In 1988, the first example of an ``Einstein ring'' was
discovered [74]. With high resolution radio observations, the extended radio
source MG1131+0456 turned out to be a ring with a diameter of
about 1.75 arcsec. The source was identified as a radio lobe at a
redshift of
, whereas the lens is a galaxy at
. Recently, a remarkable observation of the Einstein ring
1938+666 was presented [91
]. The infrared HST image shows an almost perfectly circular ring
with two bright parts plus the bright central galaxy. The
contours agree very well with the MERLIN radio map (see
Figure
15).
By now about a half dozen cases have been found that qualify as Einstein rings [128].
Their diameters vary between 0.33 and about 2 arcseconds. All
of them are found in the radio regime, some have optical or
infrared counterparts as well. Some of the Einstein rings are not
really complete rings, but they are ``broken'' rings with one or
two interruptions along the circle. The sources of most Einstein
rings have both an extended and a compact component. The latter
is always seen as a double image, separated by roughly the
diameter of the Einstein ring. In some cases monitoring of the
radio flux showed that the compact source is variable. This gives
the opportunity to measure the time delay and the Hubble constant
in these systems.
The Einstein ring systems provide some advantages over the
multiply-imaged quasar systems for the goal to determine the lens
structure and/or the Hubble constant. First of all the extended
image structure provides many constraints on the lens. A lens
model can be much better determined than in cases of just two or
three or four point-like quasar images. Einstein rings thus help
us to understand the mass distribution of galaxies at moderate
redshifts. For the Einstein ring MG 1654+561 it was found [97] that the radially averaged surface mass density of the lens was
fitted well with a distribution like
, where
lies between
(an isothermal sphere would have exactly
!); there was also evidence found for dark matter in this lensing
galaxy.
Second, since the diameters of the observed rings (or the separations of the accompanying double images) are of order one or two arcseconds, the expected time delay must be much shorter than the one in the double quasar Q0957+561 (in fact, it can be arbitrarily short, if the source happens to be very close to the point caustic). This means one does not have to wait so long to establish a time delay (but the source has to be variable intrinsically on even shorter time scales...).
The third advantage is that since the emitting region of the radio flux is presumably much larger than that of the optical continuum flux, the radio lightcurves of the different images are not affected by microlensing. Hence the radio lightcurves between the images should agree with each other very well.
Another interesting application is the (non-)detection of a central image in the Einstein rings. For singular lenses, there should be no central image (the reason is the discontinuity of the deflection angle). However, many galaxy models predict a finite core in the mass distribution of a galaxy. The non-detection of the central images puts strong constraints on the size of the core radii.
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Gravitational Lensing in Astronomy
Joachim Wambsganss http://www.livingreviews.org/lrr-1998-12 © Max-Planck-Gesellschaft. ISSN 1433-8351 Problems/Comments to livrev@aei-potsdam.mpg.de |