The impact of UV variability on the abundance of bright galaxies at z 9

JWST observations have revealed a population of galaxies bright enough that potentially challenge standard galaxy formation models in the Lambda cold dark matter (Lambda CDM) cosmology. Using a minimal empirical framework, we investigate the influence of variability on the rest-frame ultra-violet (UV) luminosity function of galaxies at z >= 9. Our study differentiates between the median UV radiation yield and the variability of UV luminosities of galaxies at a fixed dark matter halo mass. We primarily focus on the latter effect, which depends on halo assembly and galaxy formation processes and can significantly increase the abundance of UV-bright galaxies due to the upscatter of galaxies in lower-mass haloes. We find that a relatively low level of variability, sigma(UV) approximate to 0.75 mag, matches the observational constraints at z approximate to 9. However, increasingly larger sigma(UV) is necessary when moving to higher redshifts, reaching at z approximate to 12 (16). This implied variability is consistent with expectations of physical processes in high-redshift galaxies such as bursty star formation and dust clearance during strong feedback cycles. Photometric constraints from JWST at z greater than or similar to 9 therefore can be reconciled with a standard Lambda CDM-based galaxy formation model calibrated at lower redshifts without the need for adjustments to the median UV radiation yield.