Quasars（类星体） are active and very powerful black holes at the centre of distant galaxies¹. The black holes are extremely massive weighing between 100 million and 10 billion solar masses and rotating around the super massive black hole 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（摩擦） each other, heating it and causing it to emit（发出，放射） light. Near to the black hole it becomes so hot that X-ray radiation is emitted, farther out ultraviolet light（紫外线） is emitted, then visible light and farthest out infrared² radiation（红外线照射） . 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.

Enormous black holes

"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（重力，引力） 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 galaxy³, so the quasar can grow into some of the most massive objects known in the universe, second to galaxies", explains Marianne Vestergaard.

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 US and Germany she 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 Marianne Vestergaard.

Witnesses to the growth of the first black holes

In two of the most distant quasars there are indications that they reside in（居住，存在于） 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 devour⁴（毁灭，凝视） gas at great haste, while the quasars with heavier black holes have more hot dust at the centre. 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 astronomers⁵ 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.

"I appears we have found what are likely primitive⁶（原始的，远古的） 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 Marianne 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".