The [CII] 158\u3cem\u3eμ\u3c/em\u3em Line Emission in High-Redshift Galaxies

Gas is a crucial component of galaxies, providing the fuel to form stars, and it is impossible to understand the evolution of galaxies without knowing their gas properties. The [CII] fine structure transition at 158 μm is the dominant cooling line of cool interstellar gas, and is the brightest of emission lines from star forming galaxies from FIR through metre wavelengths, almost unaffected by attenuation. With the advent of ALMA and NOEMA, capable of detecting [CII]-line emission in high-redshift galaxies, there has been a growing interest in using the [CII] line as a probe of the physical conditions of the gas in galaxies, and as a star formation rate (SFR) indicator at z ≥ 4. In this paper, we have used a semi-analytical model of galaxy evolution (G.A.S.) combined with the photoionisation code CLOUDY to predict the [CII] luminosity of a large number of galaxies (25 000 at z ≃ 5) at 4 ≤ z ≤ 8. We assumed that the [CII]-line emission originates from photo-dominated regions. At such high redshift, the CMB represents a strong background and we discuss its effects on the luminosity of the [CII] line. We studied the L[CII]–SFR and L[CII]–Zg relations and show that they do not strongly evolve with redshift from z = 4 and to z = 8. Galaxies with higher [CII] luminosities tend to have higher metallicities and higher SFRs but the correlations are very broad, with a scatter of about 0.5 and 0.8 dex for L[CII]–SFR and L[CII]–Zg, respectively. Our model reproduces the L[CII]–SFR relations observed in high-redshift star-forming galaxies, with [CII] luminosities lower than expected from local L[CII]–SFR relations. Accordingly, the local observed L[CII]–SFR relation does not apply at high-z (z ≳ 5), even when CMB effects are ignored. Our model naturally produces the [CII] deficit (i.e. the decrease of L[CII]/LIR with LIR), which appears to be strongly correlated with the intensity of the radiation field in our simulated galaxies. We then predict the [CII] luminosity function, and show that it has a power law form in the range of L[ CII] probed by the model (1 × 107–2 × 109 L⊙ at z = 6) with a slope α = −1. The slope is not evolving from z = 4 to z = 8 but the number density of [CII]-emitters decreases by a factor of 20×. We discuss our predictions in the context of current observational estimates on both the differential and cumulative luminosity functions

Metal enrichment in a semi-analytical model, fundamental scaling relations, and the case of Milky Way galaxies

Gas flows play a fundamental role in galaxy formation and evolution, providing the fuel for the star formation process. These mechanisms leave an imprint in the amount of heavy elements. Thus, the analysis of this metallicity signature provides additional constraint on the galaxy formation scenario. We aim to discriminate between four different galaxy formation models based on two accretion scenarios and two different star formation recipes. We address the impact of a bimodal accretion scenario and a strongly regulated star formation recipe. We present a new extension of the eGalICS model, which allows us to track the metal enrichment process. Our new chemodynamical model is applicable for situations ranging from metal-free primordial accretion to very enriched interstellar gas contents. We use this new tool to predict the metallicity evolution of both the stellar populations and gas phase. We also address the evolution of the gas metallicity with the star formation rate (SFR). We then focus on a sub-sample of Milky Way-like galaxies. We compare both the cosmic stellar mass assembly and the metal enrichment process of such galaxies with observations and detailed chemical evolution models. Our models, based on a strong star formation regulation, allow us to reproduce well the stellar mass to gas-phase metallicity relation observed in the local universe. However, we observe a systematic shift towards high masses. Our $Mstar-Zg-SFR relation is in good agreement with recent measurements: our best model predicts a clear dependence with the SFR. Both SFR and metal enrichment histories of our Milky Way-like galaxies are consistent with observational measurements and detailed chemical evolution models. We finally show that Milky Way progenitors start their evolution below the observed main sequence and progressively reach this observed relation at z = 0.Comment: 22 pages, 11 figure

Towards a new modelling of gas flows in a semi-analytical model of galaxy formation and evolution

