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Title: Is there a Supermassive Black Hole at the Centre of the Milky Way?
Authors: Mark J. Reid (1) ((1)Harvard--Smithsonian Centre for Astrophysics)

This review outlines the observations that now provide an overwhelming scientific case that the centre of our Milky Way Galaxy harbours a supermassive black hole. Observations at infrared wavelength trace stars that orbit about a common focal position and require a central mass (M) of 4 million solar masses within a radius of 100 Astronomical Units. Orbital speeds have been observed to exceed 5,000 km/s. At the focal position there is an extremely compact radio source (Sgr A*), whose apparent size is near the Schwarzschild radius (2GM/c˛ ). This radio source is motionless at the ~1 km/s level at the dynamical centre of the Galaxy. The mass density required by these observations is now approaching the ultimate limit of a supermassive black hole within the last stable orbit for matter near the event horizon.

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The neighbourhood around a black hole seems like no place to raise a star. Violent gravitational forces can rip gas clouds apart, making it hard for stars to condense. But astronomers have spotted evidence of very young stars in a ring of gas close to the heart of the Milky Way, where a massive black hole is thought to reside.
These protostars, 6 to 20 light years from the galaxy's centre, are shrouded by so much gas and dust that they can't be seen with telescopes. Instead, Farhad Yusef-Zadeh of Northwestern University in Evanston, Illinois, and colleagues found them through their radio signals.

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Massive Star Formation
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Title: Massive Star Formation in the Molecular Ring Orbiting the Black Hole at the Galactic Center
Authors: F. Yusef-Zadeh, J. Braatz, M. Wardle, D. Roberts

A ring of dense molecular gas extending 2-7 pc orbits the supermassive black hole Sgr A* at the centre of our Galaxy. Using the Green Bank Telescope, we detected water maser lines and both narrow (0.35 km/s) and broad (30 - 50 km/s) methanol emission from the molecular ring. Two of the strongest methanol lines at 44 GHz are confirmed as masers by interferometric observations. These class I methanol masers are collisionally excited and are signatures of early phases of massive star formation in the disk of the Galaxy, suggesting that star formation in the molecular ring is in its early phase. Close inspection of the kinematics of the associated molecular clumps in the HCN (J=1-0) line reveals broad red-shifted wings indicative of disturbance by protostellar outflows from young (few times 10^4 yr), massive stars embedded in the clumps. The thermal methanol profile has a similar shape, with a narrow maser line superimposed on a broad, red-shifted wing. Additional evidence for the presence of young massive protostars is provided by shocked molecular hydrogen and a number of striking ionised and molecular linear filaments in the vicinity of methanol sources suggestive of 0.5-pc scale protostellar jets. Given that the circumnuclear molecular ring is kinematically unsettled and thus is likely be the result of a recent capture, the presence of both methanol emission and broad, red-shifted HCN emission suggests that star formation in the circumnuclear ring is in its infancy.

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Title: First VLTI infrared spectro-interferometry on GCIRS 7 - Characterising the prime reference source for Galactic center observations at highest angular resolution
Authors: J.-U. Pott, A. Eckart, A. Glindemann, S. Kraus, R. Schodel, A. M. Ghez, J. Woillez, G. Weigelt

 Investigating the environment of the massive black hole SgrA* at the centre of the Galaxy requires the highest angular resolution available to avoid source confusion and to study the physical properties of the individual objects. GCIRS7 has been used as wavefront and astrometric reference. Our studies investigate, for the first time, its properties at 2&10um using VLTI/AMBER and MIDI. We aim at analysing the suitability of IRS7 as an IF-phase-reference for the upcoming generation of dual-field facilities at optical interferometers. We observed with (R~30) and 50m (proj.) baseline, resulting in 9 and 45mas resolution for NIR and MIR, resp. The first K-band fringe detection of a GC star suggests that IRS7 could be marginally resolved at 2um, which would imply that the photosphere of the supergiant is enshrouded by a molecular and dusty envelope. At 10um, IRS7 is strongly resolved with a visibility of approximately 0.2. The MIR is dominated by moderately warm (200 K), extended dust, mostly distributed outside of a radius of about 120 AU (15 mas) around the star. A deep 9.8-silicate absorption in excess of the usual extinction law with respect to the NIR extinction has been found. This confirms recent findings of a relatively enhanced, interstellar 9.8-silicate absorption with respect to the NIR extinction towards another star in the central arcsec, suggesting an unusual dust composition in that region. Our VLTI observations show that interferometric NIR phase-referencing experiments with mas resolution using IRS7 as phase-reference appear to be feasible, but more such studies are required to definitely characterize the close environment around this star. We demonstrate that interferometry is required to resolve the innermost environment of stars at the Galactic centre.

