Explaining JWST counts with galaxy formation models

TL;DR

This paper demonstrates that the observed galaxy counts from JWST in the near-infrared are well explained by the GALFORM galaxy formation model, highlighting the roles of luminosity function features, redshift effects, and galaxy types.

Contribution

It provides a detailed explanation of the galaxy count features observed by JWST using the GALFORM model, linking them to galaxy properties and evolutionary effects.

Findings

The break in galaxy counts is due to the luminosity function, volume effects, and k-corrections.

Ellipticals dominate the bright end, spirals the faint end, with a characteristic stellar mass of ~10^10 M_sun.

Most counts at m_AB < 28 are from galaxies with redshift z < 2.

Abstract

A distinct power-law break is apparent m_AB approximately 21 in the deep Near-Infrared PEARLS-JWST galaxy counts. The break becomes more pronounced at longer wavelengths, with the counts slope flattening smoothly with apparent magnitude in the shortest band used at 0.9 microns, trending towards an increasingly broken slope by the longest wavelength passband of JWST NIRCam, 4.4 microns. This behaviour is remarkably well predicted by the GALFORM semi-analytical model of galaxy formation. We use the model to diagnose the origin of this behaviour. We find that the features that are responsible for the break are: 1) the inherent break in the luminosity function; 2) the change in the volume element with redshift and 3) the redshift-dependent nature of the k-correction. We study the contribution to these effects by early and late-type galaxies, using as a proxy for morphology the bulge-to-total stellar mass ratio. We find that the way in which ellipticals populate the bright end of the luminosity function while spirals dominate the faint end is preserved in the galaxy number counts, with a characteristic stellar mass at the break of approximately 10^10 M_sun. We also find that the shape of the number counts is mainly driven by galaxies with relatively low redshift (z < 2) for the PEARLS observational limit of m_AB < 28. We give a comprehensive description of why the galaxy number counts in the near-infrared PEARLS-JWST observation look the way they do and which population of galaxies is dominant at each apparent magnitude.