Constraints on the Faint End of the Galaxy Stellar Mass Function at z ? 4-8 from Deep JWST Data

We analyze a sample of 3300 galaxies between redshifts z similar or equal to 3.5 and z similar or equal to 8.5 selected from James Webb Space Telescope (JWST) images in the Hubble Ultra Deep Field and UKIDSS Ultra Deep Survey field, including objects with stellar masses as low as similar or equal to 10(8)M(circle dot) up to z similar or equal to 8. The depth and wavelength coverage of the JWST data allows us, for the first time, to derive robust stellar masses for such high-z, low stellar mass galaxies on an individual basis. We compute the galaxy stellar mass function, after complementing our sample with ancillary data from CANDELS to constrain the GMSF at high stellar masses (M >M-& lowast;). Our results show a steepening of the low stellar mass end slope (alpha) with redshift, with alpha = -1.61 +/- 0.05 at z similar or equal to 4 and alpha = -1.98 +/- 0.14 at z similar or equal to 7. We also observe an evolution of the normalization phi* from z similar or equal to 7 to z similar or equal to 4, with phi*(Z similar or equal to 4)/phi(& lowast;)(Z similar or equal to 7)=130(-50)(+210). Our study incorporates a novel method for the estimation of the Eddington bias, which takes into account its possible dependence both on stellar mass and redshift, while allowing for skewness in the error distribution. We finally compute the resulting cosmic stellar mass density and find a flatter evolution with redshift than previous studies.