# Can the Hanbury-Brown and Twiss effect be used to measure the size of composite objects like galaxies?

I know that the Hanbury-Brown and Twiss effect can be used to measure the size of stars. Can it also be used to measure the size of galaxies?

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In theory, yes, In practice its a lot trickier...

Within a simple interferometric measurement (1 baseline) You are only measuring the spatial frequencies parallel to that baseline, with a nice symmetric star you can assume a simple physical model of a circle and whatever angular diameter you recover that would be your 'stellar'diameter. With extended sources such as galaxies the physical model you assume for image reconstruction is fairly arbritrary and would therefore take a lot of processing time.

Also several orientations of the baseline (or simultaneous over $>1$ baselines) would be required to find the angular extent in different directions...

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For objects significantly off the ecliptic plane, do you not have two independent baselines? (i.e. measure every three months) – Emilio Pisanty Jul 24 '12 at 12:10

I assume you mean measure the apparent angular diameter of a galaxy too distant to be resolved, in which case I would say yes. (If you can resolve it, the HBT effect is unnecessary, of course!) First of all, stars are extended objects, just like galaxies, so there should be no fundamental difference. Secondly, the Wiki article discusses summing over different photon sources within an overall object, with the net result being that your detectors have to be sufficiently close together... "for a given angular diameter desired to be detected," I assume.

I don't know off the top of my head how stellar apparent angular diameters compare with very distant galaxies, but if anything, I would guess stars would be smaller.

The only wrinkle I can think of is that a star's image ought to be very nearly perfectly circular while a galaxy's need not be, but I think you should probably still get an average or effective diameter, even when your source is non-circular.

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(2) & (3) are both generally correct. The HBT effect is in essence a property of detection, not emission (the sources are incoherent). H&B observed Sirius, which at 8.6 light years (ly) distance and 1.7 times bigger than the sun, is 6 milli-arcseconds (mas). A 140,000 ly diameter galaxy (like Andromeda) at 10 billion ly has an angular size of 2900 mas. The original Sirius observation had a 6m baseline, so the galactic observation's HBT correlations would occur over a baseline of 1.2 cm--a much more difficult experiment to do-- not to mention the sparsity of photons. At 10Bly, the Andromeda galaxy would be magnitude 21.4, compared to Sirius's -1.46; that is, 1.4 billion times dimmer, and hence, 1.4 billion fewer photons.

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