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There seem to be two distinct modes of global star formation. An inefficient, quiescent mode that is self-regulated by feedback occurs in galactic disks. A violent, efficient starburst mode is triggered and fueled by merger-induced tidal torques, and accounts for the formation of the massive stellar spheroids. The disk mode of star formation most likely also accounts for the low mass spheroids, which are generated dynamically via secular evolution of disks.

Semi-analytic galaxy formation modelling has not yet incorporated the full richness of star formation phenomenology. There are five critical issues currently confronting semi-analytic theory, and improvements in star formation modelling and the dynamical coupling of baryonic and non-baryonic matter will be necessary to address most of them.


I am indebted to my group at Oxford for many discussions of galaxy formation, and in particular for conversations pertinent to this review with Julien Devriendt, Ignacio Ferreras, Adrianne Slyz and James Taylor. Part of this review was completed at the Institut d'Astrophysique de Paris, where I am grateful to the Director, Bernard Fort, for his kind hospitality. I also thank N. Prantzos for useful discussions.

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