Stellar Mass Growth in the First Galaxies: Theory and Observation

Abstract: We compare the growth in stellar mass of galaxies in the $6<z\<12$ epoch with predictions of a semi-analytic galaxy formation model - Galacticus. In contrast to diverse and controversial results that compare models and data for the \emph{luminosity} evolution of galaxies -- reported in an abundance of papers, we find very good, unambiguous agreement in the more fundamental quantity of stellar mass - measured from JWST observations - and Galacticus predictions. Specifically, we find good agreement for the shape of the integrated stellar mass as a function of redshift without any adjustment of parameters, and in \emph{amplitude} as well, when 'feedback' is lowered by a factor of 3 compared to that required to match later-universe models and data. This result emerged from detailed investigation of the claim by Dressler et al. that bursts of star formation dominated the growth in stellar mass, specifically, that half of the galaxies with stellar mass growth of $M_* > 2\times10^8 \mathrm{M}\odot$ in the epoch $8<z<6$ had less than $M*<\times10^8 \mathrm{M}_\odot$ prior to $z = 8$. Here too we find agreement between models and data, namely that these ~100 Myr 'bursts' had strong in situ growth at $z\le8$, or showed (in Galacticus) substantial stellar and/or gas-rich mergers, and 30-40 Myr 'starbursts' as are common in $z<3$ galaxies. We note that, if a theoretical simulation is unable to pass the test of matching the growth of stellar mass, any success in reproducing the luminosity function is meaningless.