The infrared emission is thermal emission from dust, energized in many cases by star formation but in some cases by an AGN. One suggested scenario was that some event, possibly a galactic merger, injected large quantities of gas and dust into the nucleus. This fueled a luminous episode of accretion onto a black hole, at first enshrouded by the dusty gas, whose dissipation revealed the AGN at optical and and ultraviolet wavelengths (Sanders et al. 1988).