A Gravitational Lens Survey

By Dr. Whitney Shane, MIRA's Charles Hitchcock Adams Fellow

When a beam of light passes close to a massive body it is deflected by gravity much as a stream of particles would be. This effect was predicted by Einstein and first observed in starlight passing close to the Sun. Recently many new examples have been found. One of the most interesting of these is the case in which a bright source of light such as a quasar is located behind a galaxy. At the very least this will deform and displace the image of the quasar. In more interesting cases we will see two or more images of the quasar as various light beams passing the galaxy on different sides are deflected toward the observer. In the ideal case, when everything is perfectly symmetrical, we should see the image of the quasar as a ring around the galaxy. The quasar is often variable in light, with changes taking place over times of weeks to years. It is interesting to monitor these changes as they are seen in the various images of the quasar so that we can determine the differences in time of transit along the various lines of sight. Uncorrelated variations tell us something about stars and even planets in distant galaxies, and differences in color indicate the presence of dust in the galaxies. Hopefully as we proceed with our research we can discuss some of these results in more detail.

At MIRA we are in an excellent position to observe these objects. Our telescope is large enough to detect many of them with reasonable exposure times. The superb seeing conditions at the Oliver Observing Station and the good quality of our mirror make it possible for us to separate the very closely spaced images that we encounter. We are able to apportion our observing time in a flexible way so that, bad weather and instrumental failures permitting, we can set up a consistent monitoring program. For these reasons we have decided to make the monitoring of gravitational lenses one of our major observing programs.

Our initial observing list contains eighteen objects, all of which have now been observed (one so recently that the processed image is not yet available). Based on this initial survey we will eliminate some of the objects and concentrate on the more promising ones. We will no doubt make additional changes as the program progresses.

We reproduce here the best images that we have thus far of the candidates for our monitoring program. Most of these images were made using the aging MIRA 2000 by 2000 pixel Axiom camera, but some were made using an ST-7 camera on loan from Dr. Arthur Babcock. All were made with a clear filter (or no filter at all), and the exposure times were between 5 and 30 minutes. They were made on various nights between May 2000 and January 2002.

Note: North is up and east is left.

Q0142-100: There are only two images, but they are among the most widely separated in our sample. The lensing galaxy has been detected, but it is very faint and blended with the weaker of the two images. This is an excellent candidate for observation.
MG0414+0534: This is an extremely faint object, but we are able to see the two strongest images (here blended into one) and, very faintly, the two other images above and to the right. The faint galaxy is somewhere in the middle.
HS0818+1227: Not much is known about this object. There are two quasar images, well separated. The weaker of the two is blended with the lensing galaxy. The image above and to the left is an unrelated star.
SB0909+523: The two quasar images are close but clearly resolved. The very faint lensing galaxy is blended with the rightmost image.
RXJ0911+0551: One quasar image is well separated on the right. Three other images and the lensing galaxy are all blended in the strong image on the left. If these could also be resolved then this would be a very interesting object.
RXJ0921+4528: This is a very poor exposure, which will have to be repeated. The brightest spot is part of the lensed image, and the faint material and the image on the left probably also are part of the system. The object is large and we should be able to resolve it easily.
FBQ0951+2635: There are two quasar images here and a very faint lensing galaxy. This image, which was made by stacking several short exposures, shows some east-west elongation, suggesting that there is hope of resolving the images.
Q0957+561: There are two widely separated components here. The lensing galaxy is blended with the lower quasar image. This is probably the easiest of all our candidates.
B1030+071: This is probably the faintest of all our candidates. The stronger of the two quasar images is just visible in the middle of the frame. Working on this object might be asking too much of our obsolescent CCD chip.
HE1104-1805: The two quasar images are seen clearly separated here. The lensing galaxy is somewhere in between them. The elongated images are probably a result of the southern declination.
PG1115+080: The two brightest quasar images and the lensing galaxy are hopelessly blended in the brightest spot here, but one of the fainter images is seen clearly separated to the upper right and the other is just visible as a faint rightward extension of the bright spot.
B1152+199: This is the only object on our list of which we do not yet have a processed image.
H1413+117: This is known as the "Cloverleaf". It is a wonderfully symmetric arrangement of four images around a faint lensing galaxy. Unfortunately it is so small that we will probably be unable to resolve it. Our best image so far shows only that it is slightly extended.
B1422+231: There are four quasar images here and a complex lensing galaxy. What we see here are two quasar images blended in the upper left spot and one separated from the others in the lower right. The fourth image is seen faintly on the left.
SBS1520+530: Only the two middle spots in the little row of four belong to the lens. The two images are well separated. The left one contains the faint lensing galaxy. A diffraction spike from a nearby bright star is seen on the right.
B1608+656: This is another very faint and small object, so we were delighted to see it so well resolved. The structure is actually quite complex and more ring-like than any of the others. Here we see three distinct components. Those at the top and the lower left are both parts of the ring.
HE2149-2745: We really have no business working on anything this far south, but the separation of the two components is so great that we seem to be able to resolve them. The star images (which are not seen on this small extract) are also elongated, but in a different direction from this composite image.
Q2237+0305: This is the famous Einstein Cross, everyone's favorite gravitational lens. The lensing galaxy is very large and nearby, and so diffuse that it is not seen at all on this little picture. The core of the galaxy must be responsible for the lensing. On our image we see all four quasar images, well enough resolved that we are sure that we can make the required measurements.

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Last updated 12/15/02 DMC