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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.
Image Type: Illustration
Science Credit: NASA, ESA, S. Toft (Niels Bohr Institute), V. Smolcic (University of Zagreb), B. Magnelli (Argelander Institute for Astronomy), A. Karim (Argelander Institute for Astronomy and Durham University), A. Zirm (Niels Bohr Institute), M. Michalowski (University of Edinburgh and Universiteit Gent), P. Capak (California Institute of Technology), K. Sheth (National Radio Astronomy Observatory), K. Schawinski (ETH Zurich), J.-K. Krogager (Niels Bohr Institute and European Southern Observatory), S. Wuyts (Max Planck Institute for Extraterrestrial Physics), D. Sanders (University of Hawaii), A. Man (Niels Bohr Institute), D. Lutz (Max Planck Institute for Extraterrestrial Physics), J. Staguhn (NASA Goddard Space Flight Center and Johns Hopkins University), S. Berta (Max Planck Institute for Extraterrestrial Physics), H. McCracken (Institut d’Astrophysique de Paris), J. Krpan (University of Zagreb), D. Riechers (Cornell University and California Institute of Technology), and G. Brammer (European Southern Observatory and STScI)
Astronomers using NASA's Hubble and Spitzer space telescopes and Europe's
Herschel Space Observatory have pieced together the evolutionary sequence of
compact elliptical galaxies that erupted and burned out early in the history of
the universe.
Enabled by Hubble's infrared imaging capabilities, astronomers have assembled
for the first time a representative spectroscopic sampling of ultra-compact,
burned-out elliptical galaxies -- galaxies whose star formation was finished
when the universe was only 3 billion years old, less than a quarter of its
current estimated age of 13.8 billion years.
The research, supported by several ground-based telescopes, solves a
10-year-old mystery about the growth of the most massive elliptical galaxies we
see today. It provides a clear picture of the formation of the most massive
galaxies in the universe, from their initial burst of star formation through
their development of dense stellar cores to their ultimate reality as giant
ellipticals.
"We at last show how these compact galaxies can form, how it happened, and
when it happened. 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," said Sune Toft of the Dark Cosmology Center at the Niels
Bohr Institute in Copenhagen, who is the leader of this study. "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."
Through the research, astronomers have determined the compact ellipticals
voraciously consumed the gas available for star formation, to the point they
could not create new stars, and then merged with smaller galaxies to form giant
ellipticals. The stars in the burned-out galaxies were packed 10 to 100 times
more densely than in equally massive elliptical galaxies seen in the nearby
universe today, and that surprised astronomers, according to Toft.
To develop the evolutionary sequence for ultra-compact, burned-out galaxies,
Toft's team assembled, for the first time, representative samples of two galaxy
populations using the rich dataset in Hubble's COSMOS (Cosmic Evolution Survey)
program.
One group of galaxies is the compact ellipticals. The other group contains
galaxies that are highly obscured with dust and undergoing rapid star formation
at rates thousands of times faster than observed in the Milky Way. Starbursts in
these dusty galaxies likely were ignited when two gas-rich galaxies collided.
These galaxies are so dusty that they are almost invisible at optical
wavelengths, but they shine bright at submillimeter wavelengths, where they were
first identified nearly two decades ago by the Submillimeter Common-User
Bolometer Array (SCUBA) camera on the James Clerk Maxwell Telescope in
Hawaii.
Toft's team started by constructing the first representative sample of
compact elliptical galaxies with accurate sizes and spectroscopic redshifts, or
distances, measured with Hubble's Cosmic Assembly Near-Infrared Deep
Extragalactic Legacy Survey (CANDELS) and 3D-HST programs. 3D-HST is a
near-infrared 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 Spitzer. This allowed for
accurate stellar age estimates, from which they concluded compact elliptical
galaxies formed in intense starbursts inside the galaxies that preceded them by
as long as two billion years.
Next, the team made the first representative sample of the most distant
submillimeter galaxies using 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, all located in Hawaii. 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 that were present early in the universe.
When Toft's team compared the samples of the two galaxy populations, it
discovered an evolutionary link between the compact elliptical galaxies and the
submillimeter galaxies. The observations show that the violent starbursts in the
dusty galaxies had the same characteristics that would have been predicted for
progenitors to the compact elliptical galaxies. Toft's team also calculated the
intense starburst activity inside the submillimeter galaxies lasted only about
40 million years before the interstellar gas supply was exhausted.
The results appear in the Jan. 29 online issue of The Astrophysical
Journal.
For related and high resolution imagery, visit:
For more information on Hubble, visit:
NASA
Guillermo Gonzalo Sánchez Achutegui
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