Modes of Star Formation along the Hubble Sequence and Beyond

I assess the similarities and differences between the star-formation modes in quiescent spiral galaxies versus those in violent starburst regions. As opposed to the quiescent star-formation mode in spiral galaxies, current empirical evidence on the star-formation processes in the extreme, high-pressure environments induced by galaxy encounters strongly suggests that star cluster formation is an important and perhaps even the dominant mode of star formation in such starburst events. The sizes, luminosities, and mass estimates of the young massive star clusters (YMCs) are entirely consistent with what is expected for young Milky Way-type globular clusters (GCs). Recent evidence lends support to the scenario that GCs, which were once thought to be the oldest building blocks of galaxies, are still forming today. One of the key unanswered questions in this field relates to their possible survival chances for a Hubble time, and thus to the potential evolutionary connection between YMCs and GCs. 1. Quiescent versus violent modes of star formation Star formation plays a major role in the evolution of galaxies. However, despite this, it is still rather poorly understood. In fact, on galaxy-wide scales, we do not have much more than a few scaling relations – such as the SchmidtKennicutt law (Schmidt 1959, Kennicutt 1998) – at our disposal. The process of star formation itself, while central to galaxy astrophysics, is one of the remaining “dark corners of the evolutionary process” (e.g., O’Connell 2005). The Schmidt-Kennicutt law, in one of its modern guises relating the (galactic) star formation density to the gas surface density (e.g., Kennicutt 1998), is remarkably successful in describing the star-formation properties of galaxies ranging from quiescent spirals (such as the Milky Way), via circumnuclear and localised star-forming environments (e.g., M83, NGC 6946), to major, galaxywide bursts of star formation (e.g., M82, the “Antennae” system) and – in the extreme – the ultraluminous infrared galaxies (ULIRGs) thought to be the remnants of major galaxy-galaxy interactions. The global star-formation rates