An international team of astronomers used a database collating observations from the world’s best telescopes, including the Subaru Telescope, to detect the signal from active supermassive black holes of dying galaxies in the early universe. The presence of these active supermassive black holes correlates with changes in the host galaxy, suggesting that a black hole may have far-reaching effects on the evolution of its host galaxy.
The galaxy we live in contains stars of different ages, including stars still forming. But in some other galaxies, known as elliptical galaxies, all the stars are older and roughly the same age. This indicates that early in their history elliptical galaxies had a period of massive star formation that ended abruptly. Why this star formation stopped in some galaxies, but in others is not well understood. One possibility is that a supermassive black hole disrupts the gas in some galaxies, creating an environment that is 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 void of space. The light we see from an object 10 billion light years away had to travel 10 billion years to reach Earth. Thus the light we see today shows us what the Milky Way looked like 10 billion years ago when the light emanated from that galaxy. So looking at distant galaxies is like looking back in time. But the distance in between also means that distant galaxies look faint, making them difficult to study.
To overcome these difficulties, an international team led by Kei Ito in Sokendai, Japan used the Cosmic Evolution Survey (COSMOS) to sample galaxies 9.5–12.5 billion light-years away. COSMOS combines data captured by the world’s leading telescopes, including the Atacama Large Millimeter/submillimeter Array (ALMA) and the Subaru Telescope. COSMOS includes radio wave, infrared light, visible light and X-ray data.
The team previously used optical and infrared data to identify two clusters of galaxies: those with ongoing star formation and those where star formation has stopped. The X-ray and radio wave data signal-to-noise ratio was too weak to identify individual galaxies. So the team combined data from different galaxies to generate high signal to noise ratio images of “average” galaxies. In the averaged images, the team confirmed both X-ray and radio emissions for galaxies without star formation. This is the first time such an emission has been detected for galaxies more than 10 billion light years away. Furthermore, the results suggest that the X-ray and radio emission can be explained by stars in the Milky Way alone, indicating the presence of an active supermassive black hole. This black hole activity signal is weak for galaxies where star formation is ongoing.
These results suggest that the sudden end in star formation in the early universe is related to increased supermassive black hole activity. More research is needed to determine the details of the relationship.
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