Jeremy Blaizot

The hierarchical formation of the brightest cluster galaxies

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 limitations of simple ways to capture their evolution. In a model where cooling flows are suppressed at late times by active galactic nucleus (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 on to 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 counterparts. 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.

A test of the nature of cosmic acceleration using galaxy redshift distortions

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 β = 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.

Modelling the galaxy bimodality: shutdown above a critical halo mass

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 M shock ∼ 10 12 Menjoy 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.

Dancing in the dark: galactic properties trace spin swings along the cosmic web

A large-scale hydrodynamical cosmological simulation, Horizon-AGN , is used to investigate the alignment between the spin of galaxies and the large-scale cosmic filaments above redshift one. The analysis of more than 150 000 galaxies with morphological diversity in a 100h −1 Mpc comoving box size shows that the spin of low-mass, rotationdominated, blue, star-forming galaxies is preferentially aligned with their neighbouring filaments. High-mass, dispersion-dominated, red, quiescent galaxies tend to have a spin perpendicular to nearby filaments. The reorientation of the spin of massive galaxies is provided by galaxy mergers which are significant in the mass build up of high-mass galaxies. We find that the stellar mass transition from alignment to misalignment happens around 3×10 10 M⊙. This is consistent with earlier findings of a dark matter mass transition for the orientation of the spin of halos (5 × 10 11 M⊙ at the same redshift from Codis et al. 2012). With these numerical evidence, we advocate a scenario in which 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 physical and morphological 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. It allows mergers to play their key role by reducing post-merger gas inflows and, therefore, keeping galaxy spins misaligned with cosmic filaments. It also promotes diversity amongst galaxy properties.

Building merger trees from cosmological N-body simulations Towards improving galaxy formation models using subhaloes

Although a fair amount of work has been devoted to growing Monte-Carlo merger trees which resemble those built from an N-body simulation, comparatively little effort has been invested in quantifying the caveats one necessarily encounters when one extracts trees directly from such a simulation. To somewhat revert the tide, this paper seeks to provide its reader with a comprehensive study of the problems one faces when following this route. The first step to building merger histories of dark matter haloes and their subhaloes is to identify these structures in each of the time outputs (snapshots) produced by the simulation. Even though we discuss a particular implementation of such an algorithm (called AdaptaHOP) in this paper, we believe that our results do not depend on the exact details of the implementation but extend to most if not all (sub)structure finders. We then highlight different ways to build merger histories from AdaptaHOP haloes and subhaloes, contrasting their various advantages and drawbacks. We find that the best approach to (sub)halo merging histories is through an analysis that goes back and forth between identification and tree building rather than one which conducts a straightforward sequential treatment of these two steps. This is rooted in the complexity of the merging trees which have to depict an inherently dynamical process from the partial temporal information contained in the collection of instantaneous snapshots available from the N-body simulation.

The Vimos VLT deep survey - Global properties of 20 000 galaxies in the IAB < 22.5 WIDE survey

