Download Gravitationally Lensed Galaxy

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National Aeronautics
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Gravitationally Lensed Galaxy
Space Weather
by John T. Clarke
Taken from:
Hubble 2010: Science Year in Review
National Aeronautics and Space Administration
www.hubblesite.org/hubble_discoveries/science_year_in_review
The full contents
of this book include Hubble science articles, an overview of
the telescope, and more. The complete volume and its component sections are
available for download online at:
Hubble
Science Year
In Review
National Aeronautics and Space Administration
Hubble 21: science Year in review
The full contents of this book include Hubble science articles, an overview of
and more.
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Hubble
2011: Science Year in Review
Hubble
Science Year
In Review
www.nasa.gov
NP-2011-12-271-GSFC
www.nasa.gov
NP-2011-12-271-GSFC
www.hubblesite.org/hubble_discoveries/science_year_in_review
HUBBLE 2011: SCIENCE YEAR IN REVIEW
The Brightest Gravitationally Lensed Galaxy
The more distant a galaxy is from Earth, the longer it takes for its light to travel across the immense expanse of space to
reach us. The visual appearance of distant galaxies, therefore, tells us about events and conditions of a bygone era—such
as the rate of star formation in the early universe, when star clusters coalesced and protogalaxies first formed. Astronomers
search continuously for the farthest of such objects in order to understand how processes in the universe evolved from a few
million years after the Big Bang to the current time.
Such remote objects are exceedingly faint and small, making it difficult to see critical details of how star formation progressed
in them. In fact, these details are usually well beyond the normal view of even Hubble ’s sensitive eye. However, when faint
and immensely distant objects are magnified naturally by means of a “gravitational lens,” Hubble can reveal details of galaxy
structure and star formation to an unprecedented level.
Albert Einstein first advanced the concept that light could be bent by gravity in his famous 1916 general theory of relativity.
He proposed the revolutionary notion that space itself was warped by massive objects. Light traveling through space near a
massive object would bend to follow the curvature of space around the object.
Observations of star positions during a solar eclipse in 1919 proved this theory correct. Stars near the darkened Sun
behaved exactly as Einstein predicted; the Sun’s gravity bent their light, causing an apparent shift in their positions. In 1937,
astronomer Fritz Zwicky applied Einstein’s theory on a larger scale. He predicted that a massive foreground object could
sufficiently warp the space around itself to act like a lens. A galaxy cluster, for example, could distort, brighten, and magnify
the light from a more distant (background) galaxy.
Most astronomers did not take this idea seriously, and the telescopes of the time were not sufficiently sensitive to prove the
idea by observing lensed galaxies. Then in 1979, the first of many gravitational lenses was discovered. Astronomers using
The startling blue arcs seen bending around galaxy cluster RCS2 032727-132623 are actually the distorted image of an immensely faint
galaxy located behind the cluster and twice as distant.
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the 2.1-meter optical wavelength telescope at the Kitt Peak National Observatory, along with additional radio-wavelength
observations, realized that two neighboring quasars exhibiting similar properties were actually two gravitationally lensed
images of the same distant quasar.
One of the most striking examples of a gravitational lens is the nearly 90-degree arc of light in the galaxy cluster RCS2
032727-132623, located in the constellation Eridanus. Magnified and distorted light from a distant galaxy creates this arc. It
is the brightest gravitationally lensed object yet discovered. In 2006, a team of astronomers using the Very Large Telescope
in Chile spectroscopically measured the galaxy’s true distance and found it to be approximately twice as far away as the
intervening galaxy cluster. The lensed galaxy appears 20 times larger and more than three times brighter than any previously
discovered gravitationally lensed galaxy.
In 2011, a team of astronomers used Hubble ’s Wide Field Camera 3 to further study and identify features of the lensed galaxy.
The team included Maryland-based astronomer Jane Rigby, Keren Sharon in Michigan, and Chicago-based researchers
Michael Gladders and Eva Wuyts. The gravitational lens stretches and distorts the distant galaxy’s light into a giant arc,
allowing the astronomers to view several individual knots of star formation in the image. Hubble ’s position above the
blurring effects of the Earth’s atmosphere, and the camera’s combination of sensitivity and resolution allowed scientists to
observe these knots in unprecedented detail.
