Introduction

Mass lost from stars in the central regions of galaxies may flow inwards to form both massive molecular clouds and a young, new stellar population having the form of a thick, rapidly rotating nuclear disc. Angular momentum is transferred from the clouds to the stars in the original spheroid, thereby spinning up the old bulge population within the central ≃ 1 kpc and increasing the general ‘boxiness’ of the underlying stellar density distribution. New stars formed by the collapse of massive molecular clouds in the nuclear disc also contribute towards the general boxiness of the central nuclear bulge.

Bulge-disc interaction

Following previous investigations into the fate of stellar mass loss in the central regions of galaxies (e.g. Bailey & Clube 1978, Bailey 1980, 1982, 1985), we assume that material flows inwards ultimately to form a dense, cold nuclear disc of molecular gas. We assume a disc of radius Rd ≃ 500 pc and mass Md ≈ 108M⊙, embedded within a nuclear bulge of radius Rn ≃ 1 kpc and mass Mn ≃ 1010M⊙, adopting a flat rotation curve. Over a Hubble time, the total stellar mass loss exceeds 10% of the original bulge mass, i.e. ≳ 109M⊙.

We also assume that the nuclear disc fragments into clouds with masses Mc ≃ 106M⊙, by analogy with the clouds in the disc of our Galaxy. These massive clouds, moving at the local circular velocity, suffer dynamical friction against the surrounding bulge stars, which causes them to spiral slowly into the galactic centre.