The size evolution of galaxy discs formed within Λ cold dark matter haloes

By means of galaxy evolutionary models, we explore the direct consequences of the Lambda cold darkmatter (Lambda CDM) cosmogony on the size evolution of galactic discs, avoiding intentionally the introduction of intermediate (uncertain) astrophysical processes. Based on the shape of the rotation curves and guided by a simplicity criterion, we adopt an average galaxy mass baryon fraction f(gal) = 0.03. In order to study general behaviours, only models with the average initial conditions are analysed. The stellar and B-band effective radii, R-star and R-B, of individual galaxies grow significantly with time (inside-out disc formation) with laws that are weakly dependent on stellar mass, M-star or luminosity, L-B. However, the change of R-star with z at a fixed M-star is slow; for z <= 2.5, R-star (M-star = constant) proportional to (1 + z)(-0.4) for a large range of masses. On the other hand, the change of RB with z at a fixed LB is strong and it resembles the RB decreasing law of the individual models; roughly R-B(L-B = const) proportional to (1 + z)(-0.85) for z less than or similar to 0.75, and proportional to (1 + z)(-1.1) for higher zs. We find also that at z = 0, R-star proportional to M-star(0.38) and R-B proportional to L-B(0.40), remaining the slopes of these relations practically the same up to z approximate to 3. Our model predictions are in reasonable agreement with observational inferences on the typical radius change with z of late-type galaxies more luminous (massive) than high values imposed by the selection effects. The models also seem to be consistent, within the large scatter, with the RB and LB values obtained from small (non-complete) samples of sub-L-star late-type galaxies with available rest-frame photometric information at different zs. The properties and evolution of the Lambda CDM haloes seem to be the main drivers of galaxy disk size evolution. Nevertheless, themodels reveal a potential difficulty in explaining the observed steepening of the R-B-L-B relation with respect to the R-star-M-star one, an effect related to the well-established colour-magnitude relation.