Bruno Guiderdoni

galics– I. A hybrid N-body/semi-analytic model of hierarchical galaxy formation

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.

Semi-analytic modelling of galaxy evolution in the IR/submm range

This paper proposes a new semi-analytic modelling of galaxy properties in the IR/submm wavelength range, which is explicitly set in a cosmological framework. We start from a description of the non-dissipative and dissipative collapses of primordial perturbations, and add star formation, stellar evolution and feedback, as well as the absorption of starlight by dust and its re-emission in the IR and submm. This type of approach has had some success in reproducing the optical properties of galaxies. We hereafter propose a simple extension to the IR/submm range. The growth of structures is followed according to the standard cold dark matter model. We assume that star formation proceeds either in a ‘quiescent’ mode, e.g., as in discs, or in a ‘burst’ mode with 10 times shorter time-scales. In order to reproduce the current data on the evolution of the comoving cosmic star formation rate and gas densities, we need to introduce a mass fraction involved in the ‘burst’ mode strongly increasing with redshift, probably reflecting the increase of interaction and merging activity. We estimate the IR/submm luminosities of these ‘mild starburst’ and ‘luminous UV/IR galaxies’, and we explore how much star formation could be hidden in heavily extinguished, ‘ultraluminous IR galaxies’ by designing a family of evolutionary scenarios which are consistent with the current status of the ‘cosmic constraints’, as well as with the IRAS 60-μm luminosity function and faint counts, but with different high-z IR luminosity densities. However, these scenarios generate a cosmic infrared background whose spectrum falls within the ±1σ range of the isotropic IR component detected by Puget et al. and revisited by Guiderdoni et al. We give predictions for the faint galaxy counts and redshift distributions at IR and submm wavelengths. The submm range is very sensitive to the details of the evolutionary scenarios. As a result, the ongoing and forthcoming observations with ISO and SCUBA (and later with SIRTF, SOFIA, FIRST and PLANCK) will put strong constraints on the evolution of galaxies at z∼1 and beyond.

MoMaF: the Mock Map Facility

We present the Mock Map Facility, a powerful tool for converting theoretical outputs of hierarchical galaxy formation models into catalogues of virtual observations. The general principle is straightforward: mock observing cones can be generated using semi-analytically post-processed snapshots of cosmological N-body simulations. These cones can then be projected to synthesize mock sky images. To this end, the paper describes in detail an efficient technique for creating such mock cones and images from the galaxies in cosmological simulations (galics) semi-analytic model, providing the reader with an accurate quantification of the artefacts it introduces at every step. We show that replication effects introduce a negative bias on the clustering signal – typically peaking at less than 10 per cent around the correlation length. We also thoroughly discuss how the clustering signal is affected by finite-volume effects, and show that it vanishes at scales larger than approximately one-tenth of the simulation box size. For the purpose of analysing our method, we show that number counts and redshift distributions obtained with galics/momaf compare well with K-band observations and the two-degree field galaxy redshift survey. Given finite-volume effects, we also show that the model can reproduce the automatic plate measuring machine angular correlation function. The momaf results discussed here are made publicly available to the astronomical community through a public data base. Moreover, a user-friendly Web interface (http://galics.iap.fr) allows any user to recover her/his own favourite galaxy samples through simple SQL queries. The flexibility of this tool should permit a variety of uses ranging from extensive comparisons between real observations and those predicted by hierarchical models of galaxy formation, to the preparation of observing strategies for deep surveys and tests of data processing pipelines.

Images - III. The evolution of the near-infrared Tully-Fisher relation over the last 6 Gyr

