Search

Aims. An unbiased and detailed characterization of the galaxy luminosity function (LF) is a basic requirement in many astrophysical issues: it is of particular interest in assessing the role of the environment in the evolution of the LF of different galaxy types.

Aims. Powerful gravitational telescopes in the form of massive galaxy clusters can be used to enhance the light collecting power over a limited field of view by about an order of magnitude in flux. This effect is exploited here to increase the depth of a survey for lensed supernovae at near-IR wavelengths.

Context. Strong lens candidates have been newly identified within the COSMOS field. We present VLT/FORS1 spectroscopic follow-up observations and HST/WFPC2 imaging of the system COSMOS 5921+0638, which exhibits quadruply lensed images and a perfect Einstein ring.

Context. Massive galaxy clusters at intermediate redshift can magnify the flux of distant background sources by several magnitudes.

Aims. We study the dependence of galaxy clustering on luminosity and stellar mass at redshifts $z\sim$[0.2–1], using the first 10K redshifts from the zCOSMOS spectroscopic survey of the COSMOS field.

Context. Future dark energy space missions such as JDEM and EUCLID are being designed to survey the galaxy population to trace the geometry of the universe and the growth of structure, which both depend on the cosmological model. To reach the goal of high precision cosmology they need to evaluate the capabilities of different instrument designs based on realistic mock catalogs of the galaxy distribution.

Context. For more than two decades we have known that galaxy morphological segregation is present in the Local Universe. It is important to see how this relation evolves with cosmic time.

The existence of strong lensing systems with Einstein radii covering the full mass spectrum, from ∼$1{-}2\arcsec$(produced by galaxy scale dark matter haloes) to >$10\arcsec$(produced by galaxy cluster scale haloes) have long been predicted. Many lenses with Einstein radii around $1{-}2\arcsec$and above $10\arcsec$have been reported but very few in between. In this article, we present a sample of 13 strong lensing systems with Einstein radii in the range $3\arcsec{-}8\arcsec$(or image separations in the range $6\arcsec{-}16\arcsec$), i.e. systems produced by galaxy group scale dark matter haloes. This group sample spans a redshift range from 0.3 to 0.8. This opens a new window of exploration in the mass spectrum, around 1013–1014$M_{\odot}$, a crucial range for understanding the transition between galaxies and galaxy clusters, and a range that have not been extensively probed with lensing techniques. These systems constitute a subsample of the Strong Lensing Legacy Survey (SL2S), which aims to discover strong lensing systems in the Canada France Hawaii Telescope Legacy Survey (CFHTLS). The sample is based on a search over 100 square degrees, implying a number density of ~0.13 groups per square degree. Our analysis is based on multi-colour CFHTLS images complemented with Hubble Space Telescopeimaging and ground based spectroscopy. Large scale properties are derived from both the light distribution of elliptical galaxies group members and weak lensing of the faint background galaxy population. On small scales, the strong lensing analysis yields Einstein radii between 2.5″ and 8″. On larger scales, strong lens centres coincide with peaks of light distribution, suggesting that light traces mass. Most of the luminosity maps have complicated shapes, implying that these intermediate mass structures may be dynamically young. A weak lensing signal is detected for 6 groups and upper limits are provided for 6 others. Fitting the reduced shear with a Singular Isothermal Sphere, we find $\sigma_{\rm SIS}\,\sim 500$km s-1with large error bars and an upper limit of ~900 km s-1for the whole sample (except for the highest redshift structure whose velocity dispersion is consistent with that of a galaxy cluster). The mass-to-light ratio for the sample is found to be $M/L_i$~ 250 (solar units, corrected for evolution), with an upper limit of 500. This compares with mass-to-light ratios of small groups (with $\sigma_{\rm SIS} \sim 300$km s-1) and galaxy clusters (with $\sigma_{\rm SIS} > 1000$km s-1), thus bridging the gap between these mass scales.
The group sample released in this paper will be complemented with other observations, providing a unique sample to study this important intermediate mass range in further detail.

Context. Strong gravitational lensing is a unique tool that can be used to accurately model the inner mass distribution of massive galaxy clusters. In particular, clusters with large Einstein radii provide a wealth of multiply imaged systems in the cluster core. Measuring the redshift of these multiple images provides strong constraints for precisely determining the shape of the central dark matter profile.

