Gravitational microlensing - The effect of random motion of individual stars in the lensing galaxy

We investigate the influence of random motion of individual stars in the lensing galaxy on the light curve of a gravitationally lensed background quasar. We compare this with the effects of the transverse motion of the galaxy. We find that three-dimensional random motion of stars with a velocity dispersion σ in each dimension is more effective in producing "peaks" in a microlensed light curve by a factor α ~ 1.3 than motion of the galaxy with a transverse velocity v_t_ = σ. This effectiveness parameter α seems to depend only weakly on the surface mass density. With an assumed transverse velocity of v_t_ = 600 km s^-1^ of the galaxy lensing the QSO 2237+0305 and a measured velocity dispersion of σ = 215 km s^-1^, the expected rate of maxima in the light curves calculated for bulk motion alone has to be increased by about 10% due to the random motion of stars. As a consequence, the average time interval {DELTA}t between two high-magnification events is smaller than the time interval {DELTA}t_bulk_, calculated for bulk motion alone, {DELTA}t~0.9{DELTA}t_bulk_.