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تسجيل مستخدم جديدObservations of the gas cloud G2 in the Galactic Center
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In 2011, we discovered a compact gas cloud (G2) with roughly three Earth masses that is falling on a near-radial orbit toward the massive black hole in the Galactic Center. The orbit is well constrained and pericenter passage is predicted for early 2014. Our data beautifully show that G2 gets tidally sheared apart due to the massive black holes force. During the next months, we expect that in addition to the tidal effects, hydrodynamics get important, when G2 collides with the hot ambient gas around Sgr A*. Simulations show that ultimately, the clouds material might fall into the massive black hole. Predictions for the accretion rate and luminosity evolution, however, are very difficult due to the many unknowns. Nevertheless, this might be a unique opportunity in the next years to observe how gas feeds a massive black hole in a galactic nucleus.
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We present new observations of the recently discovered gas cloud G2 currently falling towards the massive black hole in the Galactic Center. The new data confirm that G2 is on a highly elliptical orbit with a predicted pericenter passage mid 2013. Th
e updated orbit has an even larger eccentricity of 0.966, an epoch of pericenter two months later than estimated before, and a nominal minimum distance of 2200 Schwarzschild radii only. The velocity gradient of G2 has developed further to 600 km/s FWHM in summer 2012. We also detect the tail of similar total flux and on the same orbit as G2 along the trajectory at high significance. No hydrodynamic effects are detected yet, since the simple model of a tidally shearing gas cloud still describes the data very well. The flux of G2 has not changed by more than 10% between 2008 and 2012, disfavoring models where additional gas from a reservoir is released to the disrupting diffuse gas component.
We present new observations and analysis of G2 - the intriguing red emission-line object which is quickly approaching the Galaxys 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. G2s 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.
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. Ionized 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 G2s 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.
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 s
cenarios: a Compact Cloud which started around the year 1995 and a Spherical Shell of gas, with an apocenter distance within the disk(s) of young stars and a radius of a few times the size of the Compact Cloud. 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, G2s 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 favors the Spherical Shell Scenario and might be a severe problem for the Compact Cloud as well as the so-called Compact Source 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. 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.
The origin, structure and evolution of the small gas cloud, G2, is investigated, that is on an orbit almost straight into the Galactic central supermassive black hole (SMBH). G2 is a sensitive probe of the hot accretion zone of Sgr A*, requiring gas
temperatures and densities that agree well with models of captured shock-heated stellar winds. Its mass is equal to the critical mass below which cold clumps would be destroyed quickly by evaporation. Its mass is also constrained by the fact that at apocenter its sound crossing timescale was equal to its orbital timescale. Our numerical simulations show that the observed structure and evolution of G2 can be well reproduced if it formed in pressure equilibrium with the surrounding in 1995 at a distance from the SMBH of 7.6e16 cm. If the cloud would have formed at apocenter in the clockwise stellar disk as expected from its orbit, it would be torn into a very elongated spaghetti-like filament by 2011 which is not observed. This problem can be solved if G2 is the head of a larger, shell-like structure that formed at apocenter. Our numerical simulations show that this scenario explains not only G2s observed kinematical and geometrical properties but also the Br_gamma observations of a low surface brightness gas tail that trails the cloud. In 2013, while passing the SMBH G2 will break up into a string of droplets that within the next 30 years mix with the surrounding hot gas and trigger cycles of AGN activity.
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