and Cosmology

3.8 Galaxies as Gravitational Lenses
Fig. 3.39. A NIR image of QSO 1115+080
is shown on the left, as observed with the
NICMOS camera on-board HST. The double
structure of image A (the left of the QSO
images) is clearly visible, although the image
separation of the two A components
is less than 0 ′′ . 5. The lens galaxy, located
in the “middle” of the QSO images, has
a much redder spectral energy distribution
than the quasar images. In the right-hand
panel, the quasar images and the lens galaxy
have been subtracted. What remains is
a nearly closed ring; the light of the galaxy
which hosts the active galactic nucleus is
imaged into an Einstein ring
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Fig. 3.40. Left: in the center of a nearby spiral galaxy, four
point-like sources were found whose spectra show strong
emission lines. This image from the CFHT clearly shows the
bar structure in the core of the lens galaxy. An HST/NICMOS
image of the center of QSO 2237+0305 is shown on the right.
The central source is not a fifth quasar image but rather the
bright nucleus of the lens galaxy
ing multiply imaged by a lens galaxy – one of the
two radio sources is imaged into four components, the
other mapped into a double image. In the NIR the radio
galaxy is visible as a complete Einstein ring. This
example shows very clearly that the appearance of the
images of a source depends on the source size: to obtain
an Einstein ring a sufficiently extended source is
needed.
At radio wavelengths, the quasar MG 1654+13 consists
of a compact central source and two radio lobes.
As we will discuss in Sect. 5.1.3, this is a very typical
radio morphology for quasars. One of the two lobes has
a ring-shaped structure, which prior to this observation
had never been observed before. An optical image of the
field shows the optical quasar at the position of the compact
radio component and, in addition, a bright elliptical
galaxy right in the center of the ring-shaped radio lobe.
This galaxy has a significantly lower redshift than the
quasar and hence is the gravitational lens responsible
for imaging the lobe into an Einstein ring.