a test for compact dark matter in clusters of galaxies
Geraint F. Lewis, PASA, 18 (2), in press.
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Introduction
After several decades of dedicated searching, the nature of cosmological dark matter still eludes us. The two main contenders have been neutrinos and massive compact objects, although recent results suggest that neutrinos are not massive enough to be cosmologically important (Fukuda et al. 1998). Large microlensing surveys have also cast doubt on the importance of compact objects, demonstrating that they are not a significant contributor to the mass budget of the Galactic halo, with only a few tens of percent of the mass of the Galactic halo in the form of compact objects (e.g. Lasserre et al. 2000). This conclusion, however, is uncertain as the dark matter in the Galactic halo may be quite lumpy (e.g. Klypin et al. 1999), with our view towards the Magellanic Clouds unfortunately representative of a under-dense line-of-sight, and hence microlensing experiments to date may not have measured the true overall density of compact objects within the halo. As clusters of galaxies represent the largest bound concentrations of matter in the Universe, their density of dark matter makes them ideal laboratories to probe its nature. Walker & Ireland (1995) proposed that compact dark matter could be detected via its gravitational microlensing influence on our view of distant quasars observed through clusters of galaxies, as this would introduce a flickering into their light curves. Tadros, Warren & Hewett (1998) undertook such a search, monitoring 
quasars behind the Virgo cluster. The vicinity of the Virgo cluster, however, makes it a very poor gravitational lens, with the optical depth in compact objects being 
, so any microlensing induced variability would be rare. Moving to more distant clusters can greatly ``improve'' the lensing geometry, significantly enhancing the microlensing optical depth, although the number of quasars expected behind such distant clusters falls rapidly. Compensating this, a moderate/large number of clusters must be investiaged to ensure a significant population of quasars for study. All of this is further exacerbated by the need to untangle microlensing variability from the intrinsic variability displayed by the majority of quasars.
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Title/Abstract Page: Gravitational microlensing of giant
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© Copyright Astronomical Society of Australia 1997