The team found additional faint images of the same lensed galaxy scattered within the foreground galaxy cluster. This
phenomenon is expected; in fact, it is commonly seen in other galaxy clusters manifesting lensed background galaxies.
By analyzing the positions of these multiple images within the cluster, the members of the team could apply mathematical
models to reconstruct the appearance of the lensed galaxy in its normal, undistorted shape and examine its characteristics.
Applying lens modeling to the recent Hubble data resulted in a more precise measurement of the magnification of the galaxy
and a more accurate estimate of the star formation rate than was possible using ground-based data.
The team’s analysis revealed that the lensed galaxy is forming new stars at a rate of 30 stars per year, which is about 30
times higher than the current rate in our own Milky Way galaxy. The Hubble images showed that these new stars are forming
within a dozen knots in the lensed galaxy. Using its Hubble -based roadmap, the team is now obtaining ground-based
spectroscopy for these star-forming knots to measure how the knots are moving relative to each other and how much they
differ in their atomic composition. This spectroscopy, when combined with the Hubble images, should reveal whether the
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HUBBLE 2011: SCIENCE YEAR IN REVIEW
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HUBBLE 2011: SCIENCE YEAR IN REVIEW
background galaxy is actually a merger of two or more galaxies, or a single galaxy with a highly unusual shape. In either
case, by combining high-quality Hubble data with the magnifying power of a strong gravitational lens, the object will be one
of the best understood from this very early epoch in the universe.
The circled objects are four distinct, distorted projections of a single background galaxy created by the lensing effect of galaxy cluster RCS2 032727132623. The inset image was reconstructed from these projections, and the small rectangle shows the calculated location of this distant galaxy. The
additional thin, long arcs seen in the larger image are other galaxies also gravitationally lensed by the cluster. These were not used to reconstruct the
source galaxy seen in the inset. Some, however, were used in calculating the overall mass of the galaxy cluster.
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Further Reading
Bartusiak, M. “Gravity’s Rainbow.” Astronomy 25, no. 8 (August 1997): 44–49.
Bennett, C. “Astrophysical Observations: Lensing and Eclipsing Einstein’s Theories.” Science 307, no. 5711 (February 11, 2005):
879–884.
MacRobert, A. “Planet Found by Gravitational Lensing.” Sky & Telescope 108, no. 1 (July 2004): 18.
Sharon, K. et al. “Source Plane Reconstruction of the Bright Lensed Galaxy RCSGA 032727-132609.” The Astrophysical Journal 746,
no. 2 (February 2012): 161–170.
Wittman, D. M. et al. “Detection of Weak Gravitational Lensing Distortions of Distant Galaxies by Cosmic Dark Matter at Large Scales.”
Nature 405, no. 6783 (May 11, 2000): 143–148.
Dr. Keren Sharon was raised near Tel Aviv, Israel. Currently she is an assistant research scientist at the
astronomy department of the University of Michigan. Prior to taking this position, she was a Kavli postdoctoral
fellow at the Kavli Institute for Cosmological Physics at the University of Chicago. She received her doctorate
in 2009 from the Tel Aviv University, where she studied supernova rates in clusters of galaxies, using both
ground- and space-based telescopes. Her current research focuses on modeling the mass distribution of
galaxy clusters using strong gravitational lensing and using these clusters as natural telescopes to study the
background universe.
Dr. Jane Rigby is an astrophysicist at the NASA’s Goddard Space Flight Center in Greenbelt, Maryland. Prior
to this position, she was a Carnegie fellow and a Spitzer fellow at the Carnegie Observatories in Pasadena,
California. Raised in rural Delaware, Dr. Rigby received her doctorate in 2006 from the University of Arizona,
and has bachelor of science degrees in physics and in astronomy from The Pennsylvania State University. Her
research focuses on black holes in the centers of galaxies and galaxies that are rapidly forming new stars. Dr.
Rigby has been principal investigator on three observing programs using the Hubble Space Telescope , and
has published results from the Spitzer Space Telescope and the Chandra X-ray Observatory, as well as the
Keck and Magellan telescopes in Hawaii and Chile.
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