We report on work to increase the number of well-measured Type Ia supernovae (SNe Ia) at high redshifts. Light curves, including high signal-to-noise HST data, and spectra of six SNe Ia that were discovered during 2001 are presented. Additionally, for the two SNe with z > 1, we present groundbased J-band photometry from Gemini and the VLT. These are among the most distant SNe Ia for which ground based near-IR observations have been obtained. We add these six SNe Ia together with other data sets that have recently become available in the literature to the Union compilation (Kowalski et al. 2008). We have made a number of refinements to the Union analysis chain, the most important ones being the refitting of all light curves with the SALT2 fitter and an improved handling of systematic errors. We call this new compilation, consisting of 557 supernovae, the Union2 compilation. The flat concordance ΛCDM model remains an excellent fit to the Union2 data with the best fit constant equation of state parameter w = −0.997 +0.050 −0.054 (stat)+0.077 −0.082 (stat + sys together) for a flat universe, or w = −1.035 +0.055 −0.059 (stat)+0.093 −0.097 (stat + sys together) with curvature. We also present improved constraints on w(z). While no significant change in w with redshift is detected, there is still considerable room for evolution in w. The strength of the constraints depend strongly on redshift. In particular, at z 1, the existence and nature of dark energy are only weakly constrained by the data.

We present ACS, NICMOS, and Keck AO-assisted photometry of 20 Type Ia supernovae (SNe Ia) from the HST Cluster Supernova Survey. The SNe Ia were discovered over the redshift interval 0.623 < z < 1.415. Fourteen of these SNe Ia pass our strict selection cuts and are used in combination with the world's sample of SNe Ia to derive the best current constraints on dark energy. Ten of our new SNe Ia are beyond redshift z = 1, thereby nearly doubling the statistical weight of HST-discovered SNe Ia beyond this redshift. Our detailed analysis corrects for the recently identified correlation between SN Ia luminosity and host galaxy mass and corrects the NICMOS zeropoint at the count rates appropriate for very distant SNe Ia. Adding these supernovae improves the best combined constraint on dark energy density, ρ DE (z), at redshifts 1.0 < z < 1.6 by 18% (including systematic errors). For a flat ΛCDM universe, we find Ω Λ = 0.729 +0.014 −0.014 (68% CL including systematic errors). For a flat wCDM model, we measure a constant dark energy equation-of-state parameter w = −1.013 +0.068 −0.073 (68% CL). Curvature is constrained to ∼ 0.7% in the owCDM model and to ∼ 2% in a model in which dark energy is allowed to vary with parameters w 0 and w a . Tightening further the constraints on the time evolution of dark energy will require several improvements, including high-quality multi-passband photometry of a sample of several dozen z > 1 SNe Ia. We describe how such a sample could be efficiently obtained by targeting cluster fields with WFC3 on HST.The updated supernova Union2.1 compilation of 580 SNe is available at http://supernova.lbl.gov/Union ⋆ is less than the mass threshold. We begin by noting that.We can then integrate this probability over all true host masses less than the threshold:⋆ )P (m true ⋆ ) up to a normalization constant found by requiring the integral to be unity when integrating over all possible true masses. P (m true ⋆ ) is estimated from the observed distribution for each type of survey. The SNLS (Sullivan et al. 2010) and SDSS (Lampeitl et al. 2010) host masses were assumed to be representative of untargeted surveys, while the mass distribution in Kelly et al. (2010) was assumed typical of nearby targeted surveys. As these distributions are approximately log-normal, we use this model for P (m true ⋆) using the mean and RMS from the log of the host masses from these surveys (with the average measurement errors subtracted in quadrature), giving log 10 P (m true ⋆ ) = N (µ = 9.88, σ 2 = 0.92 2 ) for untargeted surveys and log 10 P (m true ⋆ ) = N (10.75, 0.66 2 ) for targeted surveys. When host mass measurements are available, P (m obs ⋆ |m true ⋆ ) is also modeled as a log-normal; when no measurement is available, a flat distribution is used.For a supernova from an untargeted survey with no host mass measurement (including supernovae presented in this paper which are not in a cluster), P (m trueis the integral of P (m true ⋆ ) up to the threshold mass: 0.55. Similarly, nearby supernovae from targeted surveys w...

