Bulge formation through disc instability -- I

Abstract

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 $f_{\mathrm{d}}$ to the total gravitational acceleration at the optical radius. The formula $B/T=0.5f_{}^{1.8}$ 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.

19 pages, 11 figures, accepted for publication in Astronomy & Astrophysics