Quasar Microlensing at High Magnification and the Role of Dark Matter: Enhanced Fluctuations and Suppressed Saddle Points

Contrary to naive expectation, diluting the stellar component of the lensing galaxy in a highly magnified system with smoothly distributed ``dark'' matter increases rather than decreases the microlensing fluctuations caused by the remaining stars. For parameters typical of a quadruply imaged QSO, saddlepoints (as opposed to minima) of the arrival time surface are much more strongly affected. For a mass ratio of smoothly distributed (dark) matter to clumpy (microlensing) matter of 4:1, a saddlepoint with a macro-magnification of mu=9.5 will spend half of its time more than a magnitude fainter than predicted. The anomalous flux ratio observed for the close pair of images in MG0414+0534 is a factor of five more likely than computed by Witt, Mao and Schechter (1995) if the dark matter fraction is as high as 93%. The magnification probability histograms for each macroimage exhibit structure that varies with the dark matter content, providing a handle on the dark matter fraction. Enhanced fluctuations can manifest themselves either in the temporal variations of a lightcurve or as flux ratio anomalies in a single epoch snapshot of a multiply imaged system. While the milli-lensing simulations of Metcalf and Madau (2001) also give larger anomalies for saddlepoints than for minima, the effect appears to be less dramatic for extended subhalos than for point masses. Furthermore, micro-lensing is distinguishable from milli-lensing because it will produce noticeable changes in the magnification on a time scale of a decade or less.

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