Galactic Plane

The plane of the Milky Way is familiar to us all as the band of billions of stars that can be seen at night. This is where most of the "stuff" of our Galaxy is, and so its little surprise that there is more radio emission when the Milky Way is overhead than when it is set. The Milky Way is an very powerful radio source at low frequencies and the Galactic background noise completely dominates the static that you hear on a shortwave radio. Here is a short .WAV file of the background hiss of the Galaxy, from Simple.

The center of our Galaxy is mostly obscured to optical astronomers by intervening dust, because of this there was actually no concensus about the location of the center until radio telescopes were developed. The center of the Galaxy lies in the constellation Sagittarius, at about 17h43m right ascension -29° declination - this passes almost directly overhead from our observing site in Narrabri, at -30° latitude.

This image shows how the output of a RadioJove receiver at Narrabri (Australia) varies over 24 hours (2005/09/05). The graph is clearly dominated by the large hump that peaks around 18 hours LST, corresponding to the transit of the central plane of the Galaxy.

The brown lines show power calibration points made with a noise source from Richard Flagg (highly recommended product by the way).

With the standard dual-dipole phased array "RadioJove" antenna we measure about 55000 kelvins when the central plane of the Galaxy is at transit but only about 20000K when it is set. Most of this 20000K still comes from sources within our Galaxy, just stuff away from the central plane, however some of the noise does come from extra-galactic objects.

Some of the noise is also made within the receiver itself, however as you can see on the graph, the receiver noise floor for a decent receiver should be small with respect to the contribution from the Galaxy at low frequencies.

The image above shows how stable the emission from the Galaxy (and the RadioJove receivers) can be over a few days!

The image below shows an animation made over the course of a year, you can see how astronomical sources rise 4 minutes earlier each day and therefore the hump from the Galaxy appears to move throughout the year. You can also see that most of the interference (little spikes of noise) happens during the day and appears to stay fixed in the middle of the image.

The radio emission we detect from the Galaxy at 20MHz is called synchrotron emission. This is caused by electrons moving around in magnetic fields rather than the thermal processes that produce, eg starlight.