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.

Virtually all massive galaxies, including our own, host central black holes ranging in mass from millions to billions of solar masses. The growth of these black holes releases vast amounts of energy that powers quasars and other weaker active galactic nuclei. A tiny fraction of this energy, if absorbed by the host galaxy, could halt star formation by heating and ejecting ambient gas. A central question in galaxy evolution is the degree to which this process has caused the decline of star formation in large elliptical galaxies, which typically have little cold gas and few young stars, unlike spiral galaxies.

We use both an HI-selected and an optically-selected galaxy sample to directly measure the abundance of galaxies as a function of their "baryonic" mass (stars + atomic gas). Stellar masses are calculated based on optical data from the Sloan Digital Sky Survey (SDSS) and atomic gas masses are calculated using atomic hydrogen (HI) emission line data from the Arecibo Legacy Fast ALFA (ALFALFA) survey. By using the technique of abundance matching, we combine the measured baryonic function (BMF) of galaxies with the dark matter halo mass function in a ΛCDM universe, in order to determine the galactic baryon fraction as a function of host halo mass. We find that the baryon fraction of lowmass halos is much smaller than the cosmic value, even when atomic gas is taken into account. We find that the galactic baryon deficit increases monotonically with decreasing halo mass, in contrast with previous studies which suggested an approximately constant baryon fraction at the low-mass end. We argue that the observed baryon fractions of low mass halos cannot be explained by reionization heating alone, and that additional feedback mechanisms (e.g. supernova blowout) must be invoked. However, the outflow rates needed to reproduce our result are not easily accommodated in the standard picture of galaxy formation in a ΛCDM universe.Stellar mass is not always the dominant baryonic component in a galaxy. In fact, the HI-

We address the origin of the ‘downsizing’ of elliptical galaxies, according to which the stars in more massive galaxies formed earlier and over a shorter period than those in less massive galaxies. We show that this could be the natural result of a shutdown of star formation in dark matter haloes above a critical mass of ∼1012 M⊙. This is demonstrated using a semi‐analytic simulation of galaxy formation within the standard hierarchical scenario of structure formation. The assumed threshold mass is motivated by the prediction of stable shock heating above this mass and the finding that such a shutdown reproduces the observed distribution of galaxies in luminosity and colour. The shutdown at a critical halo mass introduces a characteristic stellar mass for the transition of galaxies into the ‘red sequence’ of the galaxy colour–magnitude diagram. Central galaxies of haloes that are more massive today have reached this mass earlier and can therefore grow further along the red sequence by dry mergers, ending up more massive and containing older stars. Small galaxies formed in haloes below the critical mass can shutdown late, when they fall into haloes above the critical mass and become satellites. While our semi‐analytic simulation that incorporates an explicit shutdown reproduces downsizing as inferred from the stellar ages of ellipticals, we explain why it is much harder to detect downsizing using the mass functions of different galaxy types.

We use three-dimensional high-resolution adaptive-mesh-refinement simulations to investigate if mechanical feedback from active galactic nucleus jets can halt a massive cooling flow in a galaxy cluster and give rise to a self-regulated accretion cycle. We start with a 3 x 10^9MSun black hole at the centre of a spherical halo with the mass of the Virgo cluster. Initially, all the baryons are in a hot intracluster medium in hydrostatic equilibrium within the dark matter's gravitational potential. The black hole accretes the surrounding gas at the Bondi rate and a fraction of the accretion power is returned into the intracluster medium mechanically through the production of jets. The accretion, initially slow (~0.0002MSun/yr), becomes catastrophic, as the gas cools and condenses in the dark matter's potential. Therefore, it cannot prevent the cooling catastrophe at the centre of the cluster. However, after this rapid phase, where the accretion rate reaches a peak of ~0.2MSun/yr, the cavities inflated by the jets become highly turbulent. The turbulent mixing of the shock-heated gas with the rest of the intracluster medium puts a quick end to this short-lived rapid-growth phase. After dropping by almost two orders of magnitudes, the black hole accretion rate stabilises at ~0.006MSun/yr, without significant variations for several billions of years, indicating that a self-regulated steady-state has been reached. This accretion rate corresponds to a negligible increase of the black hole mass over the age of the Universe, but is sufficient to create a quasi-equilibrium state in the cluster core.Comment: 12 pages, 5 figure

