Einstein Ring in Distant Universe (Forwarded)

ESO Education and Public Relations Dept.

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Contacts

Rémi Cabanac
Canada-France-Hawaii Telescope
Hawaii, USA
Phone: +1 808 885 3163

Chris Lidman
ESO, Chile
Phone: +56 55 43 5311 or 56 2 463 3106

David Valls-Gabaud
Observatoire Midi-Pyrenees, Toulouse, France
Phone : +33 5 61 33 29 46

Helmut Jerjen
Mount stromlo Observatory, Australia
Phone : +61 2 6125 80 38

For immediate release: 30 June 2005

ESO Press Photo 20/05

Einstein Ring in Distant Universe

Astronomers Using the VLT Discover Bright Cosmic Mirage Far Away

Using ESO's Very Large Telescope, Rémi Cabanac and his
European colleagues have discovered an amazing cosmic
mirage, known to scientists as an Einstein Ring. This cosmic
mirage, dubbed FOR J0332-3557, is seen towards the southern
constellation Fornax (the Furnace), and is remarkable on at
least two counts. First, it is a bright, almost complete
Einstein ring. Second, it is the farthest ever found.

ESO PR Photo 20a/05 ESO PR Photo 20b/05
Deep Image of a Region Zoom-in on the Newly Found
in Fornax (FORS/VLT) Einstein Ring (FORS/VLT)

Caption: ESO PR Photo 20a/05 is a composite image taken
in two bands (B and R) with VLT/FORS1 of a small portion
of the sky (field-of-view 7x7' or 1/15th of the area of
the full moon). The faintest object seen in the image has
a magnitude 26, that is, it is 100 million times fainter
than what can be observed with the unaided eye. The bright
elliptical galaxy on the lower-left quadrant is a dwarf
galaxy part of a large nearby cluster in the Fornax
constellation. As for all deep images of the sky, this
field shows a variety of objects, the brightest ponctual
sources being stars from our Galaxy. By far the field is
dominated by thousands of faint background galaxies the
colours of which are related to the age of their dominant
stellar population, their dust content and their distance.
The newly found Einstein ring is visible in the top right
part of the image.

ESO PR Photo 20b/05 zooms-in on the position of the newly
found cosmic mirage.

ESO PR Photo 20c/05
Einstein Ring in Distant Universe (FORS/VLT)

Caption of ESO PR Photo 20c/05:The left image is magnified
and centred on the newly discovered Einstein ring. The
image quality ("seeing") of the R-band image is exceptional
(0.5") and the image reveals the lensing system in stunning
details. The central dot is the lens, a quiescent massive
galaxy that distort the light emitted by background sources.
The large arc surrounding the central lens is a part of the
Einstein-ring created by a background source finely aligned
with the lens. The reddish colour indicates that the
redshift of the system is very large. FORS2 spectroscopy
of the lensing system yield a redshift close to 1 for the
lens (we see the lens as it was when the universe was half
its present size), and a record-breaking redshift z=3.8 for
a background source of such brightness, hence we see the
object (a star forming galaxy) as it was when the universe
was only 12% of its present age. The lensing model indicates
that the light of the source is magnified at least 13 times.
The right panel shows the reconstructed image based on the
model of the lens and the source, showing the ring to
extend over 3/4 of a circle.

"There are only a very few optical rings or arcs known, and
even less so in which the lens and the source are at large
distance, i.e. more than about 7,000 million light-years away
(or half the present age of the Universe)", says Rémi Cabanac,
former ESO Fellow and now working at the Canada-France-Hawaii
Telescope. "Moreover, very few are nearly complete", he adds.

But in the case of this newly found cosmic ring, the images
show it to extend to almost 3/4 of a circle. The lensing
galaxy is located at a distance of about 8,000 million
light-years from us, while the source galaxy whose light
is distorted, is much farther away, at 12,000 million light-
years. Thus, we see this galaxy as it was when the universe
was only 12% of its present age. The lens magnifies the
source almost 13 times.

The observations reveal the galaxy acting as a lens to be a
rather quiet galaxy, 40,000 light-years wide, with an old
stellar population. The far away lensed galaxy, however, is
extremely active, having recently experienced bursts of star
formation. It is a compact galaxy, 7,000 light-years across.

"Because the gravitational pull of matter bends the path
of light rays, astronomical objects -- stars, galaxies and
galaxy clusters -- can act like lenses, which magnify and
severely distort the images of galaxies behind them,
producing weird pictures as in a hall of mirrors", explains
Chris Lidman (ESO), co-discover of the new cosmic mirage.

In the most extreme case, where the foreground lensing galaxy
and the background galaxy are perfectly lined up, the image
of the background galaxy is stretched into a ring. Such an
image is known as an Einstein ring, because the formula for
the bending of light, first described in the early twentieth
century by Chwolson and Link, uses Albert Einstein's theory
of General Relativity.

Gravitational lensing provides a very useful tool with which
to study the Universe. As "weighing scales", it provides a
measure of the mass within the lensing body, and as a
"magnifying glass", it allows us to see details in objects
which would otherwise be beyond the reach of current
telescopes.

From the image, co-worker David Valls-Gabaud (CFHT), using
state-of-the-art modelling algorithms, could deduce the mass
of the galaxy acting as a lens -- it is almost one million
million suns.

More information

The paper describing this research has been published as a
Letter to the Editor in Astronomy and Astrophysics, volume
436, L21-L25 ("Discovery of a high-redshift Einstein ring",
by R.A. Cabanac, D. Valls-Gabaud, A.O. Jaunsen, C. Lidman,
and H. Jerjen). The paper is available for download in PDF
format from the A&A web site.

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