New Scientist Space - 28 April 2010
Mass transport by buoyant bubbles in galaxy clustersBLACK holes blowing huge bubbles may explain the lack of star formation in the cores of galaxy clusters.
Gas at the centre of galaxy clusters should be cooling as it loses energy; this would allow nearby material to compress the gas and create ideal conditions for making stars. But giant "bubbles" of hot, low-density gas may be dragging away the cool gas, say Edward Pope of the University of Victoria in British Columbia, Canada, and colleagues. These form when a black hole belches out jets of hot plasma, which are then pushed out of the core by surrounding denser gas.
Pope and his colleagues studied filaments of material - some up to 30,000 light years long - trailing in the wake of such bubbles in the Perseus cluster. They calculated that the filaments have their shape because they are made of dragged cold gas. By removing so much gas from near the core, the bubbles make star formation there less likely, the team say. Their paper will appear in Monthly Notices of the Royal Astronomical Society.
The research is a new take on an old problem, says Brian McNamara of the University of Waterloo in Ontario, Canada. "They have their own angle on it, which is pretty interesting."
- arXiv.org > astro-ph > arXiv:1004.2050v2 > (Submitted: 12 Apr 2010 (v1), Last Revised: 14 Apr 2010 (v2))
We investigate the effect of three important processes by which AGN-blown bubbles transport material: drift, wake transport and entrainment. The first of these, drift, occurs because a buoyant bubble pushes aside the adjacent material, giving rise to a net upward displacement of the fluid behind the bubble. For a spherical bubble, the mass of upwardly displaced material is roughly equal to half the mass displaced by the bubble, and should be ~ 10^{7-9} solar masses depending on the local ICM and bubble parameters. We show that in classical cool core clusters, the upward displacement by drift may be a key process in explaining the presence of filaments behind bubbles. A bubble also carries a parcel of material in a region at its rear, known as the wake. The mass of the wake is comparable to the drift mass and increases the average density of the bubble, trapping it closer to the cluster centre and reducing the amount of heating it can do during its ascent. Moreover, material dropping out of the wake will also contribute to the trailing filaments. Mass transport by the bubble wake can effectively prevent the build-up of cool material in the central galaxy, even if AGN heating does not balance ICM cooling. Finally, we consider entrainment, the process by which ambient material is incorporated into the bubble. Abridged