Dual Signatures of Bursty Star Formation in High-Redshift UV Luminosity Functions

The UV luminosity function (UVLF) encodes key information about galaxy formation. The slowly evolving bright-end UVLFs at z 10 have made UV variability from bursty star formation a promising explanation, but the impact of such variability need not be restricted to the bright end. Motivated by ultra-deep measurements of the z 7 UVLF, we investigate whether UV variability can provide a unified interpretation of the low-mass star formation efficiency (SFE) at z 7 and the abundance of UV-bright galaxies at z 12. Using abundance matching, we show that a steep faint-end UVLF at z 7 admits a range of interpretations. For modest, mass-independent UV variability, the inferred median SFE starts flattening below M h about10 10.5 M. If UV variability instead grows with decreasing halo mass---a possibility motivated by simulations and observations---the inferred median SFE is steeper and lower by a factor of about three at about10 9 M, consistent with stronger feedback suppression in low-mass halos. Calibrated to the common target z 7 UVLF, these two scenarios diverge strongly when extrapolated to 12 z 17 under redshift-independent UV variability and SFE prescriptions. Strongly mass-dependent M UV scatter links the steep z 7 faint-end slope and high z 12 bright-end abundance as dual signatures of burstiness in similarly low-mass halos. Stronger variability makes bright galaxies contribute a larger fraction of the UV luminosity density, whereas weaker variability favors faint sources, despite their similar predicted reionization histories. At fixed M UV, UV variability broadens the halo mass distribution and lowers the clustering bias, which provides a useful diagnostic of burstiness for JWST and Roman.