Using the light from 67 extremely distant quasars, astronomers have calculated
that cosmic dawn, the period in which the first stars began to form, ended 1.1
billion years after the big bang.
We now know when cosmic dawn ended. For a period of roughly 100 million
years in the early universe, starting about 380,000 years after the big
bang, the cosmos was completely dark. Then, stars and galaxies began to
form, emitting light and ionising the intergalactic hydrogen gas in a
process called reionisation, or cosmic dawn. It ended, with all of the
hydrogen ionised, 1.1 billion years after the big bang.
Sarah Bosman at the Max Planck Institute for Astronomy in Germany calculated
this date using the light from 67 quasars – extremely bright objects powered
by supermassive black holes. The quasars were observed using the Very Large
Telescope in Chile and the W. M. Keck Observatory in Hawaii, and all of them
are far enough away that we know they must have formed within about 1
billion years of the big bang.
As the light from the quasars travelled towards Earth, different wavelengths
would have been absorbed by neutral hydrogen and ionised hydrogen. Taking
into account the universe’s constant expansion, Bosman and her team analysed
the dark absorption lines in the light’s spectrum to determine when it
stopped travelling through neutral hydrogen and started encountering only
ionised hydrogen in the space between galaxies instead.
“Reionisation has this bubble-like structure where galaxies clear out these
big bubbles around themselves”, says Bosman. Reionisation isn’t complete
until all of those bubbles merge and the hydrogen gas is ionised across the
sky, at the locations of all of the quasars. “We can tell it’s the end of
reionisation when all the quasars agree – it’s ionised everywhere.”
The date they found was 1.1 billion years after the big bang, which is 200
million years later than previous estimates. That means that the first
generation of stars and galaxies, which drove reionisation, may be closer
and therefore easier to observe than cosmologists thought.
“The history of the universe has gone through many phases between the big
bang and now, and now we’re really starting to trace all these phases,” says
Bosman. “The next step is to go to earlier times and link the reionisation
information to the galaxies that are causing it, so we can really see the
galaxies destroy the gas.”
Reference:
Monthly Notices of the Royal Astronomical Society, in
press,
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Space & Astrophysics