Over a period of 16 years, astronomers tracked stars as they orbited the Milky Way's central region, which is thought to harbour a colossal black hole. One star, called S2, was observed over its complete 15.8-year-long orbit. The star approached the black hole to within one light day, which is only about five times the distance between Neptune and the Sun. (Courtesy of ESO)
Astronomers believe they have come up with concrete proof for the existence of black holes. Ever since Albert Einstein came up with his general theory of relativity, black holes has been central to our knowledge of the Universe. Now experts say they have shown that the theoretical phenomenon, whose gravitational pull is thought to hold galaxies together, exist "beyond any reasonable doubt". The team of scientists spent 16 years studying the existence of a super massive black hole thought to be at the centre of our galaxy, the Milky Way. While the black hole itself is invisible to the eye, the team proved its existence by tracking the motions of 28 stars circling around it. Just as swirling leaves caught in a gust of wind can provide clues about air currents, so the stars' movements reveal information about forces at work at the galactic centre.
There is a giant black hole at the centre of our galaxy, a study has confirmed. German astronomers tracked the movement of 28 stars circling the centre of the Milky Way, using the European Southern Observatory in Chile. The black hole is four million times heavier than our Sun, according to the paper in The Astrophysical Journal.
Two separate telescopes in Chile picked up the same black hole flare recently, allowing them to see for the first time what it looks like when superheated gas orbits the black hole's event horizon as it is being devoured. The black hole in question is the Milky Way's own supermassive Sagittarius A*, with a mass of about four million times that of the Sun.
Title: Measuring Distance and Properties of the Milky Way's Central Supermassive Black Hole with Stellar Orbits Authors: A. M. Ghez, S. Salim, N. N. Weinberg, J. R. Lu, T. Do, J. K. Dunn, K. Matthews, M. Morris, S. Yelda, E. E. Becklin, T. Kremenek, M. Milosavljevic, J. Naiman
We report new precision measurements of the properties of our Galaxy's supermassive black hole. Based on astrometric (1995-2007) and radial velocity (2000-2007) measurements from the W. M. Keck 10-meter telescopes, a fully unconstrained Keplerian orbit for the short period star S0-2 provides values for Ro of 8.0+-0.6 kpc, M_bh of 4.1+-0.6x10^6 Mo, and the black hole's radial velocity, which is consistent with zero with 30 km/s uncertainty. If the black hole is assumed to be at rest with respect to the Galaxy, we can further constrain the fit and obtain Ro = 8.4+-0.4 kpc and M_bh = 4.5+-0.4x10^6 Mo. More complex models constrain the extended dark mass distribution to be less than 3-4x10^5 Mo within 0.01 pc, ~100x higher than predictions from stellar and stellar remnant models. For all models, we identify transient astrometric shifts from source confusion and the assumptions regarding the black hole's radial motion as previously unrecognised limitations on orbital accuracy and the usefulness of fainter stars. Future astrometric and RV observations will remedy these effects. Our estimates of Ro and the Galaxy's local rotation speed, which it is derived from combining Ro with the apparent proper motion of Sgr A*, (theta0 = 229+-18 km/s), are compatible with measurements made using other methods. The increased black hole mass found in this study, compared to that determined using projected mass estimators, implies a longer period for the innermost stable orbit, longer resonant relaxation timescales for stars in the vicinity of the black hole and a better agreement with the M_bh-sigma relation.
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.
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.
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.
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.
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.