The VVDS-Wide survey has been designed to trace the large-scale distribution of galaxies at z ~ 1 on comoving scales reaching ~100~h-1 Mpc, while providing a good control of cosmic variance over areas as large as a few square degrees. This is achieved by measuring redshifts with VIMOS at the ESO VLT to a limiting magnitude IAB = 22.5, targeting four independent fields with sizes of up to 4 deg2 each. We discuss the survey strategy which covers 8.6 deg2 and present the general properties of the current redshift sample. This includes 32 734 spectra in the four regions, covering a total area of 6.1 deg2 with a sampling rate of 22 to 24%. This paper accompanies the public release of the first 18 143 redshifts of the VVDS-Wide survey from the 4 deg2 contiguous area of the F22 field at RA = 22^h. We have devised and tested an objective method to assess the quality of each spectrum, providing a compact figure-of-merit. This is particularly effective in the case of long-lasting spectroscopic surveys with varying observing conditions. Our figure of merit is a measure of the robustness of the redshift measurement and, most importantly, can be used to select galaxies with uniform high-quality spectra to carry out reliable measurements of spectral features. We also use the data available over the four independent regions to directly measure the variance in galaxy counts. We compare it with general predictions from the observed galaxy two-point correlation function at different redshifts and with that measured in mock galaxy surveys built from the Millennium simulation. The purely magnitude-limited VVDS Wide sample includes 19 977 galaxies, 304 type I AGNs, and 9913 stars. The redshift success rate is above 90% independent of magnitude. A cone diagram of the galaxy spatial distribution provides us with the current largest overview of large-scale structure up to z ~ 1, showing a rich texture of over- and under-dense regions. We give the mean N(z) distribution averaged over 6.1 deg2 for a sample limited in magnitude to IAB = 22.5. Comparing galaxy densities from the four fields shows that in a redshift bin Δz = 0.1 at z ~ 1 one still has factor-of-two variations over areas as large as ~ 0.25 deg2. This level of cosmic variance agrees with that obtained by integrating the galaxy two-point correlation function estimated from the F22 field alone. It is also in fairly good statistical agreement with that predicted by the Millennium simulations. The VVDS WIDE survey currently provides the largest area coverage among redshift surveys reaching z ~ 1. The variance estimated over the survey fields shows explicitly how clustering results from deep surveys of even 1 deg2 size should be interpreted with caution. The survey data represent a rich data base to select complete sub-samples of high-quality spectra and to study galaxy ensemble properties and galaxy clustering over unprecedented scales at these redshifts. The redshift catalog of the 4 deg2 F22 field is publicly available at http://cencosw.oamp.fr.

Lyman-alpha emission properties of simulated galaxies: interstellar medium structure and inclination effects

Aims. This paper is the first of a series investigating Lyman-alpha (hereafter Ly ) radiation transfer through hydrodynamical simulations of galaxy formation. Its aim is to assess the impact of the interstellar medium (ISM) physics on Ly radiation transfer and to quantify how galaxy orientation with respect to the line of sight alters observational signatures. Methods. We compare the results of Ly radiation transfer calculations through the ISM of a couple of idealized galaxy simulations in a dark matter halo of 10 10 M . In the first one, G1, this ISM is modeled using physics typical of large scale cosmological hydrodynamics simulations of galaxy formation, where gas is prevented from radiatively cooling below 10 4 K. In the second one, G2, gas is allowed to radiate away more of its internal energy via metal lines and consequently fragments into dense star-forming clouds. Results. First, as expected, the small-scale structuration of the ISM plays a determinant role in shaping a galaxy’s Ly properties. The artificially warm, and hence smooth, ISM of G1 yields an escape fraction of 50% at the Ly line center, and produces symmetrical double-peak profiles. On the contrary, in G2, most young stars are embedded in thick star-forming clouds, and the result is a 10 times lower escape fraction. G2 also displays a stronger outflowing velocity field, which favors the escape of red-shifted photons, resulting in an asymmetric Ly line. Second, the Ly properties of G2 strongly depend on the inclination at which it is observed: From edge-on to face-on, the line goes from a double-peak profile with an equivalent width of 5A to a 15 times more luminous red-shifted asymmetric line with EW 90A. Conclusions. The remarkable discrepancy in the Ly properties we derived for two ISM models raises a fundamental issue. In e ect, it demonstrates that Ly radiation transfer calculations can only lead to realistic properties in simulations where galaxies are resolved into giant molecular clouds. Such a stringent requirement translates into severe constraints both in terms of ISM physics modeling and numerical resolution, putting these calculations out of reach of current large scale cosmological simulations. Finally, we find inclination e ects to be much stronger for Ly photons than for continuum radiation. This could potentially introduce severe biases in the selection function of narrow-band Ly emitter surveys, and in their interpretation, and we predict these surveys could indeed miss a significant fraction of the high-z galaxy population.

