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

TL;DR

This study investigates the unusual dark matter halo profiles of isolated, gas-rich ultradiffuse galaxies, revealing discrepancies with standard models and simulations, and highlighting challenges for galaxy formation theories.

Contribution

It provides the first detailed analysis of dark matter halos in isolated gas-rich UDGs, showing they have atypically large radii and low concentrations, challenging existing galaxy formation models.

Findings

UDGs have lower-than-median halo concentrations.

Large $R_{max}$ halos are overabundant in simulations.

Inner densities of some UDGs are lower than simulated predictions.

Abstract

We analyze circular velocity profiles of seven ultradiffuse galaxies (UDGs) that are isolated and gas-rich. Assuming that the dark matter halos of these UDGs have a Navarro-Frenk-White (NFW) density profile or a Read density profile (which allows for constant-density cores), 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 kiloparsecs. 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.