We present an extended version of the semi-analytical model, GalICS. Like its predecessor, eGalICS applies a post-treatment of the baryonic physics on pre-computed dark-matter merger trees extracted from an N-body simulation. We review all the mechanisms that affect, at any given time, the formation and evolution of a galaxy in its host dark-matter halo. We mainly focus on the gas cycle from the smooth cosmological accretion to feedback processes. To follow this cycle with high accuracy we introduce some novel prescriptions: i) a smooth baryonic accretion with two phases: a cold mode and a hot mode built on the continuous dark-matter accretion. In parallel to this smooth accretion, we implement the standard photoionisation modelling to reduce the input gas flow on the smallest structures. ii) a complete monitoring of the hot gas phase. We compute the evolution of the core density, the mean temperature and the instantaneous escape fraction of the hot atmosphere by considering that the hot gas is in hydrostatic equilibrium in the dark-matter potential well, and by applying a principle of conservation of energy on the treatment of gas accretion, supernovae and super massive black hole feedback iii) a new treatment for disc instabilities based on the formation, the migration and the disruption of giant clumps. The migration of such clumps in gas-rich galaxies allows to form pseudo-bulges. The different processes in the gas cycle act on different time scales, and we thus build an adaptive time-step scheme to solve the evolution equations. The model presented here is compared in detail to the observations of stellar-mass functions, star formation rates, and luminosity functions, in a companion paper

G.A.S. II: Dust extinction in galaxies; Luminosity functions and InfraRed eXcess

19 pages, 18 figures, Accepted by A&AInternational audienceDust is a crucial component of the interstellar medium of galaxies. The presence of dust strongly affects the light produced by stars within a galaxy. As these photons are our main information vector to explore the stellar mass assembly and therefore understand a galaxy's evolution, modeling the luminous properties of galaxies and taking into account the impact of the dust is a fundamental challenge for semi-analytical models.We present the complete prescription of dust attenuation implemented in the new semi-analytical model: G.A.S. This model is based on a two-phase medium originating from a physically motivated turbulent model of gas structuring (G.A.S. I paper). Dust impact is treated by taking into account three dust components: Polycyclic Aromatic Hydrocarbons, Very Small Grains, and Big Grains. All three components evolve in both a diffuse and a fragmented/dense gas phase. Each phase has its own stars, dust content and geometry. Dust content evolves according to the metallicity of it associated phase.The G.A.S. model is used to predict both the UV and the IR luminosity functions from $z=9.0$ to $z=0.1$. Our two-phase ISM prescription catches very well the evolution of UV and IR luminosity functions. We note a small overproduction of the IR luminosity at low redshift ($z<0.5$). We also focus on the Infrared-Excess (IRX) and explore its dependency with the stellar mass, UV slope, stellar age, metallicity and slope of the attenuation curves. Our model predicts large scatters for relations based on IRX, especially for the IRX-$\beta$ relation. Our analysis reveals that the slope of the attenuation curve is more driven by absolute attenuation in the FUV band than by disk inclination. We confirm that the age of the stellar population and the slope of the attenuation curve can both shift galaxies below the fiducial star-birth relation in the IRX-$\beta$ diagram

