No assembly required: mergers are mostly irrelevant for the growth of low-mass dwarf galaxies

We investigate the merger histories of isolated dwarf galaxies based on a suite of 15 high-resolution cosmological zoom-in simulations, all with masses of M_halo ≈ 10^{10} M_{⊙} (and M_\star ∼ 10^5-10^7 M_{⊙}) at z= 0, from the Feedback in Realistic Environments project. The stellar populations of these dwarf galaxies at z= 0 are formed essentially entirely `in situ': over 90{ per cent} of the stellar mass is formed in the main progenitor in all but two cases, and all 15 of the galaxies have > 70{ per cent} of their stellar mass formed in situ. Virtually all galaxy mergers occur prior to z∼ 3, meaning that accreted stellar populations are ancient. On average, our simulated dwarfs undergo five galaxy mergers in their lifetimes, with typical pre-merger galaxy mass ratios that are less than 1:10. This merger frequency is generally comparable to what has been found in dissipationless simulations when coupled with abundance matching. Two of the simulated dwarfs have a luminous satellite companion at z= 0. These ultra-faint dwarfs lie at or below current detectability thresholds but are intriguing targets for next-generation facilities. The small contribution of accreted stars makes it extremely difficult to discern the effects of mergers in the vast majority of dwarfs either photometrically or using resolved-star colour-magnitude diagrams (CMDs). The important implication for near-field cosmology is that star formation histories (SFHs) of comparably massive galaxies derived from resolved CMDs should trace the build-up of stellar mass in one main system across cosmic time as opposed to reflecting the contributions of many individual SFHs of merged dwarfs.