1. 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

2. 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

3. Lynden Bell (visiting Caltech)
Nature 223, (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

4. 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

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

6. 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

7. 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

8. 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

9. 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

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

11. 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

12. 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

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

14. 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

15. 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

16. 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

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

18. 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

19. 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