* Astronomy

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RE: Galactic Center Cloud G2

Blobrana · L

Title: Pericenter passage of the gas cloud G2 in the Galactic Center
Authors: Stefan Gillessen, Reinhard Genzel, Tobias K Fritz, Frank Eisenhauer, Oliver Pfuhl, Thomas Ott, Marc Schartmann, Alessandro Ballone, Andreas Burkert

We have further followed the evolution of the orbital and physical properties of G2, the object currently falling toward the massive black hole in the Galactic Center on a near-radial orbit. New, very sensitive data were taken in April 2013 with NACO and SINFONI at the ESO VLT . The 'head' of G2 continues to be stretched ever further along the orbit in position-velocity space. A fraction of its emission appears to be already emerging on the blue-shifted side of the orbit, past pericenter approach. Ionised gas in the head is now stretched over more than 15,000 Schwarzschild radii RS around the pericenter of the orbit, at ~ 2000 RS ~ 20 light hours from the black hole. The pericenter passage of G2 will be a process stretching over a period of at least one year. The Brackett-{\gamma} luminosity of the head has been constant over the past 9 years, to within ± 25%, as have the line ratios Brackett-{\gamma} / Paschen-{\alpha} and Brackett-{\gamma} / Helium-I. We do not see any significant evidence for deviations of G2's dynamical evolution, due to hydrodynamical interactions with the hot gas around the black hole, from a ballistic orbit of an initially compact cloud with moderate velocity dispersion. The constant luminosity and the increasingly stretched appearance of the head of G2 in the position-velocity plane, without a central peak, is not consistent with several proposed models with continuous gas release from an initially bound zone around a faint star on the same orbit as G2.

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Black hole bonanza possible as immense gas cloud passes

A vast and hidden field of small black holes predicted to be near the centre of our galaxy could be revealed as a giant gas cloud passes by.
The G2 cloud is as large as our Solar System, and bound for a "supermassive" black hole at the Milky Way's core.
On the way, it should encounter many black holes just tens of km across.
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Blobrana · L

Title: Keck Observations of the Galactic Center Source G2: Gas Cloud or Star?
Authors: K. Phifer, T. Do, L. Meyer, A. M. Ghez, G. Witzel, S. Yelda, A. Boehle, J. R. Lu, M. R. Morris, E. E. Becklin, K. Matthews

We present new observations and analysis of G2 - the intriguing red emission-line object which is quickly approaching the Galaxy's central black hole. The observations were obtained with the laser guide star adaptive optics systems on the W. M. Keck I and II telescopes and include spectroscopy (R ~ 3600) centered on the Hydrogen Br-gamma line as well as K' (2.1 micrometer) and L' (3.8 micrometer) imaging. Analysis of these observations shows the Br-gamma line emission has a positional offset from the L' continuum. This offset is likely due to background source confusion at L'. We therefore present the first orbital solution derived from Br-gamma line astrometry, which when coupled with radial velocity measurements, results in a later time of closest approach (2014.21 ± 0.14), closer periastron (130 AU, 1900Rs), and higher eccentricity (0.9814 ± 0.0060) compared to a solution using L' astrometry. The new orbit casts doubt on previous associations of G2 and a low surface brightness "tail". It is shown that G2 has no K' counterpart down to K' ~ 20 mag. G2's L' continuum and the Br-gamma line-emission is unresolved in almost all epochs; however it is marginally extended in our highest quality Br-gamma data set from 2006 and exhibits a clear velocity gradient at that time. While the observations altogether suggest that G2 has a gaseous component which is tidally interacting with the central black hole, there is likely a central star providing the self-gravity necessary to sustain the compact nature of this object.

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Title: Location of the bow shock ahead of cloud G2 at the Galactic Center
Authors: A. Sadowski, R. Narayan, L. Sironi, F. Ozel

We perform detailed magnetohydrodynamic simulations of interaction between the gas cloud G2 with the accretion flow around the Galactic Center black hole Sgr A*. We take as our initial conditions a steady-state, converged solution of the accretion flow we obtained using the general-relativistic magnetohydrodynamic code HARM. Using the observed parameters for the cloud's orbit, we compute the interaction of the cloud with the ambient gas and identify the shock structure that forms ahead of the cloud. We show that for many configurations, the cloud front crossed the orbit pericenter around January 2013 and that the first signatures of the shock, i.e., synchrotron radio emission from electrons accelerated at the bow shock, should already be visible. We argue that the radio emission is likely to reach peak values in February/March 2013.

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Compact Cloud G2

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RE: Galactic Center Cloud G2

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Title: Recommendations for the Optimal NRAO Response to The G2 Encounter with Sagittarius A*
Authors: Geoffrey C. Bower (chair), Heino Falcke, Stefan Gillessen, Dan Marrone, Ramesh Narayan, Mike Nowak, Jurgen Ott, Feryal Ozel, Mark Reid, Farhad Yusef-Zadeh

At the request of NRAO Director Fred Lo and Assistant Director Dale Frail, an ad hoc committee was convened to discuss the optimal response of NRAO to the impact of the G2 cloud with the Sgr A* accretion region (Gillessen et al. 2012). The committee held a two-hour telecon on 8 June 2012 and then produced this brief report. The committee was unanimous in their enthusiasm for the opportunity that the en- counter provides for probing the astrophysics of a unique and important astrophysical source. Theoretical estimates point to a good chance of detecting radio emission from the bow shock of G2 as it ploughs through the ambient hot medium near the pericenter.

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Blobrana · L

Title: Simulations of the Origin and Fate of the Galactic Center Cloud G2
Authors: M. Schartmann, A. Burkert, Ch. Alig, S. Gillessen, R. Genzel, F. Eisenhauer, T. K. Fritz

We investigate the origin and fate of the recently discovered gas cloud G2 close to the Galactic Center. Our hydrodynamical simulations focussing on the dynamical evolution of the cloud in combination with currently available observations favor two scenarios: a Compact Cloud which started around the year 1995 and an extended Spherical Shell of gas, with an apocenter distance within the disk(s) of young stars. The former is able to explain the detected signal of G2 in the position-velocity diagram of the Br gamma emission of the year 2008.5 and 2011.5 data. The latter can account for both, G2's signal as well as the fainter extended tail-like structure G2t seen at larger distances from the black hole and smaller velocities. In contrast, gas stripped from a compact cloud by hydrodynamical interactions is not able to explain the location of the detected G2t emission in the observed position-velocity diagrams. This might be a severe problem for the so-called Compact Source Scenario and favours the Spherical Shell Scenario. From these first idealized simulations we expect a roughly constant feeding of the supermassive black hole through a nozzle-like structure over a long period, starting shortly after the closest approach in 2013.51 for the Compact Cloud. If the matter accretes in the hot accretion mode, we do not expect a significant boost of the current activity of Sgr A* for the Compact Cloud model, but a boost of the average infrared and X-ray luminosity by roughly a factor of 80 for the Spherical Shell scenario with order of magnitude variations on a timescale of a few months. Assuming that a part of the gas is accreted in cold disk mode, even higher boost factors can be reached. The near-future evolution of the cloud will be a sensitive probe of the conditions of the gas distribution in the milli-parsec environment of the massive black hole in the Galactic Center.

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