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We present a study of high-redshift star-forming galaxies, focusing on Lymanalpha Emitters, which are among the most promising candidates for driving cosmic reionization, yet difficult to observe due to their faintness. To address this, we leverage strong gravitational lensing by foreground galaxy clusters to amplify otherwise undetectable sources. Using spectroscopic data from VLT/MUSE, complemented by HST and JWST imaging, we measure the lensing-corrected Lyman-alpha luminosity function and trace the redshift evolution of Ly α photon escape fractions up to redshift ∼6.7. Systematic uncertainties—particularly those related to lensing magnification and source selection—are quantified and commented upon. Our results highlight the significant role of faint LAEs in cosmic reionization and demonstrate the power of combining gravitational lensing with advanced observational facilities. This work was conducted at the Department of Astrophysics, Vietnam National Space Center, in collaboration with the Laboratoire d’Astrophysique de Marseille, as part of the MUSE collaboration.

A glimpse of dark energy Cosmologists can tell from observing distant supernovae that the Universe is undergoing a phase of accelerated expansion, but the physical cause remains a mystery. The favoured explanation requires huge amounts of invisible 'dark energy', distributed across the Universe, forcing expansion via gravitational repulsion. A new survey of redshift distortions of thousands of faint galaxies provides hints as to the nature of dark-energy induced cosmic acceleration. The distortion at a redshift of z = 0.8 is consistent with the standard cosmological-constant model with low matter density and flat geometry. The current error bars are too large to distinguish among alternative origins for the accelerated expansion, but the next generation of galactic surveys, more powerful and far-reaching than the current crop, should provide much tighter constraints on the properties of dark energy. A measurement of the radial anisotropy at a redshift z ≅ 0.8 that is consistent with the standard cosmological-constant model with low matter density and flat geometry is reported, although error bars are still too large to distinguish among alternative origins for the accelerated expansion. Observations of distant supernovae indicate that the Universe is now in a phase of accelerated expansion 1 , 2 the physical cause of which is a mystery 3 . 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 4 , 5 , 6 , 7 , 8 . Although they produce similar expansion rates, different models predict measurable differences in the growth rate of large-scale structure with cosmic time 9 . 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 10 . 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 11 , 12 , 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.

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Well red A survey of the distant Universe has revealed a much larger population of star-forming galaxies at high cosmological redshift than was previously known. A count of 970 galaxies with redshifts between 1.4 and 5 is a several-fold increase on previous estimates. Knowledge of the number of galaxies present in the Universe at each epoch is an important constraint on models of galaxy evolution. To understand the evolution of galaxies, we need to know as accurately as possible how many galaxies were present in the Universe at different epochs 1 . Galaxies in the young Universe have hitherto mainly been identified using their expected optical colours 2 , 3 , 4 , but this leaves open the possibility that a significant population remains undetected because their colours are the result of a complex mix of stars, gas, dust or active galactic nuclei. Here we report the results of a flux-limited I-band survey of galaxies at look-back times of 9 to 12 billion years. We find 970 galaxies with spectroscopic redshifts between 1.4 and 5. This population is 1.6 to 6.2 times larger than previous estimates 2 , 3 , 4 , with the difference increasing towards brighter magnitudes. Strong ultraviolet continua (in the rest frame of the galaxies) indicate vigorous star formation rates of more than 10–100 solar masses per year. As a consequence, the cosmic star formation rate representing the volume-averaged production of stars is higher than previously measured at redshifts of 3 to 4.

With the new generation of spectrographs, integral field spectroscopy is becoming a widely used observational technique. The Integral Field Unit (IFU) of the Visible Multi–Object Spectrograph (VIMOS) on the ESO VLT allows sampling of a field as large as \\documentclass{aastex} \\usepackage{amsbsy} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{bm} \\usepackage{mathrsfs} \\usepackage{pifont} \\usepackage{stmaryrd} \\usepackage{textcomp} \\usepackage{portland,xspace} \\usepackage{amsmath,amsxtra} \\usepackage[OT2,OT1]{fontenc} \\newcommand\\cyr{ \\renewcommand\\rmdefault{wncyr} \\renewcommand\\sfdefault{wncyss} \\renewcommand\\encodingdefault{OT2} \\normalfont \\selectfont} \\DeclareTextFontCommand{\\textcyr}{\\cyr} \\pagestyle{empty} \\DeclareMathSizes{10}{9}{7}{6} \\begin{document} \\landscape $54^{\\prime \\prime }\\times 54^{\\prime \\prime }$ \\end{document} , covered by 6400 fibers coupled with microlenses. We present here the methods of the data‐processing software that has been developed to extract the astrophysical signal of faint sources from the VIMOS IFU observations. We focus on the treatment of the fiber‐to‐fiber relative transmission and the sky subtraction, and the dedicated tasks we have built to address the peculiarities and unprecedented complexity of the data set. We review the automated process we have developed under the VIPGI data organization and reduction environment (Scodeggio et al.2005), along with the quality control performed to validate the process. The VIPGI IFU data‐processing environment has been available to the scientific community to process VIMOS IFU data since 2003 November.

