Well, readers, when we try to think closely at this word, the first idea we get is that there is a galaxy somewhere in the universe that is emitting radio waves. This notion is partially correct. Well, you might be thinking, Why is it partially correct? Hang on your minds first; we will try to understand the history behind their discovery. 

Many people believe that the study of extraterrestrial radio waves started after World War II when we began to point our radars from detecting enemy aircraft to the interstellar sky. Well, that is not the case. Karl Jansky, who worked at Bell Telephone Laboratories, studied these extraterrestrial radio waves during the early 1930s. In 1933, he concluded that he received radio waves at 15m wavelengths were of extraterrestrial origin. He even found out that most of the intensity is coming from the center of the Milky Way. In 1940, Grote Reber drew the first radio map of the Milky way galaxy. Not only this, Grote even found two new regions in the night sky, Cassiopeia & Cygnus, from where he received intense radio sources. 

In 1946, three gentlemen named J.S. Hey, S.J Parsons & J.W. Phillips detected more discrete radio sources. They used the Yagi antenna, which had a beamwidth of 6 by 15. With this, they were able to draw a contour map. So now, what is a contour map? Well, a map made up of contour lines. If my answer is this, then I certainly deserve to be hanged!!! Let's suppose these lines are a function with two variables, the curve that will try to unite such points with an equal value under the function. Hence if you draw this curve, you will see that it is joining points with equal values, and such kinds of lines are called contour maps. So let's come back to the contour map drawn by our three gentlemen. They identified two regions of intense radio emission: one from the galactic center and the other from the constellation Cygnus. Hey, and others were able to find out radio fluctuations from the Cygnus. They concluded that this radio source must be small (tiny in this case, they mean star-like objects) & hence such kinds of objects are called discrete radio sources. 

English & Australians were pioneers in the discovery of these radio objects. J.L Pawsey and his coworkers mounted an aerial system on a cliff at a seashore and then used the sea surface as their reflector for Lloyd's mirror interferometer. Martin Ryle & Graham Smith (not the cricketer) of England used a radio equivalent of Michelson's optical interferometer. Using these instruments, they calculated the angular width of the radio source they were observing in terms of wavelength 𝜆 and d which the distance between the aerial systems by using the simple formula of 𝜃 = 𝜆 /𝑑 

Two teams, one headed by Martin Ryle, created ground-breaking radio telescope devices and utilized them to precisely locate and image faint radio sources. The other team was led by John Bolton, which determined that discrete radio sources were galaxies or supernova remnants rather than stars. Both of them studied Cygnus A, and they set up an upper limit of 8 arc for the angular width of Cygnus A.

In 1948, Bolton did cutting-edge work by doing a radio survey of the night sky using a sea interferometer. He discovered at least six radio sources, including Taurus A, Coma A, Hercules A & Centaurus A. Not only this, our radio astronomer friends in England & Australia raised the number of observed radio sources to thousands. Russia, the U.S.A, Canada, the Netherlands, Japan & India established essential research centers, which increased the number of discovered radio sources. Readers, these discoveries sound fascinating. However, it would be a great addition to our fascination if we knew their nature. This can be done by optical estimation & distance estimation. In this article, we will mainly focus on the distance estimation part. Now whatever I will say next might sound bizarre. However, many radio astronomers at that time were confused about whether these radio objects were present in our solar system or beyond it. This idea of the location of the radio objects was finally struck down by Ryle & Smith. They showed that radio sources like Cygnus A & Cassiopeia A had to lie beyond the solar system. Bolton and his co-workers discovered the Taurus A (Crab Nebula), Virgo A & Centaurus A (external galaxies NGC 4486 & NGC 5128), which puts down the idea of these radio objects present in the solar system.

During the 1950s, when Rock & Roll and classic pop music were on the rise, Walter Baade & Rudolf Minkowski made the optical identification of these radio sources and found Cygnus A's accurate location. Baade & Minkowski also detected more radio sources such as Puppis A, NGC 1275, etc., optically.

Here is a million-dollar question: Can we classify these discrete radio objects?

 The answer is yes!!! 

J.H Oort, G. Westerbout and their contemporary B.Y. Mills came up with a classification system that put these radio objects into two groups: the galactic radio sources & extragalactic radio sources. Further, Baade & Minkowski divided each category into two more types of radio sources; these sources were divided into (a) Supernova remnants such as Taurus A, Puppis A, etc. and (b) hot H2 regions (massive molecular clouds made out of H2 and some other molecules responsible for star formation). Then we have the extragalactic radio sources divided into (a) normal galaxies like M31, M101, etc. and (b) strong radio sources primarily based in the elliptical galaxies (these galaxies are generally elliptical, and when we observe them, they look ellipsoidal), NGC 5128, NGC 1316, etc. come under this category.

Origins

Finally, after talking so much about the origins of these radio sources, I will tell why these galaxies are called radio galaxies. The name suggests that strong radio sources emit more radio energy than normal galaxies. Even you can say that the energy emitted in the former sources in radio wavelengths is more than the optical wavelength; hence they are called radio galaxies. 

Radio galaxies emit radiation in the form of synchrotron emission. How do we know that?

 It’s because of the intense polarization nature of this emission. 

Magnetobremsstrahlung radiation, no don't worry! It’s just another name for synchrotron emission discovered by Floyd Haber. It is emitted when a charged particle gets accelerated in a direction that is perpendicular to its velocity direction. Such kind of radiation has a clear sign of polarization. So, you can now understand why radiation from radio galaxies has a strong polarization.

