Origin of Bulges

Summary

Insight into the origin of bulges is sought in this review only from the properties of their stellar populations. Evidence concerning the age of the Galactic bulge stellar population is reviewed first, then the case of the bulge of M31 is discussed. The similarity of bulges and ellipticals is then illustrated, inferring that the problems of the origin of bulges and of the origin of ellipticals may well be one and the same: i.e. the origin of galactic spheroids. In this mood, the current evidence concerning the age of the dominant stellar populations of early-type galaxies is then reviewed, both for low- as well as high-redshift galaxies, and both for cluster as well as field ellipticals. All reported evidence argues for the bulk of the stars in galactic spheroids having formed at high redshift, with only minor late additions and a small dependence on environment. An attempt is made to evaluate how current formation scenarios can account for this observational evidence. The role of spheroids in the cosmic star formation and metal enrichment history is also briefly discussed. Finally, some critical questions are asked, answers to which may help our further understanding of the formation and evolution of galactic spheroids.

Introduction

Much on our speculations on how bulges originated depends on what we believe about when they formed. Some scenarios prefer bulges to be young, or middle age, late comers anyway.