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NGC 5128
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NGC 5128: Mysterious Cosmic Objects Erupting in X-rays Discovered

ngc5128.gif

 This image shows the location of a remarkable source that dramatically flares in X-rays unlike any ever seen. Along with another similar source found in a different galaxy, these objects may represent an entirely new phenomenon, as reported in our latest press release.
These two objects were both found in elliptical galaxies, NGC 5128 (also known as Centaurus A) shown here and NGC 4636. In this Chandra X-ray Observatory image of NGC 5128, low, medium, and high-energy X-rays are coloured red, green, and blue, and the location of the flaring source is outlined in the box to the lower left.

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NGC5128
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Title: Observational Evidence for a Dark Side to NGC5128's Globular Cluster System
Author: Matthew Taylor, Thomas Puzia, Matias Gomez, Kristin Woodley

We present a study of the dynamical properties of 125 compact stellar systems (CSSs) in the nearby giant elliptical galaxy NGC5128, using high-resolution spectra (R~26,000) obtained with VLT/FLAMES. Our results provide evidence for a new type of star cluster, based on the CSS dynamical mass scaling relations. All radial velocity (v_r) and line-of-sight velocity dispersion (sigma_los) measurements are performed with the penalized pixel fitting (ppxf) technique, which provided sigma_ppxf estimates for 115 targets. The sigma_ppxf estimates are corrected to the 2D projected half-light radii, sigma_{1/2}, as well as the cluster cores, sigma_0, accounting for observational/aperture effects and are combined with structural parameters, from high spatial resolution imaging, in order to derive total dynamical masses (M_dyn) for 112 members of NGC5128's star cluster system. In total, 89 CSSs have dynamical masses measured for the first time along with the corresponding dynamical mass-to-light ratios (Upsilon_V^dyn). We find two distinct sequences in the Upsilon_V^dyn - M_dyn plane, which are well approximated by power laws of the forms Upsilon_V^dyn ~ M_dyn^0.33±0.04 and Upsilon_V^dyn - M_dyn^0.79±0.04. The shallower sequence corresponds to the very bright tail of the globular cluster luminosity function (GCLF), while the steeper relation appears to be populated by a distinct group of objects which require significant dark gravitating components such as central massive black holes and/or exotically concentrated dark matter distributions. This result would suggest that the formation and evolution of these CSSs are markedly different from the "classical" globular clusters in NGC5128 and the Local Group, despite the fact that these clusters have luminosities similar to the GCLF turn-over magnitude.~We include a thorough discussion of myriad factors potentially influencing our measurements.

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Posts: 128727
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RE: Centaurus A
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Title: The outer filament of Centaurus A as seen by MUSE
Author: F. Santoro, J.B.R. Oonk, R. Morganti, T.A. Oosterloo, G. Tremblay

We investigate signatures of a jet-interstellar medium (ISM) interaction using optical integral-field observations of the so-called outer filament near Centaurus A, expanding on previous results obtained on a more limited area. Using the Multi Unit Spectroscopic Explorer (MUSE) on the VLT during science verification, we observed a significant fraction of the brighter emitting gas across the outer filament. The ionized gas shows complex morphology with compact blobs, arc-like structures and diffuse emission. Based on the kinematics, we identified three main components. The more collimated component is oriented along the direction of the radio jet. The other two components exhibit diffuse morphology together with arc-like structures also oriented along the radio jet direction. Furthermore, the ionization level of the gas is found to decrease from the more collimated component to the more diffuse components. The morphology and velocities of the more collimated component confirm our earlier results that the outer filament and the nearby HI cloud are likely partially shaped by the lateral expansion of the jet. The arc-like structures embedded within the two remaining components are the clearest evidence of a smooth jet-ISM interaction along the jet direction. This suggests that, although poorly collimated, the radio jet is still active and has an impact on the surrounding gas. This result indicates that the effect on the ISM of even low-power radio jets should be considered when studying the influence Active Galactic Nuclei can have on their host galaxy.

