Formation of Stars and Clusters over Cosmological Time
The concept that stars are currently forming began some 60 years ago with the key observation of expanding OB associations. Now we see that these associations are an intermediate scale in a cascade of hierarchical structures that begins on the ambient Jeans length close to a kiloparsec in size and continues down to the interiors of clusters, perhaps even to binary and multiple stellar systems. The origin of this structure lies with the dynamical nature of cloud and star formation, driven by supersonic turbulence and interstellar gravity. Dynamical star formation is also relatively fast compared to the timescale for cosmic accretion, and in this limit of rapid gas consumption, the star formation rate essentially keeps up with the accretion rate, whatever that rate is, leading to a sequence of near-equilibrium states during galaxy formation and evolution. In these states, most of the large-scale and statistical properties of galaxies and of star formation inside them depend on the rate of cosmic accretion, which is a function of galaxy mass and epoch. This simplification of star formation on a cosmic scale allows successful simulations and modeling even when the details of star formation on the scale of molecular clouds is unclear. Dynamical star formation also helps to explain the formation of bound clusters, which require a local efficiency that exceeds the average by more than an order of magnitude. Efficiency increases with density in a hierarchically structured gas. Cluster formation should vary with environment as the relative degree of cloud self-binding varies, and this depends mostly on the ratio of the interstellar velocity dispersion to the galaxy rotation speed. As this ratio increases, galaxies become more clumpy, thicker, and have more tightly bound star-forming regions. The formation of old globular clusters is understood in this context, with the metal-rich and metal-poor globulars forming in high-mass and low-mass galaxies, respectively, because of the galactic mass-metallicity relation. This relation is a result of the accretion equilibrium. Metal-rich globulars remain in the disks and bulge regions where they formed, while metal-poor globulars get captured as parts of satellite galaxies and end up in today’s spiral galaxy halos. Blue globulars in the disk could have formed very early when the whole Milky Way had a low mass.