Explaining the "too massive" high-redshift galaxies in JWST data: numerical study of three effects and a simple relation

Abstract

The James Webb Space Telescope has discovered high luminosity galaxies that appear to be "too many" and "too massive" compared to predictions of the Standard LCDM cosmology, suggesting that star formation in the early universe is more rapid than previously anticipated. In this paper we examine in detail the following three effects which can instead provide alternative explanations for these observations: (1) a "top heavy" initial mass function (IMF) for the stars, (2) a variety of star formation histories (constant, exponentially decreasing, and peaked star formation rates), and (3) a variety of initial metallicities. Due to any of these three effects, galaxies of a given luminosity in JWST may be interpreted as having a larger stellar mass than they actually do. Our results are obtained using the Pegase stellar population code, and are presented as the ratio of the modified star formation efficiency relative to the fiducial one (which uses a Salpeter IMF and constant star formation rate). As an example, if the high-mass end of the IMF goes as $M^{-1.35}$, the star formation efficiency and inferred stellar galactic mass could be lower by a factor of $\sim 10$ than in the fiducial case. Our examination (keeping the star formation rate constant) of a top-heavy IMF with slope $\alpha$ leads to a simple relation that is a good approximation to the numerical results, $\epsilon(\alpha) \approx \epsilon_{\rm fid}e^{2.66(\alpha -2.35)}$. Since there are more low mass galaxies than high mass galaxies, these effects may result in a large number of seemingly overly massive galaxies compared to the expectations. Thus, the effects studied in this paper may explain both puzzling observations regarding high luminosity galaxies in JWST: the apparently overly massive galaxies as well as the profusion of apparently high mass galaxies.