As telescopes became more advanced and powerful, astronomers were able to discover more and more distant galaxies. These are some of the earliest galaxies to form in our universe, which began moving away from us as the universe expanded. The greater the distance, the faster a galaxy appears to be receding from us. Interestingly, we can estimate how fast a galaxy is moving and when it formed based on how “redshifted” its emission appears. This is similar to a phenomenon called the “Doppler effect,” where objects moving away from an observer emit light that appears shifted to longer wavelengths (hence the term “redshift”).
The Atacama Large Millimeter/submillimeter Array (ALMA) telescope in the middle of the Atacama Desert in Chile is particularly well suited to 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 from the University of Tsukuba, Japan, Professor Richard S. Ellis from University College London and Dr. Nicolas Laporte, a research fellow at the University of Cambridge, UK, has observed redshift emissions from a distant galaxy, MACS1149-JD1 (hereafter JD1), which has led her to some interesting conclusions.
“In addition to finding high redshift, namely very distant galaxies, studying their inner motion of gas and stars is a motivation to understand the process of galaxy formation in the earliest possible universe,” explains Ellis. The results of their study were published in The Letters of the Astrophysical Journal.
Galaxy formation begins with the accumulation of gas and continues with the formation of stars from that gas. Over time, star formation progresses outward from the center, a galactic disc develops, and the galaxy takes on a specific shape. As star formation continues, newer stars form in the rotating disk while older stars remain in the central portion. By studying the age of stellar objects and the movement of stars and gas in the galaxy, it is possible to determine the evolutionary stage of the galaxy.
In a series of observations over a two-month period, the astronomers successfully measured small differences in “redshift” from position to position within the galaxy and found that JD1 meets the criterion for a rotationally dominated galaxy. Next, they modeled the galaxy as a rotating disk and found that it reproduced the observations very well. The calculated rotation speed was about 50 kilometers per second, which was compared to the rotation speed of the Milky Way disk of 220 kilometers per second. The team also measured JD1’s diameter at just 3,000 light-years, much smaller than that of the Milky Way at 100,000 light-years across.
The significance of their finding is that JD1 is by far the most distant, and therefore earliest, source found to date showing a rotating disk of gas and stars. This, combined with similar measurements of closer systems in the research literature, has allowed the team to describe the gradual evolution of rotating galaxies over more than 95% of our cosmic history.
In addition, the mass estimated from the galaxy’s spin rate was consistent with the stellar mass previously estimated from the galaxy’s spectral signature and came mostly from “mature” stars that formed about 300 million years ago. “This shows that the stellar population in JD1 formed at an even earlier epoch in the cosmic age,” says Hashimoto.
“The spin rate of JD1 is much slower than in galaxies of later epochs and in our galaxy, and it is likely that JD1 is in an early stage of developing a spin motion,” says Inoue. Using 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!
Apr 9, 2022