Comparison of the VIMOS-VLT Deep Survey with the Munich semi-analytical model

Aims: Our aim is to perform the same colour - density analysis on galaxy mock samples as was carried out on a 5 h-1 Mpc scale using the VIMOS-VLT Deep Survey (VVDS), and to compare the results from these mock samples with observed data. This allows us to test galaxy evolution in the model and to understand the relation between the studied environment and the underlying dark matter distribution. Methods: We used galaxy mock catalogues with the same flux limits as the VVDS-Deep (IAB <= 24) survey (Cmocks), constructed using a semi-analytic model for galaxy evolution applied to the Millennium Simulation. From each Cmock, we extracted a sub-sample of galaxies mimicking the VVDS observational strategy (Omocks). We then computed the B-band luminosity function LF and the colour - density relation in the mock samples using the same methods as employed for the VVDS data. Results: We find that the B-band LF in mock samples roughly agrees with the observed LF, but at 0.2 < z < 0.8 the faint-end slope of the model LF is steeper than the observed one. Computing the LF for early- and late-type galaxies separately, we show that mock samples have an excess of faint early-type galaxies and of bright late-type galaxies compared with the data. We find that the colour - density relation in Omocks agrees excellently with that in Cmocks. This suggests that the VVDS observational strategy does not introduce any severe bias to the observed colour - density relation. At z ~ 0.7, the colour - density relation in mock samples agrees qualitatively with observations, with red galaxies residing preferentially in high densities. However, the strength of the colour - density relation in mock samples does not vary within 0.2 < z < 1.5, while the observed relation flattens with increasing redshift and possibly inverts at z ~ 1.3. We argue that the lack of evolution in the colour - density relation in the model cannot be due only to inaccurate prescriptions for the evolution of satellite galaxies, but indicates that the treatment of the central galaxies has also to be revised. Conclusions: The reversal of the colour - density relation can be explained by wet mergers between young galaxies, producing a starburst event. This should be seen on group scales, where mergers are frequent, with possibly some residual trend on larger scales. This residual is found in observations at z = 1.5 on a scale of ~5 h-1 Mpc, but not in the model, suggesting that the treatment of physical processes influencing both satellites and central galaxies in models should be revised. A detailed analysis would be desirable on small scales as well, which requires flux limits fainter than those of the VVDS data