Bulge formation through disc instability -- I

TL;DR

This paper uses simulations to explore how disc instabilities lead to pseudobulge formation, revealing relationships between galaxy morphology, bulge-to-total mass ratios, and disc-halo parameters.

Contribution

It introduces a formula linking bulge-to-total ratio to disc contribution, validated against observations and integrated into galaxy formation models.

Findings

Higher disc-to-halo mass ratios increase bar prominence.

Final B/T correlates with disc's gravitational contribution at optical radius.

The B/T formula accurately fits simulation data and matches observational trends.

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 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.