The Perseus cluster is one of the most luminous X-ray clusters known, and has an unusually high radio luminosity as well (Gisler and Miley, 1979). It may represent an extreme in the evolution of the gas component in clusters.
Optically, Perseus is an L cluster (Bahcall, 1974a); the brightest galaxies form a chain oriented roughly east-west (see Figure 1c). The very prominent and unusual galaxy NGC1275 (Figure 20) is located near the east end of this chain. Together, Perseus, the clusters A262 and A347, and some smaller groups form the Perseus supercluster, which is also elongated in an east-west direction (Gregory et al., 1981). On a moderately large scale, the X-ray emission from Perseus also appears to be elongated in the same direction as the galaxy distribution (Wolff et al., 1974, 1976; Cash et al., 1976; Malina et al., 1976; Branduardi-Raymont et al., 1981). The X-ray emission is peaked on NGC1275 and becomes more spherically symmetric near this galaxy (see Figures 21 and 22). Oddly, the center of the extended X-ray emission in Perseus appears to lie slightly to the east of NGC1275, while the galaxy distribution is centered to the west (Figure 21).
Figure 20. An optical photograph of the spectacular galaxy NGC1275 in the Perseus cluster from Lynds (1970), copyright 1970 AURA, Inc., the National Optical Astronomy Observatories, Kitt Peak. The photograph was taken with a filter sensitive to the H emission line, and shows the prominent optical emission line filaments around this galaxy.
The X-ray emission from Perseus has been observed out to large distances ( 2.5°) from the cluster core (Nulsen et al., 1979; Nulsen and Fabian, 1980; Ulmer et al., 1980a), although the brightness which is observed is that expected from models of the inner X-ray emission. Unlike Coma, Perseus does not appear to have a significant radio halo (Gisler and Miley, 1979; Hanisch and Erickson, 1980; Birkinshaw, 1980). In the outer portions of the cluster, the radio galaxy NGC1265 is the classic example of a head-tail radio galaxy (see Section 4.3). The distortion in the radio morphology of the source indicates that it is passing at high velocity through moderately dense intracluster gas (Owen et al., 1978).
Figure 21. The X-ray surface brightness of the Perseus cluster of galaxies, observed by Branduardi-Raymont et al. (1981) with the IPC on the Einstein satellite. Contours of constant X-ray surface brightness are shown superimposed on the optical image of the cluster. The center of the galaxy distribution in the cluster is shown as a dashed circle, while the centroid of the extended X-ray emission is the ×. The peak in the X-ray surface brightness is centered on the galaxy NGC1275.
Perseus was the first cluster to have an X-ray line detected in its spectrum (Mitchell et al., 1976). The gas temperature in Perseus, derived from its spectrum (Mushotzky et al., 1981), is smaller than one would expect, given the very large line-of-sight velocity dispersion in the cluster (equation 4.10) or the extent of the gas distribution observed (equation 4.6). However, recent work on the galaxy spatial and velocity distribution by Kent and Sargent (1983) has reduced the discrepancy considerably.
The galaxy NGC1275 in the core of the Perseus cluster is one of the most unusual objects in the sky; it occupies a role in extragalactic astronomy similar to the position of the Crab Nebula (which it visually resembles) in galactic astronomy. The visual appearance of the galaxy (Figure 20) is dominated by a complex network of filaments, which show an emission line spectrum (Kent and Sargent, 1979). These filaments form two distinct velocity systems (Rubin et al., 1977, 1978): a 'low velocity' system with the same velocity as the stars in NGC1275, and a 'high velocity' system with a radial velocity about 3000 km/s higher. Strangely enough, 21 cm absorption line measurements indicate that the high velocity system lies between us and NGC1275 (Rubin et al., 1977). Thus the velocity difference cannot be explained as a difference in the Hubble velocity due to differences in the distance to the two systems. At the moment, the leading suggestion appears to be that the high velocity gas is in a spiral or irregular galaxy that is accidentally superposed on NGC1275 and that just happens to be moving towards the center of the Perseus cluster at 3000 km/s. It is possible that this intervening galaxy is in the outer parts of the cluster or supercluster and is falling into the cluster on a nearly radial orbit. Alternatively, Hu et al. (1983) suggest that the high velocity system is a spiral galaxy that is actually colliding with the X-ray emitting gas around NGC1275 (Section 5.7.3). Unfortunately, no stellar component of this possible intervening galaxy has ever been convincingly detected (Kent and Sargent, 1979; but see Adams, 1977), and the galaxy seems to have very little neutral hydrogen, given its strong line emission (van Gorkom and Ekers, 1983).
Although the spatial distribution of stellar light from NGC1275 is that of a giant elliptical galaxy (Oemler, 1976), the galaxy has very blue colors and an A-star stellar spectrum (Kent and Sargent, 1979), suggesting ongoing or recent star formation. The nucleus of the galaxy contains a very compact, highly variable, powerful nonthermal source of radiation with a spectrum that extends from radio to hard X-rays. Because of its line emission, filaments, blue color, and nuclear emission, NGC1275 was classified as a Seyfert galaxy, although Seyferts are generally spiral galaxies and are not usually located in compact cluster cores.
The diffuse cluster X-ray emission from Perseus is very strongly peaked on the position of NGC1275 (Fabian et al., 1981a; Branduardi-Raymont et al., 1981; Figures 21 and 22). This is in addition to the highly variable, very hard X-ray point source associated with the nucleus of NGC1275 (Primini et al., 1981; Rothschild et al., 1981). The gas around NGC1275 has a positive temperature gradient dTg / dr > 0 (Ulmer and Jernigan, 1978), which is not expected if the gas is hydrostatic and not cooling. High resolution X-ray spectra of the region about NGC1275 show the presence of significant quantities of cool gas in this region (Canizares et al., 1979; a href="Sarazin_refs.html#549" target="ads_dw">Mushotzky et al., 1981). Together, the X-ray surface brightness, X-ray spectra, and temperature gradients are best explained if gas is cooling onto NGC1275 at a rate of 400 M / yr (Section 5.7).
Figure 22. A higher resolution X-ray image of the Perseus cluster emission around the galaxy NGC1275, from Branduardi-Raymont et al. (1981) and Fabian et al. (1984), using the HRI on the Einstein satellite. Contours of constant X-ray surface brightness are shown superimposed on the optical image of the galaxy NGC1275.
This cooling flow can provide an explanation of all the unusual properties of NGC1275 except the high velocity filaments. As the gas continues to cool through lower temperatures, it could produce the optical line emission seen in the low velocity filaments. The line emission becomes filamentary because the cooling is thermally unstable (Fabian and Nulsen, 1977; Mathews and Bregman, 1978; Cowie et al., 1980; Section 5.7.3). If a small fraction of the accreted mass reaches the nucleus, it could power the nonthermal nuclear emission. Over the age of the cluster of 1010 yr, roughly 3 × 1012 M would have been accreted. There is no reasonable reservoir for this much mass except in low mass star formation, which may be favored under the physical conditions in accretion flows (Fabian et al., 1982b; Sarazin and O'Connell, 1983). Ongoing star formation would explain the blue color and A-star spectrum of NGC1275. Moreover, if accretion has been going on for the lifetime of the cluster, the entire luminous mass of NGC1275 might be due to accretion (Fabian et al., 1982b; Sarazin and O'Connell, 1983; Wirth et al., 1983).