Measuring dark halos around ancient quasars

At the center of every galaxy is a supermassive black hole. Beyond a certain size, these become active, emitting large amounts of radiation, and are called quasars. It is thought that these are activated by the presence of massive dark matter (DMH) around the galaxy, directing matter to the center, feeding the black hole.

A team consisting of researchers from the University of Tokyo have, for the first time, examined hundreds of ancient quasars and found that this behavior is very consistent throughout history. This is surprising, as many large systems show variation in the life of the universe, so how a quasar works can have an impact on the evolution of the universe. Their research is published in the The Astrophysical Journal.

Measuring the severity of DMHs is not easy; it is a famously rare substance, if substance is the right word to use, given the nature of the dark universe is unknown. We only know that it exists because of its gravitational influence on massive structures such as galaxies. Thus, dark matter can only be measured by considering its gravitational effects on matter. This includes how it can attract an object or affect its movement, or by using lensing (bending of light) of objects behind a suspected dark spot.

The challenge becomes greater at longer distances, given how weak light from distant, and therefore ancient, events can be. But this did not stop Professor Nobunari Kashikawa from the Department of Astronomy, and his team, from trying to answer an old question in astronomy: How are black holes born, and how do they grow?

Researchers are particularly interested in exploring this in relation to supermassive black holes, the supermassive ones, that exist at the heart of every galaxy. This would be very difficult to study if it weren’t for the fact that some grow so large that they start producing incredibly powerful jets of matter or particles of radiation that in any case become what we call quasars. These are so powerful that even over long distances, we can now see them using modern technology.

“We measured for the first time the average mass of dark matter around an active black hole in the universe about 13 billion years ago,” Kashikawa said.

“We find that the DMH mass of quasars remains the same at about 10 billion times the mass of our sun. Such measurements have been made for recent DMH quasars, and those measurements are remarkably similar to what we see in ancient quasars. It is interesting because it suggests that there is a visible DMH mass. a quasar is active, whether it happened billions of years ago or now.”

Quasars at long distances appear faint, as the light that left them long ago was scattered, absorbed by intervening matter, and stretched to almost invisible infrared wavelengths due to the expansion of the universe over time.

So Kashikawa and his team, whose project started in 2016, used a lot of sky research involving a range of different instruments, the main one being the Subaru Telescope of Japan, located in the US state of Hawaii.

“The improvement has allowed Subaru to see further than before, but we can learn more by expanding the project’s international outlook,” Kashikawa said.

“The US-based Vera C. Rubin Observatory and the space-based Euclid satellite, launched by the EU in 2023, will search a large area of ​​the sky and find more DMH about quasars. We can build a complete picture of the relationship between galaxies and and supermassive black holes. That could help inform our theories about how black holes form and grow.