The First Aperture Synthesis Images of the Galactic Centre Sgr A * at 30 The First Aperture Synthesis Images of the Galactic Centre Green Bank W.Va 25 March 2004 Ron Ekers CSIRO, Australia For www version remove animations on slide 15

Caltech 1968 QSO were now well accepted as AGN at cosmological distances Energy source most likely to be gravitational Black holes were not only credible, but becoming fashionable! Sgr A* at 30 - March 2004

Lynden Bell (visiting Caltech) Nature 223, 690-694 (1969) Strong evolution of QSO's (radio sources) hence many dead quasars in the local universe implies many quiescent black holes (107-8 Mo) Expect a few in the local group. Look for evidence in our galactic centre need small scale - existing maps were 3' resolution (10pc) need a spectral feature to determine enclosed mass emission thought to be partly thermal so try recombination lines. Sgr A* at 30 - March 2004

Sgr A in the 60’s Mills Cross (1967?) Clark & Hogg (1965) 408MHz Alec Little Clark & Hogg (1965) Saw fringes on 2 element interferometer at Green Bank, no details Not known to us at the time Sgr B2 Sgr A Sgr A* at 30 - March 2004

OVRO 90 foot 130 foot Sgr A* at 30 - March 2004

Caltech-OVRO 1970 Use OVRO interferometer and try for recomb lines no previous high resolution observations of Sgr A and no interferometric detection of a recomb lines New 130 foot antenna 1km east of the 90’ prototype for Caltech's version of the VLA NB the NRAO 3 element was a VLA test bed 130'+90' gave 6" resolution at 5GHz (0.3pc) Ekers and Lynden Bell Astrophys Lett 9 (1971) Archival material from the 1970 Sgr A* at 30 - March 2004

OVRO – Sgr A visibility OVRO 90’ + 130’ 3500’ E-W Jan 1970 Original plot from archives Sgr A* 0.6Jy Sgr A* at 30 - March 2004

What did we see in 1970? Very complex visibility function Small scale structure was too weak for recomb line search Complex fine structure on a scales down to 5” Model fitting First use of Hogbom’s CLEAN algorithm Sgr A* at 30 - March 2004

OVRO - 1970 Ekers and Lynden-Bell Astrophys Lett 9 (1971) Ridge to East and position of point source (a) which is SgrA* were correctly modeled Scale and position matched Sgr A* at 30 - March 2004

VLA - 1983 Scale and position matched Sgr A* at 30 - March 2004

What about Sgr A* ? Fine structure is complex but model is dominated by a point source at the position of Sgr A* coincidence of point source with the IR cusp was emphasized T > 2,000 K thermal considered the most like explanation since the Tb lower limit didn’t require non-thermal processes. Lynden-Bell quote (letter in archives) I think that black holes have so far overrun any observational foundation that may once have been seen in them that we ought to say strongly that we see no evidence that calls out for any such explanation. So the paper concluded Although stimulated by the black hole idea our observations are most simply explained in terms of young stars and giant HII regions Sgr A* at 30 - March 2004

Was Sgr A* first detected at Caltech in 1970? Con Not well separated from other fine structure The lower limit in brightness was only 2000K Not claimed as evidence of a black hole (or AGN) Preferred explanation was a compact HII region Pro Looking for a black hole Sgr A* was clearly seen with correct flux and position Positional agreement with the IR cusp at the galactic centre emphasized Resolves the Downes & Goss v Balick & Brown conflict! Sgr A* at 30 - March 2004

Cambridge 1 mile telescope Downes and Martin Nature, 233 112-114 (1971) Strip distribution clearly shows SgrA* First clear separation of SgrA East and West Sgr A* at 30 - March 2004

NRAO 1974 3 element interferometer The need for good UV coverage Southern lobe doesn’t exist – it is a faint southerly extension of West B&S note added in proof to acknowledge that SgrA south didn’t exist Balick and Sanders (1974) 2” resolution Overuse of Clean algorithm Is the Southern lobe real? Sgr A* at 30 - March 2004

WORST UV OVRO WSRT Sgr A* at 30 - March 2004 Combining WSRT and OVRO data OVRO data is hidden behind white mask – use animation Sgr A* at 30 - March 2004

Sgr A at 5GHz - WORST Westerbork + OVRO Ekers, Goss, Schwarrz, Downes, Rogstad A&A 43, 159 (1975) Scale and position matched Sgr A* at 30 - March 2004

VLA Scale and position matched Sgr A* at 30 - March 2004

Sgr A at 5GHz - WORST Westerbork + OVRO Confirmed SgrA* Ekers, Goss, Schwarrz, Downes, Rogstad A&A 43, 159 (1975) Confirmed SgrA* Visibility 0.6Jy No variability 1973-4

Sgr A in the VLA era Sgr A* at 30 - March 2004 Replace SgrA* by smaller image, try to match colors Sgr A* at 30 - March 2004