Astronomers Discover Six-Image Gravitational Lens

An international team of astronomers has used the National Science
Foundation’s Very Long Baseline Array (VLBA) radio telescope and NASA’s
Hubble Space Telescope (HST) to discover the first gravitational lens in
which the single image of a very distant galaxy has been split into six
different images. The unique configuration is produced by the gravitational
effect of three galaxies along the line of sight between the more-distant
galaxy and Earth.

“This is the first gravitational lens with more than four images of the
background object that is produced by a small group of galaxies rather than
a large cluster of galaxies,” said David Rusin, who just received his Ph.D.
from the University of Pennsylvania. “Such systems are expected to be
extremely rare, so this discovery is an important stepping stone. Because
this is an intermediate case between gravitational lenses produced by single
galaxies and lenses produced by large clusters of galaxies, it will give us
insights we can’t get from other types of lenses,” Rusin added.

The gravitational lens, called CLASS B1359+154, consists of a galaxy more
than 11 billion light-years away in the constellation Bootes, with a trio of
galaxies more than 7 billion light-years away along the same line of sight.
The more-distant galaxy shows signs that it contains a massive black hole
at its core and also has regions in which new stars are forming. The
gravitational effect of the intervening galaxies has caused the light and
radio waves from the single, more-distant galaxy to be “bent” to form six
images as seen from Earth. Four of these images appear outside the triangle
formed by the three intermediate galaxies and two appear inside that triangle.

“This lens system is a very interesting case to study because it is more
complicated than lenses produced by single galaxies, and yet simpler than
lenses produced by clusters of numerous galaxies,” said Chris Kochanek of
the Harvard-Smithsonian Center for Astrophysics (CfA). “When we understand
this system, we will have a much clearer picture of how galaxies are changed
by being part of a bigger cluster of galaxies,” he added.

B1359+154 was discovered in 1999 by the Cosmic Lens All-Sky Survey, an
international collaboration of astronomers who use radio telescopes to
search the sky for gravitational lenses. Images made by the NSF’s Very Large
Array in New Mexico and by Britain’s MERLIN radio telescope showed six
objects suspected of being gravitational-lens images, but the results were
inconclusive. Rusin and his team used the VLBA and HST in 1999 and 2000 to
make more-detailed studies of B1359+154. The combination of data from the
VLBA and HST convinced the astronomers that B1359+154 actually consists of
six lensed images of a single background galaxy. The VLBA images were made
from data collected during observations at a radio frequency of 1.7 GHz.

“This is a great example of modern, multi-wavelength astronomy,” said Rusin.
“We need the radio telescopes to detect the gravitational lenses in the
first place, then we need the visible-light information from Hubble to show
us additional detail about the structure of the system.”

Armed with the combined VLBA and HST data about the positions and
brightnesses of the six images of the background galaxy as well as the
positions of the three intermediate galaxies, the astronomers did computer
simulations to show how the gravitation of the three galaxies could produce
the lens effect. They were able to design a computer model of the system
that, in fact, produces the six images seen in B1359+154.

“Our computer model certainly is not perfect, and we need to do more
observations of this system to refine it, but we have clearly demonstrated
that the three galaxies we see can produce a six-image lens system,” said
Martin Norbury, a graduate student at Jodrell Bank Observatory in Britain.
“We think this work will give us an excellent tool for studying much-denser
clusters of galaxies and the relationships of the individual cluster
galaxies to the ‘halo’ of dark matter in which they are embedded,” he added.

Clusters of galaxies are known to produce gravitational lenses with up to
eight images of a single background object. However, the number of galaxies
in such a cluster makes it difficult for astronomers to decipher just how
their gravitational effects have combined to produce the multiple images.
Researchers hope to be able to understand the lensing effect well enough
to use the lenses to show them how galaxies, gas and unseen dark matter
in clusters are distributed. A system such as B1359+154, with only three
galaxies involved in the lensing, can help astronomers learn how complex
gravitational lenses work.

“The next big step is to use HST to see the pattern of rings produced by the
galaxy surrounding the black hole. We already see hints of them, but with the
upgrades to HST in the next servicing mission we should be able to trace it
completely both to pin down the structure of the lens and to have an
enormously magnified image for studying the distant host galaxy,” Kochanek
said.

In addition to Rusin, Kochanek and Norbury, the researchers are: Emilio Falco
of the CfA; Chris Impey of Steward Observatory at the University of Arizona;
Joseph Lehar of the CfA; Brian McLeod of the CfA; Hans-Walter Rix of the
Max Planck Institute for Astronomy in Germany; Chuck Keeton of Steward
Observatory; Jose Munoz of the Astrophysical Institute of the Canaries in
Tenerife, Spain; and Chien Peng of Steward Observatory. The team published
its results in the Astrophysical Journal.

The VLBA is a system of 10 radio-telescope antennas that work together as a
single astronomical instrument. The antennas are spread across the United
States, from Hawaii in the west to the U.S. Virgin Islands in the east.
A radio telescope system more than 5,000 miles across, the VLBA produces
extremely detailed images.

The National Radio Astronomy Observatory is a facility of the National Science
Foundation, operated under cooperative agreement by Associated Universities,
Inc. The Space Telescope Science Institute is operated by the Association of
Universities for Research in Astronomy, Inc., for NASA,, under contract with
the Goddard Space Flight Center, Greenbelt, MD. The Hubble Space Telescope
is a project of international Cooperation between NASA and the European
Space Agency.

* Images to Accompany This Story

http://www.aoc.nrao.edu/pr/sixlens/lensgraphics.html

IMAGE CAPTIONS:

[Image 1]
Hubble Space Telescope image of the gravitational lens system. The white
objects are the multiple lensed images of the background galaxy, more than
11 billion light-years away. The orange objects are images of the three
galaxies some 7 billion light-years away whose gravity is “bending” the
light from the background galaxy to produce the multiple images.

CREDIT: Rusin et al., STScI. NRAO/AUI/NSF

[Image 2]
Very Long Baseline Array (VLBA) radio image of the gravitational lens system.
The darkest (brightest) objects are the six lensed images of the background
galaxy. Other objects seen in this 1.7 GHz radio image are not part of the
gravitational lens system.

CREDIT: Rusin et al., STScI. NRAO/AUI/NSF

[Image 3]
HST image of the gravitational lens system, with labels indicating the
individual components. A, B, C, D, E and F are the lensed images of the
background galaxy. G, G’ and G” are the three lensing galaxies.

CREDIT: Rusin et al., STScI. NRAO/AUI/NSF

Contact:

Dave Finley, National Radio Astronomy Observatory

(505) 835-7302

dfinley@nrao.edu

Ray Villard, Space Telescope Science Institute

(410) 338-4514

villard@stsci.edu

David Aguilar, Harvard-Smithsonian Center for Astrophysics

(617) 495-7462

daguilar@cfa.harvard.edu