  We study the effect of heavy impurities on the dynamics of supercooled
liquids. When a small fraction of particles in the supercooled liquid is made
heavier, they exhibit slower dynamics than the original particles and also make
the overall system slower. If one looks at the overlap correlation function to
quantify dynamics in the system, it has different behavior for the heavy and
the light particles. In particular, at the relaxation time of the overall
system, the degree of relaxation achieved by the heavier particles is lesser on
average than that achieved by the light particles. This difference in
relaxation however, goes down drastically as a crossover temperature, $T_0$, is
crossed. Below this crossover temperature, particles in the system have similar
relaxation times irrespective of their masses. This crossover temperature
depends on the fraction of the heavy particles and their masses. Next, we
isolate the effect of mass heterogeneity on the dynamics of supercooled liquids
and find that its effect increases monotonically with temperature. We also see
that the development of dynamical heterogeneity with decreasing temperature is
less dramatic for the system with impurities than for the pure system. Finally,
the introduction of heavy impurities can be seen as a way of reducing the
kinetic fragility of a supercooled liquid.
