Resolving 3D Disk Orientation using High-Resolution Images: New
  Constraints on Circumgalactic Gas Inflows
  We constrain gas inflow speeds in star-forming galaxies with color gradients
consistent with inside-out disk growth. Our method combines new measurements of
disk orientation with previously described circumgalactic absorption in
background quasar spectra. Two quantities, a position angle and an axis ratio,
describe the projected shape of each galactic disk on the sky, leaving an
ambiguity about which side of the minor axis is tipped toward the observer.
This degeneracy regarding the 3D orientation of disks has compromised previous
efforts to measure gas inflow speeds. We present HST and Keck/LGSAO imaging
that resolves the spiral structure in five galaxies at redshift $z\approx0.2$.
We determine the sign of the disk inclination for four galaxies, under the
assumption that spiral arms trail the rotation. We project models for both
radial infall in the disk plane and circular orbits onto each quasar sightline.
We compare the resulting line-of-sight velocities to the observed velocity
range of Mg II absorption in spectra of background quasars, which intersect the
disk plane at radii between 69 and 115 kpc. For two sightlines, we constrain
the maximum radial inflow speeds as 30-40 km s$^{-1}$. We also rule out a
velocity component from radial inflow in one sightline, suggesting that the
structures feeding gas to these growing disks do not have unity covering
factor. We recommend appropriate selection criteria for building larger samples
of galaxy--quasar pairs that produce orientations sensitive to constraining
inflow properties.
