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Accurate shape measurements are essential to infer cosmological parameters from large area weak gravitational lensing studies. The compact diffraction-limited point spread function (PSF) in space-based observations is greatly beneficial, but its chromaticity for a broadband observation can lead to new subtle effects that could hitherto be ignored: the PSF of a galaxy is no longer uniquely defined and spatial variations in the colours of galaxies result in biases in the inferred lensing signal. Taking Euclid as a reference, we show that this colour gradient bias (CG bias) can be quantified with high accuracy using available multicolour Hubble Space Telescope (HST) data. In particular we study how noise in the HST observations might impact such measurements and find this to be negligible. We determine the CG bias using HST observations in the F606W and F814W filters and observe a correlation with the colour, in line with expectations, whereas the dependence with redshift is weak. The biases for individual galaxies are generally well below 1 per cent, which may be reduced further using morphological information from the Euclid data. Our results demonstrate that CG bias should not be ignored, but it is possible to determine its amplitude with sufficient precision, so that it will not significantly bias the weak lensing measurements using Euclid data. [ABSTRACT FROM AUTHOR]

Gravitational lensing has long been considered as a valuable tool to determine the total mass of galaxy clusters. The shear profile, as inferred from the statistics of ellipticity of background galaxies, allows us to probe the cluster intermediate and outer regions, thus determining the virial mass estimate. However, the mass sheet degeneracy and the need for a large number of background galaxies motivate the search for alternative tracers which can break the degeneracy among model parameters and hence improve the accuracy of the mass estimate. Lensing flexion, i.e. the third derivative of the lensing potential, has been suggested as a good answer to the above quest since it probes the details of the mass profile.We investigate here whether this is indeed the case considering jointly using weak lensing, magnification and flexion.We use a Fisher matrix analysis to forecast the relative improvement in the mass accuracy for different assumptions on the shear and flexion signal-to- noise (S/N) ratio also varying the cluster mass, redshift, and ellipticity. It turns out that the error on the cluster mass may be reduced up to a factor of ∼2 for reasonable values of the flexion S/N ratio. As a general result, we get that the improvement in mass accuracy is larger for more flattened haloes, but it extracting general trends is difficult because of the many parameters at play. We nevertheless find that flexion is as efficient as magnification to increase the accuracy in both mass and concentration determination. [ABSTRACT FROM AUTHOR]

Context. The statistics of peaks in weak lensing convergence maps is a promising tool for investigating both the properties of dark matter haloes and constraining the cosmological parameters. Aims. We study how the number of detectable peaks and its scaling with redshift depend upon the cluster's dark matter halo profiles and use peak statistics to constrain the parameters of the mass-concentration (MC) relation. We investigate which constraints the Euclid mission can set on the MC coefficients taking degeneracies with the cosmological parameters into account, too. Methods. To this end, we first estimated the number of peaks and its redshift distribution for different MC relations and found that the steeper the mass dependence and the greater the normalisation, the larger the number of detectable clusters, with the total number of peaks changing up to 40% depending on the MC relation. We then performed a Fisher matrix forecast of the errors on the MC relation parameters, as well as on cosmological parameters. Results. We find that peak number counts detected by Euclid can determine the normalization Av, the mass Bv, redshift Cv slopes, and intrinsic scatter σv of the MC relation to an unprecedented accuracy, which is σ(Av)/Av = 1%, σ(Bv)/Bv = 4%, σ(Cv)/Cv = 9%, and σ(σv)/σv = 1% if all cosmological parameters are assumed to be known. If we relax this severe assumption, constraints are degraded, but remarkably good results can be restored by setting only some of the parameters or combining peak counts with Planck data. This precision can give insight into competing scenarios of structure formation and evolution and into the role of baryons in cluster assembling. Alternatively, for a fixed MC relation, future peak counts can perform as well as current BAO and SNeIa when combined with Planck. [ABSTRACT FROM AUTHOR]

Euclid is the next ESA mission devoted to cosmology. It aims at observing most of the extragalactic sky, studying both gravitational lensing and clustering over ~15,000 square degrees. The mission is expected to be launched in year 2020 and to last six years. The sheer amount of data of different kinds, the variety of (un)known systematic effects and the complexity of measures require efforts both in sophisticated simulations and techniques of data analysis. We review the mission main characteristics, some aspects of the the survey and highlight some of the areas of interest to this meeting. [ABSTRACT FROM PUBLISHER]

