The farthest cosmic lens yet found, a massive
elliptical galaxy, is shown in the inset image at left. The galaxy existed 9.6
billion years ago and belongs to the galaxy cluster, IRC 0218.
Image Credit:
NASA and ESA
Astronomers using NASA's Hubble Space Telescope have unexpectedly discovered
the most distant galaxy that acts as a cosmic magnifying glass. Seen here as it
looked 9.6 billion years ago, this monster elliptical galaxy breaks the previous
record-holder by 200 million years.
These "lensing" galaxies are so massive that their gravity bends, magnifies,
and distorts light from objects behind it, a phenomenon called gravitational
lensing. Finding one in such a small area of the sky is so rare that you would
normally have to survey a region hundreds of times larger to find just one.
The object behind the cosmic lens is a tiny spiral galaxy undergoing a rapid
burst of star formation. Its light has taken 10.7 billion years to arrive here
and seeing this chance alignment at such a great distance from Earth is a rare
find. Locating more of these distant lensing galaxies will offer insight into
how young galaxies in the early universe build themselves up into the massive
dark-matter-dominated galaxies of today. Dark matter cannot be seen, but it
accounts for the bulk of the universe's matter.
"When you look more than 9 billion years ago in the early universe, you don't
expect to find this type of galaxy lensing at all," explained lead researcher
Kim-Vy Tran of Texas A&M University in College Station. "It's very difficult
to see an alignment between two galaxies in the early universe. Imagine holding
a magnifying glass close to you and then moving it much farther away. When you
look through a magnifying glass held at arm's length, the chances that you will
see an enlarged object are high. But if you move the magnifying glass across the
room, your chances of seeing the magnifying glass nearly perfectly aligned with
another object beyond it diminishes."
Team members Kenneth Wong and Sherry Suyu of Academia Sinica Institute of
Astronomy & Astrophysics (ASIAA) in Taipei, Taiwan, used the gravitational
lensing from the chance alignment to measure the giant galaxy's total mass,
including the amount of dark matter, by gauging the intensity of its lensing
effects on the background galaxy's light. The giant foreground galaxy weighs 180
billion times more than our sun and is a massive galaxy for its time. It is also
one of the brightest members of a distant cluster of galaxies, called IRC
0218.
"There are hundreds of lens galaxies that we know about, but almost all of
them are relatively nearby, in cosmic terms," said Wong, first author on the
team's science paper. "To find a lens as far away as this one is a very special
discovery because we can learn about the dark-matter content of galaxies in the
distant past. By comparing our analysis of this lens galaxy to the more nearby
lenses, we can start to understand how that dark-matter content has evolved over
time."
The team suspects the lensing galaxy continued to grow over the past 9
billion years, gaining stars and dark matter by cannibalizing neighboring
galaxies. Tran explained that recent studies suggest these massive galaxies gain
more dark matter than stars as they continue to grow. Astronomers had assumed
dark matter and normal matter build up equally in a galaxy over time, but now
know the ratio of dark matter to normal matter changes with time. The newly
discovered distant lensing galaxy will eventually become much more massive than
the Milky Way and will have more dark matter, too.
Tran and her team were studying star formation in two distant galaxy
clusters, including IRC 0218, when they stumbled upon the gravitational lens.
While analyzing spectrographic data from the W.M. Keck Observatory in Hawaii,
Tran spotted a strong detection of hot hydrogen gas that appeared to arise from
a giant elliptical galaxy. The detection was surprising because hot hydrogen gas
is a clear signature of star birth. Previous observations showed that the giant
elliptical, residing in the galaxy cluster IRC 0218, was an old, sedate galaxy
that had stopped making stars a long time ago. Another puzzling discovery was
that the young stars were at a much farther distance than the elliptical galaxy.
Tran was very surprised, worried and thought her team made a major mistake with
their observations.
The astronomer soon realized she hadn't made a mistake when she looked at the
Hubble images taken in blue wavelengths, which revealed the glow of fledgling
stars. The images, taken by Hubble's Advanced Camera for Surveys and the Wide
Field Camera 3, revealed a blue, eyebrow-shaped object next to a smeared blue
dot around the massive elliptical. Tran recognized the unusual features as the
distorted, magnified images of a more distant galaxy behind the elliptical
galaxy, the signature of a gravitational lens.
To confirm her gravitational-lens hypothesis, Tran's team analyzed Hubble
archival data from two observing programs, the 3D-HST survey, a near-infrared
spectroscopic survey taken with the Wide Field Camera 3, and the Cosmic Assembly
Near-infrared Deep Extragalactic Legacy Survey, a large Hubble deep-sky program.
The data turned up another fingerprint of hot gas connected to the more distant
galaxy.
The distant galaxy is too small and far away for Hubble to determine its
structure. So, team members analyzed the distribution of light in the object to
infer its spiral shape. In addition, spiral galaxies are more plentiful during
those early times. The Hubble images also revealed at least one bright compact
region near the center. The team suspects the bright region is due to a flurry
of star formation and is most likely composed of hot hydrogen gas heated by
massive young stars. As Tran continues her star-formation study in galaxy
clusters, she will be hunting for more signatures of gravitational lensing.
The team's results appeared in the July 10 issue of The Astrophysical Journal
Letters.
The Hubble Space Telescope is a project of international cooperation between
NASA and the European Space Agency. NASA's Goddard Space Flight Center in
Greenbelt, Maryland, manages the telescope. The Space Telescope Science
Institute (STScI) in Baltimore conducts Hubble science operations. STScI is
operated for NASA by the Association of Universities for Research in Astronomy,
Inc., in Washington.
For images and more information about Hubble, visit:
http://www.nasa.gov/hubble
NASA
Guillermo Gonzalo Sánchez Achutegui
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