June 21, 2017
Compact, Disk Galaxy
New Clues to How Modern-Day
Astronomers combined the power of a "natural lens"
in space with the capability of NASA's Hubble Space
Telescope to make a surprising discovery - the first
example of a compact yet massive, fast-spinning,
disk-shaped galaxy that stopped making stars only a
few billion years after the big bang.
Researchers say that finding such a galaxy so early
in the history of the universe challenges the
current understanding of how massive galaxies form
Astronomers expected to see a chaotic ball of stars
formed through galaxies merging together.
they saw evidence that the stars were born in a
The galaxy, called MACS 2129-1, is considered "dead"
because it is no longer making stars.
This new insight is forcing astronomers to rethink
their theories of how galaxies burn out early on and
evolve into local elliptical-shaped galaxies.
"Perhaps we have been blind to the fact that
early 'dead' galaxies could in fact be disks,
simply because we haven't been able to resolve
them," said study leader Sune Toft of the Dark
Cosmology Center at the Niels Bohr Institute,
University of Copenhagen.
By combining the power of a "natural lens" in space with the
capability of NASA's
Hubble Space Telescope, astronomers made a
surprising discovery - the first example of a compact yet massive,
fast-spinning, disk-shaped galaxy that stopped making stars only a
few billion years after
Finding such a galaxy early in the history of the universe
challenges the current understanding of how massive galaxies form
and evolve, say researchers.
When Hubble photographed the galaxy, astronomers expected to see a
chaotic ball of stars formed through galaxies merging together.
Instead, they saw evidence that the stars were born in a
This is the first direct observational evidence that at least some
of the earliest so-called "dead" galaxies - where star formation
stopped - somehow evolve from a Milky Way-shaped disk into the giant
elliptical galaxies we see today.
This is a surprise because elliptical galaxies contain older stars,
while spiral galaxies typically contain younger blue stars. At least
some of these early "dead" disk galaxies must have gone through
They not only changed
their structure, but also the motions of their stars to make a shape
of an elliptical galaxy.
"This new insight may
force us to rethink the whole cosmological context of how
galaxies burn out early on and evolve into local
elliptical-shaped galaxies," said study leader Sune Toft of the
Dark Cosmology Center at the Niels Bohr Institute, University of
"Perhaps we have been
blind to the fact that early "dead" galaxies could in fact be
disks, simply because we haven't been able to resolve them."
Previous studies of
distant dead galaxies have assumed that their structure is similar
to the local elliptical galaxies they will evolve into.
assumption in principle requires more powerful space telescopes than
are currently available. However, through the phenomenon known as
"gravitational lensing," a massive, foreground cluster of galaxies
acts as a natural "zoom lens" in space by magnifying and stretching
images of far more distant background galaxies.
By joining this natural
lens with the resolving power of Hubble, scientists were able to see
into the center of the dead galaxy.
Acting as a "natural telescope" in space,
the gravity of the extremely massive foreground galaxy cluster MACS
magnifies, brightens, and distorts the far-distant background galaxy
shown in the top box.
The middle box is a blown-up view of the gravitationally lensed
In the bottom box is a reconstructed image, based on modeling,
that shows what the galaxy would look like if the galaxy cluster
were not present.
The galaxy appears red because it is so distant
that its light is shifted into the red part of the spectrum.
The remote galaxy is three times as massive as the Milky Way but
only half the size. Rotational velocity measurements made with the
European Southern Observatory's Very Large Telescope (VLT)
showed that the disk galaxy is spinning more than twice as fast as
the Milky Way.
Using archival data from the Cluster Lensing And Supernova survey
with Hubble (CLASH), Sune Toft and his team were able to
determine the stellar mass, star-formation rate, and the ages of the
Why this galaxy stopped forming stars is still unknown. It may be
the result of an active galactic nucleus, where energy is gushing
from a supermassive black hole.
This energy inhibits star
formation by heating the gas or expelling it from the galaxy. Or it
may be the result of the cold gas streaming onto the galaxy being
rapidly compressed and heated up, preventing it from cooling down
into star-forming clouds in the galaxy's center.
But how do these young, massive, compact disks evolve into the
elliptical galaxies we see in the present-day universe?
mergers," Toft said.
"If these galaxies
grow through merging with minor companions, and these minor
companions come in large numbers and from all sorts of different
angles onto the galaxy, this would eventually randomize the
orbits of stars in the galaxies.
You could also
imagine major mergers. This would definitely also destroy the
ordered motion of the stars."
The findings (A
Massive, Dead Disk Galaxy in the Early Universe) are
published in the June 22 issue of the journal Nature.
Toft and his team hope to
use NASA's upcoming James Webb Space Telescope (JWST) to look for a larger
sample of such galaxies.
The Hubble Space
Telescope is a project of international cooperation between NASA and
ESA (European Space Agency). NASA's Goddard Space Flight Center in
Greenbelt, Maryland, manages the telescope.
The Space Telescope
Science Institute (STScI) in Baltimore, Maryland, conducts Hubble
STScI is operated for NASA by the Association of
Universities for Research in Astronomy, Inc., in Washington, D.C.
The Very Large Telescope is a telescope facility operated by the
European Southern Observatory on Cerro Paranal in the Atacama Desert
of Northern Chile.
ESA, and S. Toft (University of Copenhagen)
NASA, ESA, M. Postman (STScI),
and the CLASH team
Ann Jenkins / Ray
Space Telescope Science Institute, Baltimore, Maryland
410-338-4488 / 410-338-4514
Dark Cosmology Center, Niels Bohr Institute,
University of Copenhagen, Copenhagen, Denmark