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Discovering Galaxies Playing Hide-and-Seek

Rieko Momose (Project Researcher*, Kavli Institute for the Physics and Mathematics of the Universe)

Kazuhiro Shimasaku, Associate Professor, Department of Astronomy

A galaxy is a huge celestial body composed of many stars. Our own Milky Way Galaxy, which contains about 100 billion stars and is 100 million times larger than our solar system, is one of them. There are countless galaxies in the universe, forming clusters here and there. Since the size of the population determines how galaxies grow, it is essential to study the distribution of galaxies at various epochs in the universe.

The distribution of galaxies in the past (distant universe) is often depicted using the Lyman-alpha (Lyα) emission lines of hydrogen atoms, which are bright in a type of ultraviolet light called "Lyα galaxies. The reason is that Lyα emission lines are easily observed in the distant universe. Recently, however, it has been reported that Lyα galaxies are distributed in areas where other common galaxies are densely clustered, and that the distribution of galaxies is not correctly traced. The possibility that Lyα galaxies are "present but invisible" has emerged as a possible cause of this.

In the case of solar or lunar eclipses, however, is it possible for such a huge object as a galaxy to be hidden from view? Actually, it is possible only for Lyα galaxies. The space between galaxies is not a vacuum. There is a gas called intergalactic gas (mainly composed of hydrogen), which absorbs Lyα emission lines in proportion to its concentration. The denser the galaxy, the more dense the gas is, so in theory, Lyα galaxies in dense areas or behind the galaxies are not visible to us. It is just like turning on a light in a dense fog, but it illuminates only the near side of the fog. However, it is difficult to show that they are there but invisible, and there was no proof that this phenomenon is actually occurring.

Figure: Schematic diagram of the distribution of high-density regions and galaxies suggested by this study. Lyα emission lines from galaxies in front of the high-density region (yellow circles) reach us and are detected as Lyα galaxies. However, Lyα emission lines from galaxies in the high-density region and galaxies immediately behind the high-density region (orange circles) are absorbed by the dense intergalactic gas in the high-density region and are not visible. In the detected galaxies (yellow circles), the gas density in the front side (observer's side) is lower than that in the back side.

In this study, we tested this possibility using a new method in a region where Lyα galaxy and intergalactic gas data are available. If the distribution of Lyα galaxies is the same as that of the gas, then the gas density should be the same in the front and the back when averaged over a large number of Lyα galaxies. However, the results showed that the concentration in the front side was lower than that in the back side. This means that the observed Lyα galaxies tend to be located in the front side of dense areas (Figure). However, the direction of observation is our own choice and has no special meaning for Lyα galaxies. Therefore, the most natural interpretation is that Lyα galaxies exist in and behind dense regions, but some of them are not visible to us due to absorption. In fact, other galaxies show no difference in gas concentration between the foreground and background. It is as if the Lyα galaxies are playing hide-and-seek with the gas. If this region were observed from the opposite side, the hidden Lyα galaxy would appear and another Lyα galaxy would be hidden instead.

Galaxy studies assume that all galaxies are visible. However, this study shows for the first time that this is not always the case. This study also revealed the limitation of the estimation of galaxy distribution by Lyα galaxies. It may be necessary to use other galaxies and intergalactic gas to find a more dense location, although this will require more effort.

The study of the relationship between galaxies and intergalactic gas is still in its infancy, and there are still many unknowns. I am looking forward to seeing what phenomena will be found next.

The results of this research have been published in R. Momose et al ., The Astrophysical Journal Letters 912, L24 (2021).

* At the time of the research: Project Researcher, Japan Society for the Promotion of Science (JSPS)

(Press release on May 7, 2021)

Published in Faculty of Science News, September 2021

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