Using the multi-integral field spectrograph GIRAFFE at VLT, we have derived the K-band Tully-Fisher relation (TFR) at z ∼ 0.6 for a representative sample of 65 galaxies with emission lines (W0(OII) ≥ 15 A). We confirm that the scatter in the z ∼ 0. 6T FR is caused by galaxies with anomalous kinematics, and find a positive and strong correlation between the complexity of the kinematics and the scatter that they contribute to the TFR. Considering only relaxed-rotating disks, the scatter, and possibly also the slope, of the TFR, do not appear to evolve with redshift. We detect an evolution of the K-band TFR zero point between z ∼ 0. 6a ndz = 0, which, if interpreted as an evolution of the K-band luminosity of rotating disks, would imply that a brightening of 0.66 ± 0.14 mag occurs between z ∼ 0. 6a ndz = 0. Any disagreement with the results of Flores et al. (2006, A&A, 455, 107) are attributed to both an improvement of the local TFR and the more detailed accurate measurement of the rotation velocities in the distant sample. Most of the uncertainty can be explained by the relatively coarse spatial-resolution of the kinematical data. Because most rotating disks at z ∼ 0.6 are unlikely to experience further merging events, one may assume that their rotational velocity, which is taken as a proxy of the total mass, does not evolve dramatically. If true, our result implies that rotating disks observed at z ∼ 0.6 are rapidly transforming their gas into stars, to be able to double their stellar masses and be observed on the TFR at z = 0. The rotating disks observed are indeed emission-line galaxies that are either starbursts or LIRGs, which implies that they are forming stars at a high rate. Thus, a significant fraction of the rotating disks are forming the bulk of their stars within 6 to 8 Gyr, in good agreement with former studies of the evolution of the mass-metallicity relationship.

IMAGES. I. Strong evolution of galaxy kinematics since z = 1

Nearly half the stellar mass of present-day spiral galaxies has formed since z = 1, and galaxy kinematics is an ideal tool to identify the underlying mechanisms responsible for the galaxy mass assembly since that epoch. Here, we present the first results of the ESO large program, “IMAGES”, which aims at obtaining robust measurements of the kinematics of distant galaxies using the multi-IFU mode of GIRAFFE on the VLT. 3D spectroscopy is essential to robustly measure the often distorted kinematics of distant galaxies (e.g., Flores et al. 2006, A&A, 455, 107). We derive the velocity fields and σ-maps of 36 galaxies at 0.4 < z < 0.75 from the kinematics of the [O ii] emission line doublet, and generate a robust technique to identify the nature of the velocity fields based on the pixels of the highest signal-to-noise ratios (S/N). Combining these observations with those of Flores et al., we have gathered a unique sample of 63 velocity fields of emission line galaxies (W0([O ii]) ≥ 15 A) at z = 0.4−0.75, which are a representative subsample of the

The optically dark side of galaxy formation

Deep optical surveys, probe the rest-frame ultraviolet luminosities of high-redshift galaxies. They can therefore be used to infer star formation rates, under assumptions about young stellar populations. Current data suggest that the global star-formation rate of the Universe peaked at a redshift of z = 1, then subsequently declined,, leading to claims that the bulk of star formation in the Universe has been seen. However, the large uncertainties inherent in correcting for ultraviolet absorption by dust associated with young stars suggest that these formation rates might be substantially underestimated in high-redshift galaxies. Here we circumvent this problem by considering the dust thermal emission at infrared (and submillimetre) wavelengths. We propose an improved determination of the long-sought cosmic infrared background (built up from the accumulated infrared light of faint galaxies along the line of sight), from which we are able to estimate the required population of high-redshift, dust-enshrouded starburst galaxies. We argue that most of the star formation at high redshift may be hidden by dust, and we define the necessary characteristics of a feasible far-infrared survey that could detect this population.

IMAGES II. A surprisingly low fraction of undisturbed rotating spiral disks at z~0.6: The morpho-kinematical relation 6 Gyrs ago

