The Raw Material of Cluster Formation: Observational Constraints

Abstract

Hydrodynamical simulations of star cluster formation trace the conversion of gas into stars and it is thus important that the initial conditions of such simulations are informed by observations of the gas in Giant Molecular Clouds (GMCs). We therefore present a brief overview of the properties of GMCs and how these are deduced observationally, focussing in particular on the three inter-related ‘Larson’ relations. We emphasise the dynamical importance of magnetic fields within GMCs, even though the observational evidence currently suggests that the fields in GMCs are somewhat ‘super-critical’ (i.e. with a mass-to-flux ratio sufficient to allow collapse). We also summarise recent progress in simulating the formation of GMCs through large scale (galaxy-wide) simulations, emphasising the evidence from these simulations that such clouds undergo important interactions with their wider environment on a timescale of a few Myr, and therefore cannot be strictly treated as isolated systems. We also review the observational evidence that GMCs are gravitationally bound and conclude that a significant fraction may be (mildly) gravitationally unbound. The implications of these for the star formation efficiency of GMCs is explored in future chapters.