Using the ESO 3.6-m telescope at La Silla (Chile), an international team
of astronomers [1] has discovered a complex cosmic mirage in the southern
constellation Crater (The Cup). This “gravitational lens” system consists
of (at least) four images of the same quasar as well as a ring-shaped
image of the galaxy in which the quasar reside – known as an “Einstein
ring”. The more nearby lensing galaxy that causes this intriguing optical
illusion is also well visible.

The team obtained spectra of these objects with the new EMMI camera
mounted on the ESO 3.5-m New Technology Telescope (NTT), also at the La
Silla observatory. They find that the lensed quasar [2] is located at a
distance of 6,300 million light-years (its “redshift” is z = 0.66 [3])
while the lensing elliptical galaxy is rougly halfway between the quasar
and us, at a distance of 3,500 million light-years (z = 0.3).

The system has been designated RXS J1131-1231 – it is the closest
gravitationally lensed quasar discovered so far.

The full text of this Press Release, with two photos (ESO PR Photos
20a-b/03) and all related links, is available at:

Cosmic mirages

The physical principle behind a “gravitational lens” (also known as a
“cosmic mirage”) has been known since 1916 as a consequence of Albert
Einstein’s Theory of General Relativity. The gravitational field of a
massive object curves the local geometry of the Universe, so light rays
passing close to the object are bent (like a “straight line” on the
surface of the Earth is necessarily curved because of the curvature of
the Earth’s surface).

This effect was first observed by astronomers in 1919 during a total
solar eclipse. Accurate positional measurements of stars seen in the dark
sky near the eclipsed Sun indicated an apparent displacement in the
direction opposite to the Sun, about as much as predicted by Einstein’s
theory. The effect is due to the gravitational attraction of the stellar
photons when they pass near the Sun on their way to us. This was a direct
confirmation of an entirely new phenomenon and it represented a milestone
in physics.

In the 1930’s, astronomer Fritz Zwicky (1898 – 1974), of Swiss nationality
and working at the Mount Wilson Observatory in California, realised that
the same effect may also happen far out in space where galaxies and large
galaxy clusters may be sufficiently compact and massive to bend the light
from even more distant objects. However, it was only five decades later,
in 1979, that his ideas were observationally confirmed when the first
example of a cosmic mirage was discovered (as two images of the same
distant quasar).

Cosmic mirages are generally seen as multiple images of a single quasar
[2], lensed by a galaxy located between the quasar and us. The number and
the shape of the images of the quasar depends on the relative positions of
the quasar, the lensing galaxy and us. Moreover, if the alignment were
perfect, we would also see a ring-shaped image around the lensing object.
Such “Einstein rings” are very rare, though, and have only been observed in
a very few cases.

Another particular interest of the gravitational lensing effect is that it
may not only result in double or multiple images of the same object, but
also that the brightness of these images increase significantly, just as it
happens with an ordinary optical lens. Distant galaxies and galaxy clusters
may thereby act as “natural telescopes” which allow us to observe more
distant objects that would otherwise have been too faint to be detected
with currently available astronomical telescopes.

Image sharpening techniques resolve the cosmic mirage better

A new gravitational lens, designated RXS J1131-1231, was serendipitously
discovered in May 2002 by Dominique Sluse, then a PhD student at ESO in
Chile, while inspecting quasar images taken with the ESO 3.6-m telescope
at the La Silla Observatory. The discovery of this system profited from
the good observational conditions prevailing at the time of the
observations. From a simple visual inspection of these images, Sluse
provisionally concluded that the system had four star-like (the lensed
quasar images) and one diffuse (the lensing galaxy) component.

Because of the very small separation between the components, of the order
of one arcsecond or less, and the unavoidable “blurring” effect caused by
turbulence in the terrestrial atmosphere (“seeing”), the astronomers used
sophisticated image-sharpening software to produce higher-resolution
images on which precise brightness and positional measurements could then
be performed (see also ESO PR 09/97). This so-called “deconvolution”
technique makes it possible to visualize this complex system much better
and, in particular, to confirm and render more conspicuous the associated
Einstein ring, cf. PR Photo 20a/03.

Identification of the source and of the lens

The team of astronomers [1] then used the ESO 3.5-m New Technology
Telescope (NTT) at La Silla to obtain spectra of the individual image
components of this lensing system. This is imperative because, like human
fingerprints, the spectra allow unambiguous identification of the observed

Nevertheless, this is not an easy task because the different images of the
cosmic mirage are located very close to each other in the sky and the best
possible conditions are needed to obtain clean and well separated spectra.
However, the excellent optical quality of the NTT combined with
reasonably good seeing conditions (about 0.7 arcsecond) enabled the
astronomers to detect the “spectral fingerprints” of both the source and
the object acting as a lens, cf. ESO PR Photo 20b/03.

