This graphic shows the evolutionary sequence in the growth of massive
elliptical galaxies over 13 billion years, as gleaned from space-based
and ground-based telescopic observations. The growth of this class of
galaxies is quickly driven by rapid star formation and mergers with
other galaxies. Credit: NASA, ESA, S. Toft (Niels Bohr Institute), and A. Feild (STScI)
Astronomers combining the power of the Hubble Space Telescope,
Spitzer and Herschel infrared space telescopes, and ground-based
telescopes have assembled a coherent picture of the formation history
of the most massive galaxies in the universe, from their initial burst
of violent star formation through their appearance as high
stellar-density galaxy cores and to their ultimate destiny as giant
ellipticals.
This solves a decade-long mystery as to how compact elliptical-shaped
galaxies that existed when the universe was only 3 billion years old,
or one-quarter of its current age of 13.8 billion years, already had
completed star formation. These compact ellipticals have now been
definitively linked directly to an earlier population of dusty
starburst galaxies that voraciously used up available gas for star
formation very quickly. Then they grew slowly through merging as the
star formation in them was quenched, and they eventually became giant
elliptical galaxies.
"This is the first time anybody has put together a representative
spectroscopic sample of ultra-compact, burned-out galaxies with the
high quality of infrared imaging of Hubble," said Sune Toft of the Dark
Cosmology Center at the Niels Bohr Institute in Copenhagen.
"We at last show how these compact galaxies can form, how it
happened, and when it happened," Toft added. "This basically is the
missing piece in the understanding of how the most massive galaxies
formed, and how they evolved into the giant ellipticals of today. This
had been a great mystery for many years because just 3 billion years
after the big bang we see that half of the most massive galaxies have
already completed their star formation."
Even more surprising, said Toft, is that these massive, burned-out
galaxies were once extremely compact, compared to similar elliptical
galaxies seen today in the nearby universe. This means that stars had
to be crammed together 10 to 100 times more densely than seen in
galaxies today. "It's comparable to the densities of stars in globular
clusters, but on the larger scale of a galaxy," said Toft.
In tying together an evolutionary sequence for these compact massive
galaxies, Toft identified their progenitors as highly dust-obscured
galaxies undergoing rapid star formation at rates that are thousands of
times faster than in our Milky Way galaxy. Starbursts in these
galaxies are likely ignited when two gas-rich galaxies collided. These
galaxies are so dusty that they are almost invisible at optical
wavelengths, but are bright at submillimeter wavelengths, where they
were first identified nearly two decades ago by the SCUBA
(Submillimeter Common-User Bolometer Array) camera on the James Clerk
Maxwell Telescope in Hawaii.
Toft's team assembled, for the first time, representative samples of
the two galaxy populations using the rich dataset in Hubble's COSMOS
(Cosmic Evolution Survey) program.
They constructed the first representative sample of compact quiescent
galaxies with accurate sizes and distances (spectroscopic redshifts)
measured from the Hubble Space Telescope's CANDELS (Cosmic Assembly
Near-Infrared Deep Extragalactic Legacy Survey) and 3D-HST programs.
3D-HST is a near-infrared Hubble spectroscopic survey to study the
physical processes that shape galaxies in the distant universe. The
astronomers combined these data with observations from the Subaru
telescope in Hawaii and NASA's Spitzer Space Telescope. This allowed
for accurate stellar age estimates, from which they concluded that
galaxies formed in intense starbursts 1 billion to 2 billion years
earlier, in the very early universe.
The team then made the first representative sample of the most
distant submillimeter galaxies using the rich COSMOS data from the
Hubble, Spitzer, and Herschel space telescopes, and ground-based
telescopes such as Subaru, the James Clerk Maxwell Telescope, and the
Submillimeter Array. This multi-spectral information, stretching from
optical light through submillimeter wavelengths, yielded a full suite of
information about the sizes, stellar masses, star-formation rates,
dust content, and precise distances of the dust-enshrouded galaxies
present early on in the universe.
When Toft's team compared the samples of these two galaxy
populations, they discovered a link between the compact elliptical
galaxies and the submillimeter galaxies observed 1 billion to 2 billion
years earlier. The observations show that the violent starburst
activity in the earlier galaxies had the same characteristics that would
have been predicted for progenitors to the compact elliptical
galaxies. The team also calculated that the intense starburst activity
only lasted about 40 million years before the interstellar gas supply
was exhausted.
CONTACT
Donna Weaver / Ray VillardSpace Telescope Science Institute, Baltimore, Md.
410-338-4493 / 410-338-4514
dweaver@stsci.edu / villard@stsci.edu
Sune Toft
Dark Cosmology Center, Niels Bohr Institute, Copenhagen, Denmark
011-45-3532-5908
sune@dark-cosmology.dk
