Sven Van Loo - April 24, 2015

Magnetic Fields and the Galactic Star Formation Rate

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 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 at a slow, inefficient rate of 2% of the local
free-fall time, the star formation rates in the hydrodynamical case 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 carried-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 IRDCs
allow us to elucidate the dominant physics processes controlling star formation in galactic
disks.