Chandra image of Perseus A cluster showing bubbles `blown' by AGN

The absence of catastrophic cooling in galaxy clusters has been explained somewhat by recent observations of galaxy clusters, revealing X-ray emission voids of up to 30 kpc in size that have been identified with buoyant, magnetized bubbles. However, the mechanism by which these outflows heat the cluster medium as a whole remains unclear --- indeed, how these bubbles continue to exist, much less maintain their observed sharp boundaries, is somewhat mysterious. I have shown that even the modest magnetic fields in the intercluster medium (ICM) may almost automatically be ramped up to dynamically important strengths, affecting both these bubbles and minor mergers. Further investigation will clarify if such effects are directly observable, and will shed light both on the large-scale hydrodynamics of these interesting system, but on the interesting small-scale plasma physics at work in these regimes.

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A projectile in a magnetized medium `draping' the field over itself, modifying its later dynamics. Movies available here. From Dursi & Pfrommer (2007).

Recent Chandra and XMM-Newton observations of galaxy cluster cooling flows have revealed X-ray emission voids of up to 30 kpc in size that have been identified with buoyant, magnetized bubbles, presumably inflated by a central Active Galactic Nucleus (AGN). This suggests a way of heating the ICM and preventing a cooling catastrophe. However, while the presence of large amounts of hot gas is suggestive, the process by which the bubble gas heats the ICM is unclear. For example, it is unclear what role thermal diffusion plays, as the ICM magnetic field could potentially be very tangled and all but completely suppress conduction of heat. Turbulence or weak shocks could also play a role. Complicating the issue of bubble/ICM interaction is that how the bubble interface remains sharp is unknown.

Recent numerical and analytic work of mine, however, (Dursi 2007, Dursi & Pfrommer 2007) have shown that for the case of a small core merging into a magnetized medium - even one quite weakly magnetized, as the β~100 that one might expect in this context - the projectile `plows up' fieldlines, building magnetic field strength up to equipartition and beyond, as shown in the figure above. Further, this magnetic field layer can have significant dynamic effects, from slowing down the projectile to partly stabilizing against instabilities.

A bubble self-disrupting as it rises. From Dursi, Robinson, et al. (2004).

These dynamical effects may be quite important, as earlier work of mine done with undergraduate research student K.~Robinson at Chicago and P.~Ricker at UIUC (Robinson, Dursi, et al. (2004)) showed that such a coherent magnetic field may be necessary for the maintenance of these bubbles, as absent any such features, a hydrodynamic bubble will shred itself in one buoyant rise time (eg in the figure above), making it very difficult to see how purely hydrodynamic flow could explain both AGN heating and the persistence of such bubbles.

Showing that this works for an over-dense core, however, is not the same as showing that this will also work for an under-dense bubble which is prone to breaking up; however, there is some evidence that it might, and our analysis has confirmed that the timescales could well be sort enough for the draping to successfully provide a protective `bubble wrap' for these objects, as well. Further, there is the intriguing possibility that these magnetized layers may be directly observable. I plan to continue this work to examine these two questions, also applying the same effect to questions of the ISM.

The work considered above includes only ideal magnetohydrodynamics (MHD), and it is inferred that, because of the magnetic field geometry, thermal conductivity across the drape will be strongly suppressed. However, quantitatively determining this requires self-consistent inclusion of the relevant microphysical thermal and magnetic diffusivities --- which are themselves open research topics in this regime. But therein lies opportunity; coupled with the possibility of the direct observation of these drapes, one may be able to constrain what the microphysics must look like, and therefore understand fairly directly the plasma physics of the intercluster medium.

L. J. Dursi, C. Pfrommer. Draping of Cluster Magnetic Fields over Bullets and Bubbles -- Morphology and Dynamic Effects, arXiv:0711.0213, ApJ submitted, 2007.

L. J. Dursi. Bubble-Wrap for Bullets: The Stability Imparted by a Thin Magnetic Layer, ApJ, 670:221, 2007.

K. Robinson, L. J. Dursi, P. M. Ricker, R. Rosner, A. C. Calder, M. Zingale, J. W. Truran, T. Linde, A. Caceres, B. Fryxell, , K. Olson, K. Riley, A. Siegel, and N. Vladimirova. Morphology of Rising Hydrodynamic and Magnetohydrodynamic Bubbles from Numerical Simulations. ApJ , 601(2):621-643, February 2004.

P. M. Ricker, K. Robinson, L. J. Dursi, R. Rosner, A. C. Calder, M. Zingale, J. W. Truran, T. Linde, A. Caceres, B. Fryxell, K. Olson, K. Riley, A. Siegel, and N. Vladimirova. Simulations of Rising Hydrodynamic and Magnetohydrodynamic Bubbles . In The Riddle of Cooling Flows in Galaxies and Clusters of Galaxies. May 31 - June 4, 2003. Edited by T. H. Reipric, J. C. Kempner, and N. Soker. , 2003.

P. M. Ricker, A. C. Calder, L. J. Dursi, B. Fryxell, D. Q. Lamb, P. MacNeice, K Olson, R. Rosner, F. X. Timmes, J. W. Truran, H. M. Tufo, and M. Zingale. Large-Scale Simulations of Clusters of Galaxies In Proceedings of the VII International Workshop on Advanced Computing and Analysis Techniques in Physics Research (ACAT 2000), P. C. Bhat and M. Kasemann, eds. (Melville, NY: AIP Press, 2001), 316

Talks

Sweeping up a Magnetic Sheath: Magnetic Draping over Moving Cores and Bubbles in Galaxy Clusters
Accretion and Explosion: the Astrophysics of Degenerate Stars, KITP, May 17 2007
[PDF] [Keynote]
A talk given while I was in residence at the KITP program on supernovae in May 2007. The talk is on work examining `magnetic draping' in galaxy clusters. Video or audio of the talk, as well as the slides, are available at the KITP website.

Buoyancy and Astrophysics Toronto Astrophysical Gas Dynamics Seminar, Oct 2004
[PDF] [OpenOffice .sxi]
Rayleigh-Taylor, RT-plus burning, and rising bubbles; an overview of some astrophysical problems involving buoyancy and why such simple problems are so surprisingly difficult to deal with.