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A gas cloud on its way towards the supermassive black hole at the Galactic Centre

Abstract

Measurements of stellar orbits1,2,3 provide compelling evidence4,5 that the compact radio source Sagittarius A* at the Galactic Centre is a black hole four million times the mass of the Sun. With the exception of modest X-ray and infrared flares6,7, Sgr A* is surprisingly faint, suggesting that the accretion rate and radiation efficiency near the event horizon are currently very low3,8. Here we report the presence of a dense gas cloud approximately three times the mass of Earth that is falling into the accretion zone of Sgr A*. Our observations tightly constrain the cloud’s orbit to be highly eccentric, with an innermost radius of approach of only 3,100 times the event horizon that will be reached in 2013. Over the past three years the cloud has begun to disrupt, probably mainly through tidal shearing arising from the black hole’s gravitational force. The cloud’s dynamic evolution and radiation in the next few years will probe the properties of the accretion flow and the feeding processes of the supermassive black hole. The kilo-electronvolt X-ray emission of Sgr A* may brighten significantly when the cloud reaches pericentre. There may also be a giant radiation flare several years from now if the cloud breaks up and its fragments feed gas into the central accretion zone.

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Figure 1: Infalling dust/gas cloud in the Galactic Centre.
Figure 2: The velocity shear in the gas cloud.
Figure 3: Test particle simulation of the orbital tidal disruption.

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Acknowledgements

This paper is based on observations at the Very Large Telescope (VLT) of the European Observatory (ESO) in Chile. We thank C. McKee and R. Klein for discussions on the cloud destruction process. J.C. acknowledges support from FONDAP, FONDECYT, Basal and VRI-PUC. A.B. acknowledges the support of the excellence cluster 'Origin and Structure of the Universe'.

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Contributions

S.G. collected and analysed the data and discovered the orbit of the gas cloud. R.G. and S.G. wrote the paper. T.K.F. detected the high proper motion and extracted the astrometric positions and the photometry. R.G., A.B. and E.Q. derived the cloud’s properties, its evolution and the estimate of the X-ray luminosity. R.G., E.Q., A.B. and C.F.G. contributed to the analytical estimates. C.A. and J.C. set up numerical simulations to check the analysis. F.E., O.P. and K.D.-E. helped in the data analysis and interpretation. T.O. provided valuable software tools.

Corresponding authors

Correspondence to S. Gillessen or R. Genzel.

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The authors declare no competing financial interests.

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Gillessen, S., Genzel, R., Fritz, T. et al. A gas cloud on its way towards the supermassive black hole at the Galactic Centre. Nature 481, 51–54 (2012). https://doi.org/10.1038/nature10652

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