  We study the shapes of spatially integrated H I emission-line profiles of
galaxies in the EAGLE simulation using three separate measures of the profile's
asymmetry. We show that the subset of EAGLE galaxies whose gas fractions and
stellar masses are consistent with those in the xGASS survey also have similar
H I line asymmetries. Central galaxies with symmetric H I line profiles
typically correspond to rotationally supported H I and stellar disks, but those
with asymmetric line profiles may or may not correspond to dispersion-dominated
systems. Galaxies with symmetric H I emission lines are, on average, more gas
rich than those with asymmetric lines, and also exhibit systematic differences
in their specific star formation rates, suggesting that turbulence generated by
stellar or AGN feedback may be one factor contributing to H I line asymmetry.
The line asymmetry also correlates strongly with the dynamical state of a
galaxy's host dark matter halo: older, more relaxed haloes host more-symmetric
galaxies than those hosted by unrelaxed ones. At fixed halo mass, asymmetric
centrals tend to be surrounded by a larger number of massive subhaloes than
their symmetric counterparts, and also experience higher rates of gas accretion
and outflow. At fixed stellar mass, central galaxies have, on average, more
symmetric H I emission lines than satellites; for the latter, ram pressure and
tidal stripping are significant sources of asymmetry.
