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Title: Thermal plasma in the giant lobes of the radio galaxy Centaurus A
Authors: S. P. O'Sullivan, I. J. Feain, N. M. McClure-Griffiths, R. D. Ekers, E. Carretti, T. Robishaw, S. A. Mao, B. M. Gaensler, J. Bland-Hawthorn, L. Stawarz

We present a Faraday rotation measure (RM) study of the diffuse, polarised, radio emission from the giant lobes of the nearest radio galaxy, Centaurus A. After removal of the smooth Galactic foreground RM component, using an ensemble of background source RMs located outside the giant lobes, we are left with a residual RM signal associated with the giant lobes. We find the most likely origin of this residual RM is from thermal material mixed throughout the relativistic lobe plasma. The alternative possibility of a thin-skin/boundary layer of magnetoionic material swept up by the expansion of the lobes is highly unlikely since it requires, at least, an order of magnitude enhancement of the swept up gas over the expected intragroup density on these scales. Strong depolarisation observed from 2.3 to 0.96 GHz also supports the presence of a significant amount of thermal gas within the lobes; although depolarisation solely due to RM fluctuations in a foreground Faraday screen on scales smaller than the beam cannot be ruled out. Considering the internal Faraday rotation scenario, we find a thermal gas number density of ~10^{-4} cm^{-3} implying a total gas mass of ~10^{10} solar masses within the lobes. The thermal pressure associated with this gas (with temperature kT ~ 0.5 keV, obtained from recent X-ray results) is approximately equal to the non-thermal pressure, indicating that over the volume of the lobes, there is approximate equipartition between the thermal gas, radio-emitting electrons and magnetic field (and potentially any relativistic protons present).

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Split-Personality Elliptical Galaxy Holds a Hidden Spiral

Most big galaxies fit into one of two camps: pinwheel-shaped spiral galaxies and blobby elliptical galaxies. Spirals like the Milky Way are hip and happening places, with plenty of gas and dust to birth new stars. Ellipticals are like cosmic retirement villages, full of aging residents in the form of red giant stars. Now, astronomers have discovered that one well-known elliptical has a split personality. Centaurus A is hiding a gassy spiral in its center.
Centaurus A isn't your typical elliptical to begin with. Its most striking feature is a dark dust lane across its middle - a sign that it swallowed a spiral galaxy about 300 million years ago.

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Blobby old galaxy boasts hidden arms

Centaurus A was facing a midlife crisis. The giant elliptical galaxy's brightest stars were old and puffy, and it had nearly run out of gas needed to create new ones. The galaxy was just a featureless blob that had lost its sparkle.
Then a chance encounter allowed boring old Centaurus A to have a fling with a younger, smaller galaxy. The event revived the elder partner, triggering a fresh round of star birth and creating one of its most notable features: a dark dust lane along its middle.
In a surprise twist, new observations show the cosmic hanky-panky also caused Centaurus A to sprout two spiral arms - something no other elliptical galaxy is known to have

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ALMA Turns its Eyes to Centaurus A

 

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A new image of the centre of the distinctive galaxy Centaurus A, made with the Atacama Large Millimetre/submillimetre Array (ALMA), shows how the new observatory allows astronomers to see through the opaque dust lanes that obscure the galaxy's centre, with unprecedented quality. ALMA is currently in its Early Science phase of observations and is still under construction, but is already the most powerful telescope of its kind. The observatory has just issued the Call for Proposals for its next cycle of observations, in which the growing telescope will have increased capabilities.
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A Deeper Look at Centaurus A

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The strange galaxy Centaurus A is pictured in a new image from the European Southern Observatory. With a total exposure time of more than 50 hours this is probably the deepest view of this peculiar and spectacular object ever created. The image was produced by the Wide Field Imager of the MPG/ESO 2.2-metre telescope at ESO's La Silla Observatory in Chile.

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 The dark heart of a cosmic collision

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Two of ESA's space observatories have combined to create a multi-wavelength view of violent events taking place within the giant galaxy of Centaurus A. The new observations strengthen the view that it may have been created by the cataclysmic collision of two older galaxies.
Centaurus A is the closest giant elliptical galaxy to Earth, at a distance of around 12 million light-years. It stands out for harbouring a massive black hole at its core and emitting intense blasts of radio waves.

