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

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

Centaurus A - Overview Nearest galaxy with bright active nucleus (3.7 Mpc – 1”=17.9 pc, 1’=1.076 kpc) Classified as an FR I radio galaxy Composite multi-band image on right taken from CXC website

Centaurus A (Chandra/ACIS-I – 570 ks )

Centaurus A Radio Montage (Morganti et al. 1999)

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).

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.

Multi-wavelength Overview

MOS keV smoothed (Gaussian) exposure corrected image of Cen A NML – all unrelated point sources removed

Chandra observation of Cen A NML

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

Results from Spectral Analysis Fit absorbed (Galactic) APEC (single temperature) models to all knots (PN+MOS1+MOS2 simultaneously). Thermal models provide acceptable fits in all cases, non- thermal models are rejected at high confidence (except for N5) -> X-ray knots are thermal. Temperature ranges from keV for knots N1-N4, somewhat higher for N5 (few keV). Elemental abundance is low (typically <0.2 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 10 7 Solar masses, thermal energy is about ergs. Lifetime (sound crossing time=diameter/sound speed) is a few Myrs. Diffuse X-ray emission along SE boundary of lobe – perhaps gas pushing the NML to the NW?

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

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 ( 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 -> 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 (10 58 ergs) compared to the inner lobes.

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 (5x10 41 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  L/nd 2  typical value for T gas =700 eV), L beamed =10 46 ergs s -1 scaled over 4   Kallman and McCray 1982, Kallman 1992  Alternatively, AGN could have been (much) more luminous in the past.

Multi-wavelength Overview

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 (3x10 6 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

Multi-wavelength Overview

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.