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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

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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