JWST catches the assembly of a <i>z</i> ∼ 5 ultra-low-mass galaxy

Using CANUCS imaging we found an apparent major merger of two $z\sim5$ ultra-low-mass galaxies ($M_\star\sim10^{7}M_\odot$ each) that are doubly imaged and magnified $\sim$12-15$\times$ by the lensing cluster MACS 0417. Both experiencing young ($\sim$100 Myr), synchronised bursts star formation with $\log({\rm sSFR/Gyr^{-1}} )\sim$1.3-1.4, yet SFRs just $\sim 0.2 M_\odot\ {\rm yr}^{-1}$. They have sub-solar ($Z\sim0.2Z_\odot$) gas-phase metallicities connected even more metal-poor star-forming bridge. The galaxy forms from will a mass at least $M_\star\sim 2\times10^7 M_\odot$, half it formed during interaction-induced starburst. More than ionizing photons produced system (before merger) been burst. This provides first detailed look involving high-$z$ type believed to be responsible for reionizing Universe. It suggests such can grow via combination obtained through mergers, merger-triggered starbursts, long-term in-situ formation. If mergers common, then starbursts could significant contributors photon budget