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Abstract:

We study numerically the formation of molecular clouds in large-scale colliding flows including self-gravity. The models emphasize the competition between the effects of gravity on global and local scales in an isolated cloud. Global gravity builds up large-scale filaments, while local gravity -- triggered by a combination of strong thermal and dynamical instabilities -- causes cores to form. The dynamical instabilities give rise to a local focusing of the colliding flows, facilitating the rapid formation of massive protostellar cores of a few 100 M$_\odot$. The forming clouds do not reach an equilibrium state, though the motions within the clouds appear comparable to ``virial''. The self-similar core mass distributions derived from models with and without self-gravity indicate that the core mass distribution is set very early on during the cloud formation process, predominantly by a combination of thermal and dynamical instabilities rather than by self-gravity.

Abstract:

We revisit the analysis of the nonlinear thin shell instability (NTSI) numerically, including magnetic fields. The magnetic tension force is expected to work against the main driver of the NTSI - namely, transverse momentum transport. However, depending on the field strength and orientation, the instability may grow. For fields aligned with the inflow, we find that the NTSI is suppressed only when the Alfvén speed surpasses the (supersonic) velocities generated along the collision interface. Even for fields perpendicular to the inflow, which are the most effective at preventing the NTSI from developing, internal structures form within the expanding slab interface, probably leading to fragmentation in the presence of self-gravity or thermal instabilities. High Reynolds numbers result in local turbulence within the perturbed slab, which in turn triggers reconnection and dissipation of the excess magnetic flux. We find that when the magnetic field is initially aligned with the flow, there exists a (weak) correlation between field strength and gas density. However, for transverse fields, this correlation essentially vanishes. In light of these results, our general conclusion is that instabilities are unlikely to be erased unless the magnetic energy in clouds is much larger than the turbulent energy. Finally, while our study is motivated by the scenario of molecular cloud formation in colliding flows, our results span a larger range of applicability, from supernova shells to colliding stellar winds. © 2007. The American Astronomical Society. All rights reserved.

Abstract:

This contribution discusses the challenges of implementing star formation and stellar feedback processes in galaxy simulations. Insufficient computational power and numerous poorly understood physical processes, force simulations to adopt sub-grid models of the interstellar medium. These may crucially bias results. We advocate for smaller (similar to kiloparsec) scale simulations of the interstellar medium to guide the development of sub-grid models in larger simulations. In this vein, I show results on ever increasing scales ranging from similar to 1 h(-1) kpc(3) ISM simulations, to a 1 h(-1) Mpc(3) simulation of a galaxy forming at evolved high redshift, to a larger cosmological volume, 6.25 h(-1) Mpc(3), down to redshift 3. We find that galactic winds can be powered by SN implemented as point explosions in high redshift galaxies simulated with sufficient spatial resolution. Galaxies at lower redshift require alternative sub-grid feedback models and we present one possible solution to generate winds from galaxies at redshift similar to 4.