Annu. Rev. Astron. Astrophys. 1982. 20: 547-85 Copyright © 1982 by Annual Reviews. All rights reserved

4.5 The Perseus Cluster

The Perseus cluster is a special example of an evolved cluster with a dominant galaxy. Its optical morphology is distinguished by a chain of bright galaxies between NGC 1275 and IC 310 (Rood & Sastry 1971, Bahcall 1974). The galaxy distribution appears flattened with an axial ratio of 1.24, elongated in the direction along the line of galaxies (Kent, private communication). Perseus has a large velocity dispersion (1282_-78⁺⁹⁵ km s^-1; Danese et al. 1980), a low spiral fraction (10%; Bahcall 1977b), and a high central density of galaxies (Bahcall 1977c), all of which suggest an evolved cluster. Also notable is the unusual galaxy NGC 1275 in the core of the cluster (see Rubin et al. 1977).

Ryle & Windram (1968) reported the existence of a halo radio source that supported the model of X-ray emission from inverse Compton scattering of microwave background photons by energetic electrons. However, Gisler & Miley (1979) and Birkinshaw (1980) have presented observations that question the radio halo's existence. In particular, Birkinshaw showed that the diffuse 15' envelope around NGC 1275 and the extension northeast of NGC 1265 could have produced a pseudo-halo in the Ryle & Windram observation.

With an X-ray luminosity of 5 x 10⁴⁴ erg s^-1 (0.5-3 keV within 0.5 Mpc radius), Perseus is one of the brightest cluster sources (Forman et al. 1972, Mushotzky et al. 1978). From the X-ray cluster luminosity function (Schwartz 1978, McKee et al. 1980), one finds that such luminous clusters are rare with a space density of ~ 10^-8 Mpc^-3. The Einstein spectroscopic observations lead to an iron abundance of 0.44 ± 0.2 (90% confidence) of the solar value for the gas around NGC 1275, which is the same as that measured for the total cluster (Mushotzky et al. 1981). Thus in Perseus there is no observational evidence for sedimentation of the heavy elements.

The Perseus cluster X-ray emission has three components, which are discussed in detail by Branduardi-Raymont et al. (1981) and Fabian et al. (1981).

1. Extended cluster emission characterized by a high temperature (6.4 ± 0.4 keV; Mushotzky & Smith 1980), which may be described by the isothermal-hydrostatic model with a core radius of 250 kpc.

2. A smaller, enhanced region of extended emission characterized by a lower temperature gas (0.6-1.4 keV; Mushotzky et al. 1981) centered on NGC 1275.

3. An unresolved source identified with the nucleus of NGC 1275, which may be the source of a hard spectral component found by Primini et al. (1981). Rothschild et al. (1981) reported variability in the hard component on a timescale of years. A nuclear component has not been reported in other evolved XD clusters.

For the X-ray cluster component (1. above), Branduardi-Raymont et al. (1981) found that the emission is elongated, with an axial ratio of 1.2 in the same direction as that for the galaxy distribution with the centroid 1.73' ± .13' east of NGC 1275. Since Bahcall (1974, 1975) showed that the galaxy distribution can be described by an isothermal sphere, the projected X-ray surface brightness may be described by the model outlined in Section 2.2. Branduardi-Raymont et al. calculated to be 1.65 ± 0.39 for a velocity dispersion of 1307 km s^-1 (Tifft 1978) and a gas temperature of 6.4 keV. However, from fitting the extended X-ray cluster component, they determined to be 0.592 ± 0.015 for a core radius of 250 kpc, in agreement with the results of Gorenstein et al. (1978). From HEAO-A2 scanning observations, Nulsen & Fabian (1980) derived a similar value ( = 0.68) for the outer region of the Perseus cluster, using a more generalized model. Some of the discrepancy in the determinations may result from uncertainties in the observed parameters resulting from asphericity, local anisotropy of the velocity distribution, or departures from equilibrium (Nulsen & Fabian 1980).

The second cluster component, the enhanced emission around NGC 1275 (Fabian et al. 1974, Wolff et al. 1976, Gorenstein et al. 1978) has been interpreted by Fabian & Nulsen (1977) to be the result of cooling accretion flows driven by the pressure of the hot cluster gas. Fabian et al. (1981) generated density and temperature profiles as a function of radius for the Perseus cluster from HRI observations. They found that beginning at about a ten arcmin radius (~ 300 kpc), the density exceeds that required for the establishment of cooling flows. Optical H filaments such as those seen around NGC 1275 may be explained by thermal instabilities in the cooling, accreting gas (Fabian & Nulsen 1977, Mathews & Bregman 1978, Cowie et al. 1980). For the Perseus cluster, Fabian et al. (1981) determined a mass-inflow rate of 200-400 M/yr, which is in good agreement with the cooling flow of ~ 300 M/yr determined by Mushotzky et al. (1981) from X-ray spectroscopic observations. This infalling material may be significant in the growth of the central galaxy in Perseus, as well as in other XD clusters.