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 lensinggalaxy in a highly magnified system with smoothly distributed ``dark'' matterincreases rather than decreases the microlensing fluctuations caused by theremaining stars. For a bright pair of images straddling a critical curve, thesaddlepoint (of the arrival time surface) is much more strongly affected thanthe associated minimum. With a mass ratio of smooth matter to microlensingmatter of 4:1, a saddlepoint with a macro-magnification of mu = 9.5 will spendhalf of its time more than a magnitude fainter than predicted. The anomalousflux ratio observed for the close pair of images in MG0414+0534 is a factor offive more likely than computed by Witt, Mao and Schechter if the smooth matterfraction is as high as 93%. The magnification probability histograms formacro-images exhibit distinctly different structure that varies with the smoothmatter content, providing a handle on the smooth matter fraction. Enhancedfluctuations can manifest themselves either in the temporal variations of alightcurve or as flux ratio anomalies in a single epoch snapshot of a multiplyimaged system. While the millilensing simulations of Metcalf and Madau alsogive larger anomalies for saddlepoints than for minima, the effect appears tobe less dramatic for extended subhalos than for point masses. Morever,microlensing is distinguishable from millilensing because it will producenoticeable changes in the magnification on a time scale of a decade or less.Comment: As accepted for publication in ApJ. 17 pages. Substantial revisions include a discussion of constant M/L models and the calculation of a "photometric" dark matter fraction for MG0414+053