Context. Morphology is the most accessible tracer of galaxies physical structure, but its interpretation in the framework of galaxy evolution still remains problematic. Its quantification at high redshift requires deep high-angular resolution imaging, which is why space data (HST) are usually employed. At $z > 1$, the HST visible cameras however probe the UV flux, which is dominated by the emission of young stars, which could bias the estimated morphologies towards late-type systems.

Aims. We report on the redshift of the lensing galaxy and of the quasar QJ 0158-4325  and on the lens model of the system.

Aims. We investigate the relationships between three main optical galaxy observables (spectral properties, colors, and morphology), exploiting the data set provided by the COSMOS/zCOSMOS survey. The purpose of this paper is to define a simple galaxy classification cube, with a carefully selected sample of ≈1000 galaxies.

Properties of dark matter haloes can be probed observationally and numerically, and comparing both approaches provides ways to constrain cosmological models. When it comes to the inner part of galaxy cluster scale haloes, interaction between the baryonic and the dark matter component is an important issue that is far from being fully understood. With this work, we aim to initiate a program coupling observational and numerical studies to probe the inner part of galaxy clusters. In this article, we apply strong lensing techniques on Abell 1703, a massive X-ray luminous galaxy cluster at $z = 0.28$. Our analysis is based on imaging data from both the space and ground in 8 bands, complemented by a spectroscopic survey. Abell 1703 is rather circular from the general shape of its multiply imaged systems and is dominated by a giant elliptical cD galaxy in its centre. This cluster exhibits a remarkable bright central ringformed by 4 images at zspec= 0.888 only 5–13″ away from the cD centre. This unique feature offers a rare lensing constrain for probing the central mass distribution. The stellar contribution from the cD galaxy (${\sim}$1.25 $\times$1012$M_{\sun}$within 30 kpc) is accounted for in our parametric mass modelling, and the underlying smooth dark matter component distribution is described using a generalized nfwprofile parametrized with a central logarithmic slope α. The rms of our mass model in the image plane is equal to 1.4″. We find that within the range where observational constraints are present (from ~20 kpc to ~210 kpc), αis equal to $1.09^{+0.05}_{-0.11}$(3σconfidence level). The concentration parameter is equal to c200~ 3.5, and the scale radius is constrained to be larger than the region where observational constraints are available (rs=$730^{\rm +15}_{-75}$kpc). The 2D mass is equal to ${M}$(210 kpc) = 2.4 $\times$1014$M_{\sun}$. However, we cannot draw any conclusions on cosmological models at this point since we lack results from realistic numerical simulations containing baryons to make a proper comparison. We advocate the need for a large sample of well observed (and well constrained) and simulated unimodal relaxed galaxy clusters in order to make reliable comparisons and to potentially provide a test of cosmological models.

Context. Gravitationally lensed quasars constitute an independent tool to derive the Hubble constant through the time-delays of their different images; they offer as well the opportunity to study the mass distribution and interstellar medium of their lensing galaxies and, through microlensing due to stars in their lensing galaxy, they also allow one to study details of the emitting source.

Aims.We study the properties and nature of extremely red galaxies (ERO, $R-K\ge5.6$) found behind two lensing clusters and compare them with other known galaxy populations.

Aims.We present a wide-field multi-color survey of a homogeneous sample of eleven clusters of galaxies for which we measure total masses and mass distributions from weak lensing. This sample, spanning a small range in both X-ray luminosity and redshift, is ideally suited to determining the normalisation of scaling relations between X-ray properties of clusters and their masses (the $M-T_{{\rm X}}$and the $M-L_{{\rm X}}$relations) and also estimating the scatter in these relations at a fixed luminosity.

Context.On the nature, redshift, stellar populations and dust properties of optically faint or non-detected extremely red objects.

Context.Gravitational lenses offer unique opportunities to explore the molecular content around active galactic nuclei at high redshift, through the magnification and gain in angular resolution.

