RSAA News of the Month: July 2005
Astronomers are being provided a rare glimpse back to the time when
galaxies were in their infancy thanks to the work of an ANU astronomer.
Dr Helmut Jerjen, based at the Mt Stromlo
Observatory in Canberra, says the partial Einstein Ring
discovered in the southern constellation of Fornax is only the
fourth of its kind ever observed. The new Ring is the furthest ever
found, is remarkably bright, and is almost complete.
An Einstein Ring is an optical phenomenon that occurs when two
galaxies are perfectly aligned along the line of sight. The gravity
of the nearer galaxy acts as a lens, distorting and magnifying the
light from its distant counterpart into the shape of a
circle. “It’s is a pure geometrical coincidence that
you have these two galaxies perfectly aligned with one
another. This can be discovered anywhere in the sky, but it is
extremely rare that the alignment and distances are right —
finding them is finding like finding needles in a haystack,”
Dr Jerjen says.
Left: Dr Helmut Jerjen
Right: VLT images of the Fornax Einstein Ring
The bright red dot at the centre of the left VLT image is
the massive galaxy doing the lensing.
The arc around this galaxy is the Einstein Ring, the focussed and
magnified light of the distant galaxy.
The right VLT image has been processed to highlight the structure
of the ring and
shows it to be about 75% complete.
The closer, lensing galaxy in the new Einstein Ring is eight billion
light years away. It is about 10 times larger than our Milky Way
and contains mostly old stars. The distant galaxy is 12 billion
light years distant, and would remain invisible if it weren’t
for the magnifying effect of the foreground galaxy. This galaxy is
much younger and is in the early stages of its life. It has just
gone through a burst of star formation and contains mostly young
stars. The lensing effect is allowing a rare insight into the early
epoch of galaxy formation in the young Universe.
“We can explore the stellar composition of this distant object,
which is a baby galaxy. At 12 billion light years from us, this
object is really located at the time when galaxies were just
forming and the Universe was about 12% of its present age. Thanks
to these magnified images, we’re able to explore parts of the
universe that really wouldn’t be accessible to us
otherwise.”
Two VLT images of the region of Fornax containing the Einstein Ring.
In the left image, the object is just visible as a faint red
"smudge" in the upper left of the image
(between the two white marker lines).
The image on the right shows a smaller area of the image and the ring
is more visible.
The faintest objects visible on the image are around 100,000,000 times
fainter than can be seen with the unaided eye.
Links to higher-resolution versions of all three ESO images are included
in their press
release.
This ring was first detected in Dr Jerjen’s observations two
years ago, but has only just been confirmed through the
collaboration of a team of international astronomers at the European Southern Observatory
(ESO). The team used ESO’s Very Large Telescope (VLT),
which is located in the northern part of Chile at one of the best
sites for optical astronomy in the world, to take spectra of this
phenomenon. "With the help of the spectra we could measure the
distance of the two objects, and we found the ring is actually at a
much greater distance than the central object," Helmut
explains.
The possibility of such rings was first predicted by Einstein’s
general relativity theory 100 years ago. One of the predictions
from the theory is that light will bend when it passes through a
strong gravitational field.
The results of this research have been published in Astronomy and
Astrophysics, vol 436, L21-25.
Members of the discovery team are:
Chris Lidman is an ex-Stromlo student, gaining his PhD in astronomy
from ANU in 1995.
Press releases from ANU and ESO can be read here (ANU)
and here
(ESO).
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