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Magnetic fields and the Galactic star formation rate

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Star formation occurs primarily in giant molecular clouds (GMCs), but at a very slow and inefficient rate, usually being restricted to a small fraction of the cloud volume, where stars form in localized clusters. This suggests that the rate limiting step controlling star formation in galactic systems is the formation of dense gas clumps in GMCs. We present numerical hydrodynamic and magnetohydrodynamic simulations of the evolution of GMCs inside a kiloparsec-scale patch of a galactic disc with a minimum resolution of ~0.5 parsec. The thermal behaviour of the gas is followed using extinction-dependent heating and cooling functions.

The hydrodynamical simulations show that energy from galactic shear and large-scale cloud motions continuously cascades down to and within GMCs. This energy drives the turbulent motions within the clouds to balance gravitational collapse. The virial parameters of the clouds remain above unity for times-scales exceeding the free-fall time of GMCs. When we implement star formation ar a slow, inefficient rate of 2% of the local free-fall time, the star formation rates within the patch are about 2 orders of magnitude larger than the observed Kennicutt-Schmidt relation due to over-production of dense gas clumps. We expect magnetic support is required to inhibit dense core formation.

To investigate this hypothesis we carry-out MHD simulations, using realistic field strengths, of the same kiloparsec-scale region. These runs indeed show that a magnetic field inhibits dense clump formation to a fraction of the rate derived from the non-magnetic models. Detailed comparison of the results of these simulations with observed GMCs and IRD Cs allow us to elucidate the dominant physics processes controlling star formation in galactic disks.

This talk is part of the DAMTP Astro Mondays series.

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