Understanding Stellar Mass-Metallicity and Size Relations in Simulated Ultrafaint Dwarf Galaxies

Reproducing the physical characteristics of ultrafaint dwarf galaxies (UFDs) in cosmological simulations is challenging, particularly with respect to stellar metallicity and galaxy size. To investigate these difficulties in detail, we conduct high-resolution simulations (M_gas ∼ 60 M_⊙, M_DM ∼ 300 M_⊙) on six UFD analogs (M_vir ∼ 10⁸-10⁹ M_⊙, M_⋆ ∼ 10³-2.1 × 10⁴ M_⊙ at z = 0). Our findings reveal that the stellar properties of the UFD analogs are shaped by diverse star-forming environments from multiple progenitor halos in the early Universe. Notably, our UFD analogs exhibit a better match to the observed mass-metallicity relation, showing higher average metallicity compared to other theoretical models, though our results remain 0.5-1 dex lower than for observed UFDs. The metallicity distribution functions (MDFs) of our simulated UFDs lack high-metallicity stars ([Fe/H]≥ −2.0) while containing low-metallicity stars ([Fe/H] < −4.0). Excluding these low-metallicity stars, our results align well with the MDFs of observed UFDs. However, forming stars with higher metallicity (−2.0 ≤ [Fe/H]_max ≤ −1.5) remains a challenge, due to the difficulty of sustaining metal enrichment during the brief star formation period before cosmic reionization. Additionally, our simulations show extended outer structures in UFDs, similar to recent Milky Way UFD observations, resulting from dry mergers between progenitor halos. To ensure consistency, we adopt the same fitting method commonly used in observations to derive the half-light radius. We find that this method tends to produce lower values compared to direct calculations and struggles to accurately describe the extended outer structures.