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