Re-ionization of the intergalactic medium occurred in the early Universe at redshift z approximate to 6-11, following the formation of the first generation of stars(1). Those young galaxies (where the bulk of stars formed) at a cosmic age of less than about 500 million years (z less than or similar to 10) remain largely unexplored because they are at or beyond the sensitivity limits of existing large telescopes. Understanding the properties of these galaxies is critical to identifying the source of the radiation that re-ionized the intergalactic medium. Gravitational lensing by galaxy clusters allows the detection of high-redshift galaxies fainter than what otherwise could be found in the deepest images of the sky(2). Here we report multiband observations of the cluster MACS J1149+2223 that have revealed (with high probability) a gravitationally magnified galaxy from the early Universe, at a redshift of z = 9.6 +/- 0.2 (that is, a cosmic age of 490 +/- 15 million years, or 3.6 per cent of the age of the Universe). We estimate that it formed less than 200 million years after the Big Bang (at the 95 per cent confidence level), implying a formation redshift of less than or similar to 14. Given the small sky area that our observations cover, faint galaxies seem to be abundant at such a young cosmic age, suggesting that they may be the dominant source for the early re-ionization of the intergalactic medium.

We present a candidate for the most distant galaxy known to date with a photometric redshift of z = 10.7(-0.4)(+0.6) (95% confidence limits; with z < 9.5 galaxies of known types ruled out at 7.2 sigma). This J-dropout Lyman break galaxy, named MACS0647-JD, was discovered as part of the Cluster Lensing and Supernova survey with Hubble (CLASH). We observe three magnified images of this galaxy due to strong gravitational lensing by the galaxy cluster MACSJ0647.7+7015 at z = 0.591. The images are magnified by factors of similar to 80, 7, and 2, with the brighter two observed at similar to 26th magnitude AB (similar to 0.15 mu Jy) in the WFC3/IR F160W filter (similar to 1.4-1.7 mu m) where they are detected at greater than or similar to 12 sigma. All three images are also confidently detected at greater than or similar to 6 sigma in F140W (similar to 1.2- 1.6 mu m), dropping out of detection from 15 lower wavelength Hubble Space Telescope filters (similar to 0.2-1.4 mu m), and lacking bright detections in Spitzer/IRAC 3.6 mu m and 4.5 mu m imaging (similar to 3.2-5.0 mu m). We rule out a broad range of possible lower redshift interlopers, including some previously published as high-redshift candidates. Our high-redshift conclusion is more conservative than if we had neglected a Bayesian photometric redshift prior. Given CLASH observations of 17 high-mass clusters to date, our discoveries of MACS0647-JD at z similar to 10.8 and MACS1149-JD at z similar to 9.6 are consistent with a lensed luminosity function extrapolated from lower redshifts. This would suggest that low-luminosity galaxies could have reionized the universe. However, given the significant uncertainties based on only two galaxies, we cannot yet rule out the sharp drop-off in number counts at z greater than or similar to 10 suggested by field searches.

We present observations of the afterglows and host galaxies of three short-duration gamma-ray bursts (GRBs): 100625A, 101219A, and 110112A. We find that GRB 100625A occurred in a z = 0.452 early-type galaxy with a stellar mass of approximate to 4.6 x 10(9) M-circle dot and a stellar population age of approximate to 0.7 Gyr, and GRB 101219A originated in a star-forming galaxy at z = 0.718 with a stellar mass of approximate to 1.4 x 10(9) M-circle dot, a star formation rate of approximate to 16 M-circle dot yr(-1), and a stellar population age of approximate to 50 Myr. We also report the discovery of the optical afterglow of GRB 110112A, which lacks a coincident host galaxy to i greater than or similar to 26 mag, and we cannot conclusively identify any field galaxy as a possible host. From afterglow modeling, the bursts have inferred circumburst densities of approximate to 10(-4)-1 cm(-3) and isotropic-equivalent gamma-ray and kinetic energies of approximate to 10(50)-10(51) erg. These three events highlight the diversity of galactic environments that host short GRBs. To quantify this diversity, we use the sample of 36 Swift short GRBs with robust associations to an environment (similar to 1/2 of 68 short bursts detected by Swift to 2012 May) and classify bursts originating from four types of environments: late-type (approximate to 50%), early-type (approximate to 15%), inconclusive (approximate to 20%), and "host-less" (lacking a coincident host galaxy to limits of greater than or similar to 26 mag; approximate to 15%). To find likely ranges for the true late- and early-type fractions, we assign each of the host-less bursts to either the late- or early-type category using probabilistic arguments and consider the scenario that all hosts in the inconclusive category are early-type galaxies to set an upper bound on the early-type fraction. We calculate most likely ranges for the late- and early-type fractions of approximate to 60%-80% and approximate to 20%-40%, respectively. We find no clear trend between gamma- ray duration and host type. We also find no change to the fractions when excluding events recently claimed as possible contaminants from the long GRB/collapsar population. Our reported demographics are consistent with a short GRB rate driven by both stellar mass and star formation.