The MUSE 3D view of the Hubble Deep Field South

We observed Hubble Deep Field South with the new panoramic integral-field spectrograph MUSE that we built and have just commissioned at the VLT. The data cube resulting from 27 h of integration covers one arcmin(2) field of view at an unprecedented depth with a 1 sigma emission-line surface brightness limit of 1 x 10(-19) erg s(-1) cm(-2) arcsec(-2), and contains similar to 90 000 spectra. We present the combined and calibrated data cube, and we performed a first-pass analysis of the sources detected in the Hubble Deep Field South imaging. We measured the redshifts of 189 sources up to a magnitude I-814 = 29.5, increasing the number of known spectroscopic redshifts in this field by more than an order of magnitude. We also discovered 26 Ly alpha emitting galaxies that are not detected in the HST WFPC2 deep broad-band images. The intermediate spectral resolution of 2.3 angstrom allows us to separate resolved asymmetric Ly alpha emitters, [O II] 3727 emitters, and C III] 1908 emitters, and the broad instantaneous wavelength range of 4500 angstrom helps to identify single emission lines, such as [O III] 5007, H beta, and H alpha, over a very wide redshift range. We also show how the three-dimensional information of MUSE helps to resolve sources that are confused at ground-based image quality. Overall, secure identifications are provided for 83% of the 227 emission line sources detected in the MUSE data cube and for 32% of the 586 sources identified in the HST catalogue. The overall redshift distribution is fairly flat to z = 6.3, with a reduction between z = 1.5 to 2.9, in the well-known redshift desert. The field of view of MUSE also allowed us to detect 17 groups within the field. We checked that the number counts of [O II] 3727 and Ly alpha emitters are roughly consistent with predictions from the literature. Using two examples, we demonstrate that MUSE is able to provide exquisite spatially resolved spectroscopic information on the intermediate-redshift galaxies present in the field. This unique data set can be used for a wide range of follow-up studies. We release the data cube, the associated products, and the source catalogue with redshifts, spectra, and emission-line fluxes.

Active galactic nuclei in cosmological simulations – I. Formation of black holes and spheroids through mergers

This is the first paper of a series on the methods and results of the Active Galactic Nuclei In Cosmological Simulations (AGNICS) project, which incorporates the physics of active galactic nuclei (AGNs) into Galaxies In Cosmological Simulations (GalICS), a galaxy formation model that combines large cosmological N-body simulations of dark matter hierarchical clustering and a semi-analytic approach to the physics of the baryons. The project explores the quasar-galaxy link in a cosmological perspective, in response to growing observational evidence for a close relation between supermassive black holes (SMBHs) and spheroids. The key problems are the quasar fuelling mechanism, the origin of the black hole (BH)-to-bulge mass relation, the causal and chronological link between BH growth and galaxy formation, the properties of quasar hosts and the role of AGN feedback in galaxy formation. This first paper has two goals. The first is to describe the general structure and assumptions that provide the framework for the AGNICS series. The second is to apply AGNICS to studying the joint formation of SMBHs and spheroids in galaxy mergers. We investigate under what conditions this scenario can reproduce the local distribution of SMBHs in nearby galaxies and the evolution of the quasar population. AGNICS contains two star formation modes: a quiescent mode in discs and a starburst mode in proto-spheroids, the latter triggered by mergers and disc instabilities. Here we assume that BH growth is linked to the starburst mode. The simplest version of this scenario, in which the BH accretion rate M. and the star formation rate in the starburst component M *burst are simply related by a constant of proportionality, does not to reproduce the cosmic evolution of the quasar population. A model in which M* ρ ζ burst M* burst , where ρ burst is the density of the gas in the starburst and ζ ≃ 0.5, can explain the evolution of the quasar luminosity function in B band and X-rays (taking into account the presence of obscured AGNs inferred from X-ray studies). The scatter and the tilt that this model introduces in the BH-to-bulge mass relation are within the observational constraints. The model predicts that the quasar contribution increases with the total bolometric luminosity and that, for a given bulge mass, the most massive BHs are in the bulges with the oldest stars.

The VIMOS-VLT Deep Survey (VVDS) - The dependence of clustering on galaxy stellar mass at z ~ 1

We have investigated the dependence of galaxy clustering on their stellar mass at z~1, using the data from the VIMOS-VLT Deep Survey (VVDS). We have measured the projected two-point correlation function of galaxies, wp(rp) for a set of stellar mass selected samples at an effective redshift =0.85. We have control and quantify all effects on galaxy clustering due to the incompleteness of our low mass samples. We find that more massive galaxies are more clustered. When compared to similar results at z~0.1 in the SDSS, we observed no evolution of the projected correlation function for massive galaxies. These objects present a stronger linear bias at z~1 with respect to low mass galaxies. As expected, massive objects at high redshift are found in the highest pics of the dark matter density field.