Radiation-driven dusty outflows from early galaxies

ABSTRACT The James Webb Space Telescope (JWST) has discovered an overabundance of UV-bright ($M_{\rm UV} \lesssim -20$), massive galaxies at $z \gtrsim 10$ in comparison to pre-JWST theoretical predictions. Among the proposed interpretations, such excess has been explained by negligible dust attenuation conditions following radiation-driven outflows launched by young stars when a galaxy goes through a super-Eddington phase. Dust opacity decreases the classical Eddington luminosity by a (boost) factor A, thus favouring the driving of outflows by stellar radiation in compact, initially dusty galaxies. Here, we compute A as a function of the galaxy stellar mass, gas fraction, galaxy size, and metallicity (a total of 8 parameters). We find that the main dependence is on metallicity and, for the fiducial model, $A \sim 1800(\mathrm{Z/Z}_\odot)/(1+N_{\rm H}/10^{23.5}\, {\rm cm^2})$. We apply such results to 20 spectroscopically confirmed galaxies at $z \gtrsim 10$ and evaluate their modified Eddington ratio. We predict that three galaxies are in the outflow phase. Their outflows have relatively low velocities ($60 {\small --}100 \, {\rm km\ s^{-1}}$), implying that they are unlikely to escape from the system. For the remaining 17 galaxies that are not currently in the outflow phase, we calculate the past evolution of the modified Eddington ratio from their star formation history. We find that 15 of them experienced an outflow phase prior to observation during which they effectively displaced their dust to larger radii. Thus, dusty outflows driven by stellar radiation appear to contribute to the observed bright UV galaxies at $z > 10$.