Bulge formation through disc instability -- I
  We use simulations to study the growth of a pseudobulge in an isolated thin
exponential stellar disc embedded in a static spherical halo. We observe a
transition from later to earlier morphological types and an increase in bar
prominence for higher disc-to-halo mass ratios, for lower disc-to-halo size
ratios, and for lower halo concentrations. We compute bulge-to-total stellar
mass ratios $B/T$ by fitting a two-component S\'ersic-exponential
surface-density distribution. The final $B/T$ is strongly related to the disc's
fractional contribution $f_{\rm d}$ to the total gravitational acceleration at
the optical radius. The formula $B/T=0.5\,f_{\rm }^{1.8}$ fits the simulations
to an accuracy of $30\%$, is consistent with observational measurements of B/T
and f_d as a function of luminosity, and reproduces the observed relation
between $B/T$ and stellar mass when incorporated into the GalICS~2.0
semi-analytic model of galaxy formation.
