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

The Astrophysical Journal (Impact Factor: 6.73). 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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    ABSTRACT: We present results from a 100 ks XMM-Newton observation of the hot gas in the Virgo cluster elliptical galaxy NGC 4472. We find a surface brightness discontinuity ~21 kpc north of the nucleus, consistent with being a contact discontinuity between two moving fluids. We also detect a >60 kpc long ram-pressure stripped tail. The pressure across the discontinuity implies an infall velocity, v infall, of 1000 km s-1 < v infall< 2200 km s–1 depending on what assumptions are made about the density and pressure of the external gas. We suggest that the NGC 4472 group is falling into a collapsing filament, which is itself falling into the Virgo cluster. The gas of a collapsing filament is rapidly decelerated as it crosses the standoff shock, but the apparent high velocity of infall for NGC 4472 could be simply due to the fact that the gravitating potential of the NGC 4472 group is unaffected by this shock. While the group falls through the shock its gas will be stripped as it passes through the stalled gas of the filament. Additionally, we find two sets of cool filamentary arms to the east and the southwest of the nucleus. One of the southwest arms is coincident with a sharp filament seen with Chandra. We interpret these arms as filaments of cool gas that have been driven out from the center of the galaxy by the buoyant evolution of a radio bubble. The age of this outburst is ~108 yr assuming that the buoyant bubble rises with a velocity of ~0.4cs = 200 km s–1; the energy of the outburst is a modest ~2 × 1056 erg.
    The Astrophysical Journal 01/2011; 727(1):41. · 6.73 Impact Factor
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    ABSTRACT: We present new and archival multi-frequency radio and X-ray data for Centaurus A obtained over almost 20 years at the Very Large Array and with Chandra, with which we measure the X-ray and radio spectral indices of jet knots, flux density variations in the jet knots, polarization variations, and proper motions. We compare the observed properties with current knot formation models and particle acceleration mechanisms. We rule out impulsive particle acceleration as a formation mechanism for all of the knots as we detect the same population of knots in all of the observations, and we find no evidence of extreme variability in the X-ray knots. We find that the most likely mechanism for all the stationary knots is a collision resulting in a local shock followed by a steady state of prolonged, stable particle acceleration, and X-ray synchrotron emission. In this scenario, the X-ray-only knots have radio counterparts that are too faint to be detected, while the radio-only knots are due to weak shocks where no particles are accelerated to X-ray emitting energies. Although the base knots are prime candidates for reconfinement shocks, the presence of a moving knot in this vicinity and the fact that there are two base knots are hard to explain in this model. We detect apparent motion in three knots; however, their velocities and locations provide no conclusive evidence for or against a faster moving "spine" within the jet. The radio-only knots, both stationary and moving, may be due to compression of the fluid.
    The Astrophysical Journal 12/2009; 708(1):675. · 6.73 Impact Factor
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    ABSTRACT: We present a spectral investigation of X-ray binaries in NGC 5128 (Cen A), using six 100 ks Chandra observations taken over two months in 2007. We divide our sample into thermally and non-thermally dominated states based on the behavior of the fitted absorption column, and present the spectral parameters of sources with L >2x10^37 erg/s. The majority of sources are consistent with being neutron star low mass X-ray binaries (NS LMXBs) and we identify three transient black hole (BH) LMXB candidates coincident with the dust lane, which is the remnant of a small late-type galaxy. Our results also provide tentative support for the apparent `gap' in the mass distribution of compact objects between ~2-5 Msol. We propose that BH LMXBs are preferentially found in the dust lane, and suggest this is because of the younger stellar population. The majority (~70-80%) of potential Roche-lobe filling donors in the Cen A halo are >12 Gyr old, while BH LMXBs require donors >1 Msol to produce the observed peak luminosities. This requirement for more massive donors may also explain recent results that claim a steepening of the X-ray luminosity function with age at Lx >= 5x10^38 erg/s for the XB population of early-type galaxies; for older stellar populations, there are fewer stars >1 Msol, which are required to form the more luminous sources.
    The Astrophysical Journal 04/2013; 766(2):88. · 6.73 Impact Factor

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