We present the X-ray properties and scaling relations of a flux-limited morphology-unbiased sample of 12 X-ray luminous galaxy clusters at redshift around 0.2 based on XMM-Newton observations. The scaled radial profiles are characterized by a self-similar behavior at radii outside the cluster cores (>$0.2 r_{500}$) for the temperature ($T\propto r^{-0.36}$), surface brightness, entropy ($S \propto r^{1.01}$), gas mass and total mass. The cluster cores contribute up to 70% of the bolometric X-ray luminosity. The X-ray scaling relations and their scatter are sensitive to the presence of the cool cores. Using the X-ray luminosity corrected for the cluster central region and the temperature measured excluding the cluster central region, the normalization agrees to better than 10% for the cool core clusters and non-cool core clusters, irrelevant to the cluster morphology. No evolution of the X-ray scaling relations was observed comparing this sample to the nearby and more distant samples. With the current observations, the cluster temperature and luminosity can be used as reliable mass indicators with the mass scatter within 20%. Mass discrepancies remain between X-ray and lensing and lead to larger scatter in the scaling relations using the lensing masses (e.g. ~40% for the luminosity–mass relation) than using the X-ray masses (<20%) due to the possible reasons discussed.

Aims.Our aim is to constrain the properties of dark matter halos inhabiting high density environments, such as is the case in massive galaxy clusters.

Aims.We present data from the CFHTLS Strong Lensing Legacy Survey (SL2S). Due to the unsurpassed combined depth, area and image quality of the Canada-France-Hawaii Legacy Survey it is becoming possible to uncover a large, statistically well-defined sample of strong gravitational lenses which spans the dark halo mass spectrum predicted by the concordance model from galaxy to cluster haloes.

Aims.We searched for diffuse intracluster light in the galaxy cluster Abell 2667 ($z=0.233$) from HST images in three broad band-filters.

We present extensive multi-color imaging and low-resolution VIMOS integral field unit (IFU) spectroscopic observations of the X-ray luminous cluster Abell 2667 ($z=0.233$). An extremely bright giant gravitational arc ($z=1.0334 \pm 0.0003$) is easily identified as part of a triple image system, and other fainter multiple images are also revealed by the Hubble Space TelescopeWide Field Planetary Camera-2 images. The VIMOS-IFU observations cover a field of view of $54'' \times 54''$and enable us to determine the redshift of all galaxies down to $V_{\rm 606}=22.5$. Furthermore, redshifts could be identified for some sources down to $V_{\rm 606}=23.2$. In particular we identify 21 members in the cluster core, from which we derive a velocity dispersion of $\sigma=960_{-120}^{+190}$km s-1, corresponding to a total mass of $7.1\pm 1.8 \times 10^{13}\,h_{70}^{-1}\,M_\odot$within a $110 \, h_{70}^{-1} $kpc (30 arcsec) radius. Using the multiple images constraints and priors on the mass distribution of cluster galaxy halos we construct a detailed lensing-mass model leading to a total mass of $2.9 \pm 0.1 \times 10^{13}\,h_{70}^{-1}\,M_\odot$within the Einstein radius (16 arcsec). The lensing mass and dynamical mass are in good agreement, although the dynamical one is much less accurate. Within a 110 $h_{70}^{-1}$kpc radius, we find a rest-frame K-band M/Lratio of $61\pm 5 \, h_{70}\,M_\odot/L_\odot$. Comparing these measurements with published X-ray analysis is, however, less conclusive. Although the X-ray temperature matches the dynamical and lensing estimates, the published NFW mass model derived from the X-ray measurement with its low concentration of $c \sim 3$cannot account for the large Einstein radius observed in this cluster. A higher concentration of ∼6 would, however, match the strong lensing measurements. These results very likely reflect the complex structure of the cluster mass distribution, underlying the importance of panchromatic studies from small to large scale in order to better understand cluster physics.

