Welcome to the new version of CaltechAUTHORS. Login is currently restricted to library staff. If you notice any issues, please email coda@library.caltech.edu
Published November 3, 2005 | Supplemental Material + Submitted
Journal Article Open

A size of ~1 AU for the radio source Sgr A* at the centre of the Milky Way


Although it is widely accepted that most galaxies have supermassive black holes at their centres, concrete proof has proved elusive. Sagittarius A* (Sgr A*), an extremely compact radio source at the centre of our Galaxy, is the best candidate for proof, because it is the closest. Previous very-long-baseline interferometry observations (at 7 mm wavelength) reported that Sgr A* is ~2 astronomical units (au) in size, but this is still larger than the 'shadow' (a remarkably dim inner region encircled by a bright ring) that should arise from general relativistic effects near the event horizon of the black hole. Moreover, the measured size is wavelength dependent. Here we report a radio image of Sgr A* at a wavelength of 3.5 mm, demonstrating that its size is ~1 au. When combined with the lower limit on its mass11, the lower limit on the mass density is 6.5 × 10^21 M_⊙ pc^-3 (where M_⊙ is the solar mass), which provides strong evidence that Sgr A* is a supermassive black hole. The power-law relationship between wavelength and intrinsic size (size proportional to wavelength^1.09) explicitly rules out explanations other than those emission models with stratified structure, which predict a smaller emitting region observed at a shorter radio wavelength.

Additional Information

© 2005 Nature Publishing Group. Received 1 July; accepted 31 August 2005. The Very Large Array and the Very Long Baseline Array are operated by the National Radio Astronomy Observatory, which is a facility of the National Science Foundation, operated under cooperative agreement by Associated Universities Inc. Z.-Q.S. acknowledges support from the One-Hundred-Talent programme of the Chinese Academy of Sciences.

Attached Files

Submitted - 0512515v2.pdf

Supplemental Material - nature04205-s1.doc

Supplemental Material - nature04205-s2.pdf


Files (410.2 kB)
Name Size Download all
28.6 kB Preview Download
141.3 kB Download
240.3 kB Preview Download

Additional details

August 19, 2023
October 23, 2023