Galaxies such as our own are constantly forming new stars, primarily in dense clouds of gas and dust. Gravity, acting to collapse the material in the clouds, is variously helped and hindered by turbulence, pressure, magnetism, and potentially feedback from previously-formed stars. The interplay of forces makes it a complex process and it is still not certain how they determine the final properties of the resulting stars and stellar clusters. In an effort to understand this, we combine observations of star forming regions with hydrodynamical and radiative transfer simulations of the process. Thus we can examine the process at the largest scales of entire galaxies in order to understand how star forming clouds are themselves formed, and at the smallest scales where we can follow the formation of individual stars.
The image above shows the results of two simulations of stars forming within a cold, dense and turbulent cloud. The stars are shown as white points. Both simulations were created from the same original setup with the cloud enshrouded by a warm envelope. The cloud shown on the right lost its envelope during its evolution as might be expected if it were exposed to an external source of feedback. As a result, the cloud on the right has formed far fewer stars than the one on the left which retained its envelope, revealing some of the star forming process's dependency on the containment effect provided by the envelope.
The interstellar medium (ISM) is the vast reservoir of gas and dust that fills the galaxy and, as it coalesces, provides material for the next round of star formation. The properties of the ISM are determined by physical processes over a range of scales. We have shown how the densest phases of the ISM, star forming clouds, are born in the interactions of gas with a galaxy's spiral arms. Stars themselves produce feedback such as radiation pressure, stellar winds, and supernovae events, which influence the physical conditions (density, temperature, chemistry, metallicity) in nearby clouds. Magnetohydrodynamic and radiative transfer simulations show how this can affect later star formation and alter the structure of the host galaxy over kiloparsec scales.
Shown is a simulation of a supernova detonation taking place within a star forming cloud. The supernova originated from a star of almost 96 solar masses located in a cluster forming within a dense filament of gas. This snapshot shows how, 86,000 years after the explosion, the cloud has been torn apart and formed low density escape channels. As a result, the stars in the main cluster, seen as the white points near the centre, are no longer accreting, while a nearby cluster to the upper right has remained embedded. Comparison with similar simulations in which earlier feedback in the form of ionization and stellar winds shows that in these cases the supernova's effect may be greatly reduced.
Star formation is a process that initiates in the large-scale dynamics of a galaxy and takes place in the small scales as individual star forming events. As the interstellar gas flows into a spiral arm, it forms a shock where the change in density, coupled to self-gravity and thermal instabilities, leads to the formation of molecular clouds. The interplay between these processes at different scales is not yet fully understood. It can be addressed with the help of high-resolution smoothed particle hydrodynamics simulations. The Lagrangian nature of the code allows us to trace the origin of the gas forming the clouds.