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

Abstract

JWST observations have revealed a population of galaxies bright enough that potentially challenge standard galaxy formation models in the $\Lambda$CDM cosmology. Using a minimal empirical framework, we investigate the influence of variability on the rest-frame ultra-violet (UV) luminosity function (UVLF) of galaxies at $z\ge 9$. Our study differentiates between the $\text{median UV radiation yield}$ and the $\text{variability of UV luminosities}$ 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, ${\sigma }_{\mathrm{UV}}\approx 0.75$ mag, matches the observational constraints at $z\approx 9$. However, increasingly larger ${\sigma }_{\mathrm{UV}}$ is necessary when moving to higher redshifts, reaching ${\sigma }_{\mathrm{UV}}\approx 2.0(2.5)\mathrm{mag}$ at $z\approx 12$ ($16$). This implied variability is consistent with expectations of physical processes in high-redshift galaxies such as bursty star formation and cycles of dust clearance. Photometric constraints from JWST at $z\gtrsim 9$ 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.

6 pages, 3 figures. Accepted and to be published on MNRAS

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