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We analyze circular velocity profiles of seven ultradiffuse galaxies (UDGs)that are isolated and gas-rich. Assuming that the dark matter halos of theseUDGs have a Navarro-Frenk-White (NFW) density profile or a Read density profile(which allows for constant-density cores), the inferred halo concentrations aresystematically lower than the cosmological median, even as low as $-0.6$ dex(about $5\sigma$ away) in some cases. Alternatively, similar fits can beobtained with a density profile that scales roughly as $1/r^2$ for radii largerthan a few kiloparsecs. Both solutions require the radius where the halocircular velocity peaks ($R_{\rm max}$) to be much larger than the medianexpectation. Surprisingly, we find an overabundance of such large-$R_{\rm max}$halos in the IllustrisTNG dark-matter-only simulations compared to the Gaussianexpectation. These halos form late and have higher spins compared to medianhalos of similar masses. The inner densities of the most extreme among theselate-forming halos are higher than their NFW counterparts, leading to a $\sim1/r^2$ density profile. However, the two well-resolved UDGs in our samplestrongly prefer lower dark matter densities in the center than the simulatedones. Comparing to IllustrisTNG hydrodynamical simulations, we also find atension in getting both low enough circular velocities and high enough halomass to accommodate the measurements. Our results indicate that the gas-richUDGs present a significant challenge for galaxy formation models.

The Odd Dark Matter Halos of Isolated Gas-rich Ultradiffuse Galaxies