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

• Nazirah Jetha1, Martin Hardcastle2, Simon
  Weston2, Arif Babul3, Ewan OSullivan4, 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.

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Heating and Cooling the IGM
The X-ray Universe, Granada 28th May 2008

• 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)

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Bubble Heating
The X-ray Universe, Granada 28th May 2008

• 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

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NGC 741 Group
The X-ray Universe, Granada 28th May 2008
What is filling the bubble?
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Possibilities
The X-ray Universe, Granada 28th May 2008

• 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.

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Defining the location of the bubble
The X-ray Universe, Granada 28th May 2008
Chandra SB profiles
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Location of the bubble
The X-ray Universe, Granada 28th May 2008

• 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 (294) kpc from the
  central galaxy.
  
• Assume bubble is inflated at the centre of the
  group, and rises buoyantly,
  

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Comparison with spectral ageing models
The X-ray Universe, Granada 28th May 2008

• 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

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Comparison with spectral ageing models
The X-ray Universe, Granada 28th May 2008

• 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)

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Comparison with spectral ageing models
The X-ray Universe, Granada 28th May 2008

• 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

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An alternative fluid?
The X-ray Universe, Granada 28th May 2008

• 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

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An alternative fluid
The X-ray Universe, Granada 28th May 2008

• 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
  

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An alternative fluid
The X-ray Universe, Granada 28th May 2008

• Cant rule out gas with
  from the X-ray
  spectrum.
  
• What about in other ghost systems?

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Other ghost systems
The X-ray Universe, Granada 28th May 2008

• 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

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Other ghost systems
The X-ray Universe, Granada 28th May 2008

• 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

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Selection effects
The X-ray Universe, Granada 28th May 2008

• 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 z0 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

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Alternative fluid (2)
The X-ray Universe, Granada 28th May 2008

• Cant 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

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Conclusions
The X-ray Universe, Granada 28th May 2008

• 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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