Radio Galaxy Zoo Talk

How do we know the radio emission from galaxies is synchrotron?

  • raynorris by raynorris scientist

    @JeanTate asked how we know the radio emission from radio galaxies is synchrotron emission. The answer is a bit long, I’m afraid.

    There are a number of exotic ways of producing continuum (as opposed to spectral line) radio emission, but by far the most common are thermal and synchrotron.

    Thermal emission is just caused by heating something, such as a solid object, or dust, or electrons. Whenever you heat something, it generates blackbody radiation ( http://en.wikipedia.org/wiki/Black-body_radiation ) over the entire electromagnetic spectrum. For example, the Sun generates most of is energy in the visible part of the electromagnetic spectrum (which is why we’ve evolved to have eyes sensitive to visible light) but also generates a fair amount of radio emission. In fact the Sun is the brightest radio source in the sky. In radio, notable sources of thermal radiation include the Sun, Moon, planets, dust, and the ionised gas (HII regions) around young stars. When the thermal radiation is from electrons (as in the HII region) it is sometimes called free-free or Bremsstrahlung radiation. You can look up any of these terms on Wikipedia. Thermal emission is usually characterised by little or no polarisation, and by a blackbody spectrum, although this can be modified by the physical properties of he source.

    Synchrotron emission is caused by fast electrons spiralling around a magnetic field, and is so named because it was first seen in the early synchrotron particle accelerators. When bright radio sources were discovered after the second world war, it was quickly realised that this emission shared the characteristics of the radio emission from synchrotron accelerators: highly polarised, and with a brightness that usually decreases with frequency. Who discovered it? Wikipedia (http://en.wikipedia.org/wiki/Synchrotron_radiation ) tells you the sad story of the dispute about who had discovered it: "In particular, the Russian physicist V.L. Ginzburg broke his relationships with I.S. Shklovsky and did not speak with him for 18 years. In the West, Thomas Gold and Sir Fred Hoyle were in dispute with H. Alfven and N. Herlofson, while K.O. Kiepenheuer and G. Hutchinson were ignored by them.” And just to cause further confusion, synchrotron emission is occasionally called “magnetic bremsstrahlung”.

    From measurements of polarisation and spectral index, we now know that the vast majority of the radio emission in the sky, both from quasars (or Active Galactic Nuclei) and from star-forming galaxies (such as most spirals) are caused by synchrotron radiation. In our own Galaxy, we can see the process up close, with cosmic ray electrons in the plane of the Galaxy spiralling around the Galactic magnetic field.

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  • JeanTate by JeanTate in response to raynorris's comment.

    Thanks very much, raynorris! 😃 Myself, I really hope you will write more "bit long" answers!! 😄

    For thermal emission, radio astronomers are detecting the long wavelength Rayleigh-Jeans tail (especially for extended sources, where the emission comes from electrons)? For the radio emission to be thermal, the source must be optically thick, right? That means you cannot detect a distant QSO 'behind' an HII region?

    As you are far away from the peak frequency (for thermal emission), radio astronomers cannot work out the temperature of such sources, from radio continuum emission alone; yes?

    How do radio astronomers define "spectral index"? However they do it, thermal sources will have (essentially) the same value (all sources will be the same hue of 'blue'), so any source which has a different spectral index (value) will be producing at least some radio emission by at least one physical process other than thermal emission (I'm ignoring all line emission)? If so, then observations of polarization would "just" be confirmation. Is one kind of observation - polarization vs multi-frequency - generally easier to do/make than the other? Do radio astronomers have really cool instruments (receivers?) you can use to get both from a single 'pointing'? I guess it's hard, unless you have an observatory like LOFAR ... or the SKA!

    One thing I've been particularly struck by, here in RGZ, is just how (radio) bright* some of the sources are, even at redshifts of 2 or more. I get the impression that you can estimate the redshift of a radio source only with considerable difficulty (from radio observations alone), except if you're confident you're observing 21-cm line emission. Is that so? Or can you estimate redshift, knowing that it's synchrotron radiation?

    Hmm, that's rather a lot of questions already; guess I'd better stop ...

    *or 'radio loud'; gotta learn to speak the lingo, here in radio-astronomy-land ...

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