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The Perseus Cluster (Abell 426): Cavities & Bubbles

30/08/2011

The Perseus Cluster, designated as either A426 or Per X-1, is the brightest X-ray cluster in the sky (Edge, Stewart & Fabian 1992). The vastness of this cluster is such that it can be described as one of the most massive objects in the known universe, containing thousands of galaxies immersed in a vast cloud of immensly hot ionized gases; known as a plasma (Edge 2001). The cluster contains the radio source 3C 84 that is currently blowing bubbles of relativistic plasma into the core of the cluster (Conselice et al. 2001; Wilman et al. 2005; Sanders et al. 2005). These are seen as holes in an X-ray image of the cluster, as they push away the X-ray emitting gas. They are known as radio bubbles, because they appear as emitters of radio waves due to the relativistic particles from within a hollow spherical structure. The galaxy NGC 1275 is located at the centre of the cluster, where the X-ray emission is brightest (Fabian et al. 2000).

Fig.1: Left ~ Perseus X-Ray image with contours of constant radio surface brightness superposed. The radio fills the two cavities to the North and South-West of the central AGN. (Fabian et al. (2001) 318: pp.65-68). Right ~ A Chandra ACIS Observation of the centre of the Perseus Cluster. Prominent features include the central AGN engine and cavities to the North, South-West and North-West of the centre. (Credit: NASA/CXC/IoA).

The active galaxy NGC 1275 resides at the centre, and the intercluster medium (ICM) shows evidence that the black hole at the centre has been periodically depositing energy into the surrounding medium of gases (Seward & Charles 2010). This deposition is so large that it has been observed to offset radiative cooling: in a sense it is too hot to cool down.

There are two cavities in the ICM medium to the north and south/south-west of NGC 1275. Figure 1 overlays X-ray and radio images and shows that the X-Ray cavities are filled with radio-emitting relativistic particles (Fabian et al. 2000; Salomé et al. 2011). It can therefore be inferred from this that accretion onto the central black hole has produced jets of material and electromagnetic energy, inflating these bubble like structures. It has been theorised that in the future these bubbles will become more buoyant than the surrounding ICM and begin a journey outwards away from the central AGN (Hatch et al. 2006).

However, the bubble to the north-west of NGC 1275, with a lack of radio emission, does not contain any relativistic particles. It is therefore assumed that this is a ghost bubble of some sort that is much older than the other two that has perhaps cooled down as the thermal conduction of the surrounding ICM is very high (Seward & Charles 2010).

This is theorised by Fabian et al. (2006) to be due a cyclic process where a cooling flow in the ICM produces cold gas and stars which sink in towards the central region of NGC 1275. Some of this material then finds it way towards the central black hole, accreted and flung out as jets pumping huge amounts of energy into the surrounding ICM. This heating then turns of the cooling effect and therefore no more material is being accreted onto the black hole. Thus, cooling flows can be turned off and regulated periodically (Fabian et al. 2006; Tucker et al. 2007).

These observations provide astrophysicists with means to understanding the evolution of structures within galaxy clusters, as the are not confined to the Perseus Cluster. The largest deposition of energy observed thus far is within the cluster MS 0735.6+7421 which also exhibits this bubble and cavity behavior. Thus, understanding the Perseus Cluster will provide a dramatic picture of the relations between AGNs within clusters and the surrounding ICM.

Journal References:

  • Edge A.C.; Stewart G.C.; Fabian A.C. (1992) Properties Of Cooling Flows In A Flux Limited Sample Of Clusters Of Galaxies. Monthly Notices Royal Astronomical Society258 (1) pp. 177-188.
  • Hatch, N.A. et al. (2006) On The Origin & Excitation Of The Extended Nebula Surrounding NGC1275Monthly Notices Royal Astronomical Society, 367 (2) pp. 433-448.
  • Salomé, P. et al. (2011) A Very Extended Molecular Web Around NGC 1275Astronomy & Astrophysics, 531 (1) Article I.D.: 85.

Suggested Further Reading:

  • Seward, F.D.; Charles, P.A. (2010) Exploring The X-Ray Universe: 2nd Edition. Cambridge University Press, Cambridge: pp. 319-320.
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