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

Bubble heating in groups and clusters: the nature of ghost

cavitiesNazirah 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.

The X-ray Universe, Granada 28th 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)

MS0735.6+7421

(NASA/CXC/Ohio U./B.McNamara)

HCG 62

NASA/CfA/J. Vrtilek et al.Hydra A

NASA/CXC/SAO


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


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 + 330 MHz VLA

What is filling the bubble?


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.


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


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,


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



• 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)

=11

100

04

000

5 x 10-10 2 x 10-9

B-field (T)B-field (T)5 x 10-10 2 x 10-9

100

04

000

=1



• 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:


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


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


An alternative fluid

• Can’t rule out gas with from the X-ray

spectrum.

• What about in other ghost systems?


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


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


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


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


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?


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

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