Cosmic Reionization Shaped Dwarf Galaxy Evolution in Predictable Ways

ABSTRACT We use the extremely high-resolution ($m_{\rm baryon}=4\rm {M}_\odot$) LYRA cosmological galaxy formation simulations of six dwarf galaxies with $M_{\rm 200c}\sim 10^9\rm {M}_\odot$ at $z=0$ to investigate their stellar assembly histories. Based on the stellar ages at $z=0$, $40\!-\!100~{{\ \rm per\ cent}}$ of their stellar mass formed by reionization, when star formation (SF) is abruptly suppressed. Depending on their halo mass evolution, some of these dwarfs reignite SF post-reionization (rejuvenators), while others remain quenched (reionization relics). However, relics can still grow by more than 50 per cent in stellar mass through mergers post-reionization. We find strong correlations between metallicity distributions and the fraction of stars formed post-reionization ($f_{\rm post-reio}^\star$), with relics having lower median $\rm [Fe/H]$ and a prominent low-metallicity tail. Moreover, the shape of the galaxies at $z=0$ also correlates with their $f_{\rm post-reio}^\star$, with rejuvenators showing more spherical stellar distribution than relics. This difference arises only post-reionization when rejuvenators become rounder with more SF activity. Similarly, the shape of dark matter (DM) haloes in the inner regions display more spherical distributions in rejuvenators than in relics. While haloes become rounder than their collision-less DM-only counterparts, this evolution is stronger in rejuvenators. Finally, we find no correlation between SF activity and the formation of shallow DM density cores in these galaxies. These predictions can be tested using upcoming observational data. In particular, our results indicate that the scatter in the mass–metallicity relation in the low-mass regime is correlated with SF histories and the shape of galaxies.