We present a first combined analysis of the morphological and dynamical properties for the Intermediate MAss Galaxy Evolution Sequence (IMAGES) sample. It is a representative sample of 52 z~0.6 galaxies with Mstell from 1.5 to 15 10^10Msun and possessing 3D resolved kinematics and HST deep imaging in at least two broad band filters. We aim at evaluate robustly the evolution of rotating spirals since z~0.6, as well as to test the different schemes for classifying galaxies morphologically. We used all the information provided by multi-band images, color maps and 2 dimensional light fitting to assign to each object a morphological class. We divided our sample between spiral disks, peculiar objects, compact objects and mergers. Using our morphological classification scheme, 4/5 of identified spirals are rotating disks and more than 4/5 of identified peculiar galaxies show complex kinematics, while automatic classification methods such as Concentration-Asymmetry and GINI-M20 severely overestimate the fraction of relaxed disk galaxies. Using this methodology, we find that the fraction of rotating spirals has increased by a factor ~ 2 during the last 6 Gyrs, a much higher fraction that found previously based on morphologies alone. These rotating spiral disks are forming stars very rapidly, doubling their stellar masses over the last 6 Gyrs, while most of their stars have been formed few Gyrs earlier, which reveals the presence of a large gas supply. Because they are likely the progenitors of local spirals, we can conjecture how their properties are evolving. Their disks show some evidence for an inside-out growth and the gas supply/accretion is not made randomly as the disk need to be stable in order to match the local disk properties.

IMAGES IV: strong evolution of the oxygen abundance in gaseous phases of intermediate mass galaxies from z ~ 0.8

Context: Intermediate mass galaxies (> 1010 M_ȯ) at z ~ 0.6 are the likeliest progenitors of the present-day, numerous population of spirals. There is growing evidence that they have evolved rap ...

Modelling high redshift Lyman-alpha Emitters

We present a new model for high redshift Lyman-Alpha Emitters (LAEs) in the cosmological context which takes into account the resonant scattering of Ly-a photons through expanding gas. The GALICS semi-analytic model provides us with the physical properties of a large sample of high redshift galaxies. We implement a gas outflow model for each galaxy based on simple scaling arguments. The coupling with a library of numerical experiments of Ly-a transfer through expanding or static dusty shells of gas allows us to derive the Ly-a escape fractions and profiles. The predicted distribution of Ly-a photons escape fraction shows that galaxies with a low star formation rate have a f_esc of the order of unity, suggesting that, for those objects, Ly-a may be used to trace the star formation rate assuming a given conversion law. In galaxies forming stars intensely, the escape fraction spans the whole range from 0 to 1. The model is able to get a good match to the UV and Ly-a luminosity function (LF) data at 3 < z < 5. We find that we are in good agreement with both the bright Ly-a data and the faint population observed by Rauch et al. (2008) at z=3. Most of the Ly-a profiles of our LAEs are redshifted by the diffusion in the outflow which suppresses IGM absorption. The bulk of the observed Ly-a equivalent width (EW) distribution is recovered by our model, but we fail to obtain the very large values sometimes detected. Predictions for stellar masses and UV LFs of LAEs show a satisfactory agreement with observational estimates. The UV-brightest galaxies are found to show only low Ly-a EWs in our model, as it is reported by many observations of high redshift LAEs. We interpret this effect as the joint consequence of old stellar populations hosted by UV-bright galaxies, and high HI column densities that we predict for these objects, which quench preferentially resonant Ly-a photons via dust extinction.

GALICS - V: Low- and high-order clustering in mock Sloan Digital Sky Surveys

We use the galics hybrid model of galaxy formation to explore the nature of galaxy clustering in the local Universe. We bring the theoretical predictions of our model into the observational plane using the momaf software to build mock catalogues which mimic Sloan Digital Sky Survey (SDSS) observations. We measure low- and high-order angular clustering statistic from these mock catalogues, after selecting galaxies the same way as for observations, and compare them directly to estimates from the SDSS data. Note that we also present the first measurements of high-order statistics on the SDSS DR1. We find that our model is in general good agreement with observations in the scale/luminosity range where we can trust the predictions. This range is found to be limited (i) by the size of the dark matter simulation used – which introduces finite volume effects at large scales – and by the mass resolution of this simulation – which introduces incompleteness at apparent magnitudes fainter than r∼ 20. We then focus on the small-scale clustering properties of galaxies and investigate the behaviour of three different prescriptions for positioning galaxies within haloes of dark matter. We show that galaxies are poor tracers of either DM particles or DM substructures, within groups and clusters. Instead, SDSS data tells us that the distribution of galaxies lies somewhat in between these two populations. This confirms the general theoretical expectation from numerical simulations and semi-analytic modelling.