Keywords

We present new observations of the field containing the z = 3.786 protocluster PC 217.96+32.3. We confirm that it is one of the largest known and most overdense high-redshift structures. Such structures are rare even in the largest cosmological simulations. We used the Mayall/MOSAIC1.1 imaging camera to image a 12 × 06 area (≈150 × 75 comoving Mpc) surrounding the protocluster's core and discovered 165 candidate Lyα emitting galaxies (LAEs) and 788 candidate Lyman Break galaxies (LBGs). There are at least two overdense regions traced by the LAEs, the largest of which shows an areal overdensity in its core (i.e., within a radius of 2.5 comoving Mpc) of 14 ± 7 relative to the average LAE spatial density ($\bar{\rho }$) in the imaged field. Further, $\bar{\rho }$ is twice that derived by other field LAE surveys. Spectroscopy with Keck/DEIMOS yielded redshifts for 164 galaxies (79 LAEs and 85 LBGs); 65 lie at a redshift of 3.785 ± 0.010. The velocity dispersion of galaxies near the core is σ = 350 ± 40 km s⁻¹, a value robust to selection effects. The overdensities are likely to collapse into systems with present-day masses of >10¹⁵ M_⊙ and >6 × 10¹⁴ M_⊙. The low velocity dispersion may suggest a dynamically young protocluster. We find a weak trend between narrow-band (Lyα) luminosity and environmental density: the Lyα luminosity is enhanced on average by 1.35× within the protocluster core. There is no evidence that the Lyα equivalent width depends on environment. These suggest that star formation and/or active galactic nucleus (AGN) activity is enhanced in the higher-density regions of the structure. PC 217.96+32.3 is a Coma cluster analog, witnessed in the process of formation.

Merging galaxy clusters leave long-lasting signatures on the baryonic and non-baryonic cluster constituents, including shock fronts, cold fronts, X-ray substructure, radio halos, and offsets between the dark matter (DM) and the gas components. Using observations from Chandra, the Jansky Very Large Array, the Giant Metrewave Radio Telescope, and the Hubble Space Telescope, we present a multiwavelength analysis of the merging Frontier Fields cluster MACS J0416.1-2403 (z = 0.396), which consists of NE and SW subclusters whose cores are separated on the sky by ∼250 kpc. We find that the NE subcluster has a compact core and hosts an X-ray cavity, yet it is not a cool core. Approximately 450 kpc south–southwest of the SW subcluster, we detect a density discontinuity that corresponds to a compression factor of ∼1.5. The discontinuity was most likely caused by the interaction of the SW subcluster with a less massive structure detected in the lensing maps SW of the subcluster's center. For both the NE and the SW subclusters, the DM and the gas components are well-aligned, suggesting that MACS J0416.1-2403 is a pre-merging system. The cluster also hosts a radio halo, which is unusual for a pre-merging system. The halo has a 1.4 GHz power of (1.3 ± 0.3) × 10²⁴ W Hz⁻¹, which is somewhat lower than expected based on the X-ray luminosity of the cluster if the spectrum of the halo is not ultra-steep. We suggest that we are either witnessing the birth of a radio halo, or have discovered a rare ultra-steep spectrum halo.

We present a velocity-dispersion-based mass calibration of the South Pole Telescope Sunyaev–Zel'dovich effect survey (SPT-SZ) galaxy cluster sample. Using a homogeneously selected sample of 100 cluster candidates from 720 deg² of the survey along with 63 velocity dispersion (σ_v) and 16 X-ray Y_X measurements of sample clusters, we simultaneously calibrate the mass-observable relation and constrain cosmological parameters. Our method accounts for cluster selection, cosmological sensitivity, and uncertainties in the mass calibrators. The calibrations using σ_v and Y_X are consistent at the 0.6σ level, with the σ_v calibration preferring ∼16% higher masses. We use the full SPT_CL data set (SZ clusters+σ_v+Y_X) to measure σ₈(Ω_m/0.27)^0.3 = 0.809 ± 0.036 within a flat ΛCDM model. The SPT cluster abundance is lower than preferred by either the WMAP9 or Planck+WMAP9 polarization (WP) data, but assuming that the sum of the neutrino masses is ∑m_ν = 0.06 eV, we find the data sets to be consistent at the 1.0σ level for WMAP9 and 1.5σ for Planck+WP. Allowing for larger ∑m_ν further reconciles the results. When we combine the SPT_CL and Planck+WP data sets with information from baryon acoustic oscillations and Type Ia supernovae, the preferred cluster masses are 1.9σ higher than the Y_X calibration and 0.8σ higher than the σ_v calibration. Given the scale of these shifts (∼44% and ∼23% in mass, respectively), we execute a goodness-of-fit test; it reveals no tension, indicating that the best-fit model provides an adequate description of the data. Using the multi-probe data set, we measure Ω_m = 0.299 ± 0.009 and σ₈ = 0.829 ± 0.011. Within a νCDM model we find ∑m_ν = 0.148 ± 0.081 eV. We present a consistency test of the cosmic growth rate using SPT clusters. Allowing both the growth index γ and the dark energy equation-of-state parameter w to vary, we find γ = 0.73 ± 0.28 and w = −1.007 ± 0.065, demonstrating that the expansion and the growth histories are consistent with a ΛCDM universe (γ = 0.55; w = −1).

