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 Λ Cold Dark Matter (ΛCDM) 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 fgal = 0.03. In order to study general behaviors, only models with the average initial conditions are analyzed. The stellar and B-band effective radii, R∗ and RB , of individual galaxies grow significantly with time (inside-out disc formation) with laws that are weakly dependent on stellar mass, M∗, or luminosity, LB . However, the change of R∗ with z at a fixed M∗ is slow; for z 6 2.5, R∗(M∗=const)∝ (1 + z) 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 RB(LB=const)∝ (1 + z) for z < ∼ 0.75, and ∝ (1 + z) for higher zs. We find also that at z = 0, R∗ ∝ M∗ 0.38 and RB ∝ L B , remaining the slopes of these relations practically the same up to z ≈ 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 seem also to be consistent, within the large scatter, with the RB and LB values obtained from small (non complete) samples of sub-L∗ late-type galaxies with available rest-frame photometric information at different zs. The properties and evolution of the ΛCDM haloes seem to be the main drivers of galaxy disk size evolution. Nevertheless, the models reveal a potential difficulty in explaining the observed steepening of the RB-LB relation with respect to the R∗-M∗ one, an effect related to the well established color-magnitude relation.