arXiv subjects

Bruno Guiderdoni

Publications and source records attributed to Bruno Guiderdoni.

At least 19 recordsLinked to original sources

arXiv2024

Exploring neutral hydrogen in the radio MOlecular Hydrogen Emission Galaxies (MOHEGs) and prospects with the SKA

The empirical studies of cold gas content serve as an essential aspect in comprehending the star formation activities and evolution in galaxies. However, it is not straightforward to understand these processes because they depend on various physical properties of the Interstellar Medium. Massive FRI/II type radio galaxies rich in molecular hydrogen with less star formation activities are known as radio Molecular Hydrogen Emission Galaxies (MOHEGs). We present a study of neutral hydrogen gas-associated radio MOHEGs at redshifts <0.2 probed via the HI 21-cm absorption line. Neutral hydrogen is detected in 70% of these galaxies, which are located at a distance of 8 - 120 kiloparsec from the neighboring galaxies. These galaxies show a scarcity of HI gas as compared to merging galaxies at similar redshifts. We found no strong correlation between N(HI), N(H), and galaxy properties, independent of whether the HI is assumed to be cold or warm, indicating that the atomic gas is probably playing no important role in star formation. The relationship between total hydrogen gas surface density and star formation surface density deviates from the standard Kennicutt-Schmidt law. Our study highlights the importance of HI studies and offers insights into the role of atomic and molecular hydrogen gas in explaining the properties of these galaxies. In the upcoming HI 21-cm absorption surveys with next-generation radio telescopes such as the Square Kilometre Array (SKA) and pathfinder instruments, it may be possible to provide better constraints to such correlations.

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arXiv2021

The MUSE Extremely Deep Field: the Cosmic Web in Emission at High Redshift

We report the discovery of diffuse extended Ly-alpha emission from redshift 3.1 to 4.5, tracing cosmic web filaments on scales of 2.5-4 comoving Mpc. These structures have been observed in overdensities of Ly-alpha emitters in the MUSE Extremely Deep Field, a 140 hour deep MUSE observation located in the Hubble Ultra Deep Field. Among the 22 overdense regions identified, 5 are likely to harbor very extended Ly-alpha emission at high significance with an average surface brightness of $\mathrm{5 \times 10^{-20} erg s^{-1} cm^{-2} arcsec^{-2}}$. Remarkably, 70% of the total Ly-alpha luminosity from these filaments comes from beyond the circumgalactic medium of any identified Ly-alpha emitters. Fluorescent Ly-alpha emission powered by the cosmic UV background can only account for less than 34% of this emission at z$\approx$3 and for not more than 10% at higher redshift. We find that the bulk of this diffuse emission can be reproduced by the unresolved Ly-alpha emission of a large population of ultra low luminosity Ly-alpha emitters ($\mathrm{<10^{40} erg s^{-1}}$), provided that the faint end of the Ly-alpha luminosity function is steep ($α\lessapprox -1.8$), it extends down to luminosities lower than $\mathrm{10^{38} - 10^{37} erg s^{-1}}$ and the clustering of these Ly-alpha emitters is significant (filling factor $< 1/6$). If these Ly-alpha emitters are powered by star formation, then this implies their luminosity function needs to extend down to star formation rates $\mathrm{< 10^{-4} M_\odot yr^{-1}}$. These observations provide the first detection of the cosmic web in Ly-alpha emission in typical filamentary environments and the first observational clue for the existence of a large population of ultra low luminosity Ly-alpha emitters at high redshift.

