The long life of ultra diffuse galaxies inside low-density dark matter haloes: the case of AGC 114905

It has long been known that, in the absence of a dark matter (DM) halo, galaxy discs tend to develop global gravitational instabilities that strongly modify their initial structure. The recent discovery of gas-rich ultra diffuse galaxies (UDGs) that seem to live in DM haloes with very low concentrations, a very atypical configuration in the standard cosmological framework, poses therefore a crucial question: is the small contribution from such DM haloes sufficient to stabilize the UDG discs? In this work we investigate this question, focusing on the extreme UDG AGC 114905, which previous works found to be unstable. Here, we revisit these studies, using idealised numerical simulations with arepo of a system composed by a stellar disc, a gas disc and a DM halo in initial equilibrium with each other and with properties based on slightly revised observational data of AGC 114905. We explore different scenarios for the DM halo and we run our simulations for 5 Gyr. We find that in all cases the stellar and the gas discs are stable and that their initial density distributions and kinematic properties remain unchanged during the course of the simulation. We discuss how the apparent discrepancy with previous works (where the UDG developed instabilities) is due to our discs being dynamically hotter and living in slightly more massive DM haloes, in accordance with the new observational constraints, previously unavailable. Our findings demonstrate that AGC 114905 (and likely other similar UDGs) can evolve unperturbed in haloes that challenge current cosmological models.