http://arxiv.org/abs/2204.05981
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.
D. Kong, M. Kaplinghat, H. Yu, et. al.
Wed, 13 Apr 22
66/73
Comments: 17 pages, 14 figures
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