NIHAO XXIV: Rotation or pressure supported systems? Simulated Ultra Diffuse Galaxies show a broad distribution in their stellar kinematics (2004.09535v2)

Abstract: 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.