We present the first results of our deep survey of lensing clusters aimed at constraining the abundance of star-forming galaxies at $z\sim6{-}10$, using lensing magnification to improve the search efficiency and subsequent spectroscopic studies. Deep near-IR photometry of two lensing clusters (A1835 and AC114) was obtained with ISAAC/VLT. These images, combined with existing data in the optical bands including HST images, were used to select very high redshift candidates at $z\ga 6$among the optical-dropouts. Photometric selection criteria have been defined based on the well-proven dropout technique, specifically tuned to target star-forming galaxies in this redshift domain. 
We have identified 18(8) first and second-category optical dropouts in A1835 (AC114), detected in more than one filter up to H(Vega) $\sim 23.8$(AB~ 25.2, uncorrected for lensing). Among them, 8(5) exhibit homogeneous SEDs compatible with star-forming galaxies at $z\ga 6$, and 5(1) are more likely intermediate-redshift EROs based on luminosity considerations. We have also identified a number of fainter sources in these fields fulfilling our photometric selection and located around the critical lines. We use all these data to make a first attempt at constraining the density of star-forming galaxies present at $6\la z \la10$using lensing clusters. Magnification effects and sample incompleteness are addressed through a careful modeling of the lensing clusters. A correction was also introduced to account for the expected fraction of false-positive detections among this photometric sample. 
It appears that the number of candidates found in these lensing fields, corrected for magnification, incompleteness and false-positive detections, is higher than the one achieved in blank fields with similar photometric depth in the near-IR. The luminosity function derived for $z\ga 6$candidates appears compatible with that of LBGs at $z\simeq 3$, without any renormalization. The turnover observed by Bouwens et al. (2005) towards the bright end relative to the $z\sim 3$LF is not observed in this sample. Also the upper limit for the UV SFR density at $z\sim6{-}10$, integrated down to $L_{1500}=0.3~L^{*}_{z=3}$, of $\rho_\star=7.4$$\times$$10^{-2}~M_{\odot}$yr-1Mpc-3is compatible with the usual values derived at $z \simeq 5{-}6$, but higher than the estimates obtained in the NICMOS Ultra Deep Field (UDF). The same holds for the upper limit of the SFR density in the $z \simeq 8{-}10$interval ($\rho_\star=1.1$$\times$10-1). This systematic trend towards the bright end of the LF with respect to blank fields could be due to field-to-field variance, a positive magnification bias from intermediate-redshift EROs, and/or residual contamination. Given the low S/Nratio of the high-zcandidates, and the large correction factors applied to this sample, increasing the number of blank and lensing fields with ultra-deep near-IR photometry is essential to obtain more accurate constraints on the abundance of $z \ga 6$galaxies.

We report on new high-resolution near-infrared images of the gravitationally lensed radio source PKS 1830-211, a quasar at $z=2.507$. These adaptive optics observations, taken with the Very Large Telescope (VLT), are further improved through image deconvolution. They confirm the presence of a second object along the line of sight to the quasar, in addition to the previously known spiral galaxy. This additional object is clearly extended in our images. However, its faint luminosity does not allow to infer any photometric redshift. If this galaxy is located in the foreground of PKS 1830-211, it complicates the modeling of this system and decreases the interest in using PKS 1830-211 as a means to determine H0via the time delay between the two lensed images of the quasar.

We present the results of observations from the Plateau de Bure IRAM interferometric array of the submillimetre (submm) galaxy SMMJ16359+6612 lying at $ z=2.516$behind the core of the massive cluster A 2218. The foreground gravitational lens produces three images with a total magnification of 45 of this faint submm galaxy, which has an intrinsic submm flux of just $f_{850~\mu{\rm m}}=0.8$mJy – placing it below the confusion limit of blank-field surveys. The substantial magnification provides a rare opportunity to probe the nature of a distant sub-mJy submm-selected galaxy, part of the population which produces the bulk of the cosmic far-infrared background at submm wavelengths. Our observations detect the CO(3–2) line in all three images, as well as the CO(7–6) line and the dust continuum at 1.3 mm for the brightest image but only at a 3σlevel. The velocity profile of the CO(3–2) line displays a double-peak profile which is well fit by two Gaussians with FWHMof 220 km s-1and separated by 280 km s-1. We estimate the dynamical mass of the system to be ~1.5 $\times$$10^{10}~M_\odot$and an H2gas mass of 4.5 $\times$$10^9~M_\odot$. We identify a spatial offset of ~$1''$between the two CO(3–2) velocity components, again benefiting from the magnification due to the foreground lens, modeling of which indicates that the offset corresponds to just ~3 kpc in projection at $z=2.516$. The spatial and velocity properties of these two components are closely related to features detected in previously published Hαspectroscopy. We propose that this source is a compact merger of two typical Lyman-break galaxies with a maximal separation between the two nuclei of about 3 kpc, although a dusty disk explanation is not excluded. This system is much less luminous and massive than other high-redshift submillimetre galaxies studied to date, but it bears a close similarity to similarly luminous, dusty starburst resulting from lower-mass mergers in the local Universe.

