What is radio-emitting gas? [closed]

Question

What is radio-emitting gas? (I think it refers to the Magellanic Stream, but I can't be convinced.)

Answer 1

Referring only to the image you present, the radio part of the image (I think this is a stacked optical and radio image) shows something called the Magellanic stream. Despite what the wikipedia page says, I believe it was discovered by Dieter (1971) using 21 cm observations of neutral hydrogen atoms.

The radio emission in the case of the picture you show arises from the so-called "spin flip" transition associated with changes in the relative spin of the proton and electron in a hydrogen atom. A configuration where the spins are anti-parallel has a lower energy than where they are parallel. (Quantum mechanics only allows these two possibilities). The energy difference is small - only $5.87 \times 10^{-6}$ eV. In hydrogen gas in space it is possible for the atom to make a transition from the higher level state to the lower level, in the course of which it emits radiation with a wavelength of 21 cm (i.e. radio waves). This is not seen in terrestrial laboratory samples because the gas is collisionally de-excited before it has the chance to emit radiation. Hence 21cm radiation is an example of a forbidden line, with a long radiative lifetime and which is only observed from the rarefied neutral hydrogen gas found in the interstellar medium.

The intrinsic width of the emitted spectral line is very narrow. This means that the observed wavelength of this 21 cm line is an excellent probe of the motion of the emitting gas, since the observed radio waves will be doppler shifted from the 21 cm wavelength at which they were emitted according to the line-of-sight velocity of the gas.

The gas in question here has been stripped from the two satellite galaxies of the Milky Way, known as the Magellanic clouds. Although the picture also shows copious 21 cm radiation arising from neutral hydrogen gas in the plane of our Milky Way.

There are many other ways that gas can be a source of radio emission. In addition to atomic and molecular transitions that give radio emission at discrete wavelengths (including the radio equivalent of lasers, called masers), there can be continuum emission caused by the acceleration of electrons in the electric fields of partially and completely ionised ions - so called bremsstrahlung or free-free emission. There is also recombination radiation (free-bound) radiation and continuum radiation emitted by electrons that are accelerated in electromagnetic fields - cyclotron radiation; synchrotron radiation (if the electrons are relativistic); curvature radiation (a variety of synchrotron radiation) or even just the Rayleigh-Jeans tail from cold blackbody emission (e.g. in the cosmic microwave background).

A decent introductory summary can be found here.

Answer 2

One type of ratio-emitting gas is ionized gas, which is penetrated by a magnetic field. Also known as a plasma, the charged particles that make up this gas will start rotating around the magnetic field lines.
Their paths will have a local radius of curvature equal to the gyroradius, which is the natural movement radius of charged particles in a magnetic field.
Then, because circular motion is accelerated motion, and accelerated charges in general emit electromagnetic radiation (EMR), the gyrating charges will emit EMR.
One can calculate that for typical magnetic field strengths in interstellar space this radiation will then be emitted in the radio-range.
The details depend a lot on where the charged particles come from, what their energies are, viewing geometry of the observer etc. but essentially it's that.