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arXiv2020

Search and analysis of giant radio galaxies with associated nuclei (SAGAN) -- I : New sample and multi-wavelength studies

We present the first results of a project called SAGAN, which is dedicated solely to the studies of relatively rare megaparsec-scale radio galaxies in the Universe, called giant radio galaxies (GRGs). We have identified 162 new GRGs primarily from the NVSS with sizes ranging from ~0.71 Mpc to 2.82 Mpc in the redshift range of ~0.03 - 0.95, of which 23 are hosted by quasars (giant radio quasars, GRQs). As part of the project SAGAN, we have created a database of all known GRGs, the GRG catalogue, from the literature (including our new sample); it includes 820 sources. For the first time, we present the multi-wavelength properties of the largest sample of GRGs. Our results establish that the distributions of the radio spectral index and the black hole mass of GRGs do not differ from the corresponding distributions of normal-sized radio galaxies (RGs). However, GRGs have a lower Eddington ratio (ER) than RGs. Using the mid-infrared data, we classified GRGs in terms of their accretion mode: either a high-power radiatively efficient high-excitation state, or a radiatively inefficient low-excitation state. We find that GRGs in high-excitation state statistically have larger sizes, stronger radio power, jet kinetic power, and higher ER than those in low-excitation state. Our analysis reveals a strong correlation between the ER and the scaled jet kinetic power, which suggests a disc-jet coupling. Our environmental study reveals that ~10% of all GRGs may reside at the centres of galaxy clusters, in a denser galactic environment, while the majority appears to reside in a sparse environment. We find that the probability of BCG as a GRG is quite low. We present new results for GRGs that range from black hole mass to large-scale environment properties. We discuss their formation and growth scenarios, highlighting the key physical factors that cause them to reach their gigantic size. Abridged.

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arXiv2020

The MUSE Hubble Ultra Deep Field Survey XIV. The evolution of the Lya emitter fraction from z=3 to z=6

The Lya emitter (LAE) fraction, X_LAE, is a potentially powerful probe of the evolution of the intergalactic neutral hydrogen gas fraction. However, uncertainties in the measurement of X_LAE are still debated. Thanks to deep data obtained with MUSE, we can measure the evolution of X_LAE homogeneously over a wide redshift range of z~3-6 for UV-faint galaxies (down to M_1500~-17.75). This is significantly fainter than in former studies, and allows us to probe the bulk of the population of high-z star-forming galaxies. We construct a UV-complete photo-redshift sample following UV luminosity functions and measure the Lya emission with MUSE using the second data release from the MUSE HUDF Survey. We derive the redshift evolution of X_LAE for M_1500 in [-21.75;-17.75] for the first time with a equivalent width range EW(Lya)>=65 A and find low values of X_ LAE<~30% at z<~6. For M_1500 in [-20.25;-18.75] and EW(Lya)<~25 A, our X_LAE values are consistent with those in the literature within 1sigma at z<~5, but our median values are systematically lower than reported values over the whole redshift range. In addition, we do not find a significant dependence of X_LAE on M_1500 for EW(Lya)>~50 A at z~3-4, in contrast with previous work. The differences in X_LAE mainly arise from selection biases for Lyman Break Galaxies (LBGs) in the literature: UV-faint LBGs are more easily selected if they have strong Lya emission, hence X_LAE is biased towards higher values. Our results suggest either a lower increase of X_LAE towards z~6 than previously suggested, or even a turnover of X_LAE at z~5.5, which may be the signature of a late or patchy reionization process. We compared our results with predictions from a cosmological galaxy evolution model. We find that a model with a bursty star formation (SF) can reproduce our observed X_LAE much better than models where SF is a smooth function of time.

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arXiv2018

The MUSE-Wide Survey: A determination of the Lyman $α$ emitter luminosity function at $3 < z < 6$