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NGC 5128
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Title: Red giants in the outer halo of the elliptical galaxy NGC 5128 / Centaurus A
Author: Sarah A. Bird, Chris Flynn, William E. Harris, Mauri Valtonen

We used VIMOS on VLT to perform V and I band imaging of the outermost halo of NGC 5128 / Centaurus A ((m - M)0=27.91±0.08), 65 kpc from the galaxy's center and along the major axis. The stellar population has been resolved to I0 \approx 27 with a 50% completeness limit of I0=24.7, well below the tip of the red-giant branch (TRGB), which is seen at I0 \approx 23.9. The surface density of NGC 5128 halo stars in our fields was sufficiently low that dim, unresolved background galaxies were a major contaminant in the source counts. We isolated a clean sample of red-giant-branch (RGB) stars extending to \approx 0.8 mag below the TRGB through conservative magnitude and colour cuts, to remove the (predominantly blue) unresolved background galaxies. We derived stellar metallicities from colours of the stars via isochrones and measured the density falloff of the halo as a function of metallicity by combining our observations with HST imaging taken of NGC 5128 halo fields closer to the galaxy center. We found both metal-rich and metal-poor stellar populations and found that the falloff of the two follows the same de Vaucouleurs' law profiles from \approx 8 kpc out to \approx 70 kpc. The metallicity distribution function (MDF) and the density falloff agree with the results of two recent studies of similar outermost halo fields in NGC 5128. We found no evidence of a "transition" in the radial profile of the halo, in which the metal-rich halo density would drop rapidly, leaving the underlying metal-poor halo to dominate by default out to greater radial extent, as has been seen in the outer halo of two other large galaxies. If NGC 5128 has such a transition, it must lie at larger galactocentric distances. 

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RE: Centaurus A
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Title: MUSE discovers perpendicular arcs in Cen A inner filament
Author: Stephen Hamer, Philippe Salomé, Francoise Combes, Quentin Salomé

Evidence of AGN interaction with the intergalactic medium is observed in some galaxies and many cool core clusters. Radio-jets are suspected to dig large cavities into the surrounding gas. In most cases, very large optical filaments (several kpc) are also seen all around the central galaxy. The origin of these filaments is still not understood. Star forming regions are sometimes observed inside the filaments and are interpreted as evidence of positive feedback (AGN-triggered star formation). Cen A is a very nearby galaxy with huge optical filaments aligned with AGN radio-jet direction. Here, we search for line ratio variations along the filaments, kinematic evidence of shock-broadend line widths and large scale dynamical structures. We observe a 1'x1' region around the inner filament of Cen A with MUSE on the VLT during the Science Verification period. The brightest lines are the Halpha, [NII], [OIII] and [SII]. MUSE shows that the filaments are made of clumpy structures inside a more diffuse medium aligned with the radio-jet axis. We find evidence of shocked shells surrounding the star forming clumps from the line profiles, suggesting the star formation in induced by shocks. The clumps line ratios are best explained by a composite of shocks and star formation illuminated by a radiation cone from the AGN. A previously undetected large arc-like structure: 3 streams running perpendicular to the main filament, kinematically, morphologically and excitationally distinct are also detected. The clear difference in the excitation of the arcs and clumps suggests that the arcs are likely outside of the radiation cone, matching the position of the filament only in projection. The 3 arcs are thus most consistent with neutral material swept along by a backflow of the jet plasma from the AGN outburst and ionised through slow shocks which continues to excite gas away from the main jet axis.

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Title: Discovery of a close pair of faint dwarf galaxies in the halo of Centaurus A
Author: D. Crnojevic (1), D. J. Sand (1), N. Caldwell (2), P. Guhathakurta (3), B. McLeod (2), A. Seth (4), J. Simon (5), J. Strader (6), E. Toloba (3) ((1) Texas Tech University, Physics Department, Lubbock, TX, USA, (2) Harvard-Smithsonian Center for Astrophysics, Cambridge, MA, USA, (3) UCO/Lick Observatory, University of California, Santa Cruz, CA, USA, (4) Department of Physics and Astronomy, University of Utah, Salt Lake City, UT, USA, (5) Observatories of the Carnegie Institution for Science, Pasadena, CA, USA, (6) Michigan State University, Department of Physics and Astronomy, East Lansing, MI, USA)

As part of the Panoramic Imaging Survey of Centaurus and Sculptor (PISCeS) we report the discovery of a pair of faint dwarf galaxies (CenA-MM-Dw1 and CenA-MM-Dw2) at a projected distance of ~90 kpc from the nearby elliptical galaxy NGC5128 (CenA). We measure a tip of the red giant branch distance to each dwarf, finding D=3.63±0.41 Mpc for CenA-MM-Dw1 and D=3.60±0.41 Mpc for CenA-MM-Dw2, both of which are consistent with the distance to NGC5128. A qualitative analysis of the color magnitude diagrams indicates stellar populations consisting of an old, metal-poor red giant branch (more than 12 Gyr, [Fe/H]~-1.7 to -1.9). In addition, CenA-MM-Dw1 seems to host an intermediate-age population as indicated by its candidate asymptotic giant branch stars. The derived luminosities (MV=-10.9±0.3 for CenA-MM-Dw1 and -8.4±0.6 for CenA-MM-Dw2) and half-light radii (rh=1.4±0.04 kpc for CenA-MM-Dw1 and 0.36±0.08 kpc for CenA-MM-Dw2) are consistent with those of Local Group dwarfs. CenA-MM-Dw1's low central surface brightness (V,0=27.3±0.1 mag/arcsec2) places it among the faintest and most extended M31 satellites. Most intriguingly, CenA-MM-Dw1 and CenA-MM-Dw2 have a projected separation of only 3 arcmin (~3 kpc): we are possibly observing the first, faint satellite of a satellite in an external group of galaxies.