Weak gravitational lensing by galaxy clusters on faint higher redshift galaxies has been traditionally used to study the cluster mass distribution and as a tool to identify clusters as peaks in the shear maps. However, it becomes soon clear that peak statistics can also be used as away to constrain the underlying cosmological model due to its dependence on both the cosmic expansion rate and the growth rate of structures. This feature makes peak statistics particularly interesting from the point of view of discriminating between General Relativity and modified gravity. Here we consider a general class of f(R) theories and compute the observable mass function based on the aperture mass statistics. We complement our theoretical analysis with a Fisher matrix forecast of the constraints that an Euclid-like survey can impose on the f(R) model parameters. We show that peak statistics alone can in principle discriminate between General Relativity and f(R)models and strongly constrain the f(R) parameters that are sensitive to the non-linear growth of structure. However, further analysis is needed in order to include possible selection function in the peaks redshift determination. [ABSTRACT FROM AUTHOR]

We model the evolution of the mean galaxy occupation of dark matter haloes over the range , using the data from the VIMOS-VLT Deep Survey. The galaxy projected correlation function was computed for a set of luminosity-limited subsamples and fits to its shape were obtained using two variants of halo occupation distribution (HOD) models. These provide us with a set of best-fitting parameters, from which we obtain the average mass of a halo and average number of galaxies per halo. We find that after accounting for the evolution in luminosity and assuming that we are largely following the same population, the underlying dark matter halo shows a growth in mass with decreasing redshift as expected in a hierarchical structure formation scenario. Using two different HOD models, we see that the halo mass grows by 90 per cent over the redshift interval . This is the first time the evolution in halo mass at high redshifts has been obtained from a single data survey and it follows the simple form seen in N-body simulations with , and . This provides evidence for a rapid accretion phase of massive haloes having a present-day mass , with a merger event occurring between redshifts of 0.5 and 1.0. Furthermore, we find that more luminous galaxies are found to occupy more massive haloes irrespective of the redshift. Finally, the average number of galaxies per halo shows little increase from redshift to ∼0.5, with a sharp increase by a factor of ∼3 from to ∼0.1, likely due to the dynamical friction of subhaloes within their host haloes. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The nature of the dwarf galaxy population as a function of location in the cluster and within different environments is investigated. We have previously described the results of a search for low surface brightness objects in data drawn from an East-West strip of the Virgo cluster (Sabatini et al(2004)) and have compared this to a large area strip outside of the cluster (Roberts et al (2004)). In this talk I compare the East-West data (sampling sub-cluster A and outward) to new data along a North-South cluster strip that samples a different region (part of sub-cluster A, and the N, M clouds) and with data obtained for the Ursa Major cluster and fields around the spiral galaxy M101. The sample of dwarf galaxies in different environments is obtained from uniform datasets that reach central surface brightness values of 26 B mag/arcsec2 and an apparent B magnitude of 21 (MB =−10 for a Virgo Cluster distance of 16 Mpc). We discuss and interpret our results on the properties and distribution of dwarf low surface brightness galaxies in the context of variuos physical processes that are thought to act on galaxies as they form and evolve. [ABSTRACT FROM PUBLISHER]

The Vimos-VLT Deep Survey is a spectroscopic survey aiming at collecting more than 50000 spectra down to a limiting magnitude $I_{\mathrm{AB}}=24$, and 100000 down to $I_{\mathrm{AB}}=22.5$, on a total of about 16 deg$^2$ without any color or morphology preselection. We present the $N(z)$ distribution up to $z\sim 5$, obtained from a purely magnitude-limited sample down to $I_{\mathrm{AB}}=24$, which is an important input to weak-lensing studies. We discuss the evolution of the galaxy luminosity function up to $z\sim 2$, which exhibits a very strong increase in the typical galaxy luminosity $\Delta M^*\simeq -2.5$ in the U band compared to the local value. Surveys like the VVDS also allow to study the galaxy bias as a function of redshift without assumption about its linearity, an assumption that we find to be violated in some cases. A low bias is found, and the linear bias is shown to increase with redshift.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html [ABSTRACT FROM PUBLISHER]

We discuss a bias present in the calculation of the global luminosity function (LF) which occurs when analysing faint galaxy samples. This effect exists as a result of the different spectral energy distributions of galaxies, which are in turn quantified by the k corrections. We demonstrate that this bias occurs because not all galaxy types are visible in the same absolute magnitude range at a given redshift and it mainly arises at high redshift as it is related to large k corrections. We use realistic simulations with observed LFs to investigate the amplitude of the bias. We also compare our results to the global LFs derived from Hubble Deep Field (HDF) North and South surveys. We conclude that, as expected, there is no bias in the global LF measured in the absolute magnitude range where all galaxy types are observable. Beyond this range the faint-end slope of the global LF can be over/underestimated depending on the adopted LF estimator. The effect is larger when the reference filter in which the global LF is measured, is far from the rest-frame filter in which galaxies are selected. The fact that LF estimators are differently affected by this bias implies that the bias is minimal when the different LF estimators give measurements consistent with one another at the faint end. For instance, we show that the estimators are discrepant in the same way both in the simulated and HDF LFs. This suggests that the HDF LFs are affected by the presently studied bias. The best way of avoiding this bias is to derive the global LF in the reference filter closest to the rest-frame selection filter. [ABSTRACT FROM AUTHOR]

