Bubble heating in groups and clusters the nature of ghost cavities
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The X-ray Universe, Granada 28 th May 2008. 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

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

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Bubble heating in groups and clusters the nature of ghost cavities

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.


Heating and cooling the igm

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)


Bubble heating

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


Ngc 741 group

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?


Possibilities

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.


Defining the location of the bubble

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


Location of the bubble

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,


Comparison with spectral ageing models

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


Comparison with spectral ageing models1

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)


Comparison with spectral ageing models2

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:


An alternative fluid

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


An alternative fluid1

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


An alternative fluid2

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?


Other ghost systems

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


Other ghost systems1

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


Selection effects

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


Alternative fluid 2

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


Conclusions

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?


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