A Gallery of Lenses (2) ...
The following figure shows a Hubble Space Telescope
gallery of images giving evidence for possible gravitational
lensing.
The centers of the lensing galaxies are actually red, but the images have been processed to
make them yellow in order to give more intensity contrast in the images.
Lens Identification
These ten cases
are identified as gravitational lenses by their observational characteristics and by
comparing with a mathematical model based on the general theory of relativity. Examples
of comparison with the model
are shown for four of the lens candidates in the adjacent right figure.
In this
figure, the actual Hubble Space Telescope observation is shown in the left frame and
a model calculation of what it should look like if it is a gravitational lens is
shown in the middle frame. The right frame, labeled "Residual", is the difference between
the
actual observation and the model calculation. This is a measure of how well the
model reproduces the observation. If it were perfect, the right frame would be blank.
Confirming the Identification
In some cases the identification of the
gravitational lens has been confirmed by detailed spectroscopic investigation.
If the various images seen when there are multiple images
are indeed just different images of the same
objects, the spectra should also be identical, detail for detail. The chance of two adjacent
objects on the celestial sphere being distinct objects (instead of lensed images of the same
object) and having exactly the same spectrum is essentially zero.
For example, the second case from the left in the second row of the top figure
has been checked using ground-based telescopes and it has been confirmed that the four blue
objects arrayed around the central galaxy are lensed reproductions of the same object.
Details of the Lensing System
The
mathematical model fit to the observation allows the details of the lensing system to be
inferred. In the example just cited,
the the red object in the center is a giant
elliptical galaxy lying at a redshift of z = 0.803
(a distance of about 7.5 billion light years). Its gravitational field serves as the
lens. The four bluish objects arrayed around it are lensed images of a quasar
that lies much further away at a redshift of 3.4. The total magnification (zoom factor) is
about 2.4 magnitudes for the distant quasar. Thus the combined light of the 4 images is
about 9.1 times brighter than the point-like
quasar source would be if it were not lensed by the intervening
mass, and each of the 4 images is about 2.3 times brighter than the original quasar.
This type of lensing configuration with 4 images is another example of an
Einstein cross.
