Fast-growing Primitive Black Holes found in Distant Quasars
|Quasars were discovered in the beginning of the 1960s as distant sources that emitted electromagnetic energy and radio waves. It was thought that it was a special type of star that emitted radio radiation, so it was given the name quasar. The word quasar is a contraction of quasi-stellar and means star-like. But, their light is very different from ordinary stars, and it was first in the beginning of the 1980s that it was discovered that they were heavy black holes in the middle of a galaxy in the very early universe. Courtesy of NASA|
Quasars are active and very powerful black holes at the center of distant galaxies. The black holes are extremely massive, weighing between 100 million and 10 billion solar masses, and rotating around these super massive black holes is a disc of gas and dust. The inner ring of the disc moves faster than the outer rings.
The movement causes the material in the rings to rub against each other, heating it and causing it to emit light. Near to the black hole, it becomes so hot that X-ray radiation is emitted, farther out ultraviolet light is emitted, then visible light and, farthest out, infrared radiation. Even though the radiation is coming from a very small area the size of our solar system, it is so powerful that it can be seen across the universe.
“Quasars are a very early stage of galaxies, a sort of baby galaxies,” explains Marianne Vestergaard, astrophysicist at the Dark Cosmology Centre at the Niels Bohr Institute at the University of Copenhagen. “Most galaxies have a massive black hole with a mass of over a million solar masses, but quasars are different. Their black holes are active and growing. The gravity of the black hole gradually pulls the surrounding material of gas and dust into the black hole, which consequently slowly grows heavier and larger. The disc of gas and dust is also resupplied with material from the galaxy, so the quasar can grow into some of the most massive objects known in the universe, second to galaxies.”
But why do quasars always resemble each other? The researchers had wondered about this and their new research revealed something else. Together with colleagues from the U.S. and Germany, Vestergaard observed 21 quasars in the distant universe, when it was only 800 million years old (it is currently 13.7 billion years old).
“With our new observations, we could see that many of them had lots of hot dust, but two quasars did not — they had no sign of hot dust, so we wondered why,” explains Vestergaard.
In two of the most distant quasars, there are indications that they reside in very young galaxy systems with less dust, where the black hole is growing rapidly. For these distant black holes, the researchers see that the quasars without hot dust have small black holes, which devour gas at great haste, while the quasars with heavier black holes have more hot dust at the center. A possible explanation is that the black holes grow in step with the formation of stars in the galaxy. An increasing number of stars give an increasing amount of dust.
The farther away in the universe one looks, the farther back in time one sees, and astronomers are hunting for galaxies where they can study the formation of the first dust in the universe. Dust has a great impact on how and how early in the history of the universe stars were formed.
“It appears we have found what are likely primitive first-generation quasars, that are born in a dust-free medium shortly after Big Bang and are now seen at somewhat stages of evolution. With these quasars showing both rapid growth in both the black hole and the amount of dust, we may have found the young galaxy systems we have long been looking for,” explains Vestergaard and adds that “it is fantastic that we are witness to this building up of black holes and the amount of heavier elements in the form of dust.”