Coplanar gas inflow can be hidden within warped galactic gas disks
  Simulations suggest that galactic gas disks can be treated as "modified
accretion disks", in which coplanar gas spirals into the inner regions of the
disk, while being consumed by star-formation and removed by outflows.
Observationally there is little evidence for such inflows within the outer
disks of galaxies. Taking realistic gas surface densities from observations,
the radial velocity of the inflow is only a few km s$^{-1}$ within two
scalelengths, but gradually increases with radius to of order 50-100 km
s$^{-1}$ at the very outer disk. The effects of this inflow on the 2-d velocity
field are examined and shown to be broadly similar to those produced by warped
disks, with twist distortions of both the kinematic major and minor axes. By
examining the twists of kinematic distortions and the spiral arms for a sample
of nearby galaxies, we find that the effect of warps are likely to dominate
over the effect of radial inflows. However, we then model mock HI velocity
fields that combine warps with inflow velocities of the strength required in
the modified accretion disks, and show that these composite systems can
actually also be very well matched by pure warped disk models, with $\sim$85\%
of the mock galaxies having a mean absolute error in the residuals of less than
10 km s$^{-1}$. This suggests that the signatures of significant radial inflows
can easily be "hidden" within the warps and that this may therefore explain the
apparent failure to detect radial inflows in galactic disks.