The IMACS Cluster Building Survey uses the wide field spectroscopic capabilities of the IMACS spectrograph on the 6.5 m Baade Telescope to survey the large-scale environment surrounding rich intermediate-redshift clusters of galaxies. The goal is to understand the processes which may be transforming star-forming field galaxies into quiescent cluster members as groups and individual galaxies fall into the cluster from the surrounding supercluster. This first paper describes the survey: the data taking and reduction methods. We provide new calibrations of star formation rates (SFRs) derived from optical and infrared spectroscopy and photometry. We demonstrate that there is a tight relation between the observed SFR per unit B luminosity, and the ratio of the extinctions of the stellar continuum and the optical emission lines. With this, we can obtain accurate extinction-corrected colors of galaxies. Using these colors as well as other spectral measures, we determine new criteria for the existence of ongoing and recent starbursts in galaxies.

We perform a strong lensing analysis of the merging galaxy cluster MACS J0416.1-2403 (M0416; z = 0.42) in recent CLASH/HST observations. We identify 70 new multiple images and candidates of 23 background sources in the range 0.7 less than or similar to z(phot) less than or similar to 6.14 including two probable high-redshift dropouts, revealing a highly elongated lens with axis ratio similar or equal to 5:1, and a major axis of similar to 100 '' (z(s) similar to 2). Compared to other well-studied clusters, M0416 shows an enhanced lensing efficiency. Although the critical area is not particularly large (similar or equal to 0.6 square'; z(s) similar to 2), the number of multiple images, per critical area, is anomalously high. We calculate that the observed elongation boosts the number of multiple images, per critical area, by a factor of similar to 2.5x, due to the increased ratio of the caustic area relative to the critical area. Additionally, we find that the observed separation between the two main mass components enlarges the critical area by a factor of similar to 2. These geometrical effects can account for the high number (density) of multiple images observed. We find in numerical simulations that only similar to 4% of the clusters (with M-vir >= 6 x 10(14) h(-1) M-circle dot) exhibit critical curves as elongated as in M0416.

Our searches in the CLASH data have found many high-redshift galaxy candidates. One object at z = 6.2 in MACS J0329-02 is quadruply lensed. Another object in MACS J1149+ 22 is at redshift z = 9.6 and AB=25.7, the first candidate at z > 9 that is bright enough for spectroscopic observations. The discovery of more than 80 galaxy candidates at z > 7 demonstrates the unique potential of galaxy cluster fields for finding highly magnified, intrinsically faint galaxies at the highest redshifts. (C) 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

The galaxy cluster MACS J0717.5+3745 (z = 0.55) is the largest known cosmic lens, with complex internal structures seen in deep X-ray, Sunyaev-Zel'dovich effect, and dynamical observations. We perform a combined weak-and strong-lensing analysis with wide-field BVR(c)i'z' Subaru/Suprime-Cam observations and 16-band Hubble Space Telescope observations taken as part of the Cluster Lensing And Supernova survey with Hubble. We find consistent weak distortion and magnification measurements of background galaxies and combine these signals to construct an optimally estimated radial mass profile of the cluster and its surrounding large-scale structure out to 5 Mpc h(-1). We find consistency between strong-lensing and weak-lensing in the region where these independent data overlap, <500 kpc h(-1). The two-dimensional weak-lensing map reveals a clear filamentary structure traced by distinct mass halos. We model the lensing shear field with nine halos, including the main cluster, corresponding to mass peaks detected above 2.5 sigma(kappa). The total mass of the cluster as determined by the different methods is M-vir approximate to (2.8 +/- 0.4) x 10(15) M-circle dot. Although this is the most massive cluster known at z > 0.5, in terms of extreme value statistics, we conclude that the mass of MACS J0717.5+3745 by itself is not in serious tension with Lambda CDM, representing only a similar to 2 sigma departure above the maximum simulated halo mass at this redshift.

