Massive
red, dead galaxy spotted in young universe
A hefty red, dead galaxy in the early universe appears to have bulked
up a bit too fast.
The galaxy, seen as it was when the
universe was only 1.65 billion years old, weighs at least three times as much
as the Milky Way, but has stopped making stars. Other galaxies at the time tend
to be much smaller and continue to churn out stars. How such a monster was made
in less than a billion years, then shut down so quickly isn’t clear, says Karl
Glazebrook of Swinburne University of Technology in Australia. Finding the
behemoth and possibly others like it may mean astronomers will have to
rethink how galaxies are built to explain why some grow up fast, while
others develop slowly, he and colleagues report in the April 6 Nature.
“The team has found an extreme
galaxy, which is exciting,” says Peter Behroozi of the University of
California, Berkeley, who was not involved in the study. The data, he says,
offer evidence for a population of inactive galaxies early in the universe, which
are extremely difficult to observe. Behroozi, however, is not convinced that
the discovery warrants a rewrite of the story of galaxy formation just yet.
“The galaxy is certainly not typical, but it is consistent with the broad
diversity of galaxies coming out of theoretical models,” he says.
Glazebrook and colleagues used the
Keck Observatory in Hawaii to study the spectrum — a catalog of light by
wavelength — of the galaxy, called ZF-COSMOS-20115. The data confirmed that the
galaxy did, in fact, exist in the early universe and revealed that it had
stopped making stars 500 million to a billion years before its age when
observed. The data also indicate the galaxy created the mass of more than a
1,000 suns each year. Typical galaxies at that time generate a mass of less
than 100 suns annually. With such a rapid star formation rate, the galaxy
probably grew to its monster mass in less than 100 million years, the
researchers say.
Perhaps the galaxy got so massive
by colliding and merging with another galaxy. Such a major merger can stir up
and compress gas to high densities, which can trigger intense star formation.
These events are typically short bursts, lasting less than 100 million years,
compared with the typically billion-year timescales for star formation in
normal galaxies. In these merger galaxies, gas gets consumed quickly because
stars are forming fast. If the galaxy runs out of gas, star formation stops.
Still, if this process happens and these massive galaxies exist, they are
expected to be rare, says astronomer Dominik Riechers of Cornell University,
who was not involved in the new study.
Such hefty galaxies should be rare
because galaxies form as matter pools together and collapses due to gravity
over cosmic time. When the universe was still young, very few regions of space
would have had deep enough wells of gravity to form massive galaxies like this
one, Riechers explains.
Glazebrook and colleagues’ earlier
work, however, hints that ZF-COSMOS-20115 may not be such an oddity. In 2014,
the team reported finding a bunch of dim, red objects, including
ZF-COSMOS-20115, which fit the profile of massive, red and dead galaxies in the
early universe (SN Online: 3/14/14). Similar studies of the other galaxies’
spectra could confirm if all of those candidates are inactive, too. If so,
astronomers will have to figure out how larger numbers of deep gravity wells
could develop to allow lots of galaxies to grow, merge and become red and dead
just a billion years after the Big Bang, 13.8 billion years ago.