We use semi-analytic techniques to study the formation and evolution of brightest cluster galaxies (BCGs). We show the extreme hierarchical nature of these objects and discuss the limits of simple ways to capture their evolution. In a model where cooling flows are suppressed at late times by AGN activity, the stars of BCGs are formed very early (50 per cent at z~5, 80 per cent at z~3) and in many small galaxies. The high star formation rates in these high-z progenitors are fuelled by rapid cooling, not by merger-triggered starbursts. We find that model BCGs assemble surprisingly late: half their final mass is typically locked-up in a single galaxy after z~0.5. Because most of the galaxies accreted onto BCGs have little gas content and red colours, late mergers do not change the apparent age of BCGs. It is this accumulation of a large number of old stellar populations -- driven mainly by the merging history of the dark matter halo itself -- that yields the observed homogeneity of BCG properties. In the second part of the paper, we discuss the evolution of BCGs to high redshifts, from both observational and theoretical viewpoints. We show that our model BCGs are in qualitative agreement with high-z observations. We discuss the hierarchical link between high-z BCGs and their local counter-parts. We show that high-z BCGs belong to the same population as the massive end of local BCG progenitors, although they are not in general the same galaxies. Similarly, high-z BCGs end-up as massive galaxies in the local Universe, although only a fraction of them are actually BCGs of massive clusters.Comment: 13 pages, 17 figures, MNRAS accepted versio

A large-scale hydrodynamical cosmological simulation, Horizon-AGN , is used to investigate the alignment between the spin of galaxies and the cosmic filaments above redshift 1.2. The analysis of more than 150 000 galaxies per time step in the redshift range 1.2 < z < 1.8 with morphological diversity shows that the spin of low-mass blue galaxies is preferentially aligned with their neighbouring filaments, while high-mass red galaxies tend to have a perpendicular spin. The reorientation of the spin of massive galaxies is provided by galaxy mergers, which are significant in their mass build-up. We find that the stellar mass transition from alignment to misalignment happens around 3 × 10 10 M ⊙ . Galaxies form in the vorticity-rich neighbourhood of filaments, and migrate towards the nodes of the cosmic web as they convert their orbital angular momentum into spin. The signature of this process can be traced to the properties of galaxies, as measured relative to the cosmic web. We argue that a strong source of feedback such as active galactic nuclei is mandatory to quench in situ star formation in massive galaxies and promote various morphologies. It allows mergers to play their key role by reducing post-merger gas inflows and, therefore, keeping spins misaligned with cosmic filaments.

Observations of distant supernovae indicate that the Universe is now in a phase of accelerated expansion the physical cause of which is a mystery. Formally, this requires the inclusion of a term acting as a negative pressure in the equations of cosmic expansion, accounting for about 75 per cent of the total energy density in the Universe. The simplest option for this 'dark energy' corresponds to a 'cosmological constant', perhaps related to the quantum vacuum energy. Physically viable alternatives invoke either the presence of a scalar field with an evolving equation of state, or extensions of general relativity involving higher-order curvature terms or extra dimensions. Although they produce similar expansion rates, different models predict measurable differences in the growth rate of large-scale structure with cosmic time. A fingerprint of this growth is provided by coherent galaxy motions, which introduce a radial anisotropy in the clustering pattern reconstructed by galaxy redshift surveys. Here we report a measurement of this effect at a redshift of 0.8. Using a new survey of more than 10,000 faint galaxies, we measure the anisotropy parameter beta = 0.70 +/- 0.26, which corresponds to a growth rate of structure at that time of f = 0.91 +/- 0.36. This is consistent with the standard cosmological-constant model with low matter density and flat geometry, although the error bars are still too large to distinguish among alternative origins for the accelerated expansion. The correct origin could be determined with a further factor-of-ten increase in the sampled volume at similar redshift.

We reproduce the blue and red sequences in the observed joint distribution of colour and magnitude for galaxies at low and high redshifts using hybrid N‐body/semi‐analytic simulations of galaxy formation. The match of model and data is achieved by mimicking the effects of cold flows versus shock heating coupled to feedback from active galactic nuclei (AGNs), as predicted by Dekel and Birnboim. After a critical epoch z∼ 3, only haloes below a critical shock‐heating mass Mshock∼ 1012 M⊙ enjoy gas supply by cold flows and form stars, while cooling and star formation are shut down abruptly above this mass. The shock‐heated gas is kept hot because being dilute it is vulnerable to feedback from energetic sources such as AGNs in their self‐regulated mode. The shutdown explains in detail the bright‐end truncation of the blue sequence at ∼L*, the appearance of luminous red‐and‐dead galaxies on the red sequence starting already at z∼ 2, the colour bimodality, its strong dependence on environment density and its correlations with morphology and other galaxy properties. Before z∼ 2–3, even haloes above the shock‐heating mass form stars by cold streams penetrating through the hot gas. This explains the bright star forming galaxies at z∼ 3–4, the early appearance of massive galaxies on the red sequence, the high cosmological star formation rate at high redshifts and the subsequent low rate at low redshifts.