An international team of astronomers has used a database combining observations from the world’s best telescopes, including the Subaru Telescope, to detect the signal of active supermassive black holes from dying galaxies in the early universe. The appearance of these active supermassive black holes correlates with changes in the host galaxy, suggesting that a black hole could have far-reaching effects on the evolution of its host galaxy.
The Milky Way galaxy includes stars of various ages, including stars still in formation. But in so-called elliptical galaxies, all the stars are old and of roughly the same age. This indicates that early in their history, elliptical galaxies experienced a period of prolific star formation that suddenly ended. Why this star formation ceased in some galaxies but not in others is not well understood. One possibility is that a supermassive black hole is disrupting the gas in some galaxies, creating an environment unsuitable for star formation.
To test this theory, astronomers look at distant galaxies. Due to the finite speed of light, it takes time for light to travel through the vacuum of space. The light we see from an object 10 billion light-years away had to travel 10 billion years to reach Earth. So the light we see today shows what the galaxy looked like when the light left this galaxy 10 billion years ago. So looking at distant galaxies is like looking back in time. But the intermediate distance also means that distant galaxies appear fainter, making it difficult to study.
To overcome these difficulties, an international team led by Kei Ito at SOKENDAI in Japan used the Cosmic Evolution Survey (COSMOS) to sample galaxies 9.5 to 12.5 billion light-years away. COSMOS combines data taken by the world’s largest telescopes, including the Atacama Large Millimeter/submillimeter Array (ALMA) and the Subaru Telescope. COSMOS includes data from radio waves, infrared light, visible light and X-rays.
The team first used optical and infrared data to identify two groups of galaxies: those where star formation is ongoing and those where star formation has stopped. The signal-to-noise ratio of the X-ray and radio wave data was too low to identify individual galaxies. The team therefore combined data from different galaxies to produce higher signal-to-noise ratio images of “average” galaxies. In the averaged images, the team confirmed X-ray and radio emissions for galaxies without star formation.
This is the first time that such emissions have been detected for distant galaxies more than 10 billion light-years away. Additionally, the results show that the X-ray and radio emissions are too strong to be explained by the galaxy’s stars alone, indicating the presence of an active supermassive black hole. This black hole activity signal is weaker for galaxies where star formation is in progress.
These results, published in The Astrophysical Journal, show that an abrupt end to star formation in the early universe is correlated with increased activity of supermassive black holes. Further research is needed to determine the details of the relationship.
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Kei Ito et al, COSMOS2020: Pervasive AGN activity of massive quiescent galaxies at 0 The Astrophysical Journal (2022). DOI: 10.3847/1538-4357/ac5aaf
Provided by the National Astronomical Observatory of Japan
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