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13.1.1. Optical Counterparts

The optical identification of the discrete radio sources is important for two reasons. First, it is not possible from the radio measurements alone to determine the distance to a radio source. Only if there is an optical identification can the redshift and, from the Hubble law, the distance be determined. In this way it is then possible to calculate the absolute radio luminosity, linear size, and energy content from measurements of radio flux density and angular structure. Second, optical as well as X-ray and infra-red studies of the radio source counterparts may give some insight into the problem of the origin of the intense radio emission.

The coordinates of radio sources may be routinely measured with an accuracy better than one second of arc, generally permitting the unambiguous association with optical counterparts as faint as about 24th magnitude, which is reached with large reflectors and modern instrumentation. Nevertheless, due to the large amount of telescope time required for systematic studies of color and spectra, only for the strongest few hundred radio sources are the optical identifications reasonably complete (Spinrad et al. 1985). But even for very weak radio sources, optical identifications are usually possible.

(a) Historical Background

The discrete sources of radio emission were first distinguished from the general background radiation as a result of their rapid amplitude variations at low frequencies, which were thought to be due to fluctuations in intrinsic intensity (Hey et al. 1946), but which we now recognize are due to scintillations in the earth's ionosphere. Considering that the dimensions of a variable source cannot greatly exceed the distance traveled by light during a characteristic variability time which is typically about one minute, it was generally thought that the discrete "variable" sources must be galactic stars; thus they were originally referred to as "radio stars."

When two of the strongest radio sources were identified by Bolton and Stanley (1949) with the nearby galaxies M87 and NGC 5128, it became clear that at least some of the discrete sources were extragalactic. Later the position of the powerful radio source Cygnus A was measured by Smith (1951) with sufficient precision to permit the identification by Baade and Minkowski (1954) with a relatively faint 15th-magnitude galaxy having redshift of 0.06. After this the extragalactic nature of the discrete sources was widely recognized, although the use of the term "radio stars" for extragalactic radio sources persisted for many years.

Other radio sources were identified with galaxies during the 1950s, but progress was slow because of the poor accuracy of the radio source positions. By the early 1960s, however, the increased use of interferometer systems led to position accuracies of the order of a few arcseconds, and many sources were identified with various galaxy types from inspection of the Palomar Sky Survey, which reaches a limiting magnitude about 20.

(b) Radio Galaxies

Galaxies which are identified with strong radio sources in the range of 1041 to 1046 ergs s-1. are generally referred to as "radio galaxies." For the most part, radio galaxies are giant ellipticals with absolute visual magnitude about -21. 1

Many intermediate-luminosity radio galaxies are found in rich clusters of galaxies. X-ray observations show that these clusters often contain a hot (108 K), relatively dense (10-3 particles per cm-3) intracluster medium. Many of the radio in rich clusters show bends or distortions apparently associated with their interaction with this medium.

Many of the more powerful radio galaxies show bright optical emission lines in their nuclei whose strength appears to be correlated with the strength of the compact radio core (Heckman et al. 1983a). Radio galaxies with narrow emission lines in their spectrum are referred to as narrow-line radio galaxies (NLRG). Typical line widths are of the order of 1000 km s-1. and include both forbidden lines of O, N, S, and Fe as well as the Balmer lines of He. Broad-line radio galaxies (BLRG) have very broad H and He features, with velocities up to 25,000 km s-1, and may also include narrow emission features as well.

Because of their bright emission lines, it is relatively easy to determine the redshift of many radio galaxies. Indeed, the most distant galaxy redshifts measured are those of radio galaxies. However, for redshifts much greater than unity, the optical K correction becomes very large. Due to the rapid decrease in the brightness of elliptical galaxies toward the blue part of the spectrum, highly redshifted radio galaxies appear very faint at visual wavelengths, but can often be observed in the near infrared.

(c) Quasars

In 1960, the relatively strong, small-diameter radio source 3C295 was identified with a 20th-magnitude galaxy having a redshift of 0.46, and was the most distant galaxy known at the time (Minkowski 1960). Continued efforts to identify distant galaxies concentrated on radio sources of small diameter and high surface brightness since these positions could be measured with high accuracy. In 1961, the small-diameter radio source 3C48 was identified with what appeared to be a 16th-magnitude stellar object. The subsequent discovery of night-to-night variations in the light intensity led to the reasonable conclusion that 3C48, unlike the other identified discrete radio sources, was indeed a true radio star in our galaxy. Soon, the optical counterparts of two other relatively strong small-diameter radio sources, 3C196 and 3C286, were also found to appear stellar, and it appeared that as many as twenty percent of all high-latitude radio sources were of this type.

Early efforts to interpret the emission line spectrum of the three known radio stars were unsuccessful although by 1962 some apparent progress was being made in associating many of the lines in the 3C48 spectrum with highly excited states of rare elements. However, lunar occultation measurements gave an accurate position of the strong compact radio source 3C273 (Hazard et al. 1963). Shortly afterward, Maarten Schmidt (1963) identified 3C273 with a 13th-magnitude stellar object, and he noted that the relatively simple spectrum could be interpreted as a redshifted (z = 0.16) Balmer series plus MgII.

