This picture from the NASA/ESA Hubble Space Telescope shows J1000+0221, the most
distant gravitational lens yet discovered. The glow at the centre of
this picture is the central regions of a normal galaxy. By chance it is
precisely aligned with a much more remote, young star-forming galaxy.
The light from the more distant object is bent around the nearer object
by its strong graviational pull to form a ring of multiple images. The
chance of finding such an exact alignment is very small, suggesting that
there may be more star-forming galaxies in the early Universe than
expected. Credit: NASA/ESA/A. van der Wel
An international team of astronomers has
found the most distant gravitational lens yet — a galaxy that, as
predicted by Albert Einstein’s general theory of relativity, deflects
and intensifies the light of an even more distant object. The discovery
provides a rare opportunity to directly measure the mass of a distant
galaxy. But it also poses a mystery: lenses of this kind should be
exceedingly rare. Given this and other recent finds, astronomers either
have been phenomenally lucky — or, more likely, they have underestimated
substantially the number of small, very young galaxies in the early
Universe.
Light is affected by gravity, and light passing a distant galaxy will
be deflected as a result. Since the first find in 1979, numerous such
gravitational lenses have been discovered. In addition to providing
tests of Einstein's theory of general relativity, gravitational lenses
have proved to be valuable tools. Notably, one can determine the mass of
the matter that is bending the light — including the mass of the
still-enigmatic dark matter, which does not emit or absorb light and can
only be detected via its gravitational effects. The lens also magnifies
the background light source, acting as a "natural telescope" that
allows astronomers a more detailed look at distant galaxies than is
normally possible.
Gravitational lenses consist of two objects: one is further away and
supplies the light, and the other, the lensing mass or gravitational
lens, which sits between us and the distant light source, and whose
gravity deflects the light. When the observer, the lens, and the distant
light source are precisely aligned, the observer sees an Einstein ring:
a perfect circle of light that is the projected and greatly magnified
image of the distant light source.
Now, astronomers have found the most distant gravitational lens yet.
Lead author Arjen van der Wel (Max Planck Institute for Astronomy,
Heidelberg, Germany) explains: "The discovery was completely by
chance. I had been reviewing observations from an earlier project when I
noticed a galaxy that was decidedly odd. It looked like an extremely
young galaxy, but it seemed to be at a much larger distance than
expected. It shouldn't even have been part of our observing programme!”
Van der Wel wanted to find out more and started to study images taken
with the Hubble Space Telescope as part of the CANDELS and COSMOS
surveys. In these pictures the mystery object looked like an old galaxy,
a plausible target for the original observing programme, but with some
irregular features which, he suspected, meant that he was looking at a
gravitational lens. Combining the available images and removing the haze
of the lensing galaxy's collection of stars, the result was very clear:
an almost perfect Einstein ring, indicating a gravitational lens with
very precise alignment of the lens and the background light source [1].
The lensing mass is so distant that the light, after deflection, has travelled 9.4 billion years to reach us [2].
Not only is this a new record, the object also serves an important
purpose: the amount of distortion caused by the lensing galaxy allows a
direct measurement of its mass. This provides an independent test for
astronomers' usual methods of estimating distant galaxy masses — which
rely on extrapolation from their nearby cousins. Fortunately for
astronomers, their usual methods pass the test.
But the discovery also poses a puzzle. Gravitational lenses are the
result of a chance alignment. In this case, the alignment is very
precise. To make matters worse, the magnified object is a starbursting
dwarf galaxy: a comparatively light galaxy (it has only about 100
million solar masses in the form of stars [3]),
but extremely young (about 10-40 million years old) and producing new
stars at an enormous rate. The chances that such a peculiar galaxy would
be gravitationally lensed is very small. Yet this is the second
starbursting dwarf galaxy that has been found to be lensed. Either
astronomers have been phenomenally lucky, or starbursting dwarf galaxies
are much more common than previously thought, forcing astronomers to
re-think their models of galaxy evolution.
Van der Wel concludes: "This has been a weird and interesting
discovery. It was a completely serendipitous find, but it has the
potential to start a new chapter in our description of galaxy evolution
in the early Universe."
Notes
[1] The two objects are aligned to better than 0.01 arcseconds — equivalent to a one millimetre separation at a distance of 20 kilometres.[2] This time corresponds to a redshift z = 1.53. This can be compared with the total age of the Universe of 13.8 billion years. The previous record holder was found thirty years ago, and it took less than 8 billion years for its light to reach us (a redshift of about 1.0).
[3] For comparison, the Milky Way is a large spiral galaxy with at least one thousand times greater mass in the form of stars than this dwarf galaxy.
Notes for editors
The Hubble Space Telescope is a project of international cooperation between ESA and NASA.
The work described here has been published as van der Wel et al.,
"Discovery of a quadruple lens in CANDELS with a record lens redshift z =
1.53" , in Astrophysical Journal Letters.
The team is composed of Arjen van der Wel, Glenn van de Ven, Michael
Maseda, Hans-Walter Rix (all Max Planck Institute for Astronomy,
Heidelberg, Germany [MPIA]), Gregory Rudnick (University of Kansas, USA;
MPIA), Andrea Grazian (INAF), Steven Finkelstein (University of Texas
at Austin, USA), David Koo, Sandra M. Faber (both University of
California, Santa Cruz, USA), Henry Ferguson, Anton Koekemoer, Norman
Grogin (all STScI, Baltimore, USA) and Dale Kocevski (University of
Kentucky, USA).
Links
Contacts
Arjen van der WelMax Planck Institute for Astronomy
Heidelberg, Germany
Tel: +49 6221 528360
Email: vdwel@mpia.de
Richard Hook
ESO, Public Information Officer
Garching bei München, Germany
Tel: +49 89 3200 6655
Cell: +49 151 1537 3591
Email: rhook@eso.org
Nicky Guttridge
Hubble/ESA
Garching bei München, Germany
Tel: +49-89-3200-6855
Email: nguttrid@partner.eso.org
Source: ESA/HUBBLE - Space Telescope
