Bursty star formation and galaxy-galaxy interactions in low-mass galaxies 1 Gyr after the Big Bang

We use CANUCS JWST/NIRCam imaging of galaxies behind the gravitationally-lensing cluster MACS J0417.5-1154 to investigate star formation burstiness in low-mass (M_⋆∼10^8 M_⊙) galaxies at z∼4.7-6.5. Our sample of 123 galaxies is selected using the Lyman break selection and photometric emission-line excess methods. Sixty per cent of the 123 galaxies in this sample have Hα-to-UV flux ratios that deviate significantly from the range of η_1500 values consistent with smooth and steady star formation histories. This large fraction indicates that the majority of low-mass galaxies is experiencing bursty star formation histories at high redshift. We also searched for interacting galaxies in our sample and found that they are remarkably common (∼40 per cent of the sample). Compared to non-interacting galaxies, interacting galaxies are more likely to have very low Hα-to-UV ratios, suggesting that galaxy-galaxy interactions enhance star formation burstiness and enable faster quenching (with timescales of ≲100 Myr) that follows the rapid rise of star formation activity. Given the high frequency of galaxy-galaxy interactions and the rapid SFR fluctuations they appear to cause, we conclude that galaxy-galaxy interactions could be a leading cause of bursty star formation in low-mass, high-z galaxies. They could thus play a significant role in the evolution of the galaxy population at early cosmological times.