The first gravitational lens, 0957+561, was found serendipitously. Soon after, however, serious directed searches began.
The usual way to search for gravitational lenses
is a brute-force approach, and it is to look at very large numbers of
background objects with a view to picking out the very small fraction which
happen to have lenses close to the line of sight. Typically, for background
objects such as quasars which lie at vast distances, about 0.2% of objects show
the multiple imaging we expect from a gravitational lens system. Because of
this low success rate, lens surveys are a formidable undertaking; a survey
carried out by Jodrell Bank and collaborating institutions (CLASS, to be described
later) has taken 10 years to complete. Over 20 years after the first lens was
discovered, only 70 galaxy-mass lens systems are known. A complete list (known as CASTLeS)
of lenses discovered by all published surveys to date, including HST pictures,
is maintained by the gravitational lens group at the Center for Astrophysics at
For a gravitational lens survey, several things are ideally needed:
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After the discovery of the first gravitational
lens, systematic searches got under way. In the radio, a collaboration between
the Massachusetts Institute of Technology (MIT) and Greenbank Observatory in
Other surveys began at about the same time. With the launch of the Hubble Space Telescope in 1990, a long-term survey began working at optical wavelengths. This has so far discovered about a dozen gravitational lenses.
A large survey, known as the Cosmic Lens All Sky Survey (CLASS) has been carried out over the last 10 years by a collaboration of Jodrell Bank Observatory (University of Manchester), and institutes in the Netherlands (Leiden Observatory, Dwingeloo Radio Observatory and Groningen University) and the USA (Caltech, the National Radio Astronomy Observatory and the University of Pennsylvania).
CLASS is a radio survey which uses the Very Large
Array (VLA) in
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1. The angular resolution of observations obtained at an
observing wavelength L with an
interferometer array of maximum dimension d (both measured in the same length units) is L/d radians. Calculate the resolution, in arcseconds, of
the CLASS VLA observations. Is this a suitable resolution for gravitational
lens searching? |
The survey targeted a class of radio source known as flat-spectrum radio sources. "Flat spectrum" implies that they emit approximately the same flux over a range of radio frequencies.
In astrophysical terms, this means that they are very compact. The physics of the emitting regions is such that only compact radio-emitting regions produce flat radio spectra; less compact regions produce radio emission which diminishes at shorter wavelengths. From our point of view, compact sources are useful because they make gravitational lensing easy to recognise; the intrinsic object is just a point and any structure detected in the VLA observations automatically labels the source as a potential gravitational lens.
This is, however, not the end of the story. 97% of objects in the VLA observations are point sources (showing no structure which would suggest multiple images) and can be discarded immediately from the search. However, the remaining 3%, as well as gravitational lenses, contain objects with some intrinsic radio structure. This happens when the core of the quasar ejects some material which can be seen at radio wavelengths, making the source appear extended.
In order to separate genuine cases of lensing from
intrinsic structure, we perform observations at higher resolution using the MERLIN array at 6cm, see Figure 3.4.
This has a maximum baseline of 220km.
2. Calculate the resolution of the MERLIN observations and confirm that they have a higher resolution than the VLA observations.
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The MERLIN observations reject about 80% of the remaining candidates by showing detail in the extended radio structure. The essential point here is that although lensing can magnify images, and thereby change the total brightness of images with respect to each other, the surface brightness, or brightness per unit area on the sky, remains constant. If we observe with higher resolution, therefore, and find that one of the images becomes weaker and more nebulous whereas the other remains bright and pointlike, we can deduce that the images cannot be gravitationally lensed versions of the same background object.
The final stage is to observe the remaining
candidates not rejected by MERLIN, at still higher resolution using the
Figure 3.5 shows the gravitational lenses discovered by the CLASS survey. The survey has just been completed and all but four of the lenses have been confirmed and published. Note that most are the simple 2-image or 4-image configurations that you saw in the lens simulations in the first section of this course.
Try
to reproduce the four-image configurations seen in the observations of Fig.
3.5 using the gravitational lens simulator introduced in the previous section. Try in
particular to obtain the interesting configurations seen in 4-image lenses
such as B1422+231 (move the source around the edges of the inner diamond
caustic for these effects) - a separate image of B1422+231 is shown in Figure
3.6 which reveals the position of the fourth image. |
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In the next section we will see how gravitational lensing can be used to probe the distribution of matter in the lensing galaxy.
1. The angular resolution of observations obtained at an
observing wavelength L with an
interferometer array of maximum dimension d (both measured in the same length units) is L/d radians. Calculate the resolution, in arcseconds, of
the CLASS VLA observations. Is this a suitable resolution for gravitational
lens searching?
Answer to question
0.21
arcseconds; yes.
The
wavelength divided by the distance is 0.036/36000.0 (remembering to convert
both L and d to metres), or 10-6 radian. This corresponds
to 0.21 arcseconds, which is smaller than the image splittings of most lenses
and therefore enough for lens searches.
2. Calculate the resolution of the MERLIN observations and confirm that they have a higher resolution than the VLA observations.
Answer to question
0.056
arcseconds
L/d in this case is 0.06/220000.0 radians, hence converting to arcseconds we have 0.06x57x3600/220000 = 0.056 arcseconds which is almost 4x larger than the VLA resolution of 0.21 arcsec.
3. What do you think is going on in 1938+666?
Answer to question
There
are two sources
The
background source is a galaxy which is ejecting two radio components either
side of the galaxy. One radio component is lensed into four images: the arc
structure together with one of the images at the bottom. The other is doubly
imaged, forming one of the other bottom images together with the point image at
the top. The Hubble Space Telescope picture shows the Einstein ring associated
with the image of the extended background galaxy. See our
press release for more details.