Bubble heating in groups and clusters: the nature of ghost cavities

1. The X-ray Universe, Granada 28th May 2008 Bubble heating in groups and clusters: the nature of ghost cavities Nazirah Jetha1, Martin Hardcastle2, Simon Weston2, Arif Babul3, Ewan O’Sullivan4, Trevor Ponman5, Somak Raychaudhury5, Jan Vrtilek6 1IRFU CEA-Saclay, 2School of Physics, University of Hertfordshire, 3Department of Physics & Astronomy, University of Victoria, 4School of Physics & Astronomy, University of Birmingham, 5Harvard-Smithsonian Center for Astrophysics.

2. The X-ray Universe, Granada 28th May 2008 HCG 62 MS0735.6+7421 Hydra A (NASA/CXC/Ohio U./B.McNamara) NASA/CfA/J. Vrtilek et al. NASA/CXC/SAO 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)

3. The X-ray Universe, Granada 28th 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

4. The X-ray Universe, Granada 28th May 2008 Brightest group galaxy (NGC 741) Companion galaxy (NGC742) Chandra X-ray & 1.4 GHz VLA contours Chandra X-ray Chandra X-ray + 330 MHz VLA X-ray bubble? NGC 741 Group What is filling the bubble?

5. The X-ray Universe, Granada 28th 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.

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

7. The X-ray Universe, Granada 28th 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,

8. The X-ray Universe, Granada 28th 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

9. The X-ray Universe, Granada 28th May 2008 =11 =1 4000 4000 1000 1000 5 x 10-10 2 x 10-9 5 x 10-10 2 x 10-9 B-field (T) B-field (T) 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)

10. The X-ray Universe, Granada 28th 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:

11. The X-ray Universe, Granada 28th 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

12. The X-ray Universe, Granada 28th 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

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

14. The X-ray Universe, Granada 28th 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

15. The X-ray Universe, Granada 28th 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

16. The X-ray Universe, Granada 28th 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

17. The X-ray Universe, Granada 28th 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

18. The X-ray Universe, Granada 28th May 2008 Conclusions • Can constrain physical conditions in ghost bubbles. • For NGC 741 -- 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?