Milky Way structure detected with the 21 cm Neutral Hydrogen Emission

Abstract : In this post we want to describe the study activity of the structure of our Galaxy made using the emission at 21 cm of neutral hydrogen, detected by the instrumentation described in the previous posts : Horn Antenna for the 21 cm Neutral-Hydrogen Line, Low-Noise SDR-Based Receiver for the 21cm Neutral-Hydrogen Line, GNURadio Software for 21cm Neutral-Hydrogen Line.


Hydrogen is the most abundant element that exists in the universe. In particular, neutral hydrogen (HI) fills most of the space between stars in the galaxy. Hydrogen is a gas and is free to move. This gas, however, should have “settled” in the gravitational potential of the galaxy (due to the stellar disk). In analogy with planetary atmospheres, we might even expect to find some kind of “galactic atmosphere” around the disk.
The study of the electromagnetic radiations emitted by these HI hydrogen clouds (emission lines) should be a powerful tool for the study of their location and movement (by Doppler effect). In particular, emissions in the radio band (RF) are very important because they are not obscured by galactic dust, as happens in the visible band.

Nature, in this case, comes to our aid because, just the neutral hydrogen has a characteristic spectral emission line in the microwave RF band, at about 21.1 cm corresponding to a frequency of 1420.40575 MHz.

Neutral (non-ionized) hydrogen consists of a single proton around which a single electron rotates. Each of the two particles has its own spin, which can be clockwise or counterclockwise. When the proton and the electron have the same direction of rotation (parallel spin) the atom has a slightly higher energy than the case in which the direction of rotation is opposite (anti-parallel spin). This is due to magnetic interactions between the two particles.
The spin-parallel to anti-parallel transition causes photon emission at a wavelength of 21.1 cm. However, this transition is “forbidden” and therefore extremely unlikely, in fact it occurs with an average decay time of 11 million years! Therefore it is practically impossible to observe this phenomenon in the laboratory. However, the number of hydrogen atoms in the interstellar medium is extremely high, despite the very low density of about 100 atoms/cm3 and this makes the emission line at 21 cm easily observable with a radio telescope.
Historically, the radio emission of neutral hydrogen at 21 cm was first theoretically predicted by Van de Hulst and in 1951 Ewen and Purcell were able to detect this emission with a horn antenna. This discovery has given considerable impetus to radio astronomy research and has allowed to highlight the spiral structure of our Galaxy and its rotation curve. Since then, numerous other organic molecules have been detected in the interstellar medium with the same method of RF emissions on precise lines.

The frequency band between about 21 cm and 18 cm is known as the “Water hole”. It is an observation frequency widely used by radio telescopes in radio astronomy. Being included between the emission lines (absorption) of the gas HI and the OH group (abundant in the interstellar medium), the spectrum between these frequencies forms a sort of “silent” channel in the background of interstellar radio noise.
It has been speculated that these frequencies could be the most logical and natural choice for interstellar communications with extraterrestrial civilizations.

The Milky Way and Galactic coordinates

Before describing the measurements made with our radio telescope, we must make a premise about our Galaxy, the Milky Way, and the coordinate system that is used to identify the positions of objects within the galactic disk.
Current astronomical knowledge tells us that the Milky Way is a barred spiral galaxy, that is, a galaxy composed of a core crossed by a bar-shaped structure from which the spiral arms branch off. The stellar disk of the Milky Way has a diameter of about 100000 light years and a thickness, in the region of the arms, of about 1000 light years. Estimates on the number of stars that make it up are varied and sometimes controversial, with an estimated 200-400 billion stars. Outside the Milky Way the galactic halo stands out, delimited by the two major satellite galaxies, the Large and Small Magellanic Cloud, whose perigalactic (the points of their orbits closest to our Galaxy) are about 180000 light years away from the Milky Way itself.

Our star, the Sun, is located in the outer part of the Galaxy, at a distance of about 8.5 kpc (about 25000 light years) from the galactic center. Most of the stars and gas are found in a thin disk that revolves around the galactic center. The Sun has a circular speed of about 220 km/s, and completes a complete revolution around the center of the Galaxy in about 240 million years.

The image below shows the Galaxy seen “from above”. The sun is found in the arm of Orion which is a secondary arm located between the arm of Perseus, more external, and the arm of Sagittarius, more internal; even more external is the Cygnus arm.

To describe the position of a star or a gas cloud in the Galaxy, it is convenient to use the so-called Galactic coordinate system, (l; b), where l is the Galactic longitude and b the Galactic latitude. The Galactic coordinate system is centered on the Sun. b = 0 corresponds to the Galactic plane. The direction b = 90° is called the North Galactic Pole. The longitude l is measured counterclockwise from the direction from the Sun toward the Galactic center. The Galactic center thus has the coordinates (l = 0; b = 0).
The Galaxy has been divided into four quadrants, labeled by roman numbers :

Quadrant I 0° < l < 90°
Quadrant II 90° < l < 180°
Quadrant III 180° < l < 270°
Quadrant IV 270° < l < 360°

Quadrants II and III contain material lying at galacto-centric radii which are always larger than the Solar orbital radius (the radius of the orbit of the Sun around the Galactic center). In Quadrant I and IV one observes mainly the inner part of our Galaxy. The images shown below provide graphic evidence of the concepts just described


To “map” the galaxy to the emission of 21 cm it is necessary to know where to point our radio telescope. The pointing direction of our horn antenna can be adjusted in Azimuth (via a compass) and in Elevation. The elevation angle is checked with a digital inclinometer.
The observation must be planned in advance. You decide the area of ​​the galaxy you want to measure and its galactic coordinates (longitude and latitude) are obtained, these coordinates are then converted into the corresponding coordinates of Azimuth and Elevation that we need to point our antenna.
This operation is greatly simplified by the open Stellarium software (it is practically indispensable). Stellarium is a virtual planetarium and allows us to easily identify the position of our galaxy, both at night and during the day and provides us with the coordinates (azimuth, equatorial and galactic) of each point on the map. It is also possible to know where the scheduled observation point will be located at any time of the day : in this way we can choose the most favorable time to carry out the observation. Observations can be made both day and night. It is generally preferable to aim the antenna with high elevation angles (close to the zenith) in order to reduce the thermal noise of the ground picked up by the antenna. The image below shows a screen shot of Stellarium.

