The oldest disk-shaped galaxy
ever spotted formed just 1.5 billion years after the Big Bang, a new study
That’s much earlier than astronomers
thought that this type of galaxy could form. Previous observations show that
disk-shaped galaxies — including sprawling, spiral systems like the Milky Way —
didn’t show up in large numbers until between 3 and 4 billion years after the
Big Bang, which occurred about 13.8 billion years ago.
This precocious galaxy’s
existence suggests that massive spiral galaxies like the Milky Way can grow up relatively quickly, astronomers report in the May 21 Nature.
By showing that a disk
galaxy could form so early in the universe’s history, the new study “challenges
the accepted paradigm for how disk galaxies form and evolve in the universe,”
says astrophysicist Rachel Somerville of the Flatiron Institute in New York
City, who was not involved in the study.
The earliest galaxies
probably were made when clumps of invisible dark matter pulled in surrounding gas and dust, forming stars and
eventually creating galaxies that were round and blobby, observations and
computer simulations suggest (SN: 1/26/18). Theorists reason that assembling
those early galaxies was a violent process that scrambled and heated gas. Since
hot gas expands, the idea goes, the first galaxies were spherical blobs because
they were too hot for the gas to settle into a disk. Only when gas has had lots
and lots of time to cool off could it collapse into bright starry disk galaxies (SN: 9/3/18), researchers thought.
In the last 15 years, however,
computer simulations have showed that cold streams of gas could sneak into ancient,
blobby galaxies, potentially making it easier for disk galaxies to arise more
To see if that process,
called the cold accretion method, actually occurs in the universe, astronomer
Marcel Neeleman and colleagues sought the earliest disk galaxies they could
find. Most early galaxies are too far away and thus too faint for Earth-based
telescopes to catch light from their stars. But sensitive radio telescopes can detect
light from even more distant quasars — blazing, white-hot disks surrounding
supermassive black holes — filtering through the galaxies’ gas (SN: 7/12/18). A bright quasar behind the
early disk galaxy, called DLA0817g, let it show up in silhouette, revealing the
galaxy’s contents and structure.
Neeleman, of the Max Planck
Institute for Astronomy in Heidelberg, Germany, and his colleagues first saw hints of DLA0817g using the Atacama Large Millimeter/submillimeter
Array in Chile, the team reported in 2017.
Follow-up observations in 2019 showed that the galaxy is rotating like a record:
Half of the galaxy’s gas is moving away from Earth, and half is moving toward us.
That motion is a sure sign that the galaxy is a cold, flat rotating disk, and
likely has a spiral shape, the scientists say.
The galaxy is also massive,
at least 72 billion times the mass of the sun. Behemoth galaxies have been spotted in the early universe before (SN: 8/7/19). Growing
a massive galaxy of any shape so quickly “is challenging enough,” says astronomer
and study coauthor J. Xavier Prochaska of the University of California, Santa
Cruz. “But the shocker is to see one in a nice spiral disk.”
The team unofficially named
the galaxy the Wolfe Disk after astrophysicist Arthur Wolfe of the University of
California, San Diego, who died in 2014. Wolfe was one of the first to suggest that disk galaxies existed in
the universe’s infancy, to widespread skepticism, says Prochaska, who was one
of Wolfe’s Ph.D. students.
“He was right, at least partially,” Prochaska says. “He deserves credit for having planted that flag against all conventional wisdom.”