Observations of the core of the massive cluster Cl 0024+1654, at a redshift $z \sim 0.39$, were obtained with the Infrared Space Observatory using ISOCAM at 6.7 μm (hereafter 7 μm) and 14.3 μm (hereafter 15 μm). Thirty five sources were detected at 15 μm and thirteen of them are spectroscopically identified with cluster galaxies. The remaining sources consist of four stars, one quasar, one foreground galaxy, three background galaxies and thirteen sources with unknown redshift. The sources with unknown redshift are all likely to be background sources that are gravitationally lensed by the cluster. The spectral energy distributions (SEDs) of twelve cluster galaxies were fit from a selection of 20 models using the program GRASIL. The ISOCAM sources have best-fit SEDs typical of spiral or starburst models observed 1 Gyr after the main starburst event. The star formation rates were obtained for cluster members. The median infrared luminosity of the twelve cluster galaxies is ~$1.0\times10^{11}$${L}_\odot$, with 10 having infrared luminosity above $9\times10^{10}$${L}_\odot$, and so lying near or above the $1\times10^{11}$${L}_\odot$threshold for identification as a luminous infrared galaxy (LIRG). The [$\ion{O}{ii}$] star formation rates obtained for 3 cluster galaxies are one to two orders of magnitude lower than the infrared values, implying that most of the star formation is missed in the optical because it is enshrouded by dust in the starburst galaxy. The cluster galaxies in general are spatially more concentrated than those detected at 15 μm. However the velocity distributions of the two categories are comparable. The colour-magnitude diagramme is given for the galaxies within the ISOCAM map. Only 20% of the galaxies that are significantly bluer than the cluster main sequence were detected at 15 μm, to the limiting sensitivity recorded. The counterparts of about half of the 15 μm cluster sources are blue, luminous, star-forming systems and the type of galaxy that is usually associated with the Butcher-Oemler effect. HST images of these galaxies reveal a disturbed morphology with a tendency for an absence of nearby companions. Surprisingly the counterparts of the remaining 15 μm cluster galaxies lie on the main sequence of the colour–magnitude diagramme. However in HST images they all have nearby companions and appear to be involved in interactions and mergers. Dust obscuration may be a major cause of the 15 μm sources appearing on the cluster main sequence. The majority of the ISOCAM sources in the Butcher-Oemler region of the colour–magnitude diagram are best fit by spiral-type SEDs whereas post-starburst models are preferred on the main sequence, with the starburst event probably triggered by interaction with one or more galaxies.
Finally, the mid-infrared results on Cl 0024+1654are compared with four other clusters observed with ISOCAM. Scaling the LIRG count in Cl 0024+1654to the clusters Abell 370, Abell 1689, Abell 2218and Abell 2390with reference to their virial radii, masses, distances, and the sky area scanned in each case, we compared the number of LIRGs observed in each cluster. The number in Abell 370is smaller than expected by about an order of magnitude, even though the two clusters are very similar in mass, redshift and optical richness. The number of LIRGs detected in each of Abell 1689, Abell 2218and Abell 2390is 0, whereas 3 were expected from the comparison with Cl 0024+1654. A comparison of the mid-infrared sources in Abell 1689and Abell 2218shows that the sources in Abell 1689are more luminous and follow the same trend identified in the comparison between Cl 0024+1654and Abell 370. These trends seem to be related to the dynamical status and history of the clusters.

In this paper we present new ESO/VLT FORS1 and ISAAC images of the fields around eight gravitationally lensed quasars: CTQ 414, HE 0230-2130, LBQS 1009-0252, B 1030+074, HE 1104-1805, B 1359+154, H 1413+117 and HE 2149-2745. When available and deep enough, HST/WFPC2 data were also used to infer the photometric redshifts of the galaxies around the quasars. The search of galaxy overdensities in space and redshift, as well as a weak-shear analysis and a mass reconstruction are presented in this paper. We find that there are most probably galaxy groups towards CTQ 414, HE 0230-2130, B 1359+154, H 1413+117 and HE 2149-2745, with masses ≤$4\times 10^{14}$$M_\odot$h-1. Considering its photometric redshift, the galaxy group discovered in the field around HE 1104-1805 is associated with the quasar rather than with the lensing potential.

