Dark-ages reionization and galaxy formation simulation – XVII. Sizes, angular momenta, and morphologies of high-redshift galaxies

We study the sizes, angular momenta, and morphologies of high-redshift galaxies, using an update of the MERAXES semi-analytic galaxy evolution model. Our model successfully reproduces a range of observations from redshifts z = 0-10. We find that the effective radius of a galaxy disc scales with ultraviolet (UV) luminosity as R-e proportional to L-UV(0.33) at z = 5-10, and with stellar mass as R-e proportional to M-*(0.24) at z = 5 but with a slope that increases at higher redshifts. Our model predicts that the median galaxy size scales with redshift as R-e proportional to (1 + z)(-m), where m = 1.98 +/- 0.07 for galaxies with (0.3-1)L-z=3* and m = 2.15 +/- 0.05 for galaxies with (0.12-0.3)L-z=3*. We find that the ratio between stellar and halo specific angular momentum is typically less than 1 and decreases with halo and stellar mass. This relation shows no redshift dependence, while the relation between specific angular momentum and stellar mass decreases by similar to 0.5 dex from z = 7 to z = 2. Our model reproduces the distribution of local galaxy morphologies, with bulges formed predominantly through galaxy mergers for low-mass galaxies, disc-instabilities for galaxies with M-* similar or equal to 10(10)-10(11.5)M(circle dot), and major mergers for the most massive galaxies. At high redshifts, we find galaxy morphologies that are predominantly bulge-dominated.