Origin of monolithic high-z galaxies and UV luminosity of mergering high-z galaxies in the cosmological model with non-standard spectrum of density perturbations

Abstract

The James Webb Space Telescope (JWST) has detected an unexpectedly large number of galaxies at redshifts $z\geq 10$ compared to the predictions of the standard $\Lambda$CDM model. One of possible explanations is the presence of an excess (bump) in the power spectrum of perturbations at the mass scale of these galaxies, which is about $10^{10}M_\odot$. This excess simultaneously shifts the epoch of cosmic reionization to significantly earlier times, in contradiction with observations. Here we show that this defect of the bump model can be avoided if the perturbation spectrum has a cutoff (suppression) at smaller scales, which can be realized by lowering the amplitude of the primordial perturbation spectrum or by considering warm dark matter. With a cutoff present, fewer low-mass halos form and, consequently, fewer stars producing ionizing UV radiation. We also derive the halo merger-rate distribution in the presence of a bump using the extended Press--Schechter (EPS) theory, and verify this distribution against direct $N$-body simulations. Based on this distribution, and under the assumption that mergers trigger starbursts, we compute the UV luminosity function of early galaxies and demonstrate its agreement with available observational data. In the model under consideration, the fraction of early galaxies forming via the monolithic mechanism (through a single large-scale collapse) is significantly increased in comparison with the standard $\Lambda$CDM model, and the first stars appear directly in halos with masses $\geq10^8M_\odot$. We refer to such stars with primordial chemical composition as ``Population~IV stars,'' to distinguish them from the evolutionary different stellar populations.