Gravitational Lensing in Astronomy

Abstract

Deflection of light by gravity was predicted by General Relativity and observationally confirmed in 1919. In the following decades, various aspects of the gravitational lens effect were explored theoretically. Among them were: the possibility of multiple or ring-like images of background sources, the use of lensing as a gravitational telescope on very faint and distant objects, and the possibility of determining Hubble's constant with lensing. It is only relatively recently, (after the discovery of the first doubly imaged quasar in 1979), that gravitational lensing has became an observational science. Today lensing is a booming part of astrophysics. In addition to multiply-imaged quasars, a number of other aspects of lensing have been discovered: For example, giant luminous arcs, quasar microlensing, Einstein rings, galactic microlensing events, arclets, and weak gravitational lensing. At present, literally hundreds of individual gravitational lens phenomena are known. Although still in its childhood, lensing has established itself as a very useful astrophysical tool with some remarkable successes. It has contributed significant new results in areas as different as the cosmological distance scale, the large scale matter distribution in the universe, mass and mass distribution of galaxy clusters, the physics of quasars, dark matter in galaxy halos, and galaxy structure. Looking at these successes in the recent past we predict an even more luminous future for gravitational lensing.

Electronic supplementary material: Supplementary material is available for this article at 10.12942/lrr-1998-12.

Figure 1

Number of papers on gravitational lensing per year over the last 35 years. This diagram is based on the October 1997 version of the lensing bibliography compiled by Pospieszalska-Surdej, Surdej and Veron [108]. The apparent drop after the year 1995 does not reflect a drop in the number of papers, but rather the incompleteness of the survey.

Figure 2

Setup of a gravitational lens situation: The lens L located between source S and observer O produces two images S₁ and S₂ of the background source.

Figure 3

The relation between the various angles and distances involved in the lensing setup can be derived for the case ≪ 1 and formulated in the lens equation (6).

Figure 4

The critical curves (upper panel) and caustics (lower panel) for an elliptical lens. The numbers in the right panels identify regions in the source plane that correspond to 1, 3 or 5 images, respectively. The smooth lines in the right hand panel are called fold caustics; the tips at which in the inner curve two fold caustics connect are called cusp caustics.

Figure 5

A recent example for the identification of the lensing galaxy in a double quasar system [44]: The left panel shows on infrared (J-band) observation of the two images of double quasar HE 1104-1825 (z_Q = 2.316, Δθ = 3.2 arcsec). The right panel obtained with some new deconvolution technique nicely reveals the lensing galaxy (at z_G = 1.66) between the quasar images. (Credits: European Southern Observatory [182].)

Figure 8

Optical Lightcurves of images Q0957+561 A and B (top panel: g-band; bottom panel: r-band). The blue curve is the one of leading image A, the red one the trailing image B. Note the steep drop that occured in December 1994 in image A and was seen in February 1996 in image B. The light curves are shifted in time by about 417 days relative to each other. (Credits: Tomislav Kundić; see also [106])

Figure 6

In this false color Hubble Space Telescope image of the double quasar Q0957+561A,B. The two images A (bottom) and B (top) are separated by 6.1 arcseconds. Image B is about 1 arcsecond away from the core of the galaxy, and hence seen “through” the halo of the galaxy. (Credits: E.E. Falco et al. (CASTLE collaboration [57]) and NASA.)

Figure 7

Radio image of Q0957+561 from MERLIN telescope. It clearly shows the two point like images of the quasar core and the jet emanating only from the Northern part. (Credits: N. Jackson, Jodrell Bank.)

Figure 9

“Micro-Images”: The top left panel shows an assumed “unlensed” source profile of a quasar. The other three panels illustrate the micro-image configuration as it would be produced by stellar objects in the foreground. The surface mass density of the lenses is 20% (top right), 50% (bottom left) and 80% (bottom right) of the critical density (cf. Equation (16)).