(Abridged) We investigate the Lyman $α$ emitter luminosity function (LAE LF) within the redshift range $2.9 \leq z \leq 6$ from the first instalment of the blind integral field spectroscopic survey MUSE-Wide. This initial part of the survey probes a region of 22.2 arcmin$^2$ in the CANDELS/GOODS-S field. The dataset provided us with 237 LAEs from which we construct the LAE LF in the luminosity range $42.2 \leq \log L_\mathrm{Lyα} [\mathrm{erg\,s}^{-1}]\leq 43.5$ within a volume of $2.3\times10^5$ Mpc$^3$. For the LF construction we utilise three different non-parametric estimators: The classical $1/V_\mathrm{max}$ method, the $C^{-}$ method, and an improved binned estimator for the differential LF. All three methods deliver consistent results, with the cumulative LAE LF being $Φ(\log L_\mathrm{Lyα} [\mathrm{erg\,s}^{-1}] = 43.5) \simeq 3\times 10^{-6}$ Mpc$^{-3}$ and $Φ(\log L_\mathrm{Lyα} [\mathrm{erg\,s}^{-1}] = 42.2) \simeq 2 \times 10^{-3}$ Mpc$^{-3}$ towards the bright- and faint-end of our survey, respectively. By employing a non-parametric statistical test, as well as by comparing the full sample to sub-samples in redshift bins, we find no supporting evidence for an evolving LAE LF over the probed redshift and luminosity range. We determine the best-fitting Schechter function parameters $α= -1.84^{+0.42}_{-0.41}$ and $\log L^* [\mathrm{erg\,s}^{-1}] = 42.2^{+0.22}_{-0.16}$ with the corresponding normalisation $\log ϕ^* [\mathrm{Mpc}^{-3}] = -2.71$. When correcting for completeness in the LAE LF determinations, we take into account that LAEs exhibit diffuse extended low surface-brightness haloes. We compare the resulting LF to one obtained where we apply a correction assuming compact point-like emission. We find that the standard correction underestimates the LAE LF at the faint end of our survey by a factor of 2.5.

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arXiv2017

The MUSE Hubble Ultra Deep Field Survey X. Ly$α$ Equivalent Widths at $2.9 < z < 6.6$

We present rest-frame Ly$α$ equivalent widths (EW) of 417 Ly$α$ emitters (LAEs) detected with Multi Unit Spectroscopic Explorer (MUSE) on the Very Large Telescope (VLT) at $2.9 < z < 6.6$ in the Hubble Ultra Deep Field. Based on the deep MUSE spectroscopy and ancillary Hubble Space Telescope (HST) photometry data, we carefully measured EW values taking into account extended Ly$α$ emission and UV continuum slopes ($β$). Our LAEs reach unprecedented depths, both in Ly$α$ luminosities and UV absolute magnitudes, from log($L_{\rm Lyα}$/erg s$^{-1}$) $\sim$41.0 to 43.0 and from Muv $\sim$ -16 to -21 (0.01-1.0 $L^{*}_{\rm z=3}$). The EW values span the range of $\sim$ 5 to 240 Å or larger, and their distribution can be well fitted by an exponential law $N = N_{\rm 0}$ exp($-$EW/$w_{\rm 0}$). Owing to the high dynamic range in Muv, we find that the scale factor, $w_{\rm 0}$, depends on Muv in the sense that including fainter Muv objects increases $w_{\rm 0}$, i.e., the Ando effect. The results indicate that selection functions affect the EW scale factor. Taking these effects into account, we find that our $w_{\rm 0}$ values are consistent with those in the literature within $1σ$ uncertainties at $2.9 < z < 6.6$ at a given threshold of Muv and $L_{\rm Lyα}$. Interestingly, we find 12 objects with EW $>200$ Å above $1σ$ uncertainties. Two of these 12 LAEs show signatures of merger or AGN activity: the weak CIV $λ1549$ emission line. For the remaining 10 very large EW LAEs, we find that the EW values can be reproduced by young stellar ages ($< 100$ Myr) and low metallicities ($\lesssim 0.02$ $Z_{\rm \odot}$). Otherwise, at least part of the Ly$α$ emission in these LAEs needs to arise from anisotropic radiative transfer effects, fluorescence by hidden AGN or quasi-stellar object activity, or gravitational cooling.