We present a catalog of galaxy clusters selected via their Sunyaev–Zel'dovich (SZ) effect signature from 2500 deg² of South Pole Telescope (SPT) data. This work represents the complete sample of clusters detected at high significance in the 2500 deg² SPT-SZ survey, which was completed in 2011. A total of 677 (409) cluster candidates are identified above a signal-to-noise threshold of ξ = 4.5 (5.0). Ground- and space-based optical and near-infrared (NIR) imaging confirms overdensities of similarly colored galaxies in the direction of 516 (or 76%) of the ξ > 4.5 candidates and 387 (or 95%) of the ξ > 5 candidates; the measured purity is consistent with expectations from simulations. Of these confirmed clusters, 415 were first identified in SPT data, including 251 new discoveries reported in this work. We estimate photometric redshifts for all candidates with identified optical and/or NIR counterparts; we additionally report redshifts derived from spectroscopic observations for 141 of these systems. The mass threshold of the catalog is roughly independent of redshift above z ∼ 0.25 leading to a sample of massive clusters that extends to high redshift. The median mass of the sample is M_500c(ρ_crit) $\sim 3.5\times 10^{14}\,M_\odot \,h_{70}^{-1}$, the median redshift is z_med = 0.55, and the highest-redshift systems are at z > 1.4. The combination of large redshift extent, clean selection, and high typical mass makes this cluster sample of particular interest for cosmological analyses and studies of cluster formation and evolution.

We present optical and infrared imaging and optical spectroscopy of galaxy clusters which were identified as part of an all-sky search for high-redshift galaxy clusters, the Massive and Distant Clusters of WISE Survey (MaDCoWS). The initial phase of MaDCoWS combined infrared data from the all-sky data release of the Wide-field Infrared Survey Explorer (WISE) with optical data from the Sloan Digital Sky Survey to select probable z ∼ 1 clusters of galaxies over an area of 10,000 deg². Our spectroscopy confirms 19 new clusters at 0.7 < z < 1.3, half of which are at z > 1, demonstrating the viability of using WISE to identify high-redshift galaxy clusters. The next phase of MaDCoWS will use the greater depth of the AllWISE data release to identify even higher redshift cluster candidates.

Spitzer UltRa Faint SUrvey Program is a joint Spitzer and Hubble Space Telescope Exploration Science program using 10 galaxy clusters as cosmic telescopes to study z ≳ 7 galaxies at intrinsically lower luminosities, enabled by gravitational lensing, than blank field surveys of the same exposure time. Our main goal is to measure stellar masses and ages of these galaxies, which are the most likely sources of the ionizing photons that drive reionization. Accurate knowledge of the star formation density and star formation history at this epoch is necessary to determine whether these galaxies indeed reionized the universe. Determination of the stellar masses and ages requires measuring rest-frame optical light, which only Spitzer can probe for sources at z ≳ 7, for a large enough sample of typical galaxies. Our program consists of 550 hr of Spitzer/IRAC imaging covering 10 galaxy clusters with very well-known mass distributions, making them extremely precise cosmic telescopes. We combine our data with archival observations to obtain mosaics with ∼30 hr exposure time in both 3.6 μm and 4.5 μm in the central 4' × 4' field and ∼15 hr in the flanking fields. This results in 3σ sensitivity limits of ∼26.6 and ∼26.2 AB magnitudes for the central field in the IRAC 3.6 and 4.5 μm bands, respectively. To illustrate the survey strategy and characteristics we introduce the sample, present the details of the data reduction and demonstrate that these data are sufficient for in-depth studies of z ≳ 7 sources (using a z = 9.5 galaxy behind MACS J1149.5+2223 as an example). For the first cluster of the survey (the Bullet Cluster) we have released all high-level data mosaics and IRAC empirical point-spread function models. In the future we plan to release these data products for the entire survey.

