Milky Way

Physics

Galaxy

Astronomy

Intergalactic star

Astrophysics

Lenticular galaxy

Star cluster

Star formation

Protostar

Star (game theory)

Spiral galaxy

Monthly Notices of the Royal Astronomical Society

Gravitationally-bound clusters that survive gas removal represent an unusual mode of star formation in the Milky Way and similar spiral galaxies. While forming, they can be distinguished observationally from unbound star formation by their high densities, virialised velocity structures, and star formation histories that accelerate toward the present, but extend multiple free-fall times into the past. In this paper we examine several proposed scenarios for how such structures might form and evolve, and carry out a Bayesian analysis to test these models against observed distributions of protostellar age, counts of young stellar objects relative to gas, and the overall star formation rate of the Milky Way. We show that models in which the acceleration of star formation is due either to a large-scale collapse or a time-dependent increase in star formation efficiency are unable to satisfy the combined set of observational constraints. In contrast, models in which clusters form in a "conveyor belt" mode where gas accretion and star formation occur simultaneously, but the star formation rate per free-fall time is low, can match the observations.

How do bound star clusters form?