The evaluation of the spectra showed that the background source is a quasar
with a redshift of z = 0.66 [3], corresponding to a distance of about
6,300 million light-years. The light from this quasar is lensed by a
massive elliptical galaxy with a redshift z = 0.3, i.e. at a distance of
3,500 million light-years or about halfway between the quasar and us. It
is the nearest gravitationally lensed quasar known to date.

Because of the specific geometry of the lens and the position of the
lensing galaxy, it is possible to show that the light from the extended
galaxy in which the quasar is located should also be lensed and become
visible as a ring-shaped image. That this is indeed the case is
demonstrated by PR Photo 20a/03 which clearly shows the presence of such
an “Einstein ring”, surrounding the image of the more nearby lensing

Micro lensing within macro lensing ?

The particular configuration of the individual lensed images observed in
this system has enabled the astronomers to produce a detailed model of
the system. From this, they can then make predictions about the relative
brightness of the various lensed images.

Somewhat unexpectedly, they found that the predicted brightnesses of the
three brightest star-like images of the quasar are not in agreement with
the observed ones – one of them turns out to be one magnitude (that is,
a factor of 2.5) brighter than expected. This prediction does not call
into question General Relativity but suggests that another effect is at
work in this system.

The hypothesis advanced by the team is that one of the images is subject
to “microlensing”. This effect is of the same nature as the cosmic mirage
– multiple amplified images of the object are formed – but in this case,
additional light-ray deflection is caused by a single star (or several
stars) within the lensing galaxy. The result is that there are additional
(unresolved) images of the quasar within one of the macro-lensed images.

The outcome is an “over-amplification” of this particular image. Whether
this is really so will soon be tested by means of new observations of
this gravitational lens system with the ESO Very Large Telescope (VLT) at
Paranal (Chile) and also with the Very Large Array (VLA) radio
observatory in New Mexico (USA).


Until now, 62 multiple-imaged quasars have been discovered, in most cases
showing 2 or 4 images of the same quasar. The presence of elongated images
of the quasar and, in particular, of ring-like images is often observed at
radio wavelengths. However, this remains a rare phenomenon in the optical
domain – only four such systems have been imaged by optical/infrared
telecopes until now.

The complex and comparatively bright system RXS J1131-1231 now discovered
is a unique astrophysical laboratory. Its rare characteristics (e.g.,
brightness, presence of a ring-shaped image, small redshift, X-ray and
radio emission, visible lens, …) will now enable the astronomers to
study the properties of the lensing galaxy, including its stellar content,
structure and mass distribution in great detail, and to probe the source
morphology. These studies will use new observations which are currently
being obtained with the VLT at Paranal, with the VLA radio interferometer
in New Mexico and with the Hubble Space Telescope.

More information
The research described in this press release is presented in a Letter to
the Editor, soon to appear in the European professional journal Astronomy
& Astrophysics (“A quadruply imaged quasar with an optical Einstein ring
candidate : 1RXS J113155.4-123155”, by Dominique Sluse et al.).

More information on gravitational lensing and on this research group can
also be found at the URL :


[1]: The team consists of Dominique Sluse, Damien Hutsemekers, and Thodori
Nakos (ESO and Institut d’Astrophysique et de Geophysique de l’Universite
de Liege – IAGL), Jean-Fran=E7ois Claeskens, Frederic Courbin, Christophe
Jean, and Jean Surdej (IAGL), Malvina Billeres (ESO), and Sergiy Khmil
(Astronomical Observatory of Shevchentko University).

[2]: Quasars are particularly active galaxies, the centres of which emit
prodigious amounts of energy and energetic particles. It is believed that
they harbour a massive black hole at their centre and that the energy is
produced when surrounding matter falls into this black hole. This type of
object was first discovered in 1963 by the Dutch-American astronomer
Maarten Schmidt at the Palomar Observatory (California, USA) and the name
refers to their “star-like” appearance on the images obtained at that time.

[3]: In astronomy, the “redshift” denotes the fraction by which the lines
in the spectrum of an object are shifted towards longer wavelengths. Since
the redshift of a cosmological object increases with distance, the
observed redshift of a remote galaxy also provides an estimate of its

Dominique Sluse
Institut d’Astrophysique et de G=E9ophysique,
UniversitE de LiEge, Belgium
Phone: +32 (0)4 366 9761