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Early universe supermassive black holes in dying galaxies

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The COSMOS survey region surrounded by images of galaxies used in this study. In these galaxies, star formation ceased around 10 billion years ago. (3-color false-color composite images combining data from the Subaru Telescope and VISTA). Credit: NAOJ

An international team of astronomers used data from the best telescopes in the world, including the Subaru Telescope, to find the signal from active supermassive black holes in dying galaxies in the early universe. Changes in the host galaxy are linked to the appearance of these active supermassive black holes. This suggests that a black hole could have a big impact on how its host galaxy changes over time.

There are stars of all ages in the Milky Way galaxy, including stars that are still forming. In elliptical galaxies, on the other hand, all of the stars are old and about the same age. This means that early in their lives, elliptical galaxies had a time when they made a lot of stars and then stopped. It is not clear why some galaxies stopped making stars but others did not. One possibility is that a supermassive black hole messes up the gas in some galaxies, making it hard for stars to form.

Astronomers look at galaxies far away to see if this theory is true. Due to the fact that light can only move at a certain speed, it takes time for light to travel through space. The light we see from an object 10 billion light-years away took 10 billion years to get to Earth. So, the light we see today shows us what the galaxy looked like 10 billion years ago, when the light left the galaxy. So looking at distant galaxies is like looking back in time. But the distance between them also makes faraway galaxies look dimmer, which makes it hard to study them.

To get around these problems, an international team led by Kei Ito at SOKENDAI in Japan used the Cosmic Evolution Survey (COSMOS) to look at galaxies 9.5–12.5 billion light-years away. COSMOS uses information from the Atacama Large Millimeter/submillimeter Array (ALMA) and the Subaru Telescope, two of the best telescopes in the world. COSMOS contains information about radio waves, infrared light, visible light, and X-rays.

The team first used optical and infrared data to separate galaxies into two groups: those where new stars are still being made and those where new stars are no longer being made. The signal-to-noise ratio of the X-ray and radio wave data was too low to be able to pick out individual galaxies. So, the team put together the data from different galaxies to make images of “average” galaxies with a better signal-to-noise ratio. In the averaged images, the team saw that the galaxies without new stars were giving off both X-rays and radio waves.

This is the first time that these kinds of emissions have been found in galaxies that are more than 10 billion light-years away. The results also show that the X-ray and radio emissions are too strong to be caused by the stars in the galaxy alone. This shows that there is an active supermassive black hole in the galaxy. This signal of black hole activity is weaker in galaxies where stars are still being made.

These results, which were published in The Astrophysical Journal, show that when star formation stopped suddenly in the early universe, supermassive black hole activity went up. We need to do more research to figure out the details of the relationship.

Further information: Kei Ito et al, COSMOS2020: Ubiquitous AGN Activity of Massive Quiescent Galaxies at 0 < z < 5 Revealed by X-Ray and Radio Stacking, The Astrophysical Journal (2022). DOI: 10.3847/1538-4357/ac5aaf

Journal information: Astrophysical Journal

Source: National Astronomical Observatory of Japan

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