Institute of Astronomy

Radio emission from black holes

Published on 09/04/2014 
Question: 

I've been reading about astronomical radio sources and I've noticed something that doesn't seem to make sense to me.
It is my understanding that gamma rays are the highest energy form of electromagnetic radiation, is it the shorter wavelength that gives it more energy?
If this is true then why do planets radiate mostly in the infrared and paradoxically, extreme energetic processes like those around supermassive black holes radiate in the radio spectrum even though infared wavelengths are shorter than radio wavelengths? 
 

You are quite correct that gamma rays are at the top end of the electromagnetic energy scale, and that a shorter wavelength corresponds to higher energy.

Part of the difference between the two scenarios you mention, radiation from planets and processes related to supermassive black holes, is what is generating the radiation.  For thermal radiation, that is light produced as a result of the heat of the object, the wavelength at which the emission peaks gets shorter as the object gets hotter.  This is why emission from Earth peaks in the infrared, while sunlight peaks in the visible.  The accretion disks around supermassive black holes are actually so hot that the radiation they produce peaks in X-rays.

Active supermassive black holes are also however often associated with strong emission in the radio, as in radio galaxies, like you've noticed.  This radio emission is much stronger than would be expected from thermal sources.  Instead, the radio waves produced by radio galaxies are the result of energetic electrons (accelerated by the black hole) spiralling around magnetic field lines.  We call this sort of radiation synchrotron radiation, so called because it was first associated with synchrotrons - devices that use powerful magnetic fields to bend beams of extremely energetic particles.  The spectrum of synchrotron radiation has a very different shape to thermal radiation. 

Page last updated: 9 April 2014 at 01:46