We have observed the cluster Abell 2218 ($z=0.175$) with ISOCAM on board the Infrared Space Observatoryusing two filters, LW2 and LW3, with reference wavelengths of 6.7 and 14.3 μm, respectively. We detected 76 sources down to 54 and 121 μJy (50% completeness levels) at 6.7 and 14.3 μm, respectively. All these sources have visible optical counterparts. We have gathered optical and near-infrared magnitudes for 60 of the 67 non-stellar optical counterparts to the ISOCAM sources, as well as redshifts for 43 of them. We have obtained acceptable and well constrained fits to the observed spectral energy distributions (SEDs) of 41 of these sources, using the “GRASIL” models of Silva et al. (1998), and have determined their total infrared luminosities (LIR's) and star formation rates (SFRs).
The SEDs of 20 (out of 27) ISOCAM cluster members are best fit by models with negligible ongoing star formation, and no major episode of star formation in the last ~1 Gyr. Their SEDs resemble those of 5-10 Gyr old early-type galaxies. A slightly higher, but still very mild, star-formation activity is found among the remaining cluster sources, which are mostly spirals. The median IR luminosity of the 27 ISOCAM cluster sources is $L_{\rm IR} = 6 \times 10^8 \, L_{\odot}.$The ISOCAM-selected cluster galaxies have indistinguishable velocity and spatial distributions from those of the other cluster galaxies, and do not contribute significantly to the Butcher-Oemler effect. If A2218 is undergoing a merger, as suggested by some optical and X-ray analyses, then this merger does not seem to affect the mid-infrared properties of its galaxies.
The SEDs of most ISOCAM-selected field sources are best fit by models with moderate ongoing star formation, with a significant fraction of their stellar mass formed in the last ~1 Gyr. Their SEDs resemble those of massive star-forming spirals or starburst galaxies, observed close to the maximum of their star formation activity, but not necessarily during the short-lived starburst event. The median redshift of these field galaxies is  $z \simeq 0.6$. Their LIR's span almost two orders of magnitudes, from ~$10^{10} \, L_{\odot}$to ~$10^{12} \, L_{\odot}$, with a median of $1.2 \times 10^{11}$(eight of the 14 field sources are LIRGs). The SFRs of these 14 ISOCAM-selected field sources range from 2 to 125 $M_{\odot} \, {\rm yr}^{-1}$, with a median value of 22 $M_{\odot} \, {\rm yr}^{-1}$.
We compare our findings with those obtained in other ISOCAM cluster and field surveys.

We present BAX, Base de Données Amas de Galaxies X (http://bax.ast.obs-mip.fr), a multi-wavelength database dedicated to X-ray clusters and groups of galaxies allowing detailed information retrieval. BAX is designed to support astronomical research by providing access to published measurements of the main physical quantities and to the related bibliographic references: basic data stored in the database are cluster/group identifiers, equatorial coordinates, redshift, flux, X-ray luminosity (in the ROSAT band) and temperature, and links to additional linked parameters (in X-rays, such as spatial profile parameters, as well as SZ parameters of the hot gas, lensing measurements, and data at other wavelengths, such as optical and radio). The clusters and groups in BAX can be queried by the basic parameters as well as the linked parameters or combinations of these. We expect BAX to become an important tool for the astronomical community. BAX will optimize various aspects of the scientific analysis of X-ray clusters and groups of galaxies, from proposal planning to data collection, interpretation and publication, from both ground based facilities like MEGACAM (CFHT), VIRMOS (VLT) and space missions like XMM-Newton, Chandra and Planck.

In this Letter we present a detailed study of the lensing configuration in the cluster Abell 2218. Four multiple-images systems with measured spectroscopic redshifts have been identified in this cluster. These multiple images are very useful to constrain accurately the mass distribution in the cluster core, but they are also sensitive to the value of the geometrical cosmological parameters of the Universe. Using a simplified maximum likelihood analysis we find $0<\Omega_{\rm M}<0.30$assuming a flat Universe, and $0<\Omega_{\rm M}<0.33$and $w<- 0.85$for a flat Universe with dark energy. Interestingly, an Einstein-de Sitter model is excluded at more than 4σ. These constraints are consistent with the current constraints derived with CMB anisotropies or supernovae studies. The proposed method constitutes an independent test of the geometrical cosmological parameters of the Universe and we discuss the limits of this method and this particular application to Abell 2218. Application of this method with more sophisticated tools and to a larger number of clusters or with more multiple images constraints, will put stringent constraints on the geometrical cosmological parameters.

