1. Massachusetts Institute of Technology (MIT) Haystack Observatory, Off Route 40, Westford, Massachusetts 01886, USA
2. Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, Massachusetts 02138, USA
3. University of California Berkeley, Department of Astronomy, 601 Campbell, Berkeley, California 94720-3411 USA
4. Arizona Radio Observatory, Steward Observatory, University of Arizona, 933 North Cherry Avenue, Tucson Arizona 85721-0065, USA
5. James Clerk Maxwell Telescope, Joint Astronomy Centre, 660 North A'ohoku Place University Park, Hilo, Hawaii 96720, USA
6. Netherlands Organization for Scientific Research, Laan van Nieuw Oost-Indie 300, NL2509 AC The Hague, The Netherlands
7. National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, Virginia 22903-2475, USA
8. Kavli Institute for Cosmological Physics, University of Chicago, 5640 South Ellis Avenue, Chicago, Illinois 60637, USA
9. CARMA, PO Box 968, Big Pine, California 93513-0968, USA
10. Caltech Submillimeter Observatory, 111 Nowelo Street, Hilo, Hawai'i 96720, USA
11. Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
12. OVRO, California Institute of Technology, 100 Leighton Lane, Big Pine, California 93513-0968, USA
13. University of California Berkeley, Space Sciences Laboratory, Berkeley, California 94720-7450, USA

Correspondence to: Sheperd S. Doeleman¹ Correspondence and requests for materials should be addressed to S.S.D. (Email: sdoeleman@haystack.mit.edu).

Abstract

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 Sun^{2, 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 scattering^{4, 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.

1. Massachusetts Institute of Technology (MIT) Haystack Observatory, Off Route 40, Westford, Massachusetts 01886, USA
2. Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, Massachusetts 02138, USA
3. University of California Berkeley, Department of Astronomy, 601 Campbell, Berkeley, California 94720-3411 USA
4. Arizona Radio Observatory, Steward Observatory, University of Arizona, 933 North Cherry Avenue, Tucson Arizona 85721-0065, USA
5. James Clerk Maxwell Telescope, Joint Astronomy Centre, 660 North A'ohoku Place University Park, Hilo, Hawaii 96720, USA
6. Netherlands Organization for Scientific Research, Laan van Nieuw Oost-Indie 300, NL2509 AC The Hague, The Netherlands
7. National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, Virginia 22903-2475, USA
8. Kavli Institute for Cosmological Physics, University of Chicago, 5640 South Ellis Avenue, Chicago, Illinois 60637, USA
9. CARMA, PO Box 968, Big Pine, California 93513-0968, USA
10. Caltech Submillimeter Observatory, 111 Nowelo Street, Hilo, Hawai'i 96720, USA
11. Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
12. OVRO, California Institute of Technology, 100 Leighton Lane, Big Pine, California 93513-0968, USA
13. University of California Berkeley, Space Sciences Laboratory, Berkeley, California 94720-7450, USA

Correspondence to: Sheperd S. Doeleman¹ Correspondence and requests for materials should be addressed to S.S.D. (Email: sdoeleman@haystack.mit.edu).

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