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Annu. Rev. Astron. Astrophys. 1992. 30:
311-358
Copyright © 1992 by Annual Reviews. All
rights reserved |
The most famous, and also still the best studied gravitational lens,
is the ``double quasar,'' Q0957+561
(Walsh et
al. 1979).
Two optical
and radio images of a z = 1.41 quasar, separated by ~ 6", are
formed on opposite sides of a brightest cluster galaxy with redshift
z = 0.36 (Figure 1). Detailed lens
modeling can account for the
observed images (e.g.
Falco et
al. 1991a)
and a 1.48 yr time lag has
been measured in the variation of the two images
(Lehar et
al. 1992a,
Press et
al. 1992a,
b).
There is one other secure double quasar -
Q0142-100 (Surdej et al. 1987), and five
candidates - Q1120+019
(Meylan &
Djorgovski 1989),
Q1208+101
(Maoz et
al. 1992,
Magain et
al. 1992),
Q1429-008
(Hewett et
al. 1989),
Q1635+267
(Djorgovski &
Spinrad 1984,
Turner et
al. 1988),
and Q2345+007
(Weedman et
al. 1982,
Tyson et
al. 1986),
where the evidence for multiple imaging is not conclusive and where
any lensing galaxies, if present, must be highly subluminous. The
radio galaxy 3C 324 may also be doubly imaged
(Le Fevre et
al. 1987).
The source Q2016+112
(Lawrence et
al. 1984,
Heflin et
al. 1991)
is compellingly argued on the basis of spectroscopic and morphological
data to be triply-imaged by two observed galaxies, but the image
geometry is not well understood (however, see
Narasimha et
al. 1987).
There are four convincing examples of quadruple imaging: 0414+053
[which, although a radio source, may not be a quasar
(Hewitt et
al. 1989;
Elston and Lawrence, personal communication)], Q1115+080 [the
``triple quasar''
(Weymann et
al. 1980,
Young et
al. 1981a)],
H1413+117 [the ``clover leaf''
(Magain et
al. 1988)],
and Q2237+031 [the ``Einstein
cross''
(Huchra et
al. 1985);
see Figure 2]. Despite the absence
of a detected lensing galaxy in 0414+053 and Q1413+117, all four examples are considered
secure because the image arrangement is a pattern quite characteristic
of a gravitational lens (see Figure 6).
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| Figure 1. The ``double quasar'' Q0957+561. (Upper
left) Radio VLA map showing the two quasar images A, B separated
by 6.1" and the lensing galaxy G1. (Lower left) Optical image
of the same field. (Right) High resolution VLBI maps of the
radio jets with size 50 milliarcsec by 100 milliarcsec. (Images
supplied courtesy J. Hewitt, R. Schild, and E. Falco.)
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