<p>Referring <em>only</em> 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 <a href="https://en.wikipedia.org/wiki/Magellanic_Stream" rel="nofollow noreferrer">Magellanic stream</a>. Despite what the wikipedia page says, I believe it was discovered by <a href="http://adsabs.harvard.edu/abs/1971A&amp;A....12...59D" rel="nofollow noreferrer">Dieter (1971)</a> using 21 cm observations of <em>neutral</em> hydrogen <em>atoms</em>.</p>

<p>The radio emission in the case of the picture you show arises from the so-called <a href="https://en.wikipedia.org/wiki/Hydrogen_line" rel="nofollow noreferrer">"spin flip"</a> 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 <a href="https://en.wikipedia.org/wiki/Forbidden_mechanism" rel="nofollow noreferrer">forbidden line</a>, with a long radiative lifetime and which is only observed from the rarefied neutral hydrogen gas found in the interstellar medium.</p>

<p>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. </p>

<p>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.</p>

<p>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 <a href="https://en.wikipedia.org/wiki/Maser" rel="nofollow noreferrer">masers</a>), there can be continuum emission caused by the acceleration of electrons in the electric fields of partially and completely ionised ions - so called <a href="https://en.wikipedia.org/wiki/Bremsstrahlung" rel="nofollow noreferrer">bremsstrahlung</a> or free-free emission. There is also recombination radiation (free-bound) radiation and continuum radiation emitted by electrons that are accelerated in electromagnetic fields - <a href="https://en.wikipedia.org/wiki/Cyclotron_radiation" rel="nofollow noreferrer">cyclotron radiation</a>; <a href="https://en.wikipedia.org/wiki/Synchrotron_radiation" rel="nofollow noreferrer">synchrotron radiation</a> (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).</p>

<p>A decent introductory summary can be found <a href="http://astronomy.swinburne.edu.au/sao/downloads/HET608-M03A02.pdf" rel="nofollow noreferrer">here</a>.</p>

CC BY-SA 3.0
2016-11-24T16:39:44.220Z
Well, now you will allow me to be picky: The question is "What is radio emitting gas?", not "What do we see here?".
AtmosphericPrisonEscape
CC BY-SA 3.0
2016-11-24T16:31:40.323Z
2016-12-15T14:56:16.390Z
2016-12-15T14:56:16.390Z
ProfRob
<p>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.<br>
Their paths will have a local radius of curvature equal to the <a href="https://en.wikipedia.org/wiki/Gyroradius" rel="nofollow noreferrer">gyroradius</a>, which is the natural movement radius of charged particles in a magnetic field.<br>
Then, because circular motion is accelerated motion, and accelerated charges in general emit electromagnetic radiation (EMR), the gyrating charges <a href="https://physics.stackexchange.com/questions/65339/how-and-why-does-accelerating-charges-radiate-electromagnetic-radiation">will emit EMR</a>.<br>
One can <a href="http://www.mathpages.com/home/kmath528/kmath528.htm" rel="nofollow noreferrer">calculate</a> that for typical magnetic field strengths in interstellar space this radiation will then be emitted in the radio-range.<br>
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.</p>

CC BY-SA 3.0
2016-11-24T14:44:38.547Z
Atomic gas emits radio waves. That is what 21cm radiation comes from.
ProfRob
CC BY-SA 3.0
2016-11-24T15:45:26.397Z
@RobJeffries: Wow, so a partial answer is reason to downvote? Drastic.
AtmosphericPrisonEscape
CC BY-SA 3.0
2016-12-15T08:40:15.403Z
I upvoted, but such a complex explanation would benefit from PARAGRAPHS to tell the reader when he should pause to think of the previous phrase... before going on to the next one. Paragraphs give a text structure, clarify it, and prompt the reader to understand a paragraph as an actual phrase, they help him navigate and read over again. Generally avoid starting a complex phrase with "Also known as a plasma" when you can start it with "The charged particles" "Also known as"... is not a start to a text, a simple, clear, conceptual phrase progression. I write unclearly myself so well done you ;)
bandybabboon
CC BY-SA 3.0
2016-12-16T08:53:44.560Z
@JanDoggen Indeed. And although I was, now I am not.
ProfRob
CC BY-SA 3.0
2016-11-24T12:41:20.660Z
2016-11-24T16:59:22.960Z
2017-04-13T12:40:15.377Z
AtmosphericPrisonEscape
<p><a href="https://i.sstatic.net/Mj8EN.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/Mj8EN.png" alt="enter image description here"></a></p>

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

CC BY-SA 3.0
2016-11-24T14:48:40.750Z
Can you add the source of your image.
ProfRob
<milky-way>
What is radio-emitting gas?
Purple Rain Kim
# What is radio-emitting gas?

(Asked by: username_0 on 2016-11-24)

[![enter image description here](https://i.sstatic.net/Mj8EN.png)](https://i.sstatic.net/Mj8EN.png)

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

**Tags:** milky-way

> **Comments:**
> **username_1**: Can you add the source of your image.  

---

## 2 answers

### Answer by username_1 on 2016-11-24. Score: 5

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](https://en.wikipedia.org/wiki/Magellanic_Stream). Despite what the wikipedia page says, I believe it was discovered by [Dieter (1971)](http://adsabs.harvard.edu/abs/1971A&A....12...59D) 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"](https://en.wikipedia.org/wiki/Hydrogen_line) 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](https://en.wikipedia.org/wiki/Forbidden_mechanism), 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](https://en.wikipedia.org/wiki/Maser)), there can be continuum emission caused by the acceleration of electrons in the electric fields of partially and completely ionised ions - so called [bremsstrahlung](https://en.wikipedia.org/wiki/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](https://en.wikipedia.org/wiki/Cyclotron_radiation); [synchrotron radiation](https://en.wikipedia.org/wiki/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](http://astronomy.swinburne.edu.au/sao/downloads/HET608-M03A02.pdf).

> **Comments:**
> **username_2**: Well, now you will allow me to be picky: The question is "What is radio emitting gas?", not "What do we see here?".  

---

### Answer by username_2 on 2016-11-24. Score: 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](https://en.wikipedia.org/wiki/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](https://physics.stackexchange.com/questions/65339/how-and-why-does-accelerating-charges-radiate-electromagnetic-radiation).  
One can [calculate](http://www.mathpages.com/home/kmath528/kmath528.htm) 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.

> **Comments:**
> **username_1**: Atomic gas emits radio waves. That is what 21cm radiation comes from.  
> **username_2**: @RobJeffries: Wow, so a partial answer is reason to downvote? Drastic.  
> **username_3**: I upvoted, but such a complex explanation would benefit from PARAGRAPHS to tell the reader when he should pause to think of the previous phrase... before going on to the next one. Paragraphs give a text structure, clarify it, and prompt the reader to understand a paragraph as an actual phrase, they help him navigate and read over again. Generally avoid starting a complex phrase with "Also known as a plasma" when you can start it with "The charged particles" "Also known as"... is not a start to a text, a simple, clear, conceptual phrase progression. I write unclearly myself so well done you ;)  
> **username_1**: @JanDoggen Indeed. And although I was, now I am not.  

---

