17 March 2010

Astronomers observe growing black holes

The most distant quasars found in the early universe, a mere 800 million years after the Big Bang, have been observed by an international team of astronomers including members from the Niels Bohr Institute at the University of Copenhagen. The findings have been published in the prestigious scientific journal Nature.

Illustration of quasar

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 got 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.

Quasars are active and very powerful black holes at the centre of distant galaxies. 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 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.

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 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”, 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.

Sky of stars

The universe is one big recycling plant.
The Big Bang only formed hydrogen and
helium – a kind of ‘virgin-gas’ of the
universe. Stars are formed from gas that
is concentrated into a large ball of hot
gas, where hydrogen atoms fuse together
into heavier elements. When the fuel is
used up, the star explodes and hurls giant dust and gas clouds out into the universe,
where they become new stars. But with
each generation more heavy elements
are built up. By analysing the composition researchers can calculate how far along
the galaxies are in their cosmic evolution.

Witnesses to growth of the first black holes

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 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 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 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”.

Article in Nature >>

Press release NASA JPL >>