Bubble heating in groups and clusters: the nature of ghost cavities Nazirah Jetha 1, Martin Hardcastle 2, Simon Weston 2, Arif Babul 3, Ewan O’Sullivan 4, Trevor Ponman 5, Somak Raychaudhury 5, Jan Vrtilek 6 1 IRFU CEA-Saclay, 2 School of Physics, University of Hertfordshire, 3 Department of Physics & Astronomy, University of Victoria, 4 School of Physics & Astronomy, University of Birmingham, 5 Harvard- Smithsonian Center for Astrophysics. The X-ray Universe, Granada 28 th May 2008

Heating and Cooling the IGM Should be cool gas in centres of groups and clusters, but is not seen (e.g. Peterson et al 2001) AGN-inflated bubbles posited as a solution. Much observational evidence for bubbles heating IGM. Bubbles found in many X-ray groups/clusters. Energetically, bubbles contain sufficient energy to counteract cooling (e.g. Bîrzan et al 2004) MS (NASA/CXC/Ohio U./B.McNamara) HCG 62 NASA/CfA/J. Vrtilek et al. Hydra A NASA/CXC/SAO The X-ray Universe, Granada 28 th May 2008

Bubble Heating Bubble is gently inflated by AGN Expands gently until it reaches pressure equilibrium. Then rises buoyantly doing further work. (e.g. Churazov et al 2001, Babul et al 2007) Bubble can persist whilst radio plasma spectrum steepens  ‘ghost bubble’ with no detected radio emission. Some have faint ‘fossil’ emission (e.g. Abell 2597, Clarke et al 2005) Others have no detectable emission even at low frequency; e.g. HCG 62, NGC 741 The X-ray Universe, Granada 28 th May 2008

NGC 741 Group Chandra X-ray Brightest group galaxy (NGC 741) Companion galaxy (NGC742) X-ray bubble? Chandra X-ray & 1.4 GHz VLA contours Chandra X-ray MHz VLA What is filling the bubble? The X-ray Universe, Granada 28 th May 2008

Possibilities A conventional radio plasma sufficiently evolved that plasma is no longer visible at any frequency. Can we place age constraints on the bubble from dynamical arguments? This can be compared with spectral age constraints on the plasma filling the bubble. Bubble lies 25 kpc in projection from NGC 741. Use X-ray observations to constrain bubble location and hence age. The X-ray Universe, Granada 28 th May 2008

Defining the location of the bubble SB(bubble) SB(undisturbed) Innermost extent of bubble Outermost extent of bubble ∆SB = 0.4±0.1 Chandra SB profiles The X-ray Universe, Granada 28 th May 2008

Location of the bubble Single  -model fit to XMM-Newton large scale SB profile to characterise undisturbed gas Model bubble as oblate spheroid displacing X-ray emitting gas. Integrate along line of sight to calculate ∆SB for bubble at a given depth. Combine with the projected distance, to give a deprojected location for the bubble. Find that the bubble is (29±4) kpc from the central galaxy. Assume bubble is inflated at the centre of the group, and rises buoyantly, The X-ray Universe, Granada 28 th May 2008

Comparison with spectral ageing models Use 1.4 GHz and 325 MHz VLA observations to place limits on flux density in cavity. Obtain inverse Compton limit from X-rays - - interesting limit -- not been done before. Fit model similar to Jaffe & Perola (1977) with varying to spectrum. Infer limits for and for equipartition and non-equipartition B fields The X-ray Universe, Granada 28 th May 2008

Comparison with spectral ageing models Equipartition B- fields  extremely low (c.f. for normal radio galaxies) can only occur for the lowest external pressures and internal B- fields (even with a large no-radiating particle contribution)  = x x B-field (T) 5 x x  =1 The X-ray Universe, Granada 28 th May 2008

Comparison with spectral ageing models Assuming that plasma has evolved from ‘normal’ radio galaxy, and synchroton radiative losses dominate (i.e. plasma is in equipartition): If plasma is not in equipartition, IC losses dominate and C.f. dynamic timescale: The X-ray Universe, Granada 28 th May 2008

An alternative fluid? Unlikely that the fluid would have evolved from a standard radio galaxy plasma. Other possibilities? Hot, tenuous gas with Bubble ought to be in pressure balance with IGM. So measure of IGM and of bubble to place limits on The X-ray Universe, Granada 28 th May 2008

An alternative fluid Extract spectrum from bubble region. This will contain contributions from bubble fluid and IGM. Fit spectrum with two MeKaL models; one fixed to, the other initially to 10 keV. Use normalisation of 2nd MeKaL model to calculate density and hence pressure of the bubble fluid. (c.f. Sanders & Fabian 2006). If bubble unstable (may be an extra non-thermal contribution too) If then bubble can exist & obtain a lower limit to The X-ray Universe, Granada 28 th May 2008

An alternative fluid Can’t rule out gas with from the X-ray spectrum. What about in other ghost systems? The X-ray Universe, Granada 28 th May 2008

Other ghost systems Sample of 10 known ghost cavity systems that have both Chandra and radio (VLA and/or GMRT) data (and velocity dispersions for the BGG). Use radio data in conjunction with IC limits to place limits on assuming a traditional radio plasma. Consider also departures from equipartition The X-ray Universe, Granada 28 th May 2008

Other ghost systems No conclusive evidence for a highly aged radio plasma or a radio plasma far from equipartition! Poor constraints from IC (X-ray) Implies that we can have a e + /e - plasma, and a low magnetic field (i.e. plasma is far from equipartition). IC flux limit Thus, selection effects important The X-ray Universe, Granada 28 th May 2008

Selection effects Bubbles detected via SB contrast. Need large SB contrast to accurately identify bubbles. Most likely to obtain this with a compact bubble in or close to the z=0 plane. IC constraints more robust from larger bubble (e.g. NGC 741) Thus is difficult to constrain parameters for a traditional plasma with this sample of ghosts The X-ray Universe, Granada 28 th May 2008

Alternative fluid (2) Can’t rule out presence of hot gas. Can estimate temperature of any potential hot gas. Selection effects work in our favour here! Know that bubble must be in ~ pressure balance So surface brightness dip indicates kT of hot gas. Find that The X-ray Universe, Granada 28 th May 2008

Conclusions Can constrain physical conditions in ghost bubbles. For NGC difficult to see how the fluid can evolve from a conventional radio plasma. Applying the same technique to a sample of ghost bubbles reveals some problems Selection effects make constraining parameters assuming a radio plasma difficult. Large bubbles like in NGC 741 pose toughest tests for models -- should look out for these in our data. Are we sure the bubble medium is a relativistic plasma? Very hot gas? Target for Simbol-X? What else could the medium be? The X-ray Universe, Granada 28 th May 2008