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10.3 Gravitational Lensing

The gravitational lensing effects of galaxies were discussed decades ago by Zwicky (1937). More recently, both gravitational lensing and microlensing events have started to reveal hints of an enormous potential for DM studies.

Subramanian and Chitre (1987) pointed out that images of gravitationally lensed quasars can be used to constrain the clumpiness within the lensing object. A clumpy mass distribution tends to amplify the optical continuum of the quasar more than the emission lines, since microlensing due to the clumps is more pronounced for the smaller continuum region. Subramanian and Chitre (1987) applied this idea to MG2016+112, a quasar that is lensed by an intervening galaxy. The degree of lensing suggests the galaxy has a dark halo. These authors find that the material in the halo is clumped on scales between 3 x 104 Msun and 3 x 107 Msun. This tentative result is consistent with supermassive black holes or the dark clusters of the scenario proposed by Carr and Lacey (1987).

A different approach was adopted by Nottale (1986) who suggested that the flaring of 0846+51W1 could be attributed to microlensing by a brown dwarf in an intervening galaxy halo. This object looks like a quasar in its normal state, but when bright it developed the spectral characteristics of a BL Lac object. Notalle (1986) suggested that microlensing would preferentially amplify the blue central region of the object, thereby swamping the characteristic quasar emission lines. The spectrum would then more closely resemble a BL Lac.

Irwin et al. (1989) reported a probable microlensing event in the gravitationally lensed quasar system 2237+0305. This object was studied by Corrigan et al. (1991) who presented light curves in a range of optical wavebands. Subsequent investigations of these light curves by Webster et al. (1991) suggested that the microlensing object might have a mass as low as 5 x 10-5 Msun, although ambiguities in interpreting the data make this a tentative result.

These findings are interesting, but more direct attempts to detect baryonic DM using microlensing have recently commenced. Paczynski (1986) amongst others emphasized the potential of this phenomenon for investigating the nature of DM. There are currently three groups who are in the early stages of conducting microlensing experiments to detect principally brown dwarfs and possibly black holes in the Milky Way halo. Both experiments exploit the fact that MAssive Compact Halos Objects (MACHOs), such as brown dwarfs or black holes, will cause gravitational microlensing if they pass in front of distant stars. The Livermore-Berkeley-Stromlo experiment is concentrating on fields in the Magellanic Clouds which provide a high density of stars (Bennet et al. 1991). The French microlensing experiment relies on Schmidt plates taken at ESO and uses similar fields (Milsztajn 1991; Ferlet et al. 1990).

A third experiment has recently started collecting data at Las Campanas Observatory (Mateo et al. 1992b). This study uses the Galactic bulge as the background stellar field and can potentially detect lensing objects in the Galactic disk and halo.

Details of the detectability of MACHOs are given by these authors and by Griest (1991) and Griest et al. (1991). Objects in the mass range between about 3 x 10-8 Msun and 200 Msun are expected to produce observable signatures, although Gould (1992) suggests this could be extended up to 106 Msun. This is of particular importance since such a range would enclose the entire allowed mass range for compact baryonic objects. Thus, these experiments have the potential for either detecting the DM in the Milky Way halo, or definitively ruling out baryonic DM as the halo material.

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