An XMM-Newton Study of the Centaurus A Northern Middle Radio Lobe

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An XMM-Newton Study of the Centaurus A Northern
Middle Radio Lobe
X-ray Universe 2008

• R. P. Kraft, W. R. Forman, M. J. Hardcastle, M.
  Birkinshaw, J. H. Croston, C. Jones, P. E. J.
  Nulsen, S. S. Murray, D. W. Worrall

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Outline of Talk

• Introduction What is the Cen A Northern Middle
  Radio Lobe and Why is it Interesting?
• Observations, Data Analysis, Results
• Interpretation
• Summary and Conclusion

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Centaurus A - Overview

• Nearest galaxy with bright active nucleus (3.7
  Mpc 117.9 pc, 11.076 kpc)
• Classified as an FR I radio galaxy
• Composite multi-band image on right taken from
  CXC website

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Centaurus A (Chandra/ACIS-I 570 ks )
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Centaurus A Radio Montage (Morganti et al. 1999)
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Cen A Northern Middle Lobe (NML)

• Cen A NML buoyant bubble from previous epoch of
  nuclear activity (Saxton et al. 2003) or has the
  NE inner lobe burst (Morganti et al. 1999)?
• An X-ray filament associated with the NML was
  first reported by Feigelson et al. (1981) based
  on an Einstein IPC observation. They argued for
  a thermal origin for the emission.
• This filament was detected in several other
  observations (ROSAT, ASCA, and EXOSAT) nature
  and origin of this X-ray emission (and the NML
  more generally) remained enigmatic.
• Radio depolarization supported thermal
  interpretation (Morganti et al. 1999).

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XMM-Newton Observation of the Cen A NML

• We observed the X-ray filament of the Cen A NML
  with XMM-Newton (40 ks) to constrain the emission
  mechanism of the filament which will give us a
  better understanding of the dynamics of the NML
  more generally.
• This was a C category observation that was
  observed!
• The bottom of the filament was also contained
  within the FOV of a 100 ks Chandra observation
  (albeit far off axis spatial resolution similar
  to XMM-Newton).
• Feedback between AGN and the ambient gas may play
  a critical role in the suppression of cluster
  cooling flows and the formation of stars (and
  galaxies) at high redshift.

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Multi-wavelength Overview
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MOS12 0.5-2.0 keV smoothed (Gaussian) exposure
corrected image of Cen A NML all unrelated
point sources removed
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Chandra observation of Cen A NML
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X-ray Contours on Radio Map (radio data taken
from Morganti et al. 1999) X-ray features
appear to be anti-coincident with radio features
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Results from Spectral Analysis

• Fit absorbed (Galactic) APEC (single temperature)
  models to all knots (PNMOS1MOS2
  simultaneously).
• Thermal models provide acceptable fits in all
  cases, non-thermal models are rejected at high
  confidence (except for N5) -gt X-ray knots are
  thermal.
• Temperature ranges from 0.4-1.0 keV for knots
  N1-N4, somewhat higher for N5 (few keV).
  Elemental abundance is low (typically lt0.2
  Solar). Chandra confirms these values for N3,
  N4, and N5.
• Knots are enormously overpressurized (factor of
  10 or more) relative to ambient ISM and the
  equipartition pressure of NML.
• Total mass of knots is about 107 Solar masses,
  thermal energy is about 1056 ergs. Lifetime
  (sound crossing timediameter/sound speed) is a
  few Myrs.
• Diffuse X-ray emission along SE boundary of lobe
  perhaps gas pushing the NML to the NW?

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Possible Interpretations

• Synchrotron or IC/CMB
• Super-bubble(s) from jet-induced star formation
• Photo-ionization from beamed nuclear flux
• Entrainment/buoyant bubble (thermal gas trunk
  Saxton et al. 2003)
• Shock-heating from supersonic inflation of NML
• Direct interaction with active jet

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Disfavored Models

• Synchrotron or IC/CMB rejected because of
  thermal spectra.
• Jet-induced star formation rejected because
  thermal energy and total mass of gas too large
  (104-5 supernovae required to create knots) and
  lack of evidence of star formation around knots
• Entrainment of gas by buoyantly rising bubble
  rejected as the equipartition pressure of the
  lobe is too low and buoyant rise time (about 170
  Myrs) too long
• Supersonic inflation of NML Knots are then
  interior to the lobe -gt requires pressure of lobe
  to be roughly equal to knot pressure. The NML
  would then be enormously overpressurized relative
  to ISM and total energy of radio lobe would be
  large (1058 ergs) compared to the inner lobes.

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Feasible Model 1 Photo-ionization

• Beamed emission from nucleus could ionize a chain
  of dense clouds.
• VLBI jet is roughly aligned with the filament (at
  least in projection).
• NML is from a previous epoch of nuclear activity
  and has perhaps stripped the HI cloud.
• Filamentary X-ray morphology could represent
  distribution of cold gas.
• Naturally explains X-ray/radio anti-correlation
  the knots are compressing the lobe.
• The NML is currently unpowered and buoyant.
• May account for sidedness of NML large scale
  gas motions

• Observed X-ray flux (5x1041 ergs s-1) from
  nucleus is far too low to ionizes these clouds at
  these distances (15-30 kpc from nucleus)
• Requires (unseen) blazar type fluxes toward NML
• For ionization parameter xL/nd2100 (typical
  value for Tgas700 eV), Lbeamed1046 ergs s-1
  scaled over 4p (Kallman and McCray 1982, Kallman
  1992).
• Alternatively, AGN could have been (much) more
  luminous in the past.

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Multi-wavelength Overview
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Feasible Model 2 - Direct Interaction with Jet

• Unseen jet shock-heating dense clouds to X-ray
  temperatures (De Young 1991, De Young et al.
  2002, Higgins et al. 1999, Wang et al. 2000).
• NE inner lobe burst (Morganti et al. 1999)
• The NE inner lobe is a channel for collimated
  energy transfer to NML
• Filamentary structure is the result of ablation
  of clouds
• Age/lifetime of X-ray knots of NML and SW inner
  lobe roughly consistent (3x106 yrs).
• Some analogy to large-scale radio features of M87
  (Owen et al. 2000).

• Jet must bend at least twice without disruption
• What caused NE lobe to POP? What caused jet to
  bend twice? External gas motions (Kraft et al.
  2008)?
• Thermal energy of the knots is a significant
  fraction (20-30) required to inflate the NML
  (i.e. shock heating of clouds must be efficient)
• Why do the NE and SW inner lobes appear to be so
  similar at GHz radio frequencies?
• Material did not originate from HI cloud jet
  would have to be proton dominated to provide
  sufficient momentum

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Multi-wavelength Overview
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Summary and Conclusions

• The X-ray emission from the filament along the SE
  boundary of the NML is thermal.
• The knots are greatly overpressurized relative to
  the NML unless the lobe is far from equipartition
  (in which case the NML is enormously
  overpressurized relative to the ambient gas)
• Model 1 hot gas clouds created by
  photoionization from beamed nuclear flux
  requires blazar-type (beamed) flux from nucleus.
• Model 2 cold clouds were shock-heated by direct
  interaction with jet - the NML is still being
  powered by a collimated outflow from the active
  nucleus in this scenario.
• Con-X could distinguish between the two
  scenarios.