ALMA Detects Most Tenuous Molecular Gas Ever Observed

ALMA Detects Most Tenuous Molecular Gas Ever Observed

Read time: 3 minutes

Have you ever looked at the world through a red piece of plastic foil, like the wrapping paper of certain candies? It’s really fun: your whole world turns red, and everything looks very different from normal. That’s because the red foil only allows red light to pass through; most other colours are blocked, or ‘absorbed’, as scientists call it.

The Universe is full of more or less similar ‘absorption systems’. Not pieces of coloured plastic foil of course, but tenuous clouds of gas. They also block certain colours of starlight, and allow other colours to pass through. So if a distant star is seen behind a tenuous cloud of gas, the colours of the star look a bit different from normal.

What is true for visible light is also true for submillimeter radiation — the type of invisible light that is observed by ALMA. Different molecules in space absorb different ‘colours’ of submillimeter radiation. So by precisely studying the submillimeter radiation of distant objects, you can discover the existence of gas clouds in front of those objects. What’s more: you can even deduce what types of molecules are present in those gas clouds.

Astronomers have studied ALMA measurements of 36 bright sources of submillimeter radiation, in search of intervening clouds of molecular gas. They found two new absorption systems, and confirmed the existence of a third gas cloud. By carefully looking at which ‘colours’ (wavelengths) are being absorbed, the astronomers detected ten different types of molecules in the gas clouds, including the rare molecule HCO – a combination of one hydrogen atom (H), one carbon atom (C) and one oxygen atom (O).

Measuring the amount of absorption made it possible to calculate the density of the intervening gas clouds. It turned out that the newly discovered absorption systems are by far the most tenuous gas clouds that have ever been discovered in our Milky Way galaxy. If they had not been seen in front of the distant sources of submillimeter radiation, they would never have been detected.

The HCO molecules in the foreground gas clouds are probably formed under the influence of energetic ultraviolet radiation from nearby stars. The new discoveries make it possible to study this process in more detail.


The 36 bright sources of submillimeter radiation studied in this research are so-called calibration sources for ALMA. For two calibration sources, known as J1717-337 and NOAO530, previously unknown absorption systems were discovered, lying somewhere between the distant source and the Earth.

The extremely tenuous gas clouds lying in front of the distant calibration sources were discovered by a group of Japanese astronomers led by Ryo Ando. Ryo is a graduate student at the University of Tokyo, working with his supervisor, professor Kotaro Kohno. The new discoveries have been published in a scientific paper in the December 2015 issue of the Publications of the Astronomical Society of Japan.