Washington: For the first time astronomers
have found a distant galaxy being magnified by a quasar acting
as a gravitational lens.
Quasars, the extraordinary luminous objects in the
distant universe, are thought to be powered by supermassive
black holes in the cores of galaxies.
A single quasar could be a thousand times brighter than
an entire galaxy of a hundred billion stars, which makes
studies of their host galaxies exceedingly difficult.
Astronomers, from the California Institute of Technology
(Caltech) and Ecole Polytechnique Federale de Lausanne (EPFL)
in Switzerland, who made this discovery said it would provide
a novel way to understand these host galaxies.
"It is a bit like staring into bright car headlights and
trying to discern the colour of their rims," said Frederic
Courbin of EPFL, the lead author of the study, published in
the journal Astronomy & Astrophysics.
Using gravitational lensing, he said, "we now can measure
the masses of these quasar host galaxies and overcome this
According to Einstein`s general theory of relativity, if
a large mass (such as a big galaxy or a cluster of galaxies)
is placed along the line of sight to a distant galaxy, the
part of the light that comes from the galaxy will split.
Because of this, an observer on Earth will see two or
more close images of the now-magnified background galaxy, the
The first such gravitational lens was discovered in 1979,
and produced an image of a distant quasar that was magnified
and split by a foreground galaxy.
Hundreds of cases of gravitationally lensed quasars are
now known. But, until the current research, carried out at W M
Keck Observatory on Hawaii`s Mauna Kea, the reverse process --
a background galaxy being lensed by the massive host galaxy of
a foreground quasar -- had never been detected.
Using gravitational lensing to measure the masses of
distant galaxies independent of their brightness was suggested
in 1936 by Caltech astrophysicist Fritz Zwicky, and the
technique has been used effectively for this purpose in recent
years. Until now, it had never been applied to measure the
masses of quasar hosts themselves.
To find the cosmic lens, the astronomers searched a large
database of quasar spectra obtained by the Sloan Digital Sky
Survey (SDSS) to select candidates for "reverse" quasar-galaxy
Follow-up observations of the best candidate -- quasar
SDSS J0013+1523, located about 1.6 billion light years away --
using the W M Keck Observatory`s 10-meter telescope, confirmed
that the quasar was indeed magnifying a distant galaxy,
located about 7.5 billion light years away.
"We were delighted to see that this idea actually works,"
said Georges Meylan, a professor of physics and leader of the
"This discovery demonstrates the continued utility of
gravitational lensing as an astrophysical tool."
"Quasars are valuable probes of galaxy formation and
evolution," said Prof S George Djorgovski, leader of the
Furthermore, he added, "discoveries of more such systems
will help us understand better the relationship between
quasars and the galaxies which contain them, and their