We present a new compilation of Type Ia supernovae (SNe Ia), a new data set of low-redshift nearby-Hubble-flow SNe, and new analysis procedures to work with these heterogeneous compilations. This ''Union'' compilation of 414 SNe Ia, which reduces to 307 SNe after selection cuts, includes the recent large samples of SNe Ia from the Supernova Legacy Survey and ESSENCE Survey, the older data sets, as well as the recently extended data set of distant supernovae observed with the Hubble Space Telescope (HST ). A single, consistent, and blind analysis procedure is used for all the various SN Ia subsamples, and a new procedure is implemented that consistently weights the heterogeneous data sets and rejects outliers. We present the latest results from this Union compilation and discuss the cosmological constraints from this new compilation and its combination with other cosmological measurements (CMB and BAO). The constraint we obtain from supernovae on the dark energy density is à ¼ 0:713 þ0:027 À0:029 (stat) þ0:036 À0:039 (sys), for a flat, ÃCDM universe. Assuming a constant equation of state parameter, w, the combined constraints from SNe, BAO, and A CMB give w ¼ À0:969 þ0:059 À0:063 (stat) þ0:063 À0:066 (sys). While our results are consistent with a cosmological constant, we obtain only relatively weak constraints on a w that varies with redshift. In particular, the current SN data do not yet significantly constrain w at z > 1. With the addition of our new nearby Hubble-flow SNe Ia, these resulting cosmological constraints are currently the tightest available.

Previously we used the Nearby Supernova Factory sample to show that SNe Ia having locally star-forming environments are dimmer than SNe Ia having locally passive environments. Here we use the Constitution sample together with host galaxy data from GALEX to independently confirm that result. The effect is seen using both the SALT2 and MLCS2k2 lightcurve fitting and standardization methods, with brightness differences of 0.094 ± 0.037 mag for SALT2 and 0.155 ± 0.041 mag for MLCS2k2 with R V = 2.5. When combined with our previous measurement the effect is 0.094 ± 0.025 mag for SALT2. If the ratio of these local SN Ia environments changes with redshift or sample selection, this can lead to a bias in cosmological measurements. We explore this -2issue further, using as an example the direct measurement of H 0 . GALEX observations show that the SNe Ia having standardized absolute magnitudes calibrated via the Cepheid period-luminosity relation using HST originate in predominately star-forming environments, whereas only ∼ 50% of the Hubble-flow comparison sample have locally star-forming environments. As a consequence, the H 0 measurement using SNe Ia is currently overestimated. Correcting for this bias, we find a value of H corr 0 = 70.6 ± 2.6 km s −1 Mpc −1 when using the LMC distance, Milky Way parallaxes and the NGC 4258 megamaser as the Cepheid zeropoint, and 68.8 ± 3.3 km s −1 Mpc −1 when only using NGC 4258. Our correction brings the direct measurement of H 0 within ∼ 1 σ of recent indirect measurements based on the CMB power spectrum.B SF = 0.094 ± 0.031 mag 1 . Since the underlying connection is with star formation rather than the Hα emission itself, we refer to this effect as the star-formation bias, or SF bias for short.R13 connected the SF bias to the host-mass step by noting that few of the Ia in the SNfactory sample occur in low-mass hosts, leading to a shift in mean brightness with host mass that is driven by the changing fraction of star formation. However, this also implies that simply correcting for the host-mass step will not

We present weak gravitational lensing analysis of 22 high-redshift (z 1) clusters based on Hubble Space Telescope images. Most clusters in our sample provide significant lensing signals and are well detected in their reconstructed two-dimensional mass maps. Combining the current results and our previous weak-lensing studies of five other high-z clusters, we compare gravitational lensing masses of these clusters with other observables. We revisit the question whether the presence of the most massive clusters in our sample is in tension with the current ΛCDM structure formation paradigm. We find that the lensing masses are tightly correlated with the gas temperatures and establish, for the first time, the lensing mass-temperature relation at z 1. For the power law slope of the M − T X relation (M ∝ T α ), we obtain α = 1.54 ± 0.23. This is consistent with the theoretical self-similar prediction α = 3/2 and with the results previously reported in the literature for much lower redshift samples. However, our normalization is lower than the previous results by 20-30%, indicating that the normalization in the M − T X relation might evolve. After correcting for Eddington bias and updating the discovery area with a more conservative choice, we find that the existence of the most massive clusters in our sample still provides a tension with the current ΛCDM model. The combined probability of finding the four most massive clusters in this sample after the marginalization over cosmological parameters is less than 1%.