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Posts: 128727
Date:
NGC 5128
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NGC 5128 (also Centaurus A, Caldwell 77, Arp 153, ESO 270-9, IRAS 13225-4245, MCG -7-28-1 and PGC 46957) is a magnitude +6.84 lenticular or elliptical galaxy located 10-16 million light-years away in the constellation Centaurus.

The galaxy was discovered by Scottish astronomer James Dunlop using a homemade 9-foot 22.86 cm (9 inch) f/12 speculum Newtonian reflector at Paramatta, New South Wales, Australia, on the 4th August 1826.

Right Ascension 13h 25m 27.6s, Declination -43° 01' 09"

Centaurus A may be described as having a peculiar morphology. As seen from Earth, the galaxy looks like a lenticular or elliptical galaxy with a superimposed dust lane. The peculiarity of this galaxy was first identified in 1847 by John Herschel, and the galaxy was included in Halton Arp's Atlas of Peculiar Galaxies (published in 1966) as one of the best examples of a "disturbed" galaxy with dust absorption. The galaxy's strange morphology is generally recognised as the result of a merger between two smaller galaxies.
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NGC 5128 (also Centaurus A, Arp 153, IRAS 13225-4245, MCG -7-28-1 and PGC 46957) is a magnitude +6.8 lenticular galaxy located 10 - 16 million light-years away in the constellation Centaurus.

The galaxy was discovered by Scottish astronomer James Dunlop using a  homemade 9-foot 22.86 cm (9 inch) f/12 speculum Newtonian reflector at Paramatta, New South Wales, Australia, on the 29th April 1826.

Right Ascension 13h 25m 27.6s, Declination -43° 01' 09" 

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RE: Centaurus A
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Title: Filaments in the southern giant lobe of Centaurus A: constraints on nature and origin from modelling and GMRT observations
Author: Sarka Wykes, Huib T. Intema, Martin J. Hardcastle, Abraham Achterberg, Thomas W. Jones, Helmut Jerjen, Emanuela Orru, Alex Lazarian, Timothy W. Shimwell, Michael W. Wise, Philipp P. Kronberg

We present results from imaging of the radio filaments in the southern giant lobe of Centaurus A using data from GMRT observations at 325 and 235 MHz, and outcomes from filament modelling. The observations reveal a rich filamentary structure, largely matching the morphology at 1.4 GHz. We find no clear connection of the filaments to the jet. We seek to constrain the nature and origin of the vertex and vortex filaments associated with the lobe and their role in high-energy particle acceleration. We deduce that these filaments are at most mildly overpressured with respect to the global lobe plasma showing no evidence of large-scale efficient Fermi I-type particle acceleration, and persist for ~ 2-3 Myr. We demonstrate that the dwarf galaxy KK 196 (AM 1318-444) cannot account for the features, and that surface plasma instabilities, the internal sausage mode and radiative instabilities are highly unlikely. An internal tearing instability and the kink mode are allowed within the observational and growth time constraints and could develop in parallel on different physical scales. We interpret the origin of the vertex and vortex filaments in terms of weak shocks from transonic MHD turbulence or from a moderately recent jet activity of the parent AGN, or an interplay of both.

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Title: The Dynamic Age of Centaurus A
Author: Jean A. Eilek

In this paper I present dynamic models of the radio source Centaurus A, and critique possible models of in situ particle reacceleration (ISR) within the radio lobes. The radio and gamma-ray data require neither homogeneous plasma nor quasi-equipartition between plasma and magnetic field; inhomogeneous models containing both high-field and low-field regions are equally likely. Cen A cannot be as young as the radiative lifetimes of its relativistic electrons, which range from a few to several tens of Myr. Two classes of dynamic models -- flow driven and magnetically driven -- are consistent with current observations; each requires Cen A to be on the order of a Gyr old. Thus, ongoing ISR must be occurring within the radio source. Alfven-wave ISR is probably occurring throughout the source, and may be responsible for maintaining the gamma-ray-loud electrons. It is likely to be supplemented by shock or reconnection ISR which maintains the radio-loud electrons in high-field regions.

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