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  'Ordinary' black hole discovered 12 million light years away

An international team of scientists have discovered an 'ordinary' black hole in the 12 million light year-distant galaxy Centaurus A. This is the first time that a normal-size black hole has been detected away from the immediate vicinity of our own Galaxy. PhD student Mark Burke will present the discovery at the National Astronomy Meeting in Manchester.
Although exotic by everyday standards, black holes are everywhere. The lowest-mass black holes are formed when very massive stars reach the end of their lives, ejecting most of their material into space in a supernova explosion and leaving behind a compact core that collapses into a black hole. There are thought to be millions of these low-mass black holes distributed throughout every galaxy. Despite their ubiquity, they can be hard to detect as they do not emit light so are normally seen through their action on the objects around them, for example by dragging in material that then heats up in the process and emits X-rays. But despite this, the overwhelming majority of black holes have remained undetected.

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Title: Triggered star-formation in the inner filament of Centaurus A
Authors: R. Mark Crockett, Stanislav S. Shabala, Sugata Kaviraj, Vincenzo Antonuccio-Delogu, Joseph Silk, Max Mutchler, Robert W. O'Connell, Marina Rejkuba, Bradley C. Whitmore, Rogier A. Windhorst

We present recent Hubble Space Telescope observations of the inner filament of Centaurus A, using the new Wide Field Camera 3 (WFC3) F225W, F657N and F814W filters. We find a young stellar population near the south-west tip of the filament. Combining the WFC3 dataset with archival Advanced Camera for Surveys (ACS) F606W observations, we are able to constrain the ages of these stars to <=10 Myrs, with best-fit ages of 1-4 Myrs. No further recent star-formation is found along the filament. 
Based on the location and age of this stellar population, and the fact that there is no radio lobe or jet activity near the star-formation, we propose an updated explanation for the origin of the inner filament. Sutherland et al. (1993) suggested that radio jet-induced shocks can drive the observed optical line emission. We argue that such shocks can naturally arise due to a weak cocoon-driven bow shock (rather than from the radio jet directly), propagating through the diffuse interstellar medium from a location near the inner northern radio lobe. The shock can overrun a molecular cloud, triggering star-formation in the dense molecular cores. Ablation and shock heating of the diffuse gas then gives rise to the observed optical line and X-ray emission. Deeper X-ray observations should show more diffuse emission along the filament.

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Title: Deep Observation of the Giant Radio Lobes of Centaurus A with the Fermi Large Area Telescope
Authors: Rui-zhi Yang, Narek Sahakyan, Emma de Ona Wilhelmi, Felix Aharonian, Frank Rieger

The detection of high energy (HE) {\gamma}-ray emission up to about 3 GeV from the giant lobes of the radio galaxy Centaurus A has been recently reported by the Fermi-LAT Collaboration based on ten months of all-sky survey observations. A data set more than three times larger is used here to study the morphology and photon spectrum of the lobes with higher statistics. The larger data set results in the detection of HE {\gamma}-ray emission (up to about 6 GeV) from the lobes with a significance of more than 10 and 20 {\sigma} for the North and the South lobe, respectively. Based on a detailed spatial analysis and comparison with the associated radio lobes, we report evidence for a substantial extension of the HE {\gamma}-ray emission beyond the WMAP radio image in the case of the Northern lobe of Cen A. We reconstruct the spectral energy distribution (SED) of the lobes using radio (WMAP) and Fermi-LAT data from the same integration region. The implications are discussed in the context of hadronic and leptonic scenarios.

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Title: Suzaku view of X-ray Spectral Variability of the Radio Galaxy Centaurus A : Partial Covering Absorber, Reflector, and Possible Jet Component
Authors: Y. Fukazawa, K. Hiragi, S. Yamazaki, M. Mizuno, K. Hayashi, K. Hayashi, S. Nishino, H. Takahashi, M. Ohno

We observed a nearby radio galaxy, the Centaurus A (Cen A), three times with Suzaku in 2009, and measured the wide-band X-ray spectral variability more accurately than the previous measurements. The Cen A was in the active phase in 2009, and the flux became higher by a factor of 1.5--2.0 and the spectrum became harder than that in 2005. The Fe-K line intensity increased by 20--30% from 2005 to 2009. The correlation of the count rate between the XIS 3--8 keV and PIN 15--40 keV band showed a complex behaviour with a deviation from a linear relation. The wide-band X-ray continuum in 2--200 keV can be fitted with an absorbed powerlaw model plus a reflection component, or a powerlaw with a partial covering Compton-thick absorption. The difference spectra between high and low flux periods in each observation were reproduced by a powerlaw with a partial covering Compton-thick absorption. Such a Compton-thick partial covering absorber was for the first time observed for the Cen A. The powerlaw photon index of the difference spectra in 2009 is almost the same as that of the time-averaged spectra in 2005, but steeper by ~0.2 than that of the time-averaged spectra in 2009. This suggests an additional hard powerlaw component with a photon index of <1.6 in 2009. This hard component could be a lower part of the inverse-Compton-scattered component from the jet, whose gamma-ray emission has recently been detected with the Fermi/LAT.

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