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Bubble heating in groups and clusters: the nature of ghost cavities Nazirah Jetha 1, Martin...
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Transcript of Bubble heating in groups and clusters: the nature of ghost cavities Nazirah Jetha 1, Martin...
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
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
The X-ray Universe, Granada 28th May 2008
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?
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.
The X-ray Universe, Granada 28th May 2008
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
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,
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
The X-ray Universe, Granada 28th May 2008
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)
=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
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:
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
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
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?
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
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
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
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
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?
The X-ray Universe, Granada 28th May 2008