Observations at the Radio Telescope

For each measurement at the radio telescope, the galactic coordinates of the observed area of ​​the sky are reported and a schematic map showing the direction of observation with bold arrows. Two graphs are presented. The first is the spectrum of the signal power and the second is the conversion of frequencies in terms of speed of movement : moving away (redshift) or approaching (blueshift) of the emitting source with respect to the observer.
The power spectrum data is obtained directly from the GNURadio program (already described in Low-Noise SDR-Based Receiver for the 21cm Neutral-Hydrogen Line) which allows you to save the data on files in csv format. The frequency range covered corresponds to the band available on the Airspy R2 SDR receiver which is 10 MHz, centered around 1420 MHz. The range displayed on the graph is actually smaller because the area of ​​interest covers a smaller range than the actual band. The y axis shows the equivalent temperature of the signal in degrees °K.
The second graph transforms frequencies into velocities, assuming that the shifts in frequency, compared to the theoretical frequency of 1420.40575 MHz, are due to the Doppler effect. The speed data obtained from the calculation is corrected, subtracting from this the contribution due to the rotation / revolution movement of the earth and the peculiar movement of the sun with respect to the nearest stars (we will return to this aspect in the next article). The data obtained is the speed of the emitting hydrogen cloud with respect to the local area of ​​the sun.

The velocity curves obtained with the radio telescope observations can be verified by comparing them with the data obtained from the 25m professional radio telescopes in Dwingeloo, Holland, and 30m in Villa Elisa, Argentina. A web interface is available that allows you to have the  velocity curve, assigned the galactic coordinates and the beam-size of the antenna (FWHM) that we use: Hi Profile Search. We compared our experimental data and verified that the correspondence is excellent assuming an FWHM value of about 20 °.

Quadrant I – Around Long 43°/ Lat 0°

In this observation we are “looking” towards the inner part of the galaxy. The main peak (rather close to 0 velocity) is most likely due to local clouds from the Orion arm that hosts the solar system. There are also contributions to positive speeds, from 50 km/s up to 100 km/s, attributable to the innermost hydrogen clouds in the galaxy which, rotating clockwise, move away from the sun area.

Quadrant I – Around Long 82°/ Lat -4°

We are now observing the galaxy at a longitude of almost 90°, that is, in a direction almost tangent to the direction of rotation. The peak of greatest intensity at almost zero relative velocity corresponds again to the local hydrogen clouds, while the two minor peaks at different velocities should correspond to the clouds on the Perseus arm and on the outermost Cygnus arm. These clouds appear approaching because the sun, in its rotation around the galactic center, is moving in that direction.

Quadrant II – Around Long 146° / Lat +1°

Now we have moved on to point the radio telescope in the direction of the second quadrant, that is, towards the outside of the galaxy. Also in this case there is the main peak corresponding to the local clouds and the two peaks corresponding to the two outer arms. However, the intensity value of these peaks has increased, a sign that the clouds we are observing are closer and their relative speeds, always approaching, have decreased, this too is consistent with the rotation pattern we are hypothesizing and which is schematized in the diagram alongside the velocity chart.

Quadrant II – Around Long 176°/ Lat +1°

For this observation we have practically placed the radio telescope on a longitude of 180°, in the opposite direction to that which points towards the center of the galaxy. In this direction we expect that the hydrogen clouds have zero relative speed because the direction of their movement is only tangential with respect to the sun. Our measurement has in fact detected a single peak with a corresponding velocity equal to 0 (obviously within the limits of the precision of our measurements).

Quadrant III – Around Long 207° (-153°) / Lat +3°

The last observation in this series was made on the third quadrant. We are pointing out of the galaxy, towards the arms of Perseus and Cygnus. By composing the velocities along the line of sight vectorially, we can see how these clouds appear moving away from the sun and in fact the measured velocities are positive (redshift).

Quadrant IV

Not visible at our latitude.

HI Distribution in the Galactic Disk

A measurement that can be easily done with the radio telescope is the detection of the intensity of the signal received as the galactic latitude varies while keeping the longitude constant (for example, for convenience, at the value of 180°). In this way, a sort of “scan” of the galactic disk is performed to approximate the density of neutral hydrogen. Of course, we assume that higher signal intensities correspond to higher quantities of neutral hydrogen. We expect the matter to be concentrated in the central part of the galactic disk, that is, for latitudes close to 0°.
The table on the left and the graph below show the results obtained from this measurement. The maximum of the signal is detected at latitude 0 °, while moving away from the galactic plane, both in one direction and the other, the values ​​of the signal decrease, until they almost cancel out for latitude>40°. However, it must be said that this measurement is very approximate since the spatial resolution of our antenna is only 10°.


The radio telescope with horn antenna at 1420MHz has proved to be an excellent tool capable of providing a great deal of information on the distribution of neutral hydrogen clouds in our galaxy. The sensitivity is good, the frequency resolution is excellent, the spatial resolution is only 10° but the hydrogen clouds are extended objects and therefore can also be studied with an instrument of this type. The next step will be the study of the rotation curve of the galaxy, also to have clues about the existence of dark matter : Measurement of the Milky Way Rotation with Doppler Shift of 21cm Emission 

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