  A minimum in stellar velocity dispersion is often observed in the central
regions of disc galaxies. To investigate the origin of this feature, known as a
{\sigma}-drop, we analyse the stellar kinematics of a high-resolution N-body +
smooth particle hydrodynamical simulation, which models the secular evolution
of an unbarred disc galaxy. We compared the intrinsic mass-weighted kinematics
to the recovered luminosity-weighted ones. The latter were obtained by
analysing synthetic spectra produced by a new code, SYNTRA, that generates
synthetic spectra by assigning a stellar population synthesis model to each
star particle based on its age and metallicity. The kinematics were derived
from the synthetic spectra as in real spectra to mimic the kinematic analysis
of real galaxies. We found that the recovered luminosity-weighted kinematics in
the centre of the simulated galaxy are biased to higher rotation velocities and
lower velocity dispersions due to the presence of young stars in a thin and
kinematically cool disc, and are ultimately responsible for the {\sigma}-drop.
