Early galaxies from the universe's distant past grew up much
faster than astronomers anticipated, according to a new study in the current
issue of Astrophysical Journal.
When the universe was two to three billion years old, or just 20 percent of its current
age, these galaxies had already formed most of their stars.
"We
expected to see that these galaxies had been in the process of star formation,"
study author Pieter van Dokkum of Yale University told SPACE.com.
The finding
is somewhat perplexing to astronomers because they know that the universe is
chock full of stars now (the Milky Way alone has 400 billion) and that very
little star
formation takes place in large galaxies today.
The stars
"had to have formed at some time," said van Dokkum. But nobody knows when.
Based on
the results of this study using the Gemini telescope in Chile, star birth in
these large galaxies must have been "both much earlier and much more intense"
than previously thought, van Dokkum said.
Scientists
think black holes in these early
galaxies' centers may have prevented stellar formation (and are suspected to be
doing the same thing in galaxies today). As these dense masses devour all the
matter around them, they shoot powerful jets of energy out into the surrounding
space. These intense
jets might heat the galaxy's gases, preventing them from condensing and
forming stars - a sort of stellar birth control.
The only
problem is that astronomers don't yet understand how the jets heat the gases.
It's possible the jets aren't responsible at all, that some other energy source
is at play, but astronomers don't have any other theories to work with.
"We don't
understand exactly how this process works," van Dokkum said. "The problem is we
don't have many other candidates."
Previous
studies have implicated black holes both in promoting and preventing stellar
formation. A study
published last month in Nature found evidence that black holes could
be blocking star formation in nearby massive galaxies, which also have few
young stars.
But a study
published in February of last year found evidence of a black hole jet triggering
the collapse of a dense gas cloud, which became a stellar nursery.
"That's
what makes it hard ─ we see evidence for both these things," van Dokkum
said.
According
to van Dokkum, the jets triggering star formation could be a short-term effect,
like a sudden "shock", that later gives way to a net effect of heating,
preventing the gases from condensing.
What
astronomers do know is that something is stifling star formation because it is
natural for galaxies to form stars since "there is a lot of gas available in
the universe," according to van Dokkum. Small galaxies, which lack their large
cousins' black holes, examined in a
recent Hubble image were full of young stars, and many small galaxies today
are still forming them.
Van Dokkum
and his team hope to look back further in time, closer to the Big Bang,
to look at the features of large galaxies then for evidence of star birth. They
also plan to reexamine the galaxies in this study to try and figure out whether
the black holes are in fact acting as "cosmic contraceptives."
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