Secular evolution and the assembly of bulges

TL;DR

This paper reviews the diverse formation mechanisms of galactic bulges, emphasizing secular evolution, gas accretion, and mergers, and discusses how these processes shape bulge properties and galaxy evolution.

Contribution

It provides a comprehensive overview of bulge formation pathways, highlighting the roles of secular processes, gas inflows, and mergers in shaping bulge diversity.

Findings

Secular evolution can produce pseudo-bulges with disk-like properties.

Cold gas accretion supports bar reformation and pseudo-bulge growth.

Major and minor mergers contribute significantly to classical bulge formation.

Abstract

Bulges are of different types, morphologies and kinematics, from pseudo-bulges, close to disk properties (Sersic index, rotation fraction, flatenning), to classical de Vaucouleurs bulges, close to elliptical galaxies. Secular evolution and bar development can give rise to pseudo-bulges. To ensure prolonged secular evolution, gas flows are required along the galaxy life-time. There is growing evidence for cold gas accretion around spiral galaxies. This can explain the bar cycle of destruction and reformation, together with pseudo-bulge formation. However, bulges can also be formed through major mergers, minor mergers, and massive clumps early in the galaxy evolution. Bulge formation is so efficient that it is difficult to explain the presence of bulgeless galaxies today.