Annu. Rev. Astron. Astrophys. 1994. 32: 277-318
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3.3 Radio

Many galaxies with cooling flows have a strong central radio source (Valentijn & Bijleveld 1983, Jones & Forman 1984, Valentijn 1988, Zhao et al 1989, Burns 1990, Ball et al 1993; see Table 1). Well-known examples are Perseus A (NGC 1275), Cygnus A, PKS0745-191, Hydra A, and Virgo A (M87), Many are FR I sources (edge-darkened), although the classic FR II Cygnus A is also in a flow (Arnaud et al 1984). Of course such radio sources can only be produced from the relativistic outflow from an active nucleus if there is a dense surrounding medium to provide a working surface. Cooling flows provide the most extensive and densest working surfaces possible. The high density of the cooling flow can cause lower power jets to produce FR I sources (De Young 1993). In the case of NGC 1275 (Böhringer et al 1993), the radio lobes have been found to displace the X-ray emitting gas. NGC 1275 also has an outer halo of radio emission which may in part be due to magnetic fields and cosmic rays compressed in the flow (Fabian & Kembhavi 1982, Soker & Sarazin 1990, Becker 1992, Tribble 1993). It is interesting that the clusters that appear not to contain simple cooling flows (e.g. Coma, A2256) instead have distorted halo or wide angle tail sources which may indicate a recent merger (Fabian & Daines 1991, Böhringer et al 1992, Tribble 1993).

There is no simple correlation between radio power and Mdot. Burns (1990) finds that 71% of a sample of cD galaxies in cooling flows are radio-loud, compared with only 23% of cDs not in flows. Some of the most massive flows do not have powerful radio sources (e.g. A478). Amorphous radio sources such as A2052 (Burns 1990, Zhao et al 1993) and PKS0745-191 (Fabian et al 1985, Baum & O'Dea 1991) appear to be peculiar to cooling flows. Optical nebulosity is common in those cooling flows with a central radio source (see Table 1), but no correlation is found between optical and radio luminosities in objects with both (Crawford et al 1994).

The polarization of the radio emission provides a diagnostic with which to study the magnetic field in the ICM. Faraday rotation and depolarization of the emission have been mapped for the extended radio sources in Cygnus A (Dreher et al 1987), M87 (Owen et al 1990), Hydra A (Taylor et al 1990), 3C295 (Perley & Taylor 1991) and A1795 (Ge & Owen 1993), showing that the magnetic field and pressure in cooling flows increase inward (such that the field exceeds 30µG) with a typical length scale for field reversal being 1-10 kpc. (The field could be much higher on smaller scales.) The implied gas pressures and radio equipartition pressures agree with those obtained from X-ray data. The lack of any strong hard X-ray emission due to inverse Compton radiation from the relativistic electrons in clusters that have halo radio sources (generally not cooling flows) shows that the magnetic field there is at least 0.1 µG (Rephaeli & Gruber 1988).

Twenty-one cm absorption has been found in a few cooling flows. NGC 1275 has a strong feature at the rest wavelength of the cluster (Crane et al 1982, Jaffe 1990). The absorption indicates a column density of NH ~ 1021 T1 cm-2, where the HI has a temperature of 10 1 K, spread over much of the inner 15 kpc. Strong limits on emission and absorption at 21 cm have been obtained on several other clusters (McNamara et al 1990; Jaffe 1991, 1992). The conversion of the limits on 21 cm optical depth (eg. tauobs < 5 x 10-4 for A2052; Jaffe 1991) to gas masses depends on assumptions about temperature, optical depth, covering fraction, and beam size (see Daines et al 1994). A simple interpretation rules out an X-ray absorbing column in which the hydrogen is wholly atomic.

CO has also been sought in cooling flows (Jaffe 1987, Bregman & Hogg 1988, Grabelsky & Ulmer 1990, O'Dea et al 1994, Braine & Dupraz 1994, Antonucci & Barvainis 1994) and apart from detections around NGC 1275 in the Perseus cluster (Lazareff et al 1989, Mirabel et al 1989), the limits are strong and rule out widespread warm CO.

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