Extra Galactic

It's very difficult to get a positive detection of an extra-galactic source if you only have one 20MHz receiver, because normally the signal is buried in the radio noise from our own Galaxy. However with an interferometer detection of extra-galactic sources becomes a routine thing.. at our sites in Australia the radio galaxies Centaurus A and Fornax A are some of the main sources we detect each day, in the interferometer output of our 20MHz "Simple's".

The theory behind it is that interferometers act as a "spatial filter", meaning that the interferometer's response depends on the angular size of the radio source. With an interferometer, some positions on the sky will give a positive interferometer response while others will give a negative response. If the radio source is larger than the distance beween adjacent positive and negative lobes, then it will start to "cancel itself out" (become resolved). Because the plane of our Galaxy is so large it becomes highly resolved with even moderate baselines.. and therefore vanishes from the interferometers view of the universe. By getting rid of the Galactic radio emission it becomes relatively easy to see the more compact radio sources that were previously hidden.

However an interferometer can do much better than just "detect" discrete radio sources, they can also tell us the exact location of the object!

The graph below shows a full fringe pattern detected by Simple at Narrabri with a 120m baseline. The fringe pattern is fairly symmetrical so it's easy to pick which lobe is the middle of the fringe pattern. Simple's response is calibrated such that we get a fringe maximum when a source is exactly at transit, so the sidereal time of this central lobe immediately tells us the Right Ascension position of the source is close to 13:24 hours.

We can use the period of the fringes to measure the declination position of the radio source. Picture that a source right on the celestial pole would just appear to sit in the same place in the sky and spin on the spot, and therefore would never move through the interferometer's beam to produce a fringe pattern. A source at the equator would whizz through the interferometers beam by comparison.

It's hard to accurately measure the period of just one fringe, so I've opted to use mark the zero crossings for better accuracy and to average over 3 fringes. The fringe timing does actually change a little as the source geometry changes, but hey this is only astrophysics, not rocket science.. 1 hour and 58 minutes for 3 fringes gives 39.33 minutes per fringe.

We can calculate the theoretical spacing of the fringes at the equator, for the 120m baseline, 15m wavelength interferometer as:

fringe_spacing = wavelength / baseline (radians)
= 15m / 120m (radians) = 7.16 (degrees)

Since this fantastic planet spins at 15 degrees per hour, we expect a source at the equator to give one fringe every:

equatorial_fringe_rate = 7.16/15 = 0.477 = 28.64 minutes.

Finally we can get the declination:

declination = inverse_cos(equatorial_fringe_rate/fringe_rate)
= inverse_cos(28.64/39.33)
= +/-43.26 [Can only be -ve declination for Narrabri]

Using this fringe pattern we have identified a radio source near 13:24 RA, -43 declination. The catalogued position for Centaurus A is 13:25.5, -43:01!! In fact Cen. A was the first discrete object we ever unambiguously detected in the first days of Simple!

Cen. A was also one of the first discrete radio sources ever discovered, by CSIRO scientists John Bolton, Gorden Stanley, and Bruce Slee, from Sydney in 1949. There's some more info about it here and here.

You can use this same technique on any distinct set of fringes in your data. Prove that your backyard telescope is really able to detect radio waves from galaxies far, far away! Be the envy of all your friends!!