As part of an ongoing effort to identify, understand and correct for astrophysics biases in the standardization of Type Ia supernovae (SN Ia) for cosmology, we have statistically classified a large sample of nearby SNe Ia into those located in predominantly younger or older environments. This classification is based on the specific star formation rate measured within a projected distance of 1 kpc from each SN location, (LsSFR). This is an important refinement compared to using the local star formation rate directly (Rigault et al. 2013; 2015), as it provides a normalization for relative numbers of available SN progenitors and is more robust against extinction by dust. We find that the SNe Ia in predominantly younger environments are ∆ Y = 0.163 ± 0.029 mag (5.7 σ) fainter than those in predominantly older environments after conventional light-curve standardization. This is the strongest standardized SN Ia brightness systematic connected to host-galaxy environment measured to date. The well-established step in standardized brightnesses between SNe Ia in hosts with lower or higher total stellar masses is smaller, at ∆ M = 0.119 ± 0.032 mag (4.5 σ), for the same set of SNe Ia. When fit simultaneously, the environment age offset remains very significant, with ∆ Y = 0.129 ± 0.032 mag (4.0 σ), while the global stellar mass step is reduced to ∆ M = 0.064 ± 0.029 mag (2.2 σ). Thus, approximately 70% of the variance from the stellar mass step is due to an underlying dependence on environment-based progenitor age. Also, we verify that using the local star formation rate alone is not as powerful as LsSFR at sorting SNe Ia into brighter and fainter subsets. Standardization using only the SNe Ia in younger environments reduces the total dispersion from 0.142 ± 0.008 mag to 0.120 ± 0.010 mag. We show that as environment ages evolve with redshift, a strong bias, especially on measurement of the derivative of the dark energy equation of state, can develop. Fortunately, data to measure and correct for this effect using our local specific star formation rate indicator is likely to be available for many next-generation SN Ia cosmology experiments.

We present observations of SCP 06F6, an unusual optical transient discovered during the Hubble Space Telescope Cluster Supernova Survey. The transient brightened over a period of ∼100 days, reached a peak magnitude of ∼21.0 in both i 775 and z 850 , and then declined over a similar timescale. There is no host galaxy or progenitor star detected at the location of the transient to a 3σ upper limit of i 775 ≥ 26.4 and z 850 ≥ 26.1, giving a corresponding lower limit on the flux increase of a factor of ∼120. Multiple spectra show five broad absorption bands between 4100Å and 6500Å and a mostly featureless continuum longward of 6500Å. The shape of the lightcurve is inconsistent with microlensing. The transient's spectrum, in addition to being inconsistent with all known supernova types, is not matched to any spectrum in the Sloan Digital Sky Survey (SDSS) database. We suggest that the transient may be one of a new class.

We present a study of the morphological fractions and color-magnitude relation (CMR) in the most distant X-ray selected galaxy cluster currently known, XMMXCS J2215.9−1738 at z = 1.46, using a combination of optical imaging data obtained with the Hubble Space Telescope Advanced Camera for Surveys, and infrared data from the Multi-Object Infrared Camera and Spectrograph, mounted on the 8.2 m Subaru telescope. We find that the morphological mix of the cluster galaxy population is similar to clusters at z ∼ 1. Within the central 0.5 Mpc, approximately ∼62% of the galaxies identified as likely cluster members are ellipticals or S0s; and ∼38% are spirals or irregulars. Therefore, early-type galaxies were already entrenched as the dominant galaxy population in at least some clusters approximately ∼ 4.5 Gyr after the big bang. We measure the CMRs for the early-type galaxies, finding that the slope in the z 850-J relation is consistent with that measured in the Coma cluster, some ∼9 Gyr earlier, although the uncertainty is large. In contrast, the measured intrinsic scatter about the CMR is more than three times the value measured in Coma, after conversion to rest-frame U−V. From comparison with stellar population synthesis models, the intrinsic scatter measurements imply mean luminosity-weighted ages for the earlytype galaxies in J2215.9−1738 of ≈3 Gyr, corresponding to the major epoch of star formation coming to an end at z f ≈ 3-5. We find that the cluster exhibits evidence of the "downsizing" phenomenon: the fraction of faint cluster members on the red sequence expressed using the Dwarf-to-Giant Ratio (DGR) is 0.32 ± 0.18 within a radius of 0.5R 200. This is consistent with extrapolation of the redshift evolution of the DGR seen in cluster samples at z < 1. In contrast to observations of some other z > 1 clusters, we find a lack of very bright galaxies within the cluster.