Article

# Evidence for Nonhydrostatic Gas Motions in the Hot Interstellar Medium of Centaurus A

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(Impact Factor: 5.99). 04/2008; 677(2):L97. DOI: 10.1086/588023
Source: arXiv

ABSTRACT

We present preliminary results from a deep (600 ks) Chandra observation of the hot interstellar medium of the nearby early-type galaxy Centaurus A. We find a surface brightness discontinuity in the gas ~3.5 kpc from the nucleus spanning a 120° arc. The temperature of the gas is 0.60 ± 0.05 keV (0.68 ± 0.10 keV) interior (exterior) to the discontinuity. The elemental abundance is poorly constrained by the spectral fits, but if the abundance is constant across the discontinuity, there is a factor of 2.3 ± 0.4 pressure jump across the discontinuity. This would imply that the gas is moving at 470 ± 100 km s−1, or Mach 1.0 ± 0.2 (1.2 ± 0.2) relative to the sound speed of the gas external (internal) to the discontinuity. Alternatively, pressure balance could be maintained if there is a large (factor of ~7) discontinuity in the elemental abundance. We suggest that the observed discontinuity is the result of nonhydrostatic motion of the gas core (i.e., sloshing) due to the recent merger. In this situation, both gas motions and abundance gradients are important in the visibility of the discontinuity. Cen A is in the late stages of merging with a small late-type galaxy, and a large discontinuity in density and abundance across a short distance demonstrates that the gas of the two galaxies remains poorly mixed, even several hundred million years after the merger. The pressure discontinuity may have had a profound influence on the temporal evolution of the kiloparsec-scale jet. The jet could have decollimated, crossing the discontinuity and thereby forming the northeast radio lobe.

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The Astrophysical Journal 03/2009; 698(2). DOI:10.1088/0004-637X/698/2/2036 · 5.99 Impact Factor
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##### Article: High-energy particle acceleration at the radio-lobe shock of Centaurus A
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ABSTRACT: We present new results on the shock around the southwest radio lobe of Centaurus A using data from the Chandra Very Large Programme observations. The X-ray spectrum of the emission around the outer southwestern edge of the lobe is well described by a single power-law model with Galactic absorption -- thermal models are strongly disfavoured, except in the region closest to the nucleus. We conclude that a significant fraction of the X-ray emission around the southwest part of the lobe is synchrotron, not thermal. We infer that in the region where the shock is strongest and the ambient gas density lowest, the inflation of the lobe is accelerating particles to X-ray synchrotron emitting energies, similar to supernova remnants such as SN1006. This interpretation resolves a problem of our earlier, purely thermal, interpretation for this emission, namely that the density compression across the shock was required to be much larger than the theoretically expected factor of 4. We estimate that the lobe is expanding to the southwest with a velocity of ~2600 km/s, roughly Mach 8 relative to the ambient medium. We discuss the spatial variation of spectral index across the shock region, concluding that our observations constrain gamma_max for the accelerated particles to be 10^8 at the strongest part of the shock, consistent with expectations from diffusive shock acceleration theory. Finally, we consider the implications of these results for the production of ultra-high energy cosmic rays (UHECRs) and TeV emission from Centaurus A, concluding that the shock front region is unlikely to be a significant source of UHECRs, but that TeV emission from this region is expected at levels comparable to current limits at TeV energies, for plausible assumed magnetic field strengths. Comment: 14 pages, 8 figures. Submitted to MNRAS. Version with full resolution figures available at http://hercules.herts.ac.uk/~jcroston/cena-shell.pdf
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