天文學家利用詹姆斯韋伯太空望遠鏡發現了迄今為止所見最遙遠的靜止星系——該星系在宇宙大爆炸後僅 7 億年就停止了恆星形成。這對現有的星系演化模型提出了挑戰，因為這些模型無法解釋如此巨大的“紅色死亡”星系如何如此早地形成。

The galaxy, named RUBIES-UDS-QG-z650, squeezes more than 0 billion solar masses into a space only 0 light-years wide, suggesting that the dense cores of today's giant elliptical galaxies may have formed very early.

In the early universe, galaxies grew by absorbing gas from the surrounding intergalactic medium and converting those gases into new stars. As the mass of galaxies increases, it can attract gas more efficiently, accelerating star formation. But this growth won't last forever. Eventually, galaxies go through a process called "quenching," in which they stop forming stars and actually stop growing.

Today, astronomers have discovered that about half of the galaxies in the universe no longer form stars. These galaxies are known as stationary galaxies, extinguished galaxies, or "red dead galaxies". They appear red because they have depleted their young, hot, blue stars, leaving only older, cooler, reddish stars.

The three spectra taken by JWST/NIRSpec are superimposed on the images taken by JWST/NIRCam, two instruments on the James Webb Space Telescope. Record galaxies are shown in the middle. It appears red in the image and its spectrum decreases to the left (short wavelength). For comparison, the top and bottom spectra (blue and purple) show typical star-forming galaxies from similar periods in cosmic history. Sources: NASA/CSA/ESA, A. Weibel, PA Oesch (University of Geneva), RUBIES Team: A. Murphy de Graaff (MPIA Heidelberg), G. Brammer (Niels Bohr Institute), DAWN JWST Archives

Stationary galaxies are especially common in the most massive galaxies, which are usually elliptical in shape. These galaxies usually take a long time to form, first accumulating a large number of stellar clusters and then star formation stops. However, what exactly caused the extinction of galaxies remains one of the biggest unanswered questions in astrophysics.

"The search for the first massive stationary galaxies (MQGs) in the early universe is crucial because it reveals the mechanisms by which they may have formed," said Pascal Oesch, associate professor in the Department of Astronomy at the Faculty of Sciences at the University of Geneva and co-author of the paper. Therefore, the search for such a system has been the main goal of astronomers for many years.

隨著技術的進步，尤其是近紅外光譜技術的發展，天文學家在越來越早的宇宙時代證實了大質量靜止星系 (MQG)。它們推斷的豐度一直難以與星系形成的理論模型相協調，因為理論模型預測此類系統需要更長的時間才能形成。借助詹姆斯韋伯太空望遠鏡 (JWST)，這種緊張關係被推至紅移 5（大爆炸後 12 億年），近年來已證實了幾個 MQG。日內瓦大學領導的新研究表明，這些星系的形成時間比之前認為的更早、更快。

In JWST Cycle 2, the Wide Area Project RUBIES (Red Unknowns: Bright Infrared Extragalactic Survey), one of the largest European-led projects for extragalactic research using the NIRSpec instrument, has acquired spectral observations of thousands of galaxies, including hundreds of newly discovered sources from earlier JWST imaging data.

詹姆斯·韋伯太空望遠鏡 (JWST) 徹底改變了我們對早期宇宙的看法，揭示了巨大的星系在宇宙大爆炸後僅 7 億年就已停止形成恆星——比預測的要早得多。其強大的紅外能力使天文學家能夠在創紀錄的距離探測和研究這些意外“死亡”的星系。圖片來源：諾斯羅普·格魯曼

在這些新光譜中，科學家發現了迄今為止發現的最遙遠的 MQG，其光譜紅移為 7.29，距離大爆炸僅約 7 億年。NIRSpec/PRISM 光譜揭示了如此年輕的宇宙中恆星群的驚人古老。光譜和圖像數據的詳細建模表明，該星系在大爆炸後的前 6 億年內形成了超過 100 億（10 10）太陽品質的恆星，然後迅速停止恆星形成，從而證實了其靜止性質。

“這個名為 RUBIES-UDS-QG-z7 的星系的發現意味著，宇宙誕生後的前 10 億年中，巨大的靜止星系數量比迄今為止任何模型預測的數量多 100 多倍，”日內瓦大學理學院天文系博士生、論文第一作者 Andrea Weibel 說道。這反過來又表明，理論模型中的關鍵因素（例如恆星風的影響，以及由恆星形成和巨大黑洞驅動的流出強度）可能需要重新審視。星系的消亡比這些模型預測的要早得多。

Finally, the physical size of RUBIES-UDS-QG-z5 is small, only about 0 light-years, which means that the stellar mass density is high, comparable to the highest central density observed in stationary galaxies (z ~0-0) with slightly lower redshifts. These galaxies are likely to evolve into the nucleus of the oldest and most massive elliptical galaxies in the local universe.

"For the first time, the discovery of RUBIES-UDS-QG-z7 provides strong evidence that the centers of some nearby massive elliptical galaxies may have existed as early as the first few hundred million years of the universe," concludes Anna de Graaff, principal investigator of the RUBIES project, postdoctoral researcher at the Max Planck Institute for Astronomy in Heidelberg, and second author of the paper.

編譯自/ScitechDaily