4. Alternative scenarios
Of course there are other possible explanations for disturbed X-ray surface brightness. We briefly discuss a few alternative scenarios below.
Assuming that the undisturbed ICM is symmetric around NGC 1275 (as was assumed in Fig. 3) one may try to attribute the observed spiral-shaped emission to the gas stripped from an infalling galaxy or group of galaxies. Stripped gas (if denser and cooler than the ICM) will be decelerated by ram pressure and will fall toward the center of the potential, producing spiral-like structure. Rather narrow and long features tentatively associated with stripped gas were observed e.g., for the NGC 4921 group in Coma (Vikhlinin et al. 1996) and NGC 4696B in the Centaurus cluster (Churazov et al. 1999). We note here that to prevent stripping at much larger radii, the gas must be very dense (e.g., comparable to the molecular content of a spiral galaxy). A crude estimate of the gas mass needed to produce the observed excess emission (assuming a uniform cylindrical feature with a length of 200 kpc and radius of 15 kpc, located 60 kpc away from NGC1275) gives values of the order of a few - . Here we adopted a density for the undisturbed ICM of at this distance from NGC 1275 following the deprojection analysis of Fabian et al. (1981) and Ettori, Fabian, and White (1999). The factor of two higher density within the feature will cause a 20-40% excess in the surface brightness. In the above estimate for the mass of hot gas in the filament, it is assumed that this medium is approximately homogeneous and in ionization equilibrium. If the medium is very clumpy, the radiative emission of the plasma would be enhanced and this would result in an overestimate of the relevant gas mass. Such clumps should be easily seen with the high angular resolution of Chandra. Also if the medium consists of turbulently mixed hot and cold plasma, the very efficient excitation of lines in cold ions by hot electrons could lead to enhanced radiation (see e.g. Böhringer and Fabian 1989, Table 4) which may lead to an overestimate of the gas mass by up to an order of magnitude. The signature of this effect is a strongly line dominated spectrum, (see e.g. Böhringer and Hartquist, 1987) which could be tested by Chandra or XMM, in particular for the important iron L-shell lines. Thus it is possible that the inferred gas mass could be lower by up to an order of magnitude which makes the stripping scenario more likely and future observations with the new X-ray observatories can help to differentiate between these interpretations.
As was suggested by Fabian et al. (1981) a large scale pressure-driven asymmetry may be expected in a thermally unstable cooling flow. This is perhaps the most natural explanation which does not invoke any additional physics. The same authors gave an estimate of the amount of neutral gas needed to explain the NW dip due to photoabsorption: excess hydrogen column density around is required to suppress the soft count rate in this region.
Yet another possibility is that the motion of NGC 1275 with respect to the ICM causes the observed substructure. As pointed out in Böhringer et al. (1993), NGC 1275 is perhaps oscillating at the bottom of the cluster potential well causing the excess emission to the east of the nucleus. Since the X-ray surface brightness peak is well centered on NGC 1275, it is clear that the galaxy drags the central part of the cooling flow as it moves in the cluster core. At a distance larger than 2-3 arcminutes from NGC 1275, the cluster potential dominates over the potential of the galaxy. The gas at this distance should be very sensitive to the ram pressure of the ambient cluster gas and might give rise to the asymmetric (and time dependent) features.
The motion of NGC 1275 could also contribute to the X-ray structure through the formation of a "cooling wake" (David et al. 1994). If NGC 1275 is moving significantly, then inhomogeneities in the cooling gas would be gravitationally focussed and compressed into a wake. The wake would mark the, possibly complex, motion of NGC 1275, as it is perturbed by galaxies passing through the cluster core. Such a feature would be cool, since it arises from overdense concentrations of gas.
Finally, one can assume that cooling gas may have some angular momentum (e.g., produced by mergers) and the observed spiral structure simply reflects slow rotation of the gas combined with non-uniform cooling. Following Sarazin et al. (1995), one can assume that this gas will preserve the direction of its angular momentum and that this infalling material would eventually feed an AGN - NGC 1275. One then might expect the radio jets to be aligned perpendicular to the rotation plane of the gas. At first glance, the "spiral" feature appears approximately face-on, suggesting that jets should be directed along the line of sight as indeed is derived from the radio observations (see Pedlar et al. 1990).
© European Southern Observatory (ESO) 2000
Online publication: April 17, 2000