A cool accretion disk around the Galactic Centre black hole
There is a supermassive black hole of mass 4 × 10^6 solar masses at the centre of the Milky Way. A large reservoir of hot (10^7 kelvin) and cooler (10^2 to 10^4 kelvin) gas surrounds it within a few parsecs. Although constraints on the amount of hot gas in the accretion zone of the black hole—that is, within 10^5 Schwarzschild radii (0.04 parsecs)—have been provided by X-ray observations, the mass in cooler gas has been unconstrained. One possible way this cooler gas could be detected is by its emission in hydrogen recombination spectral lines. Here we report imaging of a 10^4-kelvin ionized gas disk within 2 × 10^4 Schwarzschild radii, using the 1.3-millimetre recombination line H30α. This emission line is double-peaked, with full velocity linewidth of about 2,200 kilometres per second. The emission is centred on the radio source Sagittarius A*, but the redshifted side is displaced 0.11 arcsec (0.004 parsecs at a distance of 8 kiloparsecs) to the northeast and the blueshifted side is displaced a similar distance to the southwest. We interpret these observations in terms of a rotating disk of mass 10^(−5) to 10^(−4) solar masses and mean hydrogen density of about 10^5 to 10^6 per cubic centimetre, with the values being sensitive to the assumed geometry. The emission is stronger than expected, given the upper limit on the strength of the Brγ spectral line of hydrogen. We suggest that the H30α transition is enhanced by maser emission.
© The Author(s), under exclusive licence to Springer Nature Limited 2019. Received: 8 August 2018; Accepted: 29 March 2019; Published online 5 June 2019. We are grateful to N. Scoville for co-writing the observing proposal and his contribution to discussions of analysis and interpretation of the data and to J. Koda and J. Ott for discussing the data analysis, looking at the data and commenting on the paper. We are grateful to A. Ciurlo, A. Ghez, M. Morris and S. Gillessen for calculating the limit on Brγ and sharing it with us and discussions, and to E. Quataert, S. Ressler and J. Lu for bringing the Brγ non-detection to our attention and discussions. We thank Y. Levin, J. Cuadra, P. Goldreich, D. Lin, J. Guillochon, S. Naoz, G. Witzel, M. Coleman, S. Philippov, S. Tremaine, A. Loeb, Z. Haiman and J. Carlstrom for discussions and comments, and the North American ALMA Science Center scientists at the National Radio Astronomy Observatory, in particular A. Remijan, C. Vlahakis, M. Lacy, S. Stierwalt, A. Moullet, E. Keller and B. Kirk for their help and advice with the observational setups and data reduction. We also thank Z. Scoville for proofreading the manuscript. E.M.M. acknowledges the Bezos Fund for providing her stipend at the Institute for Advanced Study, D. and B. Groce for encouragement and for supporting her as a Groce Fellow at Caltech; and the NRAO Student Observational Support programme for supporting two years of her graduate studies. A.P. is supported by NASA through Einstein Postdoctoral Fellowship grant number PF5-160141 awarded by the Chandra X-ray Center, which is operated by the Smithsonian Astrophysical Observatory for NASA under contract NAS8-03060. ALMA is a partnership of the ESO (representing its member states), the NSF (USA) and the NINS (Japan), together with the NRC (Canada) and the NSC and the ASIAA (Taiwan) and the KASI (Republic of Korea), in cooperation with the Republic of Chile. The Joint ALMA Observatory is operated by the ESO, the AUI/NRAO and the NAOJ. The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc. Author Contributions: E.M.M. was the principal investigator of the observing proposal, analysed the observational data, conducted theoretical calculations, produced the figures, and wrote most of the paper. E.S.P. conducted theoretical calculations and made major contributions to the interpretation of the observational results and to writing the paper. A.P. modelled the observed spectra, wrote the modelling section, and contributed to interpretation of the observational results. R.D.B. made a substantial contribution to the interpretation of the observational results. All co-authors commented on the manuscript. Data availability: This paper makes use of the following ALMA data: ADS/JAO.ALMA #2015.1.00311.S. The data are publicly available from the ALMA archive at https://almascience.nrao.edu. Code availability: We used the Common Astronomy Software Applications package (CASA) for the data reduction and analysis. We used Astropy, Python and Mathematica for plotting and data analysis. For the modelling of the spectra, we used the proprietary Code for AGN Reverberation and Modelling of Emission Lines (CARAMEL). The key result presented in this paper is observational. The results of the CARAMEL modelling are not critical for interpretation of the observational data and therefore we are not releasing the code with this paper. The authors declare no competing interests. Nature thanks Andreas Burkert and Reinhard Genzel for their contribution to the peer review of this work.
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