ABSTRACT The widely varying dwarf galaxy counts in different environments provide a strong challenge to standard hierarchical clustering models. The luminosity function is not universal, but seems to be strongly dependent upon environment. In this paper we describe an automated procedure for detecting and measuring very low surface brightness (LSB) features in deep CCD data. We apply this procedure to large-area CCD survey fields of the Virgo cluster. We show that there are many more faint (-10 ≥M[sub B]≥-14) LSB galaxies than would be predicted from extrapolation of the Virgo cluster catalogue luminosity function. Over our limited range of measurement, the faint-end slope of the luminosity function becomes α=-1.6 . The luminosity function is flatter in the inner regions of the cluster than it is in the outer regions. Although these galaxies contribute a small fraction of the total stellar light of the cluster, they may contribute significantly to the mass in galaxies if they have large mass-to-light ratios similar to those recently measured for Local Group dwarf galaxies. [ABSTRACT FROM AUTHOR]

We present the results of a redshift survey of intercluster galaxies in the central region of the Shapley Concentration supercluster, aimed at determining the distribution of galaxies in between obvious overdensities. Our sample is formed by 442 new redshifts, mainly in the b[sub J] magnitude range 17–18.8. Adding the data from our redshift surveys on the A3558 and A3528 complexes, which are close to the geometrical centre of this supercluster, we obtain a total sample of ∼2000 radial velocities. The average velocity of the observed intercluster galaxies in the Shapley Concentration appears to be a function of their (α, δ) position, and it can be fitted by a plane in the three-dimensional space (α, δ, v): the distribution of the galaxy distances around the best-fitting plane is described by a Gaussian with dispersion 3.8 h[sup -1] Mpc. Using the 1440 galaxies of our total sample in the magnitude range 17–18.8, we reconstruct the density profile in the central part of the Shapley Concentration; moreover, we detect another significant overdensity at ∼30 000 km s[sup -1] (dubbed S300). We estimate the total overdensity of galaxies, and the mass and dynamical state of these structures, and discuss the effect of considering a bias between the galaxy distribution and the underlying matter. The estimated total overdensity of galaxies of these two structures is (N/N)∼11.3 on a scale of 10.1 h[sup -1] Mpc for the Shapley Concentration, and (N/N)∼2.9 on a scale of 24.8 h[sup -1] Mpc for S300. If light traces the mass distribution, the corresponding masses are 1.4×10[sup 16]Ω[sub o] h[sup -1] M[sub ⊙] and 5.1×10[sup 16]Ω[sub o] h[sup -1] M[sub ⊙] for the Shapley Concentration and S300, respectively. A dynamical analysis suggests that, if light traces mass and Ω[sub o]=1, the Shapley Concentration has already reached its turnaround radius and has started to collapse: the final collapse will happen in ∼3×10[sup 9] h[sup -1] yr. We find an indication that the value of the bias between clusters and galaxies in the Shapley Concentration is higher that that reported in literature, confirming the impression that this supercluster is very rich in clusters. Finally, from the comparison with some theoretical scenarios, we find that the existence of the Shapley Concentration is more consistent with the predictions of the models with a matter density parameter <1, such as open CDM and ΛCDM. [ABSTRACT FROM AUTHOR]

Deals with a long term study on the Shapley Concentration in order to describe its dynamical state and to determine its mass. Overview of the MEFOS redshift survey; Wedge diagram obtained adding all the available redshifts in the region of the core of the Shapley Concentration.

Presents some preliminary results concerning the ESO slice project, a galaxy redshift survey. Observations and data reduction; Large-scale properties.

The Euclid space mission proposes to survey 15000 square degrees of the extragalactic sky during 6 years, with a step-and-stare technique. The scheduling of observation sequences is driven by the primary scientific objectives, spacecraft constraints, calibration requirements and physical properties of the sky. We present the current reference implementation of the Euclid survey and on-going work on survey optimization. [ABSTRACT FROM PUBLISHER]

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. Galaxies in the young Universe have hitherto mainly been identified using their expected optical colours, 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, 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. [ABSTRACT FROM AUTHOR]