The compactness of ultra-faint dwarf galaxies: A new challenge?

Abstract

So far, numerical simulations of ultra-faint dwarf galaxies (UFDs) have failed to properly reproduce the observed size–luminosity relation. In particular, no hydrodynamical simulation run has managed to form UFDs with a half-light radius as small as 30 pc, as seen in observations of several UFD candidates. We tackle this problem by developing a simple but numerically clean and powerful method in which predictions of the stellar content of UFDs from ΛCDM cosmological hydrodynamical simulations are combined with very high-resolution dark-matter-only runs. This method allows us to trace the buildup history of UFDs and to determine the impact of the merger of building-block objects on their final size. We find that, while no UFDs more compact than 20 pc can be formed, slightly larger systems are only reproduced if all member stars originate from the same initial mini-halo. However, this imposes that (i) the total virial mass is smaller than 3 × 108 M⊙, and (ii) the stellar content prior to the end of the reionisation epoch is very compact (< 15 pc) and strongly gravitationally bound, which is a challenge for current hydrodynamical numerical simulations. If initial stellar building blocks are larger than 35 pc, the size of the UFD will extend to 80 pc. Finally, our study shows that UFDs keep strong imprints of their buildup history in the form of elongated or extended stellar halos. Those features can erroneously be interpreted as tidal signatures.

Deep Hubble Space Telescope Photometry of Large Magellanic Cloud and Milky Way Ultrafaint Dwarfs: A Careful Look into the Magnitude–Size Relation

“…Despite the overlap of simulations with the brighter UFDs, most simulations are unable to produce analogs of the most compact observed UFDs (r h ∼ 30 pc; e.g., Revaz 2023). Rather, they are creating UFDs with half-light radii up to an order of magnitude larger than observed.…”

“…Rather, they are creating UFDs with half-light radii up to an order of magnitude larger than observed. Revaz (2023) speculates that this could be due to resolution limits, spurious numerical heating, or improper feedback prescriptions, although the widespread nature of the pattern suggests there is no singular factor. In their own work using DM-only simulations, Revaz (2023) found that minihalo mergers played a large role in the expansion of the simulated UFD sizes.…”