  Metal pollution in white dwarf atmospheres is likely to be a signature of
remnant planetary systems. Most explanations for this pollution predict a sharp
decrease in the number of polluted systems with white dwarf cooling age.
Observations do not confirm this trend, and metal pollution in old (1-5 Gyr)
white dwarfs is difficult to explain. We propose an alternative,
time-independent mechanism to produce the white dwarf pollution. The orbit of a
wide binary companion can be perturbed by Galactic tides, approaching close to
the primary star for the first time after billions of years of evolution on the
white dwarf branch. We show that such a close approach perturbs a planetary
system orbiting the white dwarf, scattering planetesimals onto star-grazing
orbits, in a manner that could pollute the white dwarf's atmosphere. Our
estimates find that this mechanism is likely to contribute to metal pollution,
alongside other mechanisms, in up to a few percent of an observed sample of
white dwarfs with wide binary companions, independent of white dwarf age. This
age independence is the key difference between this wide binary mechanism and
others mechanisms suggested in the literature to explain white dwarf pollution.
Current observational samples are not large enough to assess whether this
mechanism makes a significant contribution to the population of polluted white
dwarfs, for which better constraints on the wide binary population are
required, such as those that will be obtained in the near future with Gaia.