In this work, we analyze the mass distribution of MACSJ1206.2-0847, particularly focusing on the halo properties of its cluster members. The cluster appears relaxed in its X-ray emission, but has a significant amount of intracluster light that is not centrally concentrated, suggesting that galaxy-scale interactions are still ongoing despite the overall relaxed state. The cluster lenses 12 background galaxies into multiple images and one galaxy at z = 1.033 into a giant arc and its counterimage. The multiple image positions and the surface brightness (SFB) distribution of the arc, which is bent around several cluster members, are sensitive to the cluster galaxy halo properties. We model the cluster mass distribution with a Navarro-Frenk-White profile and the galaxy halos with two parameters for the mass normalization and the extent of a reference halo assuming scalings with their observed near-infrared light. We match the multiple image positions at an rms level of 0 ''.85 and can reconstruct the SFB distribution of the arc in several filters to a remarkable accuracy based on this cluster model. The length scale where the enclosed galaxy halo mass is best constrained is about 5 effective radii-a scale in between those accessible to dynamical and field strong-lensing mass estimates on the one hand and galaxy-galaxy weak-lensing results on the other hand. The velocity dispersion and halo size of a galaxy with m(160W),(AB) = 19.2 and M-B,M-Vega = -20.7 are sigma = 150 km s(-1) and r approximate to 26 +/- 6 kpc, respectively, indicating that the halos of the cluster galaxies are tidally stripped. We also reconstruct the unlensed source, which is smaller by a factor of similar to 5.8 in area, demonstrating the increase in morphological information due to lensing. We conclude that this galaxy likely has star-forming spiral arms with a red (older) central component.

We present a quintuply lensed z similar to 6 candidate discovered in the field of the galaxy cluster RXC J2248.7-4431 (z similar to 0.348) targeted within the Cluster Lensing and Supernova survey with Hubble (CLASH) and selected in the deep Hubble Space Telescope (HST) frontier fields survey. Thanks to the CLASH 16-band HST imaging, we identify the quintuply lensed z similar to 6 candidate as an optical dropout in the inner region of the cluster, the brightest image having mag(AB) = 24.8 +/- 0.1 in the f105w filter. We perform a detailed photometric analysis to verify its high-z and lensed nature. We get as photometric redshift z(ph) similar to 5.9, and given the extended nature and NIR colours of the lensed images, we rule out low-z early-type and galactic star contaminants. We perform a strong lensing analysis of the cluster, using 13 families of multiple lensed images identified in the HST images. Our final best model predicts the high-z quintuply lensed system with a position accuracy of 0.8 arcsec. The magnifications of the five images are between 2.2 and 8.3, which leads to a delensed UV luminosity of L-1600 similar to 0.5L(1600)* at z=6. We also estimate the UV slope from the observed NIR colours, finding a steep beta = -2.89 +/- 0.38. We use singular and composite stellar population SEDs to fit the photometry of the high-z candidate, and we conclude that it is a young (age <300 Myr) galaxy with mass of M similar to 10(8) M-circle dot, subsolar metallicity (Z < 0.2 Z(circle dot)) and low dust content (A(V) similar to 0.2-0.4).

A pressureless scenario for the dark matter (DM) fluid is a widely adopted hypothesis, despite the absence of direct observational evidence. According to general relativity, the total mass-energy content of a system shapes the gravitational potential well, but different test particles perceive this potential in different ways depending on their properties. Cluster galaxy velocities, being << c, depend solely on the gravitational potential, whereas photon trajectories reflect the contributions from the gravitational potential plus a relativistic-pressure term that depends on the cluster mass. We exploit this phenomenon to constrain the equation of state (EoS) parameter of the fluid, primarily DM, contained in galaxy clusters. We use complementary information provided by the kinematic and lensing mass profiles of the galaxy cluster MACS 1206.2-0847 at z = 0.44, as obtained in an extensive imaging and spectroscopic campaign within the Cluster Lensing And Supernova survey with Hubble. The unprecedented high quality of our data set and the properties of this cluster are well suited to determine the EoS parameter of the cluster fluid. Since baryons contribute at most 15% to the total mass in clusters and their pressure is negligible, the EoS parameter we derive describes the behavior of the DM fluid. We obtain the most stringent constraint on the DM EoS parameter to date, w = (p(r) + 2p(t))/(3 c(2) rho) = 0.00 +/- 0.15 (stat) +/- 0.08 (syst), averaged over the radial range 0.5Mpc <= r <= r(200), where p(r) and p(t) are the radial and tangential pressure, and rho is the density. We plan to further improve our constraint by applying the same procedure to all clusters from the ongoing Cluster Lensing And Supernova Survey with Hubble-Very Large Telescope program.