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Capturer le début de la rotation des galaxies dans l’univers primordial

Les astronomes détectent la rotation de galaxie la plus éloignée de tous les temps, suggérant une étape initiale de développement du mouvement de rotation.

Les astronomes ont pu détecter de plus en plus de galaxies lointaines à mesure que les télescopes sont devenus plus avancés et plus puissants. Étant donné que la lumière se déplace à une vitesse finie et que nous voyons les objets tels qu’ils étaient lorsque la lumière a été émise, plus quelque chose est éloigné, plus nous voyons loin dans le temps. Par conséquent, ces galaxies très éloignées sont parmi les premières galaxies à se former dans notre univers, qui ont commencé à s’éloigner de nous à mesure que l’univers s’étendait.

En fait, plus la distance est grande, plus une galaxie semble s’éloigner rapidement de nous. Fait intéressant, nous pouvons estimer à quelle vitesse une galaxie se déplace, et à son tour, quand elle s’est formée en fonction de la façon dont son émission apparaît “décalée vers le rouge”. Ce décalage vers le rouge est similaire à un phénomène appelé “effet Doppler”, où les objets s’éloignant d’un observateur émettent la lumière qui semble décalée vers des longueurs d’onde plus longues (d’où le terme “décalage vers le rouge”) à l’observateur.

Situé au milieu du désert d’Atacama au Chili, l’Atacama Large Millimeter/submillimeter Array ([{” attribute=””>ALMA) telescope is especially well-suited for observing such redshifts in galaxy emissions. Recently, an international research team observed redshifted emissions of a distant galaxy, MACS1149-JD1 (hereafter JD1), which has led them to some fascinating 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 team of international researchers included Professor Akio Inoue and graduate 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 research fellow at the University of Cambridge, UK. The findings of their study have been published in The Astrophysical Journal Letters.

Rotation Signature of Distant Galaxy Hints at Early Formation Epoch

After the Big Bang came the earliest galaxies. Due to the expansion of the universe, these galaxies are receding away from us. This causes their emissions to be redshifted (shifted towards longer wavelengths). By studying these redshifts, it is possible to characterize the “motion” within the galaxies as well as their distance. In a new study, astronomers at Waseda University have now revealed a likely rotational motion of one such distant galaxy. Credit: Waseda University

Galaxy formation starts with the accumulation of gas and proceeds with the formation of stars from that gas. Over 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. It is possible to determine the stage of evolution the galaxy has reached by studying the age of the stellar objects and the motion of the stars and gas in the galaxy.

After 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. They found that JD1 satisfied the criterion for a galaxy dominated by rotation. Next, the scientists modeled the galaxy as a rotating disk and discovered that it reproduced the observations very well. The calculated rotational speed was about 50 kilometers per second (110,000 miles per hour), which was compared to the rotational speed of the Milky Way disk of 220 kilometers per second (500,000 miles per hour). 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!

Reference: “Black Hole to Photosphere: 3D GRMHD Simulations of Collapsars Reveal Wobbling and Hybrid Composition Jets” by Ore Gottlieb, Matthew Liska, Alexander Tchekhovskoy, Omer Bromberg, Aretaios Lalakos, Dimitrios Giannios and Philipp Mösta, 29 June 2022, The Astrophysical Journal Letters.
DOI: 10.3847/2041-8213/ac7530

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