The Origin of Exponential Star-forming Disks
  The disk components of galaxies generally show an exponential profile
extending over several scale lengths, both in mass and star-formation rate, but
the physical origin is not well understood. We explore a physical model in
which the galactic gas disk is viewed as a "modified accretion disk" in which
coplanar gas inflow, driven by viscous stresses in the disk, provides the fuel
for star formation, which progressively removes gas as it flows inwards. We
show that magnetic stresses from magneto-rotational instability are the most
plausible source of the required viscosity, and construct a simple physical
model to explore this. A key feature is to link the magnetic field strength to
the local star-formation surface density, $B_{\rm tot} \propto \Sigma_{\rm
SFR}^\alpha$. This provides a feed-back loop between star-formation and the
flow of gas. We find that the model naturally produces stable steady-state
exponential disks, as long as $\alpha \sim$ 0.15, the value indicated from
spatially-resolved observations of nearby galaxies. The disk scale-length
$h_{\rm R}$ is set by the rate at which the disk is fed, by the normalization
of the $B_{\rm tot}-\Sigma_{\rm SFR}$ relation and by the circular velocity of
the halo. The angular momentum distribution of the gas and stars within the
disk is a consequence of the transfer of angular momentum that is inherent to
the operation of an accretion disk, rather than the initial angular momentum of
the inflowing material. We suggest that magnetic stresses likely play a major
role in establishing the stable exponential form of galactic disks.
