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Bulge formation through disc instability I. Stellar discs

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 Sérsic-exponential surface-density distribution. The ﬁnal B / T is strongly related to the disc’s fractional contribution f d to the total gravitational acceleration at the optical radius. The formula B / T = 0 . 5 f 1 . 8 d 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 G al ICS 2.0 semi-analytic model of galaxy formation.