As telescopes have become more advanced and powerful, astronomers have been able to detect more and more distant galaxies. These are some of the earliest galaxies to form in our universe that began to recede away from us as the universe expanded. In fact, the more the distance, the faster a galaxy appears to move away from us. Interestingly, we can estimate how fast a galaxy is moving, and in turn, when it was formed based on how “redshifted” its emission appears. This is similar to a phenomenon called “Doppler effect,” where objects moving away from an observer emit the light that appears shifted towards longer wavelengths (hence the term “redshift”) to the observer.
The Atacama Large Millimeter/submillimeter Array (ALMA) telescope located in the midst of the Atacama Desert in Chile is particularly well-suited for observing such redshifts in galaxy emissions. Recently, a team of international researchers including Professor Akio Inoue and graduate student Tsuyoshi Tokuoka from Waseda University, Japan, Dr. Takuya Hashimoto at University of Tsukuba, Japan, Professor Richard S. Ellis at University College London, and Dr. Nicolas Laporte, a research fellow at the University of Cambridge, UK, has observed redshifted emissions of a distant galaxy, MACS1149-JD1 (hereafter JD1), which has led them to some interesting conclusions. “Beyond finding high-redshift, namely very distant, galaxies, studying their internal motion of gas and stars provides motivation for understanding the process of galaxy formation in the earliest possible universe,” explains Ellis. The findings of their study have been published in The Astrophysical Journal Letters.
Galaxy formation begins with the accumulation of gas and proceeds with the formation of stars from that gas. With time, star formation progresses from the center outward, a galactic disk develops, and the galaxy acquires a particular shape. As star formation continues, newer stars form in the rotating disk while older stars remain in the central part. By studying the age of the stellar objects and the motion of the stars and gas in the galaxy, it is possible to determine the stage of evolution the galaxy has reached.
Conducting a series of observations over a period of two months, the astronomers successfully measured small differences in the “redshift” from position to position inside the galaxy and found that JD1 satisfied the criterion for a galaxy dominated by rotation. Next, they modeled the galaxy as a rotating disk and found that it reproduced the observations very well. The calculated rotational speed was about 50 kilometers per second, which was compared to the rotational speed of the Milky Way disk of 220 kilometers per second. The team also measured the diameter of JD1 at only 3,000 light-years, much smaller than that of the Milky Way at 100,000 light-years across.
The significance of their result is that JD1 is by far the most distant and, therefore, earliest source yet found that has a rotating disk of gas and stars. Together with similar measurements of nearer systems in the research literature, this has allowed the team to delineate the gradual development of rotating galaxies over more than 95% of our cosmic history.
Furthermore, the mass estimated from the rotational speed of the galaxy was in line with the stellar mass previously estimated from the galaxy's spectral signature, and came predominantly from that of “mature” stars that formed about 300 million years ago. “This shows that the stellar population in JD1 formed at an even earlier epoch of the cosmic age,” says Hashimoto.
“The rotation speed of JD1 is much slower than those found in galaxies in later epochs and our Galaxy and it is likely that JD1 is at an initial stage of developing a rotational motion,” says Inoue. With the recently launched James Webb Space Telescope, the astronomers now plan to identify the locations of young and older stars in the galaxy to verify and update their scenario of galaxy formation.
New discoveries are surely on the horizon!
Authors: Tsuyoshi Tokuoka,1 Akio K. Inoue,1,2 Takuya Hashimoto,3 Richard S. Ellis,4 Nicolas Laporte,5,6 Yuma Sugahara,2,7Hiroshi Matsuo,7Yoichi Tamura,8Yoshinobu Fudamoto,2,7Kana Moriwaki,9Guido Roberts-Borsani,10Ikkoh Shimizu,11Satoshi Yamanaka,12Naoki Yoshida,9,13,14,15Erik Zakrisson,16 and Wei Zheng17.
1 Department of Pure and Applied Physics, Waseda University
2 Waseda Research Institute for Science and Engineering, Waseda University
3 Tomonaga Centre for the History of the Universe (TCHoU), University of Tsukuba
4 Department of Physics & Astronomy, University College London
5 Kavli Institute for Cosmology, University of Cambridge
6 Cavendish Laboratory, University of Cambridge
7 National Astronomical Observatory of Japan
8 Division of Particle and Astrophysical Science, Graduate School of Science, Nagoya University
9 Department of Physics, the University of Tokyo
10 Department of Physics and Astronomy, University of California
11 Department of Literature, Shikoku Gakuin University
12 General Education Department, National Institute of Technology, Toba College
13 Kavli Institute for the Physics and Mathematics of the Universe (WPI), UT Institutes for Advanced Study, The University of Tokyo
14 Research Centre for the Early Universe, School of Science, The University of Tokyo
15 Institute for Physics of Intelligence, School of Science, The University of Tokyo
16 Observational Astrophysics, Department of Physics and Astronomy, Uppsala University
17 Department of Physics and Astronomy, Johns Hopkins University
About Waseda University
Located in the heart of Tokyo, Waseda University is a leading private research university that has long been dedicated to academic excellence, innovative research, and civic engagement at both the local and global levels since 1882. The University ranks number one in Japan in international activities, including the number of international students, with the broadest range of degree programs fully taught in English. To learn more about Waseda University, visit https://www.waseda.jp/top/en.
The Astrophysical Journal Letters
Method of Research
Subject of Research
Possible Systematic Rotation in the Mature Stellar Population of a z = 9.1 Galaxy