3.1. General radio properties?
The association between radio sources and clusters of galaxies was first made by Mills (1960) and van den Bergh (1961b). Radio emission from clusters has been reviewed recently by Robertson (1985). A brief review of the radio properties of clusters relevant to their X-ray emission will be given in this section. First, the general radio luminosities and spectra will be summarized. Second, possible correlations between the radio and X-ray emissions of clusters will be discussed. Third, two classes of radio source morphology (head-tail radio sources and cluster halo sources) which are unique to the cluster environment will be described. Fourth, possible observations of the effect of the intracluster medium on radio emission due to the cosmic blackbody background or background source will be reviewed. Finally, the use of the 21 cm hyperfine line to detect neutral hydrogen gas in clusters will be briefly discussed.
Figure 7 shows a contour plot of the radio emission in the Perseus cluster, superimposed on an optical photograph. The very strong radio emission from the central galaxy NGC1275 (Section 4.5.2) and the two head-tail radio galaxies NGC1265 and IC310 are shown.
Figure 7. A radio map of the Perseus cluster of galaxies from Gisler and Miley (1979). Contours of constant radio surface brightness at 610 MHz are shown superimposed on the optical image of the cluster. Note the very strong source associated with the galaxy NGC1275 (the highest contours associated with this source have been removed), and the two head-tail radio sources associated with NGC1265 and IC310. The rings are diffraction features due to NGC1275 and are not real.
The radio emission from clusters of galaxies (as well as most other extragalactic objects) is synchrotron emission due to the interaction of a nonthermal population of relativistic electrons (with a power-law energy distribution) with a magnetic field (Robertson, 1985). Such nonthermal synchrotron emission generally has a spectrum in which the intensity I (erg / cm2-Hz-s) is well represented as a power-law over a wide range of frequencies ,
where r is the radio spectral index. Typical extragalactic radio sources have r 0.8. Most of the radio emission from clusters is due to discrete sources, which can be associated with individual galaxies within the cluster. The properties of the nonthermal radio emission from radio galaxies have been reviewed recently by Miley (1980).
Radio emission from Abell clusters at a frequency of 1400 MHz has been surveyed by Owen (1975), Jaffe and Perola (1975), and Owen et al. (1982). At lower frequencies there are a number of older surveys (e.g., Fomalont and Rogstad, 1966), as well as an extensive list based on the 4C Cambridge survey (Slingo, 1974a, b; Riley, 1975; McHardy, 1978b). These observations suggested that only sources detected in the inner portions (within 1/3 Abell radii) of the Abell clusters were likely to belong to the cluster, the rest being background objects. Observations at higher frequencies have been made by Andernach et al. (1980, 1981), Haslam et al. (1978), and Waldthausen et al. (1979). The more distant Abell clusters were searched for radio emission by Fanti et al. (1983), while the richest Abell clusters were observed by Birkinshaw (1978). Jaffe (1982) surveyed a sample of high redshift clusters, and found evidence of evolution in the radio luminosity of clusters for the range of redshifts 0.25 < z < 0.95.
In discussing the radio luminosity functions of clusters, it is important to distinguish the luminosity function of galaxies in clusters from the luminosity function of the cluster as a whole. The radio emission from clusters is mainly due to sources associated with individual radio galaxies. About 20% of the nearby strong radio galaxies are located in rich clusters of galaxies (McHardy, 1979). This appears to be mainly due to the fact that strong radio emission is primarily associated with giant elliptical galaxies, which occur preferentially in clusters. A galaxy of a given morphology (elliptical, for example) and optical luminosity apparently has the same radio luminosity function whether inside or outside of a cluster (Jaffe and Perola, 1976; Auriemma et al., 1977; Guindon, 1979; McHardy, 1979). The radio luminosity function of the whole cluster can be fit as the result of superposing the luminosity function of an average of 5 radio galaxies per cluster (Owen, 1975). The cluster radio luminosity function does not appear to depend strongly on richness for the Abell clusters (Riley, 1975; Owen, 1975; McHardy, 1979).
There does appear to be a correlation between cluster radio emission and cluster morphology. Owen found that the more evolved RS types (cD,B,C, and L; see Section 2.5) have stronger radio emission. McHardy (1979) found that more evolved BM types (I,I-II; see Section 2.5) tend to contain stronger radio galaxies, at least at low frequencies.
Powerful radio sources are found most often near the cluster center (McHardy, 1979). They are usually associated with optically dominant galaxies, which often have multiple nuclei and are often cD galaxies (Guthie, 1974; McHardy 1974, 1979; Riley, 1975).
Cluster radio sources generally have steeper radio spectra (values of r 1) than radio sources in the field (Costain et al., 1972; Baldwin and Scott, 1973; Slingo, 1974a, b; McHardy, 1979). The steepness of the spectrum (the value of r) increases with cluster richness and decreases as the BM type increases (Roland et al., 1976; McHardy, 1979). The steepest radio spectra (r 1.3) in clusters are associated with radio sources in optically dominant galaxies (Riley, 1975).
De Young (1972) claimed that cluster radio galaxies were generally smaller than those in the field; this claim was not supported by larger surveys (Hooley, 1974; McHardy, 1979). Owen and Rudnick (1976a) found that cluster radio sources generally had extended emission; unresolved sources seen towards clusters are usually background objects. Guindon (1979) found that, while the average size of double or triple radio sources in clusters was the same as those in the field, clusters did lack the very largest sources.
Differences in the morphology of cluster radio sources and field sources are discussed in Sections 3.3 and 3.4 below.
Recently, the radio emission properties of poor clusters have been studied. In general, the sources in poor clusters also have steep spectra and show some of the same morphological distortions found in rich cluster sources (Burns and Owen, 1977). As in rich clusters, the strongest radio emission is associated with optically dominant galaxies near the cluster center, and these sources have especially steep spectra (White and Burns, 1980; Burns et al., 1981b; Hanisch and White, 1981). There is no apparent difference between the radio emission of poor and rich cD clusters beyond the direct effect of richness (Burns et al., 1980).