Non-circular motions and the diversity of dwarf galaxy rotation curves
  We use mock interferometric HI measurements and a conventional tilted-ring
modelling procedure to estimate circular velocity curves of dwarf galaxy discs
from the APOSTLE suite of {\Lambda}CDM cosmological hydrodynamical simulations.
The modelling yields a large diversity of rotation curves for an individual
galaxy at fixed inclination, depending on the line-of-sight orientation. The
diversity is driven by non-circular motions in the gas; in particular, by
strong bisymmetric fluctuations in the azimuthal velocities that the
tilted-ring model is ill-suited to account for and that are difficult to detect
in model residuals. Large misestimates of the circular velocity arise when the
kinematic major axis coincides with the extrema of the fluctuation pattern, in
some cases mimicking the presence of kiloparsec-scale density 'cores', when
none are actually present. The thickness of APOSTLE discs compounds this
effect: more slowly-rotating extra-planar gas systematically reduces the
average line-of-sight speeds. The recovered rotation curves thus tend to
underestimate the true circular velocity of APOSTLE galaxies in the inner
regions. Non-circular motions provide an appealing explanation for the large
apparent cores observed in galaxies such as DDO 47 and DDO 87, where the model
residuals suggest that such motions might have affected estimates of the inner
circular velocities. Although residuals from tilted ring models in the
simulations appear larger than in observed galaxies, our results suggest that
non-circular motions should be carefully taken into account when considering
the evidence for dark matter cores in individual galaxies.