Formation & Evolution des galaxies par l'approche semi-analytique

Les modèles semi-analytiques (SAMs) constituent aujourd'hui le meilleur outils d'analyse et d'étude pour la formation et l'évolution des galaxies individuels mais également des regroupements de galaxies appelés amas. Alors qu'ils reproduisent avec succès les fonctions de masse stellaire, de corrélation à deux points, de luminosité des galaxies locales (z=0), ils échouent dans les prédictions des propriétés des galaxies plus jeunes, à plus haut décalage vers le rouge. Et ce d'autant plus que la masse stellaire est faible. Ces inconsistances entre les modèles et les observations démontrent que l'histoire de l'assemblage des ces galaxies, en relation avec l'accrétion de gaz, la formation stellaire et leurs halos de matière noire n'est pas bien comprise. Dans cette thèse, nous introduisons une nouvelle version du modèle semi-analytique GalICS et nous l'utilisons pour explorer l'impact, sur la formation stellaire des galaxies à faible masse, de la rétroaction des supernovae et des trous noirs supermassifs ainsi que des processus de photo-ionisation. Ces deux mécanismes sont communément utilisés pour réduire la formation de nouvelles étoiles dans les galaxies peu massives. Nous montrons que, même appliqué avec de très fortes efficacités, ces deux processus ne peuvent pas expliquer simultanément les fonctions de masse, de luminosité et la relation entre masse stellaire et masse des halos de matière noire pour les galaxies évoluant à grand décalage spectral. Suite à ce constat, nous introduisons deux recettes ad-hoc pour la formation stellaire. Dans un premier temps nous appliquons une forte modification de l'efficacité de formation stellaire en relation directe avec la masse de matière noire de leur halo hôte. Cette première approche conduit à de bons résultats, en particulier dans le régime des faibles masses stellaires mais il présente, par construction un profond désaccord avec la loi de formation stellaire observées par Kennicutt. Pour cela, nous introduisons une seconde modification, plus profonde, basée sur l'existence d'une composante de gaz, évoluant en périphérie des premiers disques galactiques, mais ne pouvant pas, pour des raisons encore mal comprises, former de nouvelles générations d'étoiles. Progressivement, ce gaz impropre à la formation stellaire est convertit, il alimente alors la formation d'étoile. L'introduction de ce nouveau réservoir, introduit un délai entre le moment ou le gaz s'effondre au centre du halo et le moment ou ce gaz. Ce nouveau modèle donne de très bons résultats mais il pose la question de l'origine de ce gaz impropre à la formation stellaire. Nous abordons dans cette thèse quelques piste de recherche dans le cadre de la formation des grandes structures peuplant notre Univers.Semi-analytical models (SAMs) are currently the best way to understand the formation of galaxies and clusters within the cosmic web dark-matter structures. While they fairly well reproduce the local stellar mass function, correlation function and luminosity function, they fail to match observations at high redshift (z>3) in most cases, particularly in the low-mass range. The inconsistency between models and observations indicates that the history of gas accretion in galaxies, within their host dark-matter halo, and the transformation of gas into stars, is not well followed. In this thesis, we introduce a new version of the GalICS model and we use it to explore the impact, on the star formation and in the low-mass range, of supernovae feedback and photo-ionization. These two mechanisms are commonly used to limit the amount of gas available to form stars.We will show that, even with a strong efficiency, these two process cannot explain the observed stellar mass function, luminosity functions, and the stellar mass versus dark matter halo mass relation. We will thus introduce two ad-hoc modifications of the standard paradigm. We propose first a strong modification of the star formation efficiency as a function of the dark matter halo mass. This model produces good results, especially on the faint end of the stellar mass function, but is, by construction, in disagreement with the well known Kennicutt star formation law. We will thus introduce a deeper change, based on a no star-forming gas component, and a new gas distribution in the galaxy discs. The reservoir in which stays the no star-forming gas generates a delay between the gas accretion and star formation. This model is in very good agreement with a large set of observations. However, it poses the question of the origin of the no star-forming gas. We will discuss its origin in the framework of the large scale disturbed dynamic of high-redshift structures.PARIS11-SCD-Bib. électronique (914719901) / SudocSudocFranceF

Galaxy stellar mass assembly: the difficulty matching observations and semi-analytical predictions

Semi-analytical models (SAMs) are currently the best way to understand the formation of galaxies within the cosmic dark-matter structures. While they fairly well reproduce the local stellar mass functions, correlation functions and luminosity functions, they fail to match observations at high redshift (z > 3) in most cases, particularly in the low-mass range. The inconsistency between models and observations indicates that the history of gas accretion in galaxies, within their host dark-matter halo, and the transformation of gas into stars, are not well followed. Hereafter, we briefly present a new version of the GalICS semi-analytical model. We explore the impacts of classical mechanisms, such as supernova feedback or photoionization, on the evolution of the stellar mass assembly. Even with a strong efficiency, these two processes cannot explain the observed stellar mass function and star formation rate distribution and some other relations. We thus introduce an ad-hoc modification of the standard paradigm, based on the presence of a \textit{no-star-forming} gas component, and a concentration of the star-forming gas in galaxy discs. The main idea behind the existence of the no-star-forming gas reservoir is that only a fraction of the total gas mass in a galaxy is available to form stars. The reservoir generates a delay between the accretion of the gas and the star formation process. This new model is in much better agreement with the observations of the stellar mass function in the low-mass range than the previous models, and agrees quite well with a large set of observations, including the redshift evolution of the specific star formation rate. However, it predicts a large fraction of no-star-forming baryonic gas, potentially larger than observed, even if its nature has still to be examined in the context of the missing baryon problem