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Radio emission has been detected from within 30 million kilometres of the colossal black hole thought to lie at our galaxy's heart.

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For years, astronomers speculated that a giant, mysterious force lay at the centre of the Milky Way, but it wasn't until four years ago that UCLA astronomer Andrea Ghez definitively showed what it was.
Using new techniques for peering into the dusty heart of the galaxy, Ghez's observations proved that scores of stars were rapidly orbiting what could only be a black hole. But it wasn't the kind of garden-variety black hole created when a star explodes and dies; it was hundreds of thousands of times as powerful -- a "supermassive" black hole, as they are now known.

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Sgr A*
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Title: Low angular momentum accretion flow model of Sgr A* activity
Authors: B. Czerny, M. Moscibrodzka, D. Proga, A. Siemiginowska

Sgr A* is a source of strongly variable emission in several energy bands. It is generally agreed that this emission comes from the material surrounding the black hole which is either falling in or flowing out. The activity must be driven by accretion but the character of accretion flow in this object is an open question. We suggest that the inflow is dominated by the relatively low angular momentum material originating in one of the nearby group of stars. Such material flows in directly towards the black hole up to the distance of order of ten Schwarzschild radii or less, where it hits the angular momentum barrier which leads naturally to a flow variability. We study both the analytical and the numerical solutions for the flow dynamics, and we analyse the radiation spectra in both cases using the Monte Carlo code to simulate the synchrotron, bremsstrahlung and the Compton scattering. Our model roughly reproduces the broad band spectrum of Sgr A* and its variability if we allow for a small fraction of energy to be converted to non-thermal population of electrons. It is also consistent (for a range of viewing angles) with the strong constraints on the amount of circumnuclear material imposed by the measurements of the Faraday rotation.

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Did the colossal black hole at the centre of the Milky Way devour its baby brother 120 million years ago? Possibly, says a team led by Warren Brown of the Smithsonian Astrophysical Observatory in Cambridge, Massachusetts, US.
The grisly suggestion comes from observations of 10 'hypervelocity' stars that are moving so fast they will eventually escape the galaxy altogether.
Since the first such star was discovered in December 2004, astronomers have suspected that the supermassive black hole at the heart of the Milky Way, which weighs 3.6 million times the Sun's mass, is responsible for catapulting the objects outwards at extreme speeds. In that scenario, a pair of stars wanders too close to a single, supermassive black hole, and one star gets captured while the other gets flung outwards at up to 4000 km/sec.
But alternative models have also been proposed, including a scenario in which a second, middleweight black hole lurks near the larger one and together, both black holes fling stars outwards.
It is too soon to say which scenario is correct. But Brown's team says the observations to date point to an intriguing possibility: that a middleweight black hole did exist near the galactic centre at one time, but was swallowed up by its larger neighbour about 120 million years ago.

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Sagittarius A*
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This sequence begins with a 400 by 900 light-year mosaic of several Chandra images of the central region of our Galaxy that reveals hundreds of white dwarf stars, neutron stars, and black holes bathed in an incandescent fog of multimillion-degree gas. The mosaic then zooms into a large region around the supermassive black hole at our Galaxy's centre, a.k.a. Sagittarius A* or Sgr A*. Marked in this field around Sgr A* are two newly discovered large lobes of multimillion-degree gas that extend for dozens of light years on either side of the black hole. The final Chandra image in this sequence is a close-up of the location of the supermassive black hole Sgr A* and an X-ray jet. This suspected jet is 1.5 light years in length and is due to high-energy particles ejected from the vicinity of the black hole.
Credit: Galactic Centre (Survey): NASA/UMass/D.Wang et al., Sgr A* (3-color & close-up): NASA/CXC/MIT/F.K.Baganoff et al.

[youtube=http://youtube.com/watch?v=7DWeXJOPcDA]

Position(2000): RA 17h 45m 40s Dec 29° 00 28

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Title: High Energy Activity of the Super-Massive Black Hole at the Galactic Centre
Authors: Andrea Goldwurm

The centre of our galaxy hosts the nearest super-massive black hole to the solar system, identified to the compact radio source Sgr A*. High energy experiments have tried in the past to detect the X/gamma-ray emission expected from the accretion of the surrounding material into this super-massive black hole. Only recently however, thanks to the new generation of telescopes, it has been possible to reveal high energy radiation associated with Sgr A* or its close environment. I will review and discuss in particular the results on the Sgr A* X-ray flares discovered with Chandra and XMM-Newton, on the central soft gamma-ray source detected with INTEGRAL and on the galactic centre TeV emission revealed by HESS.

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