galics – III. Properties of Lyman‐break galaxies at a redshift of 3

This paper illustrates how mock observational samples of high‐redshift galaxies with sophisticated selection criteria can be extracted from the predictions of galics , a hybrid model of hierarchical galaxy formation that couples the outputs of large cosmological simulations and semi‐analytic recipes, to describe dark matter collapse and the physics of baryons. As an example of this method, we focus on the properties of Lyman‐break galaxies at redshift z ∼ 3 (hereafter LBGs) in a Λ cold dark matter (ΛCDM) cosmology. With the momaf software package described in a companion paper, we generate a mock observational sample with selection criteria as similar as possible to those implied in the actual observations of z ∼ 3 LBGs by Steidel, Pettini & Hamilton. We need to introduce an additional ‘maturity’ criterion to circumvent subtle effects due to mass resolution in the simulation. We predict a number density of 1.15 arcmin −2 at R ≤ 25.5 , in good agreement with the observed number density 1.2 ± 0.18 arcmin −2 . Our model allows us to study the efficiency of the selection criterion to capture z ∼ 3 galaxies. We find that the colour contours designed from models of spectrophotometric evolution of stellar populations are able to select more ‘realistic’ galaxies issued from models of hierarchical galaxy formation. We quantify the fraction of interlopers (12 per cent) and the selection efficiency (85 per cent), and we give estimates of the cosmic variance. We then study the clustering properties of our model LBGs. They are hosted by haloes with masses ∼1.6 × 10 12  M ⊙ , with a linear bias parameter that decreases with increasing scale from b = 5 to 3. The amplitude and slope of the two‐dimensional correlation function is in good agreement with the data. We investigate a series of physical properties: ultraviolet (UV) extinction (a typical factor 6.2 at 1600 Å), stellar masses, metallicities and star formation rates, and we find them to be in general agreement with observed values. The model also allows us to make predictions at other optical and infrared/submillimetre wavelengths, that are easily accessible though queries to a web‐interfaced relational data base. Looking into the future of these LBGs, we predict that 75 per cent of them end up as massive ellipticals and lenticulars today, even though only 35 per cent of all our local ellipticals and lenticulars are predicted to have a LBG progenitor. In spite of some shortcomings that come from our simplifying assumptions and the subtle propagation of mass resolution effects, this new ‘mock observation’ method clearly represents a first step toward a more accurate comparison between hierarchical models of galaxy formation and real observational surveys.