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arXiv2017

The MUSE Hubble Ultra Deep Field Survey: I. Survey description, data reduction and source detection

We present the MUSE Hubble Ultra Deep Survey, a mosaic of nine MUSE fields covering 90\% of the entire HUDF region with a 10-hour deep exposure time, plus a deeper 31-hour exposure in a single 1.15 arcmin2 field. The improved observing strategy and advanced data reduction results in datacubes with sub-arcsecond spatial resolution (0.65 arcsec at 7000 A) and accurate astrometry (0.07 arcsec rms). We compare the broadband photometric properties of the datacubes to HST photometry, finding a good agreement in zeropoint up to mAB=28 but with an increasing scatter for faint objects. We have investigated the noise properties and developed an empirical way to account for the impact of the correlation introduced by the 3D drizzle interpolation. The achieved 3 sigma emission line detection limit for a point source is 1.5 and 3.1 10-19 erg.s-1.cm-2 for the single ultra-deep datacube and the mosaic, respectively. We extracted 6288 sources using an optimal extraction scheme that takes the published HST source locations as prior. In parallel, we performed a blind search of emission line galaxies using an original method based on advanced test statistics and filter matching. The blind search results in 1251 emission line galaxy candidates in the mosaic and 306 in the ultradeep datacube, including 72 sources without HST counterparts (mAB>31). In addition 88 sources missed in the HST catalog but with clear HST counterparts were identified. This data set is the deepest spectroscopic survey ever performed. In just over 100 hours of integration time, it provides nearly an order of magnitude more spectroscopic redshifts compared to the data that has been accumulated on the UDF over the past decade. The depth and high quality of these datacubes enables new and detailed studies of the physical properties of the galaxy population and their environments over a large redshift range.

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arXiv2016

MUSE Deep-Fields: The Lya Luminosity Function in the Hubble Deep Field South at 2.91 < z < 6.64

We present the first estimate of the Lyα luminosity function using blind spectroscopy from the Multi Unit Spectroscopic Explorer, MUSE, in the Hubble Deep Field South. Using automatic source-detection software, we assemble a homogeneously-detected sample of 59 Lyα emitters covering a flux range of -18.0 < log10 (F) < -16.3 (erg s^-1 cm^-2), corresponding to luminosities of 41.4 < log10 (L) < 42.8 (erg s^-1). As recent studies have shown, Lyα fluxes can be underestimated by a factor of two or more via traditional methods, and so we undertake a careful assessment of each object's Lyα flux using a curve-of-growth analysis to account for extended emission. We describe our self-consistent method for determining the completeness of the sample, and present an estimate of the global Lyα luminosity function between redshifts 2.91 < z < 6.64 using the 1/Vmax estimator. We find the luminosity function is higher than many number densities reported in the literature by a factor of 2 - 3, although our result is consistent at the 1σ level with most of these studies. Our observed luminosity function is also in good agreement with predictions from semi-analytic models, and shows no evidence for strong evolution between the high- and low-redshift halves of the data. We demonstrate that one's approach to Lyα flux estimation does alter the observed luminosity function, and caution that accurate flux assessments will be crucial in measurements of the faint end slope. This is a pilot study for the Lyα luminosity function in the MUSE deep-fields, to be built on with data from the Hubble Ultra Deep Field which will increase the size of our sample by almost a factor of 10.

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arXiv2015

Lyman-α Emitters in the context of hierarchical galaxy formation: predictions for VLT/MUSE surveys

