Limits on the microlens mass function of Q2237+0305
Abstract
Gravitational microlensing at cosmological distances is potentially a powerful tool for probing the mass functions of stars and compact objects in other galaxies. In the case of multipli-imaged quasars, microlensing data has been used to determine masses of the microlenses (eg. Lewis and Irwin 1996; Schmidt and Wambsganss 1998). However such measurements have relied on an assumed transverse velocity for the lensing galaxy. Since the measured mass scales with the square of the transverse velocity, published mass limits are quite uncertain. We have properly constrained this uncertainty in previous work by treating the transverse velocity as a variable, and shown that limits can be placed on its value (Wyithe, Webster and Turner 1999b). The distribution of light curve derivatives allows us to treat the relative rates of microlensing due to proper motions of microlenses, the orbital stream motion of microlenses and the bulk galactic transverse velocity quantitatively (Wyithe, Webster and Turner 1999a). By demanding that the microlensing rate due to the motions of microlenses in the bulge is the minimum that should be observed we determine lower limits of the average mass of stars and compact objects in the bulge of Q2237+0305. If microlenses in the bulge are assumed to move in an orbital stream this limit is 0.004-0.022$M_{\odot}$. However, if the microlenses are assumed to move according to an isotropic velocity dispersion then a larger lower limit of 0.02-0.11$M_{\odot}$ is obtained. A significant contribution of Jupiter mass compact objects to the mass distribution of the bulge is therefore unambiguously ruled out in this case.Keywords
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