The Discovery of SgrA *

W.M.Goss1, Robert L.Brown1, K.Y.Lo1

1 National Radio Astronomy Observatory


The compact radio source associated with the galactic center was discovered by Balick and Brown at 11 and 3.7 cm in February 1974 using the Green Bank 35 km radio link interferometer of the National Radio Astronomy Observatory. Bruce Balick and Bob Brown published this result in the 1974 , December 1 Astrophysical Journal. The motivation was to search for high brightness radio components in HII regions. A high brightness radio source (Tb > 107 K with a size < 0.1 '' was inferred. The paper concludes :''The unusual nature of the sub-arcsecond structure and its positional coincidence with the inner 1-pc core of the galactic nucleus strongly suggests that this structure is physically associated with the galactic center (in fact , defines the galactic center).'' Other groups were also either observing the galactic center or planning to observe an AGN like component at the galactic center ( Downes, Martin, Ekers , Lynden-Bell, Goss). However , the angular resolution was insufficient in several observations due to the confusion from Sgr A West. Downes and Goss had submitted a Green Bank radio link proposal in mid 1972 but never got around to carrying out the observations due to a variety of circumstances. K.Y.Lo and collaborators were carrying out VLBI observations at 6 cm over a baseline of 228 km in 1973 and 1974. Due to interstellar scattering the source was broadened to a size of about 0.05 arc sec , comparable to the resolution of the single baseline interferometer. Thus the source was over-resolved . The first VLBI detection was made over a 242 km baseline in California (OVRO to Goldstone) at 3.7 cm in May 1975 by K.Y. Lo and collaborators.

In 1976, Davies et al used components of the early MERLIN array in the UK to measure a large angular size at 1 and 1.7 GHz , inferring a 2 relation for the broadening. These authors proposed interstellar scattering caused by turbulence in the interstellar electrons. The first determination of the elongated scattering size was made in 1985 by Lo et al using Mk III VLBI.

Various authors tried to name the source in the early 1980's. Examples of names that were soon forgotten were GCCRS ( "Galactic Center Compact Radio Source" by Reynolds and McKee in 1980 ) and Sgr A (cn) ( for "compact non-thermal " by Backer and Sramek) in the autumn of 1982. None of these names had any staying power . Bob Brown invented the name SgrA * since he was looking for a description of the "exciting source " for the cluster of HII regions observed in early VLA images at 6 cm. This name appeared for the first time in the 1982, November 1 issue of the Astrophycial Journal : " Precessing Jets in Sagittarius A : Gas Dynamics in the Central Parsec of the Galaxy" by Robert L. Brown. As far as we can tell only one single publication ( The Annual Reivew of Astronomy and Astrophysics in 2001 , Melia and Falcke article entitled " The Supermassive Black Hole at the Galactic Center ") has ever given credit to Bob Brown for the name that is universally accepted.

In the 1970's, long range communications often occurred by sending letters through the post or by telephone ! As we know email only became common two decades later. We have been able to recreate much of the complex story by using the extensive archives of Dennis Downes and Bob Brown and the excellent memories of one of the authors of this paper. We have also obtained photos of some of the telescopes and authors ( early 1970's ).