Kavli Affiliate: Mark Vogelsberger
| First 5 Authors: Xuejian Shen, Mark Vogelsberger, Michael Boylan-Kolchin, Sandro Tacchella, Rahul Kannan
| Summary:
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 $zgeq 9$. Our study differentiates between the $textit{median UV
radiation yield}$ and the $textit{variability of UV luminosities}$ 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, $sigma_{rm UV} approx 0.75$ mag, matches the observational
constraints at $zapprox 9$. However, increasingly larger $sigma_{rm UV}$ is
necessary when moving to higher redshifts, reaching $sigma_{rm UV} approx
2.0,(2.5),{rm mag}$ at $zapprox 12$ ($16$). 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 $zgtrsim 9$ 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.
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