We report the first likely spectroscopic confirmation of a $z\sim 10.0$galaxy from our ongoing search for distant galaxies with ISAAC/VLT. Galaxy candidates at $z \ga 7$are selected from ultra-deep JHKs images in the core of gravitational lensing clusters for which deep optical imaging is also available, including HST data. The object reported here, found behind Abell 1835, exhibits a faint emission line detected in the Jband, leading to $z=10.0$when identified as Lyα, in excellent agreement with the photometric redshift determination. Redshifts $z < 7$are very unlikely for various reasons we discuss. The object is located on the critical lines corresponding to $z=9$to 11. The magnification factor μranges from 25 to 100. For this object we estimate ${\it SFR}({\rm Ly}\alpha) \sim (0.8{-}2.2)~M _{\odot}$yr-1and ${\it SFR}({\rm UV}) \sim (47{-}75)~M _{\odot}$yr-1, both uncorrected for lensing. The steep UV slope
indicates a young object with negligible dust extinction. SED fits with young low-metallicity stellar population models yield (adopting $\mu=25$) a lensing corrected stellar mass of $M_\star \sim 8 \times 10^6~M _{\odot}$, and luminosities of $2 \times 10^{10}~L _{\odot}$, corresponding to a dark matter halo of a mass of typically $M_{\rm tot} \ga 5 \times 10^8~M _{\odot}$. In general our observations show that under excellent conditions and using strong gravitational lensing direct observations of galaxies close to the “dark ages” are feasible with ground-based 8-10 m class telescopes.

We report the discovery of an unusual emission line galaxy at redshift $z=1.68$found from near-IR imaging and spectroscopic observations with ISAAC/VLT of a faint gravitationally lensed R-band drop-out behind the lensing cluster Abell 1835. From the emission lines of [O iii] $\lambda\lambda$4959, 5007, and Hßthis galaxy shows a moderate to high excitation and line equivalent widths typical of relatively metal-poor H iigalaxies. Its apparent Jmagnitude translates to an absolute B-band magnitude $M_B \sim -16.4$once corrected for a gravitational magnification of 1.8 mag. This makes it one of the faintest intermediate redshift galaxies known so far. From the presence of emission lines and the available VRIJHKphotometry we determine constraints on its dominant stellar population. The only viable fit found is for a fairly young (~6-9 Myr) burst suffering from a considerable extinction ($A_V \sim 1.2{-}1.8$mag). We find that this object resembles strongly H iigalaxies and intermediate redshift compact emission line galaxies, albeit suffering from a larger extinction than usual. We also discuss the possible contamination introduced by such R-band drop-out galaxies in searches for $z \ga 5$galaxies.

ESA's Infrared Space Observatory (ISO) was used to perform a deep survey with ISOCAM through three massive gravitationally lensing clusters of galaxies. The total area surveyed depends on source flux, with nearly seventy square arcminutes covered for the brighter flux levels in maps centred on the three clusters Abell 370, Abell 2218 and Abell 2390. We present maps and photometry at 6.7 μm (hereafter 7 μm) and 14.3 μm (hereafter 15 μm), showing a total of 145 mid-infrared sources and the associated source counts. At 15 μm these counts reach the faintest level yet recorded. Almost all of the sources have been confirmed on more than one infrared map and all are identified with counterparts in the optical or near-infrared. Detailed models of the three clusters have been used to correct for the effects of gravitational lensing on the background source population. Lensing by the clusters increases the sensitivity of the survey, and the weakest sources have lensing corrected fluxes of 5 and 18 μJy at 7 and 15 μm, respectively. Roughly 70% of the 15 μm sources are lensed background galaxies. Of sources detected only at 7 μm, 95% are cluster galaxies for this sample. Of fifteen SCUBA sources within the mapped regions of the three clusters seven were detected at 15 microns. The redshifts for five of these sources lie in the range 0.23 to 2.8, with a median value of 0.9.
Flux selected subsets of the field sources above the 80% and 50% completeness limits were used to derive source counts to a lensing corrected sensitivity level of 30 μJy at 15 μm, and 14 μJy at 7 μm. The source counts, corrected for the effects of completeness, contamination by cluster sources and lensing, confirm and extend earlier findings of an excess by a factor of ten in the 15 μm population with respect to source models with no evolution. The observed mid-infrared field sources occur mostly at redshifts between 0.4 and 1.5.
For the counts at 7 μm, integrating in the range 14 μJy to 460 μJy, we resolve $(0.49\pm0.2)\times10^{-9}$W m-2sr-1of the infrared background light into discrete sources. At 15 μm we include the counts from other extensive ISOCAM surveys to integrate over the range 30 μJy to 50 mJy, reaching two to three times deeper than the unlensed surveys to resolve $(2.7\pm0.62)\times10^{-9}$W m-2sr-1of the infrared background light. These values correspond to 10% and 55%, respectively, of the upper limit to the infrared background light, derived from photon-photon pair production of the high energy gamma rays from BL-Lac sources on the infrared background photons. However, the recent detections of TeV gamma rays from the $z=0.129$BL Lac H1426+428 suggest that the value for the 15 μm background reported here is already sufficient to imply substantial absorption of TeV gamma rays from that source.