We present a catalog of galaxy cluster candidates, selected through their Sunyaev–Zel'dovich (SZ) effect signature in the first 720 deg² of the South Pole Telescope (SPT) survey. This area was mapped with the SPT in the 2008 and 2009 austral winters to a depth of ∼18 μK_CMB-arcmin at 150 GHz; 550 deg² of it was also mapped to ∼44 μK_CMB-arcmin at 95 GHz. Based on optical imaging of all 224 candidates and near-infrared imaging of the majority of candidates, we have found optical and/or infrared counterparts for 158, which we then classify as confirmed galaxy clusters. Of these 158 clusters, 135 were first identified as clusters in SPT data, including 117 new discoveries reported in this work. This catalog triples the number of confirmed galaxy clusters discovered through the SZ effect. We report photometrically derived (and in some cases spectroscopic) redshifts for confirmed clusters and redshift lower limits for the remaining candidates. The catalog extends to high redshift with a median redshift of z = 0.55 and maximum confirmed redshift of z = 1.37. Forty-five of the clusters have counterparts in the ROSAT bright or faint source catalogs from which we estimate X-ray fluxes. Based on simulations, we expect the catalog to be nearly 100% complete above M₅₀₀ ≈ 5 × 10¹⁴ M_☉ h⁻¹₇₀ at z ≳ 0.6. There are 121 candidates detected at signal-to-noise ratio greater than five, at which the catalog purity is measured to be 95%. From this high-purity subsample, we exclude the z < 0.3 clusters and use the remaining 100 candidates to improve cosmological constraints following the method presented by Benson et al. Adding the cluster data to CMB + BAO + H₀ data leads to a preference for non-zero neutrino masses while only slightly reducing the upper limit on the sum of neutrino masses to ∑m_ν < 0.38 eV (95% CL). For a spatially flat wCDM cosmological model, the addition of this catalog to the CMB + BAO + H₀ + SNe results yields σ₈ = 0.807 ± 0.027 and w = −1.010 ± 0.058, improving the constraints on these parameters by a factor of 1.4 and 1.3, respectively. The larger cluster catalog presented in this work leads to slight improvements in cosmological constraints from those presented by Benson et al. These cosmological constraints are currently limited by uncertainty in the cluster mass calibration, not the size or quality of the cluster catalog. A multi-wavelength observation program to improve the cluster mass calibration will make it possible to realize the full potential of the final 2500 deg² SPT cluster catalog to constrain cosmology.

We use weak gravitational lensing to measure the masses of five galaxy clusters selected from the South Pole Telescope (SPT) survey, with the primary goal of comparing these with the SPT Sunyaev–Zel'dovich (SZ) and X-ray-based mass estimates. The clusters span redshifts 0.28 < z < 0.43 and have masses M₅₀₀ > 2 × 10¹⁴ h⁻¹M_☉, and three of the five clusters were discovered by the SPT survey. We observed the clusters in the g'r'i' passbands with the Megacam imager on the Magellan Clay 6.5 m telescope. We measure a mean ratio of weak-lensing (WL) aperture masses to inferred aperture masses from the SZ data, both within an aperture of R_{500, SZ} derived from the SZ mass, of 1.04 ± 0.18. We measure a mean ratio of spherical WL masses evaluated at R_{500, SZ} to spherical SZ masses of 1.07 ± 0.18, and a mean ratio of spherical WL masses evaluated at R_{500, WL} to spherical SZ masses of 1.10 ± 0.24. We explore potential sources of systematic error in the mass comparisons and conclude that all are subdominant to the statistical uncertainty, with dominant terms being cluster concentration uncertainty and N-body simulation calibration bias. Expanding the sample of SPT clusters with WL observations has the potential to significantly improve the SPT cluster mass calibration and the resulting cosmological constraints from the SPT cluster survey. These are the first WL detections using Megacam on the Magellan Clay telescope.

We present the first results of our spectroscopic follow-up of 6.5 < z < 10 candidate galaxies behind clusters of galaxies. We report the spectroscopic confirmation of an intrinsically faint Lyman break galaxy (LBG) identified as a z_850LP-band dropout behind the Bullet Cluster. We detect an emission line at λ = 9412 Å at >5σ significance using a 16 hr long exposure with FORS2 VLT. Based on the absence of flux in bluer broadband filters, the blue color of the source, and the absence of additional lines, we identify the line as Lyα at z = 6.740 ± 0.003. The integrated line flux is f = (0.7 ± 0.1 ± 0.3) × 10⁻¹⁷ erg⁻¹ s⁻¹ cm⁻² (the uncertainties are due to random and flux calibration errors, respectively) making it the faintest Lyα flux detected at these redshifts. Given the magnification of μ = 3.0 ± 0.2 the intrinsic (corrected for lensing) flux is f^int = (0.23 ± 0.03 ± 0.10 ± 0.02) × 10⁻¹⁷ erg⁻¹ s⁻¹ cm⁻² (additional uncertainty due to magnification), which is ∼2–3 times fainter than other such measurements in z ∼ 7 galaxies. The intrinsic H_160W-band magnitude of the object is $m^{\rm int}_{H_{\rm 160W}}=27.57\pm 0.17$, corresponding to 0.5 L* for LBGs at these redshifts. The galaxy is one of the two sub-L* LBG galaxies spectroscopically confirmed at these high redshifts (the other is also a lensed z = 7.045 galaxy), making it a valuable probe for the neutral hydrogen fraction in the early universe.

We report the discovery of an IR-selected massive galaxy cluster in the IRAC Deep Cluster Survey (IDCS). We present new data from the Hubble Space Telescope and the W. M. Keck Observatory that spectroscopically confirm IDCS J1426.5+3508 at z = 1.75. Moreover, the cluster is detected in archival Chandra data as an extended X-ray source, comprising 53 counts after the removal of point sources. We calculate an X-ray luminosity of L_{0.5 − 2 keV} = (5.4 ± 1.2) × 10⁴⁴ erg s⁻¹ within r = 60 arcsec (∼1 Mpc diameter), which implies $M_{200,L_x} = (5.3 \pm 1.6) \times 10^{14}$ M_☉. IDCS J1426.5+3508 appears to be an exceptionally massive cluster for its redshift.