The first compound gravitational lens?
In 1991 the source PKS 1830-211 was identified as an unusually strong, ~
10 Jy, radio Einstein ring (gravitational lens). But it now appears that
there may be two separate lensing galaxies involved, not one. This would
make PKS 1830-211 the first compound gravitational lens.
PKS 1830-211 (MERLIN image at 5 GHz)
PKS 1830-211 is certainly an extragalactic gravitational lens, but
it lies in a crowded and obscured field close to the Galactic Centre, and
neither the lensing galaxy nor the lensed object has been identified optically.
In June 1995 a search was made for radio absorption lines associated with
the source, using the Parkes telescope equipped with the wideband receiver
built for Project Phoenix. A single, weak absorption line was found at 1.19
GHz. The spectrum showed no other features, but the line had all the characteristics
of HI absorption. If interpreted as HI absorption, the line gave a redshift
for the absorbing object ­p; presumably the lensing galaxy ­p; of
0.19. Observations with the Australia Telescope Compact Array confirmed
the detection of the line and showed that the absorption was centred on
However, in June 1995 Wiklind and Combes observed PKS 1830-211 with
SEST (the Swedish-ESO Submillimeter Telescope) and detected several molecular
absorption features, which gave a redshift of 0.89. This they presumed to
be the redshift of the lens.
No single intervening galaxy can account for the two redshifts. It appears,
therefore, that there are two galaxies along the line of sight to the background
source, one 4.5 times more distant than the other, both contributing to
the lensed image.
If this were so then the degree of absorption would vary across the
lensed image. To test this idea J. Lovell et al. made a spectral-line
VLBI image of the source at 1.19 GHz, using the Australia Telescope Compact
Array in its phased-array mode for maximum sensitivity, and the Mopra, Parkes
and Hobart antennas. The resulting image, with two absorption profiles,
appears on the opposite page. The absorber covers the north-east component
of the source.
There is other evidence to corroborate the hypothesis of a compound
lens. Higher-resolution VLBI images, made over three years, show that the
Einstein ring's two compact components have significantly different morphologies.
This is hard to explain if they are simply twin images produced by a single
J. Lovell, P.
McCulloch (Uni. Tasmania); R. Gough, D. Jauncey, E. King, J. Reynolds,
A. Tzioumis (ATNF);
R. Preston (JPL)