  We use a suite of idealised N-body simulations to study the impact of
spurious heating of star particles by dark matter particles on the kinematics
and morphology of simulated galactic discs. We find that spurious collisional
heating leads to a systematic increase of the azimuthal velocity dispersion
($\sigma_\phi$) of stellar particles and a corresponding decrease in their mean
azimuthal velocities ($\overline{v}_\phi$). The rate of heating is dictated
primarily by the number of dark matter halo particles (or equivalently, by the
dark matter particle mass at fixed halo mass) and by radial gradients in the
local dark matter density along the disc; it is largely insensitive to the
stellar particle mass. Galaxies within haloes resolved with fewer than $\approx
10^6$ dark matter particles are particularly susceptible to spurious
morphological evolution, irrespective of the total halo mass (with even more
particles required to prevent heating of the galactic centre). Collisional
heating transforms galactic discs from flattened structures into rounder
spheroidal systems, causing them to lose rotational support in the process. It
also affects the locations of galaxies in standard scaling relations that link
their various properties: at fixed stellar mass, it increases the sizes of
galaxies, and reduces their mean stellar rotation velocities and specific
angular momenta. Our results urge caution when extrapolating simulated galaxy
scaling relations to low masses where spurious collisional effects can bias
their normalisation, slope and scatter.
