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.

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.

This is the first paper of a series that describes the methods and basic results of the GALICS model (Galaxies In Cosmological Simulations). GALICS is a hybrid model for hierarchical galaxy formation studies, combining the outputs of large cosmological N-body simulations with simple, semi-analytic recipes to describe the fate of the baryons within dark matter haloes. The simulations produce a detailed merging tree for the dark matter haloes, including complete knowledge of the statistical properties arising from the gravitational forces. We intend to predict the overall statistical properties of galaxies, with special emphasis on the panchromatic spectral energy distribution emitted by galaxies in the ultraviolet/optical and infrared/submillimetre wavelength ranges.In this paper, we outline the physically motivated assumptions and key free parameters that go into the model, comparing and contrasting with other parallel efforts. We specifically illustrate the success of the model in comparison with several data sets, showing how it is able to predict the galaxy disc sizes, colours, luminosity functions from the ultraviolet to far infrared, the Tully-Fisher and Faber-Jackson relations, and the fundamental plane in the local Universe. We also identify certain areas where the model fails, or where the assumptions needed to succeed are at odds with observations, and pay special attention to understanding the effects of the finite resolution of the simulations on the predictions made. Other papers in this series will take advantage of different data sets available in the literature to extend the study of the limitations and predictive power of GALICS, with particular emphasis put on high-redshift galaxies.

The interplay between cosmic gas accretion on to galaxies and galaxy mergers drives the observed morphological diversity of galaxies. By comparing the state-of-the-art hydrodynamical cosmological simulations Horizon-AGN and Horizon-noAGN, we unambiguously identify the critical role of active galactic nuclei (AGN) in setting up the correct galaxy morphology for the massive end of the population. With AGN feedback, typical kinematic and morpho-metric properties of galaxy populations as well as the galaxy-halo mass relation are in much better agreement with observations. Only AGN feedback allows massive galaxies at the centre of groups and clusters to become ellipticals, while without AGN feedback those galaxies reform discs. It is the merger-enhanced AGN activity that is able to freeze the morphological type of the post-merger remnant by durably quenching its quiescent star formation. Hence morphology is shown to be driven not only by mass but also by the nature of cosmic accretion: at constant galaxy mass, ellipticals are galaxies that are mainly assembled through mergers, while discs are preferentially built from the in situ star formation fed by smooth cosmic gas infall.

We investigate the alignment of the spin of dark matter haloes relative (i) to the surrounding large-scale filamentary structure, and (ii) to the tidal tensor eigenvectors using the Horizon 4π dark matter simulation which resolves over 43 million dark matter haloes at redshift zero. We detect a clear mass transition: the spin of dark matter haloes above a critical mass M s 0 ≈ 5(±1) × 10 12 M tends to be perpendicular to the closest large-scale filament (with an excess probability of up to 12 per cent), and aligned with the intermediate axis of the tidal tensor (with an excess probability of up to 40 per cent), whereas the spin of low-mass haloes is more likely to be aligned with the closest filament (with an excess probability of up to 15 per cent). Furthermore, this critical mass is redshift-dependent, scaling as M s crit (z) ≈ M s 0 (1 + z) −γ s with γ s = 2.5 ± 0.2. A similar fit for the redshift evolution of the tidal tensor transition mass yields M t 0 ≈ 8(±2) × 10 12 M and γ t = 3 ± 0.3. This critical mass also varies weakly with the scale defining filaments.We propose an interpretation of this signal in terms of large-scale cosmic flows. In this picture, most low-mass haloes are formed through the winding of flows embedded in misaligned walls; hence, they acquire a spin parallel to the axis of the resulting filaments forming at the intersection of these walls. On the other hand, more massive haloes are typically the products of later mergers along such filaments, and thus they acquire a spin perpendicular to this direction when their orbital angular momentum is converted into spin. We show that this scenario is consistent with both measured excess probabilities of alignment with respect to the eigendirections of the tidal tensor, and halo merger histories. On a more qualitative level, it also seems compatible with 3D visualization of the structure of the cosmic web as traced by 'smoothed' dark matter simulations or gas tracer particles. Finally, it provides extra support to the disc-forming paradigm presented by Pichon et al. as it extends it by characterizing the geometry of secondary infall at high redshift.

We have studied ∼ 2100 early-type galaxies in the SDSS DR3 which have been detected by the GALEX Medium Imaging Survey (MIS), in the redshift range 0 < z < 0.11. Combining GALEX U V photometry with corollary optical data from the SDSS, we find that, at a 95 percent confidence level, at least ∼ 30 percent of galaxies in this sample have U V to optical colours consistent with some recent star formation within the last Gyr. In particular, galaxies with a N U V − r colour less than 5.5 are very likely to have experienced such recent star formation, taking into account the possibility of a contribution to N U V flux from the UV upturn phenomenon. We find quantitative agreement between the observations and the predictions of a semi-analytical ΛCDM hierarchical merger model and deduce that early-type galaxies in the redshift range 0 < z < 0.11 have ∼ 1 to 3 percent of their stellar mass in stars less than 1 Gyr old. The average age of this recently formed population is ∼ 300 to 500 Myrs. We also find that 'monolithically' evolving galaxies, where recent star formation can be driven solely by recycled gas from stellar mass loss, cannot exhibit the blue colours (N U V − r < 5.5) seen in a significant fraction (∼ 30 percent) of our observed sample.

We develop a subgrid model for the growth of supermassive black holes (BHs) and their associated active galactic nucleus (AGN) feedback in hydrodynamical cosmological simulations. This model transposes previous attempts to describe BH accretion and AGN feedback with the smoothed particle hydrodynamics (SPH) technique to the adaptive mesh refinement framework. It also furthers their development by implementing a new jet‐like outflow treatment of the AGN feedback which we combine with the heating mode traditionally used in the SPH approach. Thus, our approach allows one to test the robustness of the conclusions derived from simulating the impact of self‐regulated AGN feedback on galaxy formation vis‐à‐vis the numerical method. Assuming that BHs are created in the early stages of galaxy formation, they grow by mergers and accretion of gas at a Eddington‐limited Bondi accretion rate. However this growth is regulated by AGN feedback which we model using two different modes: a quasar‐heating mode when accretion rates on to the BHs are comparable to the Eddington rate, and a radio‐jet mode at lower accretion rates which not only deposits energy, but also deposits mass and momentum on the grid. In other words, our feedback model deposits energy as a succession of thermal bursts and jet outflows depending on the properties of the gas surrounding the BHs. We assess the plausibility of such a model by comparing our results to observational measurements of the co‐evolution of BHs and their host galaxy properties, and check their robustness with respect to numerical resolution. We show that AGN feedback must be a crucial physical ingredient for the formation of massive galaxies as it appears to be able to efficiently prevent the accumulation of and/or expel cold gas out of haloes/galaxies and significantly suppress star formation. Our model predicts that the relationship between BHs and their host galaxy mass evolves as a function of redshift, because of the vigorous accretion of cold material in the early Universe that drives Eddington‐limited accretion on to BHs. Quasar activity is also enhanced at high redshift. However, as structures grow in mass and lose their cold material through star formation and efficient BH feedback ejection, the AGN activity in the low‐redshift Universe becomes more and more dominated by the radio mode, which powers jets through the hot circumgalactic medium.

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