The VLT Multi Unit Spectroscopic Explorer (MUSE) integral-field spectrograph can detect Lyα emitters (LAE) in the redshift range $2.8 \lesssim z \lesssim 6.7$ in a homogeneous way. Ongoing MUSE surveys will notably probe faint Lyα sources that are usually missed by current narrow-band surveys. We provide quantitative predictions for a typical wedding-cake observing strategy with MUSE based on mock catalogs generated with a semi-analytic model of galaxy formation coupled to numerical Lyα radiation transfer models in gas outflows. We expect $\approx$ 1500 bright LAEs ($F_{Lyα}$ $\gtrsim$ $10^{-17}$ erg s$^{-1}$ cm$^{-2}$) in a typical Shallow Field (SF) survey carried over $\approx$ 100 arcmin$^2$, and $\approx$ 2,000 sources as faint as $10^{-18}$ erg s$^{-1}$ cm$^{-2}$ in a Medium-Deep Field (MDF) survey over 10 arcmin$^2$. In a typical Deep Field (DF) survey of 1 arcmin$^2$, we predict that $\approx$ 500 extremely faint LAEs ($F_{Lyα}$ $\gtrsim$ $4 \times 10^{-19}$ erg s$^{-1}$ cm$^{-2}$) will be found. Our results suggest that faint Lyα sources contribute significantly to the cosmic Lyα luminosity and SFR budget. While the host halos of bright LAEs at z $\approx$ 3 and 6 have descendants with median masses of $2 \times 10^{12}$ and $5 \times 10^{13}$ $M_{\odot}$ respectively, the faintest sources detectable by MUSE at these redshifts are predicted to reside in halos which evolve into typical sub-$L^{*}$ and $L^{*}$ galaxy halos at z = 0. We expect typical DF and MDF surveys to uncover the building blocks of Milky Way-like objects, even probing the bulk of the stellar mass content of LAEs located in their progenitor halos at z $\approx$ 3.

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arXiv2015

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.

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arXiv2015

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.

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arXiv2012

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

[abridged] Aims. The aim of this paper is to assess the impact of the interstellar medium (ISM) physics on Lyman-alpha (Lya) radiation transfer and to quantify how galaxy orientation with respect to the line of sight alters observational signatures. Methods. We compare the results of Lya radiation transfer calculations through the ISM of a couple of idealized galaxy simulations with different ISM models. Results. First, the small-scale structuration of the ISM plays a determinant role in shaping a galaxys Lya properties.The artificially warm, and hence smooth, ISM of G1 yields an escape fraction of 50 percent at the Lya 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 Lya line. Second, the Lya 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 -5 Angstrom to a 15 times more luminous red-shifted asymmetric line with EW 90 Angstrom. Conclusions. Lya radiation transfer calculations can only lead to realistic properties in simulations where galaxies are resolved into giant molecular clouds, putting these calculations out of reach of current large scale cosmological simulations. Finally, we find inclination effects to be much stronger for Lya photons than for continuum radiation. This could potentially introduce severe biases in the selection function of narrow-band Lya emitter surveys, which could indeed miss a significant fraction of the high-z galaxy population.

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arXiv2012

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.

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arXiv2007

Dust Distribution during Reionization

The dust produced by the first generation of stars will be a foreground to cosmic microwave background. In order to evaluate the effect of this early dust, we calculate the power spectrum of the dust emission anisotropies and compare it with the sensitivity limit of the Planck satellite. The spatial distribution of the dust is estimated through the distribution of dark matter. At small angular scales ($\ell \gtrsim 1000$) the dust signal is found to be noticeable with the Planck detector for certain values of dust lifetime and production rates. The dust signal is also compared to sensitivities of other instruments. The early dust emission anisotropies are finally compared to those of local dust and they are found to be similar in magnitude at mm wavelengths.

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arXiv2006

LeMoMaF: Lensed Mock Map Facility

We present the Lensed Mock Map Facility (LeMoMaF), a tool designed to perform mock weak lensing measurements on numerically simulated chunks of the universe. Coupling N-body simulations to a semi-analytical model of galaxy formation, LeMoMaF can create realistic lensed images and mock catalogues of galaxies, at wavelengths ranging from the UV to the submm. To demonstrate the power of such a tool we compute predictions of the source-lens clustering effect on the convergence statistics, and quantify the impact of weak lensing on galaxy counts in two different filters. We find that the source-lens clustering effect skews the probability density function of the convergence towards low values, with an intensity which strongly depends on the redshift distribution of galaxies. On the other hand, the degree of enhancement or depletion in galaxy counts due to weak lensing is independent of the source-lens clustering effect. We discuss the impact on the two-points shear statistics to be measured by future missions like SNAP and LSST. The source-lens clustering effect would bias the estimation of sigma_8 from two point statistics by 2% -5%. We conclude that accurate photometric redshifts for individual galaxies are necessary in order to quantify and isolate the source-lens clustering effect.