The Spitzer Extended Deep Survey (SEDS) is a very deep infrared survey within five well-known extragalactic science fields: the UKIDSS Ultra-Deep Survey, the Extended Chandra Deep Field South, COSMOS, the Hubble Deep Field North, and the Extended Groth Strip. SEDS covers a total area of 1.46 deg 2 to a depth of 26 AB mag (3σ ) in both of the warm Infrared Array Camera (IRAC) bands at 3.6 and 4.5 μm. Because of its uniform depth of coverage in so many widely-separated fields, SEDS is subject to roughly 25% smaller errors due to cosmic variance than a single-field survey of the same size. SEDS was designed to detect and characterize galaxies from intermediate to high redshifts (z = 2-7) with a built-in means of assessing the impact of cosmic variance on the individual fields. Because the full SEDS depth was accumulated in at least three separate visits to each field, typically with six-month intervals between visits, SEDS also furnishes an opportunity to assess the infrared variability of faint objects. This paper describes the SEDS survey design, processing, and publicly-available data products. Deep IRAC counts for the more than 300,000 galaxies detected by SEDS are consistent with models based on known galaxy populations. Discrete IRAC sources contribute 5.6 ± 1.0 and 4.4 ± 0.8 nW m −2 sr −1 at 3.6 and 4.5 μm to the diffuse cosmic infrared background (CIB). IRAC sources cannot contribute more than half of the total CIB flux estimated from DIRBE data. Barring an unexpected error in the DIRBE flux estimates, half the CIB flux must therefore come from a diffuse component.

We study the growth of galaxy masses, via gas accretion and galaxy mergers. We introduce a toy model that describes (in a single equation) how much baryonic mass is accreted and retained into galaxies as a function of halo mass and redshift. In our model, the evolution of the baryons differs from that of the dark matter because 1) gravitational shock heating and AGN jets suppress gas accretion mainly above a critical halo mass of M shock ∼ 10 12 M ; 2) the intergalactic medium after reionisation is too hot for accretion onto haloes with circular velocities v circ < ∼ 40 km s −1 ; 3) stellar feedback drives gas out of haloes, mainly those with v circ < ∼ 120 km s −1 . We run our model on the merger trees of the haloes and sub-haloes of a high-resolution dark matter cosmological simulation. The galaxy mass is taken as the maximum between the mass given by the toy model and the sum of the masses of its progenitors (reduced by tidal stripping). Designed to reproduce the present-day stellar mass function of galaxies, our model matches fairly well the evolution of the cosmic stellar density. It leads to the same z = 0 relation between central galaxy stellar and halo mass as the one found by abundance matching and also as that previously measured at high mass on SDSS centrals. Our model also predicts a bimodal distribution (centrals and satellites) of stellar masses for given halo mass, in very good agreement with SDSS observations. The relative importance of mergers depends strongly on stellar mass (more than on halo mass). Massive galaxies with m stars > m crit ∼ Ω b /Ω m M shock ∼ 10 11 M acquire most of their final mass through mergers (mostly major and gas-poor), as expected from our model's shutdown of gas accretion at high halo masses. However, although our mass resolution should see the effects of mergers down to m stars 10 10.6 h −1 M , we find that mergers are rare for m stars 10 11 h −1 M . This is a consequence of the curvature of the stellar vs. halo mass relation set by the physical processes of our toy model and found with abundance matching. So gas accretion must be the dominant growth mechanism for intermediate and low mass galaxies, including dwarf ellipticals in clusters. The contribution of galaxy mergers terminating in haloes with mass M halo < M shock (thus presumably gas-rich) to the mass buildup of galaxies is small at all masses, but accounts for the bulk of the growth of ellipticals of intermediate mass (∼10 10.5 h −1 M ), which we predict must be the typical mass of ULIRGs.

We have used GADGET2 to simulate the formation of an elliptical galaxy in a cosmological dark matter halo with mass 3x10^12M_Sun/h. Using a stellar population synthesis model has allowed us to compute magnitudes, colours and surface brightness profiles. We have included a model to follow the growth of a central black hole and we have compared the results of simulations with and without feedback from AGNs. We have studied the interplay between cold gas accretion and merging in the development of galactic morphologies, the link between colour and morphology evolution, the effect of AGN feedback on the photometry of early type galaxies, the redshift evolution in the properties of quasar hosts, and the impact of AGN winds on the chemical enrichment of the intergalactic medium (IGM). We have found that the early phases of galaxy formation are driven by the accretion of cold filamentary flows, which form a disc at the centre of the dark matter halo. When the dark matter halo is sufficiently massive to support the propagation of a stable shock, cold accretion is shut down, and the star formation rate begins to decline. Mergers transform the disc into an elliptical galaxy, but also bring gas into the galaxy. Without a mechanism that removes gas from the merger remnants, the galaxy ends up with blue colours, atypical for its elliptical morphology. AGN feedback can solve this problem even with a fairly low heating efficiency. We have also demonstrated that AGN winds are potentially important for the metal enrichment of the IGM a high redshift.(abridged)Comment: 19 pages and 17 figures, accepted to MNRAS ID: MN-07-1954-MJ.R1 . For high resolution images please check following link: http://www.aip.de/People/AKhalatyan/COSMOLOGY/BHCOSMO