NIHAO XXIV: Rotation or pressure supported systems? Simulated Ultra
  Diffuse Galaxies show a broad distribution in their stellar kinematics
  In recent years a new window on galaxy evolution opened, thanks to the
increasing discovery of galaxies with a low surface brightness, such as Ultra
Diffuse Galaxies (UDGs). The formation mechanism of these systems is still a
much debated question, and so are their kinematical properties. In this work,
we address this topic by analyzing the stellar kinematics of isolated UDGs
formed in the hydrodynamical simulation suite NIHAO. We construct projected
line-of-sight velocity and velocity dispersion maps to compute the projected
specific angular momentum, $\lambda_{\rm R}$, to characterize the kinematical
support of the stars in these galaxies. We found that UDGs cover a broad
distribution, ranging from dispersion to rotation supported galaxies, with
similar abundances in both regimes. The degree of rotation support of simulated
UDGs correlates with several properties such as galaxy morphology, higher HI
fractions and larger effective radii with respect to the dispersion supported
group, while the dark matter halo spin and mass accretion history are similar
amongst the two populations. We demonstrate that the alignment of the infalling
baryons into the protogalaxy at early $z$ is the principal driver of the $z$=0
stellar kinematic state: pressure supported isolated UDGs form via mis-aligned
gas accretion while rotation supported ones build-up their baryons in an
ordered manner. Accounting for random inclination effects, we predict that a
comprehensive survey will find nearly half of field UDGs to have rotationally
supported stellar disks.