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arXiv2006

The infrared compactness-temperature relation for quiescent and starburst galaxies

IRAS observations show the existence of a correlation between the infrared luminosity Lir and dust temperature Td in star-forming galaxies, in which larger Lir leads to higher dust temperature. The Lir-Td relation is commonly seen as reflecting the increase in dust temperature in galaxies with higher star formation rate. Even though the correlation shows a significant amount of dispersion, a unique relation has been commonly used to construct spectral energy distributions of galaxies in distant universe studies, such as source number counting or photometric redshift determination. In this work, we introduce a new parameter, namely the size of the star-forming region Rir and lay out the empirical and modelled relation between the global parameters Lir, Td and Rir of IR-bright non-AGN galaxies. IRAS 60-to-100um color is used as a proxy for the dust temperature and the 1.4GHz radio contiuum emission for the infrared spatial distribution. The analysis has been carried out on two samples. The first one is made of the galaxies from the 60um flux-limited IRAS Revised Bright Galaxy Samples which have a reliable RC size estimate from the VLA follow-ups of the IRAS Bright Galaxy Samples. The second is made of the sources from the 170um ISOPHOT Serendipity Sky Survey which are resolved by the NVSS or FIRST surveys. We show that the dispersion in the Lir-Td diagram can be reduced to a relation between the infrared surface brightness and the dust temperature, a relation that spans 5 orders of magnitude in surface brightness. We explored the physical processes giving rise to the Sir-Td relation, and show that it can be derived from the Schmidt law, which relates the star formation rate to the gas surface density.

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arXiv2006

Accretion, feedback and galaxy bimodality: a comparison of the GalICS semi-analytic model and cosmological SPH simulations

We compare the galaxy population of an SPH simulation to those predicted by the GalICS semi-analytic model and a stripped down version without supernova and AGN feedback. The SPH simulation and the no-feedback GalICS model make similar predictions for the baryonic mass functions of galaxies and for the dependence of these mass functions on environment and redshift. The two methods also make similar predictions for the galaxy content of dark matter haloes as a function of halo mass and for the gas accretion history of galaxies. Both the SPH and no-feedback GalICS models predict a bimodal galaxy population at z=0. The "red'' sequence of gas poor, old galaxies is populated mainly by satellite systems while, contrary to observations, the central galaxies of massive haloes lie on the "blue'' star-forming sequence as a result of continuing hot gas accretion at late times. Furthermore, both models overpredict the observed baryonic mass function, especially at the high mass end. In the full GalICS model, supernova-driven outflows reduce the masses of low and intermediate mass galaxies by about a factor of two. AGN feedback suppresses gas cooling in large haloes, producing a sharp cut-off in the baryonic mass function and moving the central galaxies of these massive haloes to the red sequence. Our results imply that the observational failings of the SPH simulation and the no-feedback GalICS model are a consequence of missing input physics rather than computational inaccuracies, that truncating gas accretion by satellite galaxies automatically produces a bimodal galaxy distribution with a red sequence, but that explaining the red colours of the most massive galaxies requires a mechanism like AGN feedback that suppresses the accretion onto central galaxies in large haloes.

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arXiv2004

The abundance of Lyman-(α) emitters in hierarchical models

We present predictions for the abundance of Ly-$α$ emitters in hierarchical structure formation models. We use the {\tt GALFORM} semi-analytical model to explore the impact on the predicted counts of varying assumptions about the escape fraction of Ly-$α$ photons, the redshift at which the universe reionised and the cosmological density parameter. A model with a fixed escape fraction gives a remarkably good match to the observed counts over a wide redshift interval. We present predictions for the expected counts in a typical observation with the Multi Unit Spectroscopic Explorer instrument proposed for the Very Large Telescope.

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