The Impact of Patchy Reionization on Ultrafaint Dwarf Galaxies

Abstract

We investigate how patchy reionization affects the star formation history (SFH) and stellar metallicity of ultrafaint dwarf galaxies (UFDs). Patchy reionization refers to varying ultraviolet background strengths depending on a galaxy’s environment. Recent observations highlight the significance of this effect on UFDs, as UFDs can have different SFHs depending on their relative position with respect to their host halo during the period of reionization. However, most cosmological hydrodynamic simulations do not consider environmental factors such as patchy reionization, and the effect of reionization is typically applied homogeneously. Using a novel approach to implement patchy reionization, we show how SFHs of simulated UFDs can change. Our cosmological hydrodynamic zoom-in simulations focus on UFD analogs with M vir ∼ 109 M ⊙, M * ≲ 105 M ⊙ at z = 0. We find that patchy reionization can weaken the effect of reionization by 2 orders of magnitude up to z = 3, enabling late star formation in half of the simulated UFDs, with quenching times ∼460 Myr later than those with homogeneous reionization. We also show that halo merger and mass assembly can affect the SFHs of simulated UFDs, in addition to patchy reionization. The average stellar iron-to-hydrogen ratio, [Fe/H], of the simulated UFDs with patchy reionization increases by 0.22–0.42 dex. Finally, our findings suggest that patchy reionization could be responsible for the extended SFHs of Magellanic UFDs compared to non-Magellanic UFDs.

Discovery and Characterization of Two Ultrafaint Dwarfs outside the Halo of the Milky Way: Leo M and Leo K

“…Witstok et al (2024) find bubble sizes of 0.1 ∼ 1 pMpc at 5.8 < z < 8 with the bubbles embedded in a completely neutral intergalactic medium. Given these results, it is possible that low-mass galaxies at different distances from a more massive galaxy may have been enveloped by the ionized bubbles over different timescales (Kim et al 2023). Hence, it is conceivable that Leo M and Leo K, as well as most of the quenched ultrafaint dwarf satellites, were within the MW-M31 ionizing bubbles at early times, while Pegasus W was farther afield and felt the impact of reionization at a somewhat later time, resulting in a very different SFH.…”

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

“…While essentially all UFDs appear to have formed the majority of their present-day stellar mass by the end of reionization (e.g., Brown et al 2014;Weisz et al 2014), UFDs that inhabited different environments during reionization could have different quenching times due to the inhomogeneity of reionization (Dawoodbhoy et al 2018;Ocvirk et al 2020;Sorce et al 2022;Kim et al 2023). As such, comparing the star formation histories (SFHs) of UFDs that inhabited different environments during reionization could offer insights into this epoch.…”