Mergers lighting the early Universe: modest star-formation enhancement without evidence for AGN or Lyα triggering

Abstract

Galaxy mergers and interactions are often invoked to explain enhanced star formation, black hole growth, and mass build-up of galaxies at later cosmic times, but their effect is poorly understood at high redshift (z > 2). We use JADES data to analyse a mass-complete sample of 1968 galaxies at z = 3 − 9 with log(M⋆/M⊙) = [8, 10], identifying major merger pairs (projected separation of 5 − 100 pkpc, mass ratio ≥1/4) using a probabilistic method. To look for signatures of enhancement in multiple physical properties, we carefully build a control sample of non-pairs that are simultaneously matched in redshift, stellar mass, isolation, and environment to the pair sample. We find a moderate enhancement in specific star-formation rate (sSFR) of a factor of 1.10 ± 0.05 at separations ≲ 20 kpc in pairs compared to their matched controls. We find that the interaction-related sSFR enhancement is most apparent at longer SFR averaging timescales (50–100 Myr), whereas the enhancement is not detected on 5—10 Myr timescales, where bursty star formation might be driven by internal processes. By averaging star formation histories, we find two distinct populations: one with monotonically rising SFRs and another with an earlier peak in SFR, potentially corresponding to different interaction stages. Finally, we find no significant excess of AGN in pairs, consistent with galaxy interactions not strongly triggering black hole activity at the separations probed (>5 kpc). Similarly, we also do not detect an excess in the fraction of Lyman-α emitters in pairs, suggesting that at the probed separations, galaxy interactions are not efficient at enhancing Lyman-α photon production and escape, which may only become important at the smallest physical scales.