Optimized Design of Survivable MPLS over Optical Transport Networks

International audienceIn this paper we study different options for the survivability implementation in MPLS over Optical Transport Networks in terms of network resource usage and configuration cost. We investigate two approaches to the survivability deployment: single layer and multilayer survivability and present various methods for spare capacity allocation (SCA) to reroute disrupted traffic. The comparative analysis shows the influence of the traffic granularity on the survivability cost: for high bandwidth LSPs, close to the optical channel capacity, the multilayer survivability outperforms the single layer one, whereas for low bandwidth LSPs the single layer survivability is more cost-efficient. For the multilayer survivability we demonstrate that by mapping efficiently the spare capacity of the MPLS layer onto the resources of the optical layer one can achieve up to 22% savings in the total configuration cost and up to 37% in the optical layer cost. Further savings (up to 9 %) in the wavelength use can be obtained with the integrated approach to network configuration over the sequential one, however, at the increase in the optimization problem complexity. These results are based on a cost model with actual technology pricing and were obtained for networks targeted to a nationwide coverage

Survivable MPLS Over Optical Transport Networks: Cost and Resource Usage Analysis

In this paper we study different options for the survivability implementation in MPLS over Optical Transport Networks (OTN) in terms of network resource usage and configuration cost. We investigate two approaches to the survivability deployment: single layer and multilayer survivability and present various methods for spare capacity allocation (SCA) to reroute disrupted traffic. The comparative analysis shows the influence of the offered traffic granularity and the physical network structure on the survivability cost: for high bandwidth LSPs, close to the optical channel capacity, the multilayer survivability outperforms the single layer one, whereas for low bandwidth LSPs the single layer survivability is more cost-efficient. On the other hand, sparse networks of low connectivity parameter use more wavelengths for optical path routing and increase the configuration cost, as compared with dense networks. We demonstrate that by mapping efficiently the spare capacity of the MPLS layer onto the resources of the optical layer one can achieve up to 22% savings in the total configuration cost and up to 37% in the optical layer cost. Further savings (up to 9 %) in the wavelength use can be obtained with the integrated approach to network configuration over the sequential one, however, at the increase in the optimization problem complexity. These results are based on a cost model with different cost variations, and were obtained for networks targeted to a nationwide coverage

The redshift evolution of the distribution of star formation among dark matter halos as seen in the infrared

Recent studies have revealed a strong correlation between the star formation rate (SFR) and stellar mass of the majority of star-forming galaxies, the so-called star-forming main sequence. An empirical modeling approach (the 2-SFM framework) that distinguishes between the main sequence and rarer starburst galaxies is capable of reproducing most statistical properties of infrared galaxies, such as number counts, luminosity functions, and redshift distributions. In this paper, we extend this approach by establishing a connection between stellar mass and halo mass with the technique of abundance matching. Based on a few simple assumptions and a physically motivated formalism, our model successfully predicts the (cross-)power spectra of the cosmic infrared background (CIB), the cross-correlation between CIB and cosmic microwave background (CMB) lensing, and the correlation functions of bright, resolved infrared galaxies measured by Herschel, Planck, ACT, and SPT. We use this model to infer the redshift distribution of CIB-anisotropies and of the CIB × CMB lensing signal, as well as the level of correlation between CIB-anisotropies at different wavelengths. We study the link between dark matter halos and star-forming galaxies in the framework of our model. We predict that more than 90% of cosmic star formation activity occurs in halos with masses between 1011.5 and 10^(13.5) M⊙. If taking subsequent mass growth of halos into account, this implies that the majority of stars were initially (at z > 3) formed in the progenitors of clusters (M_h(z = 0) > 10^(13.5) M⊙), then in groups (10^(12.5) < M_h(z = 0) < 10^(13.5) M⊙) at 0.5 < z < 3, and finally in Milky-Way-like halos (10^(11.5) < M_h(z = 0) < 10^(12.5) M⊙) at z < 0.5. At all redshifts, the dominant contribution to the SFR density stems from halos of mass ~10¹² M⊙, in which the instantaneous star formation efficiency – defined here as the ratio between SFR and baryonic accretion rate – is maximal (~70%). The strong redshift-evolution of SFR in the galaxies that dominate the CIB is thus plausibly driven by increased accretion from the cosmic web onto halos of this characteristic mass scale. Material (effective spectral energy distributions, differential emissivities of halos, relations between M_h and SFR) associated to this model is available at http://irfu.cea.fr/Sap/Phocea/Page/index.php?id=537