Very deep ESO/VLT FORS1 and ISAAC images, as well as HST NICMOS2 data are used to infer the nature of the quasar pair LBQS 1429-0053 A and B, either a binary quasar or a doubly-imaged lensed quasar. Direct search of a putative lensing galaxy is unsuccessful down to $R=27$, $J=24$, $Ks=22.5$and $H=22.5$. Moreover, no galaxy overdensity close to the quasar pair is found. A weak shear analysis of the FORS1 R-band 6.8´ $\times$6.8´  field also fails at detecting any concentration of dark matter more massive than $\sigma=500$km s-1and weakens the hypothesis of a dark lens. The only sign of a possible lens consists in a group of 5 objects having colors consistent with galaxies at $z \sim 1$, within a radius of 5´´  from the quasar pair. Considering this group as the lensing potential does not allow to reproduce the image position and flux ratio of LBQS 1429-0053 A and B. Our deep R-band image shows a blue, previously unknown, extended object at the position of LBQS 1429-0053 A, which is consistent with either being the lensed quasar A host, or being an intervening galaxy at lower redshift. Unless future very deep optical images demonstrate that this object is actually the lensed host of LBQS 1429-0053, we conclude that there is very little evidence for LBQS 1429-0053 being lensed. Therefore, we are led to declare LBQS 1429-0053 A and B a genuine binary quasar.

We present VLT/ISAAC near-infrared spectroscopy of two gravitationally-lensed $z \sim 1.9$galaxies, A2 and S2, located behind the cluster AC 114. Thanks to large magnification factors, we have been successful in detecting rest-frame optical emission lines (from [O ii]λ3727 to ${\rm H{\alpha}}$+[N ii]λ6584) in star-forming galaxies 1 to 2 mag fainter than in previous studies of Lyman break galaxies (LBGs) at $z \sim 3$. From the ${\rm H{\alpha}}$luminosity, we estimate star formation rates (SFRs) of 30 and 15 $M_{\odot}$yr-1for S2 and A2 respectively. These values are 7 to 15 times higher than those inferred from the UV continuum flux at 1500 Å without dust extinction correction. In setting ${\textit SFR}_{\rm H\alpha}\sim {\textit SFR}_{\rm UV}$, one derives extinction coefficients $E(B-V) \sim 0.3$for S2 and $E(B-V) \sim 0.4$for A2. The behavior of S2 and A2 in terms of O/H and N/O abundance ratios are very different, and they are also different from typical LBGs at $z \sim 3$. S2 is a low-metallicity object ($Z \sim 0.03$$Z_{\odot}$) with a low N/O ratio, similar to those derived in the most metal-poor nearby Hiigalaxies. In contrast, A2 is a high-metallicity galaxy ($Z \sim 1.3$$Z_{\odot}$) with a high N/O abundance ratio, similar to those derived in the most metal-rich starburst nucleus galaxies. The line-of-sight velocity dispersions, derived from emission line widths, are 55 and 105 km s-1, yielding a virial mass of 0.5 and $2.4 \times 10^{10}$$M_{\odot}$, for S2 and A2 respectively. Thanks to the gravitational amplification, the line profiles of S2 are spatially resolved, leading to a velocity gradient of $\pm 240$km s-1, which yields a dynamical mass of ${\sim} 1.3 \times 10^{10}$$M_{\odot}$within the inner 1 kpc radius. Combining these new data with the sample of LBGs at $z \sim 3$, including the lensed galaxy MS 1512-cB58, which is the only LBG for which physical properties have been determined with similar accuracy, we conclude that these three galaxies exhibit different physical properties in terms of abundance ratios, SFRs, $M/L_{\rm B}$and reddening. High-redshift galaxies of different luminosities could thus have quite different star formation histories.