The impact of UV variability on the abundance of bright galaxies at $z geq 9$. (arXiv:2305.05679v1 [astro-ph.GA])

The impact of UV variability on the abundance of bright galaxies at $z geq 9$. (arXiv:2305.05679v1 [astro-ph.GA])
<a href="http://arxiv.org/find/astro-ph/1/au:+Shen_X/0/1/0/all/0/1">Xuejian Shen</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Vogelsberger_M/0/1/0/all/0/1">Mark Vogelsberger</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Boylan_Kolchin_M/0/1/0/all/0/1">Michael Boylan-Kolchin</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Tacchella_S/0/1/0/all/0/1">Sandro Tacchella</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Kannan_R/0/1/0/all/0/1">Rahul Kannan</a>

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.

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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