The cores of most galaxies are thought to harbour supermassive black holes, which power galactic nuclei by converting the gravitational energy of accreting matter into radiation1. Sagittarius A* (Sgr A*), the compact source of radio, infrared and X-ray emission at the centre of the Milky Way, is the closest example of this phenomenon, with an estimated black hole mass that is 4,000,000 times that of the Sun2,3. A long-standing astronomical goal is to resolve structures in the innermost accretion flow surrounding Sgr A*, where strong gravitational fields will distort the appearance of radiation emitted near the black hole. Radio observations at wavelengths of 3.5 mm and 7 mm have detected intrinsic structure in Sgr A*, but the spatial resolution of observations at these wavelengths is limited by interstellar scattering4,5,6,7. Here we report observations at a wavelength of 1.3 mm that set a size of microarcseconds on the intrinsic diameter of Sgr A*. This is less than the expected apparent size of the event horizon of the presumed black hole, suggesting that the bulk of Sgr A* emission may not be centred on the black hole, but arises in the surrounding accretion flow.
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High-frequency VLBI work at MIT Haystack Observatory is supported by grants from the National Science Foundation. The Submillimeter Array is a joint project between the Smithsonian Astrophysical Observatory and the Academia Sinica Institute of Astronomy and Astrophysics. We thank G. Weaver for the loan of a frequency reference from Johns Hopkins University Applied Physics Labs; J. Davis for use of GPS equipment; I. Diegel, R. Vessot, D. Phillips and E. Mattison for assistance with hydrogen masers; the NASA Geodesy Program for loan of the CARMA Hydrogen Maser; D. Kubo, J. Test, P. Yamaguchi, G. Reiland, J. Hoge and M. Hodges for technical assistance; M. Gurwell for SMA calibration data; A. Kerr and A. Lichtenberger for contributions at ARO/SMT; A. Broderick, V. Fish, A. Loeb and I. Shapiro for discussions; and the staff at all participating facilities.
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Doeleman, S., Weintroub, J., Rogers, A. et al. Event-horizon-scale structure in the supermassive black hole candidate at the Galactic Centre. Nature 455, 78–80 (2008). https://doi.org/10.1038/nature07245
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