Science & Technology

Why astronomers use different radio bands to observe the universe

21cm view of the Pin Wheel Galaxy (M33). The colored rainbow is due to the rotation of the galaxy, and the radio waves change due to the Doppler shift. Credit: NRAO / AUI / NSF

The radio light will be iridescent. These colors are found in radio bands, and each band has a story that tells a story about the universe.

Radio astronomyists see the universe in several wavelength ranges that we call bands. Ultra-large arrays (VLAs) use wavelengths in the range of 4 meters to less than 1 centimeter. Atacama Large Millimeter / Submillimeter Wave Array (Alma) Uses a radio band in the range of a few centimeters to a third of a millimeter. But why do radio telescopes use such a wide wavelength range? The answer lies in the many ways an object emits radio waves and how this light interacts with gas and dust in the interstellar space.

Long wavelengths, such as those found in band 4 of the VLA, are usually produced by ionized gases.You can see where it’s hot plasma Located in our galaxy. These long wavelengths are also useful because most neutral gases are transparent at these wavelengths. This means that little light is absorbed as this light travels through space. Light of shorter wavelengths is often emitted by specific atoms or molecules. One of the most important of these is the 21-centimeter line emitted by neutral hydrogen. This wavelength is one of the best ways to observe the distribution of matter in the galaxy, as hydrogen is by far the most abundant element in the universe.

Wavelengths in the range of 10 cm to 20 cm are particularly suitable for radio sky surveys such as the VLA Sky Survey (VLASS). Radio galaxies are particularly bright in this range, as are jets emitted by supermassive black holes. Black Hole.. By scanning the sky at these wavelengths, VLASS captured images of approximately 10 million sources.

A radio galaxy powered by a black hole discovered by VLASS

A radio galaxy powered by a black hole discovered by VLASS. Credit: NRAO / AUI / NSF

Light with a wavelength of 1-2 centimeters is often Synchrotron radiation.. When electrons pass through a strong magnetic field at high speed, the magnetic field moves them in a tight spiral along the lines of magnetic force. Because of this, they emit radio waves. Synchrotron radiation is especially useful for mapping magnetic fields near black holes. Another process that emits light in this range is Maser Or microwave laser. We are most familiar with simple laser pointers that emit coherent red light, but in interstellar space, water pockets can emit coherent light with a wavelength of 1.3 centimeters. These water masers emit light of very specific wavelengths and can be used to measure the rate at which the universe expands.

Radio wavelengths on the order of millimeters are especially useful for studying cold gases and dust. Dust in the interstellar space emits light with wavelengths on the order of its size. Much of this dust is about 1 millimeter in size, so it is the wavelength that emits the most light. These short wavelengths can be difficult to observe because our atmosphere absorbs much of the light at these wavelengths. However, they are also extremely important for the study of young planetary systems. ALMA has been able to capture gas and dust disks around young stars and has seen how gaps form within these disks as young planets begin to form. It is revolutionizing our understanding of how exoplanets are formed.

HL Tauri protoplanetary disk

Image of the ALMA Observatory of the young star HL Tau and its protoplanetary disk. One of the best images of planet formation to date, this image reveals multiple rings and gaps that signal the presence of emerging planets as they clear their orbits free of dust and gas. Credits: ALMA (ESO / NAOJ / NRAO); C. Brogan, B. Saxton (NRAO / AUI / NSF)

But perhaps one of the more interesting radio bands is ALMA’s band 6, which captures light with wavelengths between 1.1 and 1.4 mm. It has been used to study how red giants generate heat and the distribution of molecules in planetary nebulae.But it was also used to create one of the most powerful radio images of recent years, the super-large one. Black Hole It is located in the center of the galaxy M87. Band 6 receivers were used by radio telescopes around the world as part of the Event Horizon Telescope (EHT) and combined the data they collected to create the first direct image of a black hole.

Radio light is invisible to our eyes, so it’s easy to think of all radio lights as the same. But the radio is filled with colors as well as the colors of visible light we can see, and radio astronomy is the most powerful when we use all the colors of that rainbow.

Why astronomers use different radio bands to observe the universe Why astronomers use different radio bands to observe the universe

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