Extreme-value modelling of the brightest galaxies at z≳9

Data from the James Webb Space Telescope have revealed an intriguing population of bright galaxies at high redshifts. In this work, we use extreme-value statistics to calculate the distribution (in UV magnitude) of the brightest galaxies in the redshift range 9 ≲ z ≲ 16. We combine the Generalised Extreme Value (GEV) approach with modelling of the galaxy luminosity function. We obtain predictions of the brightest galaxies for a suite of luminosity functions, including the Schechter and double power law functions, as well as a model parametrised by the stellar formation efficiency f_*. We find that the JWST data is broadly consistent with f_* of 5%-10%, and that the brightest galaxy at z∼16 will have M_ UV≈ -23.5^0.8_0.4. If f_* is dependent on halo mass, we predict M_ UV≈ -22.5^0.5_1.5 for such an object. We show that extreme-value statistics not only predicts the magnitude of the brightest galaxies at high redshifts, but may also be able to distinguish between models of star formation in high-redshift galaxies.