The signature of primordial black holes in the dark matter halos of
  galaxies
  The aim of this paper is to investigate the claim that stars in the lensing
galaxy of a gravitationally lensed quasar system can always account for the
observed microlensing of the individual quasar images. A small sample of
gravitationally lensed quasar systems was chosen where the quasar images appear
to lie on the fringe of the stellar distribution of the lensing galaxy. As with
most quasar systems, all the individual quasar images were observed to be
microlensed. The surface brightness of the lensing galaxy at the positions of
the quasar images was measured from HST frames, and converted to stellar
surface mass density. The surface density of smoothly distributed dark matter
at the image positions was obtained from lensing models of the quasar systems
and applied to the stellar surface mass density to give the optical depth to
microlensing. This was then used to assess the probability that the stars in
the lensing galaxy could be responsible for the observed microlensing. The
results were supported by microlensing simulations of the star fields around
the quasar images combined with values of convergence and shear from the
lensing models. Taken together, the probability that all the observed
microlensing is due to stars was found to be ~0.0003. Errors resulting from
surface brightness measurement, mass-to-light ratio and the contribution of the
dark matter halo do not significantly affect this result. It is argued that the
most plausible candidates for the microlenses are primordial black holes,
either in the dark matter halos of the lensing galaxies, or more generally
distributed along the lines of sight to the quasars.
