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P. E. J. Nulsen
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Recent data have radically altered the X-ray perspective on cooling flow clusters. X-ray spectra show that very little of the hot intracluster medium is cooler than about 1 keV, despite having short cooling times. In an increasing number of cooling flow clusters, the lobes of a central radio source are found to have created cavities in the hot gas. Generally, the cavities are not overpressured relative to the intracluster gas, but act as buoyant bubbles of radio emitting plasma that drive circulation as they rise, mixing and heating the intracluster gas. All this points to the radio source, i.e. an active galactic nucleus, as the heat source that prevents gas from cooling to low temperatures. However, heating due to bubbles alone seems to be insufficient, so the energetics of cooling flows remain obscure. We briefly review the data and theory supporting this view and discuss the energetics of cooling flows.
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Recent observations of the interactions between radio sources and the X-ray-emitting gas in cooling flows in the cores of clusters of galaxies are reviewed. The radio sources inflate bubbles in the X-ray gas, which then rise buoyantly outward in the clusters transporting energy to the intracluster medium (ICM). The bright rims of gas around the radio bubbles are cool, rather than hot, and do not show signs of being strongly shocked. Energy deposited into the ICM over the lifetime of a cluster through several outbursts of a radio source helps to account for at least some of the gas that is missing in cooling flows at low temperatures.
Collisional self-interactions occurring in protostellar jets give rise to strong shocks, the structure of which can be affected by radiative cooling within the flow. To study such colliding flows, we use the AstroBEAR AMR code to conduct hydrodynamic simulations in both one and three dimensions with a power law cooling function. The characteristic length and time scales for cooling are temperature dependent and thus may vary as shocked gas cools. When the cooling length decreases sufficiently rapidly the system becomes unstable to the radiative shock instability, which produces oscillations in the position of the shock front; these oscillations can be seen in both the one and three dimensional cases. Our simulations show no evidence of the density clumping characteristic of a thermal instability, even when the cooling function meets the expected criteria. In the three-dimensional case, the nonlinear thin shell instability (NTSI) is found to dominate when the cooling length is sufficiently small. When the flows are subjected to the radiative shock instability, oscillations in the size of the cooling region allow NTSI to occur at larger cooling lengths, though larger cooling lengths delay the onset of NTSI by increasing the oscillation period.
We present results from deep Chandra and XMM-Newton observations of the relaxed X-ray luminous galaxy cluster Abell 2204. We detect metallicity inhomogeneities in the intracluster medium on a variety of distance scales, from a ~12 kpc enhancement containing a few times 10^7 Msun of iron in the centre, to a region at 400 kpc radius with an excess of a few times 10^9 Msun. Subtracting an average surface brightness profile from the X-ray image yields two surface brightness depressions to the north and south of the cluster. Their morphology is similar to the cavities observed in cluster cores, but they have radii of 240 kpc and 160 kpc and have a total enthalpy of 2x10^62 erg. If they are fossil radio bubbles, their buoyancy timescales imply a total mechanical heating power of 5x10^46 erg/s, the largest such bubble heating power known. More likely, they result from the accumulation of many past bubbles. Energetically this is more feasible, as the enthalpy of these regions could combat X-ray cooling in this cluster to 500 kpc radius for around 2 Gyr. The core of the cluster also contains five to seven ~4 kpc radius surface brightness depressions that are not associated with the observed radio emission. If they are bubbles generated by the nucleus, they are too small to balance cooling in the core by an order of magnitude. However if the radio axis is close to the line of sight, projection effects may mask more normal bubbles. Using RGS spectra we detect a FeXVII line. Spectral fitting reveals temperatures down to ~0.7 keV; the cluster therefore shows a range in X-ray temperature of at least a factor of 15. The quantity of low temperature gas is consistent with a mass deposition rate of 65 Msun/yr.
Several galaxy clusters are known to present multiple and misaligned pairs of cavities seen in X-rays, as well as twisted kiloparsec-scale jets at radio wavelengths. It suggests that the AGN precessing jets play a role in the formation of the misaligned bubbles. Also, X-ray spectra reveal that typically these systems are also able to supress cooling flows, predicted theoretically. The absence of cooling flows in galaxy clusters has been a mistery for many years since numerical simulations and analytical studies suggest that AGN jets are highly energetic, but are unable to redistribute it at all directions. We performed 3D hydrodynamical simulations of the interaction between a precessing AGN jet and the warm intracluster medium plasma, which dynamics is coupled to a NFW dark matter gravitational potential. Radiative cooling has been taken into account and the cooling flow problem was studied. We found that precession is responsible for multiple pairs of bubbles, as observed. The misaligned bubbles rise up to scales of tens of kiloparsecs, where the thermal energy released by the jets are redistributed. After $sim 150$ Myrs, the temperature of the gas within the cavities is kept of order of $sim 10^7$ K, while the denser plasma of the intracluster medium at the central regions reaches $T sim 10^5$ K. The existence of multiple bubbles, at diferent directions, result in an integrated temperature along the line of sight much larger than the simulations of non-precessing jets. This result is in agreement with the observations. The simulations reveal that the cooling flows cessed $sim 50 - 70$ Myr after the AGN jets are started.
We are engaged in an investigation of the relationship between the properties of BCG candidates and X-ray characteristics of their host clusters for a flux-limited sample of ~250 ACO clusters from the ROSAT all-sky survey. We aim to search for the convergence scale of bulk streaming flows within the ~300 Mpc defined by this sample. X-ray selection provides significant advantages over previous optically selected samples. No R-band magnitude-structure correlation is present in this sample. Furthermore, no correlation between R-band magnitude of the BCG candidate and X-ray luminosity of the host cluster is evident. The resultant scatter of 0.34 mag. is larger than in (corrected) optically selected samples. Hence, attempts to recover Local Group peculiar velocity vectors with respect to inertial frames defined by such samples via standard candle methods may have limited sensitivity.
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