Young, Dead, Compact, Disk Galaxy Surprises Astronomers, Offers New Clues to How Modern-Day Elliptical Galaxies Formed
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 the big bang.
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 pancake-shaped disk.
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 major
makeovers. 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 Copenhagen,
Denmark. "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. Confirming this 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.
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), Toft and his team were able to determine the
stellar mass, star-formation rate, and the ages of the stars.
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? "Probably
through 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 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 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 science operations. 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.
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Ann Jenkins / Ray Villard
Space Telescope Science Institute, Baltimore, Maryland
410-338-4488 / 410-338-4514
jenkins@stsci.edu / villard@stsci.edu
Sune Toft
Dark Cosmology Center, Niels Bohr Institute,
University of Copenhagen, Copenhagen, Denmark
sune@dark-cosmology.dk