A reinspection of the 3C48 spectrum indicated that if the bright line at 3832 Å was identified with the MgII line at 3727 Åin the rest frame, its redshift would be 0.37. Other lines in the 3C48 spectrum could then be identified with OII, NeIII, and NeIV. Additional spectra of other similar objects led to the identification of CIII, CIV, and finally Lyalpha, permitting much larger redshifts to be easily measured. The word quasar is now most often used to describe the entire class of highly redshifted quasi-stellar objects or QSOs.

Assuming that the measured redshifts are cosmological and the distance is given by the Hubble law with H = 100 km s-1. Mpc-1, then the absolute visual magnitudes of quasars range from about -24 to -31. Thus, at optical wavelengths, quasars are up to a few hundred times brighter than the most luminous galaxies. Some relatively low redshift (nearby) quasars, which are strong radio sources, appear to be surrounded by a faint fuzz whose dimensions, color, and brightness are typical of giant elliptical galaxies, thus supporting the idea that quasars are the extremely active nuclei of galaxies. Further down the optical (as well as radio) luminosity function are the Type II Seyfert galaxies, which have relatively bright nuclei with broad emission lines and absolute magnitudes of -20 to -23, the so-called Markarian galaxies, which were originally isolated on the basis of their large UV excess, on photographic plates, and the blazars and BL Lacs. The literature is not always consistent on nomenclature. We will use the term AGN to describe galaxies with prominent (active) nuclei and quasar for objects where the starlike component dominates although there may be a faint underlying galaxy.

The optical spectrum of quasars is clearly nonthermal with a typical spectral index alpha ~ - 1 [(flux density) propto (frequency)alpha] which may continue to the near infrared (2.2 µm as well as to X-ray wavelengths (see Figure 13.2). Because quasars have an ultra-violet excess compared with the spectra of galaxies, moderate redshifts will cause quasars to appear blue when measured by multi-color photometry or when the color is estimated from the "red" and "blue" plates of the Palomar Sky Survey. This property has proved useful in making optical identifications of radio sources with quasars, without taking individual spectra, but it is not infallible. For very large redshifts, the color may appear neutral or even "red" when compared with stars and galaxies, so the identification of high-redshift quasars depends on position coincidence alone and requires both radio and optical position accuracies of the order of an arcsecond.

Figure 2

Figure 13.2. Spectrum of the quasar 3C345 from the radio through the ultraviolet part of the spectrum. Adopted from Bregman et al. (1986) with additional data from the radio literature.

Generally, quasar spectra show intense broad emission lines characteristic of a highly ionized gas with T ~ 104 K and ne ~ 108 cm-3, line widths corresponding to velocities of 10,000 km s-1 or more, and dimensions of the order of a parsec. The most commonly observed lines are those of Lyalpha (1216 Å), CIII (1909 Å), CIV (1549 Å), MgII (2798 Å), OIII (4363 Å, 4959 Å, 5007 Å, and the hydrogen Balmer series.

In addition, there is a larger narrow-line emission region with densities ne < 107 cm-3 producing forbidden emission lines. When examined with sufficient spectral resolution, most quasars also show numerous narrow absorption lines, but the unambiguous identification of quasar absorption lines is complicated by the presence.of multiple redshift systems. In some cases the absorption redshift is close to the emission redshift, and these are believed to be intrinsic to the quasar or associated with its parent cluster. In other cases, the absorption redshift is much less than the emission redshift, and these are thought to originate in intervening clouds lying along the line of sight.

A relatively small number of compact radio sources are identified with optical objects which appear stellar, are highly variable at optical as well as at radio wavelengths, have a nonthermal optical spectrum, often very steep, with no emission or absorption lines, and are often strongly polarized at optical and radio wavelengths. These are frequently referred to as BL Lac objects (the prototype object is BL Lacerte), or blazars. The relation between quasars and blazars as well as the more classical elliptical radio galaxies has been the subject of much research and debate (see Wolfe 1978). A commonly discussed model considers the BL Lac objects as quasars with enhanced continuum emission which overrides the emission line spectrum. This picture is supported by the detection of faint emission lines in a few BL Lac objects at the time when the strength of the continuum emission is near a minimum.

Not all quasars are strong radio sources. Optical surveys using objective prisms to spot the characteristic bright emission line spectra of quasars or surveys of UV excess objects show that radio-quiet quasars are about ten times more numerous than the radio-loud quasars. Quasars with very broad emission lines, in particular, do not seem to be strong radio sources. High-sensitivity radio observations indicate that most optically selected quasars however are weak radio sources. The underlying or "host" galaxies of the radio-quiet quasars appear to be spirals, whereas the "hosts" of radio-loud quasars are probably elliptical galaxies. Quasars which are strong radio sources are usually strong X-ray sources.

1 H = 100 km s-1 Mpc-1. Back.

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