 Now coming back to emission by radio galaxies, synchrotron emission implies that the plasma producing these radio waves contains electrons as well as protons and positrons. Still, the problem is that we have no way to find the particle content from the radiation itself. Also, for your information, Synchrotron radiation is not only limited to radio wavelengths. If a radio source can accelerate particles to high enough energies, characteristics detected in radio wavelengths may also be visible in infrared, optical, ultraviolet, or even X-ray wavelengths! 

There are also other emission processes like inverse Compton radiation that are happening along with synchrotron emission. To distinguish the latter from the former, we use polarization & continuum spectrum, but it is a challenging process. 

How to identify Radio objects? 

If you want to study a thing, you have to look at them. One must keep in mind that these radio objects are pretty far away and, I mean, far away. Consider NGC 5128. It is almost 60000 light-years away, but the good thing is that their emission lines are solid & sharp in their spectra. I am emphasizing that these sources are very far away. You need telescopes with high power of angular resolution & radiosensitivity, but surprise, there is a problem! If you have a telescope with a single antenna to increase its angular resolution, you need to increase its diameter. So, what is the problem, Devang? To satisfy radio astronomers, we require a telescope whose diameter is almost 1.5 km. The crown for being the largest telescope globally goes to the Five Hundred Meter Aperture Spherical Telescope in China, whose diameter is 500 meters. So, if you want to build such a telescope that is more massive than this one, Good Luck!!

You might be thinking now then how to study these radio objects, what to do?

 Radio astronomers did find out the solution to achieve higher angular resolution without building giant telescopes.

 Those are: 

(a) Radio Interferometer. 

(b) Aperture Synthesis.

(c) Lunar occultation.

Radio Interferometer

 First, let us talk about the interferometer technique Astronomers link these arrays of smaller telescopes together to create a giant aperture telescope. An interferometer is a telescope array of this type. The distance between the telescopes, rather than the size of the individual telescopes, determines the interferometer's resolution. 

As a result, we may construct an interferometer with a single, hundreds-of-meter-long telescope’s resolution. An interferometer records the interference pattern (or interference fringes) formed when light from two or more telescopes is combined instead of collecting photographs of the targeted objects. When light waves interact constructively, interference fringes form, resulting in a pattern of light and dark bands. The amplitude of the interference fringes encodes information about the object's size, shape, and brightness distribution. 

To produce interference fringes, each telescope's source light must travel the same distance, down to a fraction of a wavelength (less than 1 micron for optical and near-infrared light). The light from the radio source will arrive at one telescope before it reaches another, as shown in the diagram below. We transport light from each telescope through vacuum tubes into a laboratory to correct for the varying lengths it has traveled to reach each telescope. 

Fourier transform is essential for interferometer because it will help improve the measurement's efficiency, accuracy, and repeatability. Also, it gives valuable information about the diameter & physical nature of radio sources. To get a good resolution (angular), we have to increase the length between the telescopes. Such high resolution is made possible by constructing a Very Long Baseline Interferometer (VLBI) which can span countries and even continents! Jodrell Bank Radio astronomy group and National Radio Astronomy Observatory use the same technique to acquire images of the radio objects.

APERTURE SYNTHESIS

 An aperture synthesis telescope consists of an array of dishes connected in pairs. As the Earth rotates, each pair traces out one ring of a much larger dish, whose diameter is equal to the maximum separation of the pairs. Some dishes may be movable to provide a broader range of partitions. This is a technique for achieving excellent angular resolution in radio astronomy. It simulates a single large aperture telescope with an array of telescopes. An aperture of any size can feasibly be generated by moving the two interferometer elements to all possible points in the desired aperture. In effect, all synthesis telescopes take advantage of the fact that the Earth's rotation sweeps out half a ring of the synthesized aperture over 12 hours (super synthesis, or Earth-rotation synthesis); the other half of the ring is then calculated from the first half's observations. The elements are only moved to sweep out consecutive rings after that. In practice, some aperture-synthesis telescopes employ several movable dishes to reduce observing time. Examples of aperture synthesis telescopes are the Multi-Element Radio-Linked Interferometer Network (MERLIN), the Ryle Telescope, the Very Large Array, and the Westerbork Synthesis Radio Telescope (Westerbork Radio Observatory). Giant Meter wave Observatory (GMRT) at Pune.

LUNAR OCCULTATION

While observing any astronomical objects you have, there can be some unwanted things that can become a problem for our observations: the Moon itself. Now,  usually Moon is a headache to optical astronomers, but the same Moon is beneficial for radio astronomers.

 Nevertheless, the question is how? 

Well, whenever the Moon makes a journey around us, it blocks the radio waves coming to us from these radio objects. Now astronomers study this change in the intensity of the radio flux coming from the source. This occultation by the Moon helps determine the location & size of the radio object.

 Even though I have discussed how helpful the Moon can be, astronomers can face more problems while using this method: 

• Only those sources that lie on the Moon's path can be studied. 

• Occultation of a particular source happens only once a year, only for a short time. 

• Moon also emits radio waves which are highly intense than the sources we are trying to study, which are very weak.

References -

• https://www.nature.com/articles/132066a0.pdf

• https://www.nature.com/articles/158234a0.pdf

• http://www.narit.or.th/files/JAHH/2014JAHHvol17/2014JAHH...17..283R.pdf

• https://adsabs.harvard.edu/full/1993AAS...182.8003S

• https://adsabs.harvard.edu/full/1999ApJ...525C.569B

• https://en.wikipedia.org/wiki/Synchrotron_radiation

• An Introduction to Astrophysics by  Baidyanath Basu