As telescopes have become more advanced and powerful, astronomers have been able to discover more and more distant galaxies. These are some of the earliest galaxies that formed in our universe that began to recede from us as the universe expanded. In fact, the longer the distance, the faster a galaxy appears to move away from us. Interestingly, we can estimate how fast a galaxy moves, and in turn, when it was formed based on how “redshifted” its emission looks. This is similar to a phenomenon called “Doppler effect”, where objects moving away from an observer emit the light that seems displaced towards longer wavelengths (hence the term “redshift”) to the observer.
The Atacama Large Millimeter / Submillimeter Array (ALMA) telescope located in the middle 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 PhD student Tsuyoshi Tokuoka from Waseda University, Japan, Dr. Takuya Hashimoto at the University of Tsukuba, Japan, Professor Richard S. Ellis at University College London, and Dr. Nicolas Laporte, a researcher at the University of Cambridge, UK, has observed redshifted emissions from a distant galaxy, MACS1149-JD1 (hereafter JD1), which has led to some interesting conclusions. “In addition to finding galaxies with high redshifts, namely very distant galaxies, studies of their inner motion of gas and stars provide motivation to understand the process of galaxy formation in the earliest possible universe,” Ellis explains. The results of their study have been published in The Astrophysical Journal Letters.
Galaxy formation begins with the accumulation of gas and continues with the formation of stars from that gas. Over time, star formation progresses from the center outwards, a galactic disk develops and the galaxy takes on a certain 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 star objects and the motion of the stars and gas in the galaxy, it is possible to determine which stage of evolution the galaxy has reached.
By conducting a series of observations over a period of two months, astronomers successfully measured small differences in the “redshift” from position to position within the galaxy and found that JD1 met the criterion of a galaxy dominated by rotation. Then they modeled the galaxy as a rotating disk and found that it reproduced the observations very well. The estimated rotational speed was about 50 kilometers per second, which was compared with the Milky Way’s rotational speed of 220 kilometers per second. The team also measured the diameter of the JD1 at only 3,000 light-years, much smaller than the Milky Way’s diameter of 100,000 light-years.
The significance of their results is that JD1 is by far the most remote and therefore the earliest source that has been found so far that has a rotating disk of gas and stars. Together with similar measurements of more detailed systems in the research literature, this has enabled the team to delineate the gradual evolution of rotating galaxies over more than 95% of our cosmic history.
In addition, the estimated mass from the galaxy’s rotational speed was in line with the star mass previously estimated from the galaxy’s spectral signature, and came largely from “mature” stars formed about 300 million years ago. “This shows that the star population in JD1 was formed at an even earlier epoch of the cosmic age,” says Hashimoto.
“The rotational speed of JD1 is much slower than that found in galaxies in later epochs and our galaxy, and it is likely that JD1 is in an initial stage of developing a rotational motion,” says Inoue. With the recently launched James Webb Space Telescope, astronomers now plan to identify the locations of young and older stars in the galaxy to verify and update their galaxy formation scenario.
New discoveries are certainly on the horizon!
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