Sgr A at 30

1
Sgr A at 30

• The First Aperture Synthesis Images of the
  Galactic Centre
• Green Bank W.Va 25 March 2004
• Ron Ekers
• CSIRO, Australia

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!

3
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.

4
Sgr A in the 60s

• Mills Cross (1967?)
• 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
5
OVRO
90 foot
130 foot
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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

7
OVRO Sgr A visibility

• OVRO 90 130
• 3500 E-W
• Jan 1970

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 Hogboms CLEAN algorithm

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

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VLA - 1983
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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 gt 2,000 K
• thermal considered the most like explanation
  since the Tb lower limit didnt 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

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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!

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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

14
NRAO 1974 3 element interferometer

• Balick and Sanders (1974)
• 2 resolution
• Overuse of Clean algorithm
• Is the Southern lobe real?

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WORST UV
OVRO
WSRT
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Sgr A at 5GHz - WORST

• Westerbork OVRO
• Ekers, Goss, Schwarrz, Downes, Rogstad
• AA 43, 159 (1975)

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SgrA VLA contours
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VLA
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Sgr A at 5GHz - WORST

• Westerbork OVRO
• Ekers, Goss, Schwarrz, Downes, Rogstad
• AA 43, 159 (1975)
• Confirmed SgrA
• Visibility 0.6Jy
• No variability 1973-4

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Sgr A in the VLA era