Figure 10

Magnification pattern in the source plane, produced by a dense field of stars in the lensing galaxy. The color reflects the magnification as a function of the quasar position: the sequence blue-green-red-yellow indicates increasing magnification. Lightcurves taken along the yellow tracks are shown in Figure 11. The microlensing parameters were chosen according to a model for image A of the quadruple quasar Q2237+0305: κ = 0.36, γ = 0.44.

Figure 11

Microlensing Lightcurve for the yellow tracks in Figure 10. The solid and dashed lines indicate relatively small and large quasar sizes. The time axis is in units of Einstein radii divided by unit velocity.

Figure 12

mpg-Movie (6.63 MB) Still pictures of microlensing caustics for three values of the surface mass density: a) κ = 0.2; b) κ = 0.5; c) κ = 0.8. The sequences are described and analysed quantitatively in [198](For video see appendix).

Figure 13

Two images of the quadruple quasar Q2237+0305 separated by three years. It is obvious that the relative brightnesses of the images change. Image B is clearly the brightest one in the left panel, whereas images A and B are about equally bright in the right panel. (Credits: Geraint Lewis.)

Figure 14

Lightcurves of the four images of Q2237+0305 over a period of almost ten years. The changes in relative brightness are very obvious. (Credits: Geraint Lewis.)

Figure 15

Einstein ring 1938+666 (from [94]): The left panel shows the radio map as contour superimposed on the grey scale HST/NiCMOS image; the right panel is a color depiction of the infrard HST/NICMOS image. The diameter of the ring is about 0.95 arcseconds. (Credits: Neal Jackson.)

Figure 16

Galaxy Cluster Abell 2218 with Giant Luminous Arcs and many arclets, imaged with the Hubble Space Telescope. The original picture can be found in [98]. (Credits: W. Couch, R. Ellis and NASA.)

Figure 17

Galaxy Cluster CL0024+1654 with multiple images of a blue background galaxy. The original picture and more information can be obtained at [70]. A scientific analysis which includes a reconstruction of the source galaxy can be found in [42]. (Credits: W.N. Colley, E. Turner, J.A. Tyson and NASA.)

Figure 18

The reconstructed mass distribution of cluster CL1358+62 from a weak lensing analysis is shown as contour lines superposed on the image taken with the Hubble Space Telescope [78]. The map is smoothed with a Gaussian of size 24 arcsec (see shaded circle). The center of the mass distribution agrees with the central elliptical galaxy. The numbers indicate the reconstructed surface mass density in units of the critical one. (Credits: Henk Hoekstra.)

Figure 19

Five snapshots of a gravitational lens situation: From left to right the alignment between lens and source gets better and better, until it is perfect in the rightmost panel. This results in the image of an “Einstein ring”.

Figure 20

Five relative tracks between background star and foreground lens (indicated as the central star) parametrized by the impact parameter u_min. The dashed line indicates the Einstein ring for the lens (after [137]).

Figure 21

Five microlensing lightcurves for the tracks indicated in Figure 20, parametrized by the impact parameter u_min. The verical axes is the magnification in astronomical magnitudes relative to the unlensed case, the horizontal axis displays the time in “normalized” units (after [137]).

Figure 22

Observed Lightcurve of a microlensing event towards the bulge of the galaxy, event OGLE #6 [190]: The I-band magnitude is plotted as a function of time (Julian days). In the top panel, the constant V — I color of the star is shown. The maximum magnification is μ = 6.9 (or 2.1 mag), the duration of the event is 8.4 days. The star has constant brightness in the following year (Credits: Andrzej Udalski.)

Figure 23

Lightcurve of a binary microlensing event towards the bulge of the galaxy, event OGLE #7 [189]: The I-band the magnitude is plotted over time (Julian days). In the top panel the constant V-I-color of the star is shown. The maximum magnification is more than 2.5 mag higher than the unlensed brightness. The duration of the event is about 80 days. The two insets at the left part show a zoom of the two peaks. The star had constant brightness in the year preceding the microlensing event (1992). A model for this event finds a mass ratio of 1.02 between the two lensing stars, and a separation of 1.14 Einstein radii. (Credits: Andrzej Udalski.)