FOREVER22: the first bright galaxies with Population III stars at redshifts z ≃ 10–20 and comparisons with JWST data

ABSTRACT We study the formation of the first galaxies in overdense regions modelled by the FORmation and EVolution of galaxies in Extremely overdense Regions motivated by SSA22 (FOREVER22) simulation project. Our simulations successfully reproduce the star formation rates and the MUV–Mstar relations of candidate galaxies at z ∼ 10–14 observed by the JWST. We suggest that the observed galaxies are hosted by dark matter haloes with Mh ≳ 1010 M⊙ and are in short-period starburst phases. On the other hand, even simulated massive galaxies in overdense regions cannot reproduce the intense star formation rates and the large stellar masses of observed candidates at z ∼ 16. Also, we show that the contribution of Population III stars to the ultraviolet (UV) flux decreases as the stellar mass increases and it is a few per cent for galaxies with Mstar ∼ 107 M⊙. Therefore, a part of the observed flux by JWST could be the light from Population III stars. Our simulations suggest that the UV flux can be dominated by Population III stars and the UV slope shows β ≲ −3 if future observations would reach galaxies with Mstars ∼ 105 M⊙ at z ∼ 20 of which the mass fraction of Population III stars can be greater than 10 per cent.