NASA's Hubble Helps Solve Mystery of Ultra-Compact, Burned-Out Galaxies
Astronomers using NASA's Hubble Space Telescope 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 NASA's Spitzer Space Telescope and 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:
http://hubblesite.org/news/2014/10For more information on Hubble, visit:
http://www.nasa.gov/hubble
