The Odd Dark Matter Halos of Isolated Gas-Rich Ultra-Diffuse Galaxies
  We analyze circular velocity profiles of seven ultra-diffuse galaxies (UDGs)
that are isolated and gas-rich. Assuming that the dark matter halos of these
UDGs have a modified Navarro-Frenk-White (NFW) density profile with a constant
density core, the inferred halo concentrations are systematically lower than
the cosmological median, even as low as $-0.6$ dex (about $5\sigma$ away) in
some cases. Alternatively, similar fits can be obtained with a density profile
that scales roughly as $1/r^2$ for radii larger than a few $\rm kpc$. Both
solutions require the radius where the halo circular velocity peaks ($R_{\rm
max}$) to be much larger than the median expectation. Surprisingly, we find an
overabundance of such large $R_{\rm max}$ halos in the IllustrisTNG dark
matter-only simulations compared to the Gaussian expectation. These halos form
late and have higher spins compared to median halos of similar masses. The
inner densities of the most extreme among these late-forming halos are higher
than their NFW counterparts, leading to a $\sim 1/r^2$ density profile.
However, the two well-resolved UDGs in our sample strongly prefer lower dark
matter densities in the center than the simulated ones. Comparing to
IllustrisTNG hydrodynamical simulations, we also find a tension in getting both
low enough circular velocities and high enough halo mass to accommodate the
measurements. Our results indicate that the gas-rich UDGs present a significant
challenge for galaxy formation models.