The VIMOS VLT Deep Survey (VVDS), designed to measure 150,000 galaxy redshifts, requires a dedicated data reduction and analysis pipeline to process in a timely fashion the large amount of spectroscopic data being produced. This requirement has lead to the development of the VIMOS Interactive Pipeline and Graphical Interface (VIPGI), a new software package designed to simplify to a very high degree the task of reducing astronomical data obtained with VIMOS (Visible Multi–Object Spectrograph), the imaging spectrograph built by the VIRMOS Consortium for the European Southern Observatory and mounted on Unit 3 (Melipal) of the VLT (Very Large Telescope) at Paranal Observatory (Chile). VIPGI provides the astronomer with specially designed VIMOS data‐reduction functions, a VIMOS‐centric data organizer, and dedicated data browsing and plotting tools, which can be used to verify the quality and accuracy of the various stages of the data‐reduction process. The quality and accuracy of the data‐reduction pipeline are comparable to those obtained using well‐known IRAF tasks, but the speed of the data‐reduction process is significantly increased, because of the dedicated nature of VIPGI. In this paper we discuss the details of the multiobject spectroscopy (MOS) data‐reduction pipeline that has been implemented in VIPGI, as applied to the reduction of some 20,000 VVDS spectra, quantitatively assessing the accuracy of the various reduction steps. We also provide a more general overview of VIPGI capabilities, a tool that can be used for the reduction of any kind of VIMOS data.

Following up on the detailed results from our searches for z~ 6–10 galaxies using gravitional lensing clusters descrived in Richard et al. (2006), we have obtained new observations, which are presented here. For a general overview of the project see Schaerer et al. (2006). First, deep HST/ACS observations in the z-band confirm 17 out of 18 high redshift candidates as optical drop-out objects; the remaining object appears faint and star-like. Second, Spitzer/IRAC images at 3.6, 4.5, 5.8 and 8.0 μm have been obtained. 11 out of 18 candidates are in “free”, uncominated regions. These 11 high-z candidates are undetected, with upper limits which are compatible with their SED if these objects are blue star-forming galaxies at high redshift (see Fig. 1). In conclusion, our new observations corroborate so far the high-z nature of the majority of the candidates.

We report the results obtained from optical and near-infrared spectroscopy and also broad-band photometry of lensed low-luminosity galaxies ($1.3\\leq z\\leq 2.3$) located in the core of lensing clusters. The amplification factor allowed us to obtain the physical properties (SFR, abundance ratios, mass, age of the burst, dust contents, etc.) of star-forming galaxies, 1 to 2 magnitudes fainter than galaxies in a blank field, using the same indicators as in studies of low-redshift galaxies. A part of this study was concentrated on the dynamics of distant lensed galaxies using Integral Field Spectroscopy (SINFONI/VLT science verification programme).

We have developed model predictions for the morphological distribution of cluster galaxies as a function of the cluster-centric distance and the local galaxy density, using a semi-analytical code. This code allows us to obtain magnitudes and colours for cluster galaxies at different redshifts, and thus to study in detail the evolution of the colour-magnitude relation of specific distant clusters.

We present new results obtained on the identification and study of high-zgalaxies seen through lensing clusters used as gravitational telescopes. The amplification effect provides a tool to study the spectrophotometric and morphological properties of such galaxies 1 to 3 magnitudes deeper with respect to field surveys. Distant sources with 1 ≤ z ≤ 7 typically are selected close to the critical lines in clusters where the mass distribution is well known, using photometric redshifts computed on a large wavelength interval, as well as lens-inversion criteria. We focus here on the recent results obtained onAC114 and MS1008-1224, two lensing clusters at z = 0.31.[PUBLICATION ABSTRACT]