Cheng-Jiun Ma

A complete sample of 12 very x-ray luminous galaxy clusters at z >0.5

We present the statistically complete and cosmologically most relevant subset of the 12 most distant galaxy clusters detected at z > 0.5 by the Massive Cluster Survey (MACS). Ten of these systems are new discoveries; only two (MACS J0018.5+1626, aka Cl 0016+1609, and MACS J0454.1-0300, aka MS 0451.6-0305) were previously known. We provide fundamental cluster properties derived from our optical and X-ray follow-up observations as well as the selection function in tabulated form to facilitate cosmological studies using this sample.

The X-ray brightest clusters of galaxies from the Massive Cluster Survey

We present a statistically complete sample of very X-ray luminous galaxy clusters detected in the MAssive Cluster Survey (MACS). This second MACS release comprises all 34 MACS clusters with nominal X-ray fluxes in excess of 2 x 10 -12 erg s -1 cm -2 (0.1-2.4 keV) in the ROSAT Bright Source Catalogue; two-thirds of them are new discoveries. Extending over the redshift range from 0.3 to 0.5, this subset complements the complete sample of the 12 most distant MACS clusters (z > 0.5) published in 2007 and further exemplifies the efficacy of X-ray selection for the compilation of samples of intrinsically massive galaxy clusters. Extensive follow-up observations with Chandra/ACIS led to three additional MACS cluster candidates being eliminated as (predominantly) X-ray point sources. For another four clusters - which, however, remain in our sample of 34 - the point-source contamination was found to be about 50 per cent. The median X-ray luminosity of 1.3 x 10 45 erg s -1 (0.1-2.4 keV, Chandra, within r 500 ) of the clusters in this subsample demonstrates the power of the MACS strategy to find the most extreme and rarest clusters out to significant redshift. A comparison of the optical and X-ray data for all clusters in this release finds a wide range of morphologies with no obvious bias in favour of either relaxed or merging systems.

HUBBLE SPACE TELESCOPE OBSERVATIONS OF A SPECTACULAR NEW STRONG-LENSING GALAXY CLUSTER: MACS J1149.5+2223 AT z = 0.544

We present Advanced Camera for Surveys observations of MACS?J1149.5+2223, an X-ray luminous galaxy cluster at z = 0.544 discovered by the Massive Cluster Survey. The data reveal at least seven multiply imaged galaxies, three of which we have confirmed spectroscopically. One of these is a spectacular face-on spiral galaxy at z = 1.491, the four images of which are gravitationally magnified by 8 ? 23. We identify this as an L (MB ?20.7), disk-dominated (B/T 0.5) galaxy, forming stars at ~6 M ? yr?1. We use a robust sample of multiply imaged galaxies to constrain a parameterized model of the cluster mass distribution. In addition to the main cluster dark matter halo and the bright cluster galaxies, our best model includes three galaxy-group-sized halos. The relative probability of this model is P(N halo = 4)/P(N halo < 4) ? 1012 where N halo is the number of cluster/group-scale halos. In terms of sheer number of merging cluster/group-scale components, this is the most complex strong-lensing cluster core studied to date. The total cluster mass and fraction of that mass associated with substructures within R ? 500 kpc, are measured to be M tot = (6.7 ? 0.4) ? 1014 M ? and f sub = 0.25 ? 0.12, respectively. Our model also rules out recent claims of a flat density profile at 7? confidence, thus highlighting the critical importance of spectroscopic redshifts of multiply imaged galaxies when modeling strong-lensing clusters. Overall our results attest to the efficiency of X-ray selection in finding the most powerful cluster lenses, including complicated merging systems.

A weak lensing mass reconstruction of the large-scale filament feeding the massive galaxy cluster MACS J0717.5+3745

We report the first weak lensing detection of a large-scale filament funnelling matter on to the core of the massive galaxy cluster MACS J0717.5+3745. Our analysis is based on a mosaic of 18 multipassband images obtained with the Advanced Camera for Surveys aboard the Hubble Space Telescope, covering an area of ˜10 × 20 arcmin2. We use a weak lensing pipeline developed for the Cosmic Evolution Survey, modified for the analysis of galaxy clusters, to produce a weak lensing catalogue. A mass map is then computed by applying a weak gravitational lensing multiscale reconstruction technique designed to describe irregular mass distributions such as the one investigated here. We test the resulting mass map by comparing the mass distribution inferred for the cluster core with the one derived from strong lensing constraints and find excellent agreement. Our analysis detects the MACS J0717.5+3745 filament within the 3σ detection contour of the lensing mass reconstruction, and underlines the importance of filaments for theoretical and numerical models of the mass distribution in the cosmic web. We measure the filaments projected length as ˜4.5 h74-1 Mpc, and its mean density as (2.92 ± 0.66) × 108 h74 Ms kpc-2. Combined with the redshift distribution of galaxies obtained after an extensive spectroscopic follow-up in the area, we can rule out any projection effect resulting from the chance alignment on the sky of unrelated galaxy group-scale structures. Assuming plausible constraints concerning the structures geometry based on its galaxy velocity field, we construct a three-dimensional (3D) model of the large-scale filament. Within this framework, we derive the 3D length of the filament to be 18 h74-1 Mpc. The filaments deprojected density in terms of the critical density of the Universe is measured as (206 ± 46) ρcrit, a value that lies at the very high end of the range predicted by numerical simulations. Finally, we study the distribution of stellar mass in the field of MACS J0717.5+3749 and, adopting a mean mass-to-light ratio of 0.73 ± 0.22 and assuming a Chabrier initial mass function, measure a stellar mass fraction along the filament of (0.9 ± 0.2) per cent, consistent with previous measurements in the vicinity of massive clusters.

The Spatial Distribution of Galaxies of Different Spectral Types in the Massive Intermediate-Redshift Cluster MACS J0717.5+3745

We present the results of a wide-field spectroscopic analysis of the galaxy population of the massive cluster MACS J0717.5+3745 and the surrounding filamentary structure ( -->z = 0.55), as part of our systematic study of the 12 most distant clusters in the MACS sample. Of 1368 galaxies spectroscopically observed in this field, 563 are identified as cluster members; of those, 203 are classified as emission-line galaxies, 260 as absorption-line galaxies, and 17 as E+A galaxies (defined by -->(H δ + H γ )/2 > 6 A and no detection of [O II] and Hβ in emission). The variation of the fraction of emission- and absorption-line galaxies as a function of local projected galaxy density confirms the well-known morphology-density relation, and becomes flat at projected galaxy densities less than ~20 Mpc−2. Interestingly, 16 out of 17 E+A galaxies lie (in projection) within the ram-pressure stripping radius around the cluster core, which we take to be direct evidence that ram-pressure stripping is the primary mechanism that terminates star formation in the E+A population of galaxy clusters. This conclusion is supported by the rarity of E+A galaxies in the filament, which rules out galaxy mergers as the dominant driver of evolution for E+A galaxies in clusters. In addition, we find that the 42 e(a) and 27 e(b) member galaxies, i.e., the dusty-starburst and starburst galaxies respectively, are spread out across almost the entire study area. Their spatial distribution, which shows a strong preference for the filament region, suggests that starbursts are triggered in relatively low-density environments as galaxies are accreted from the field population.

AN X-RAY/OPTICAL STUDY OF THE COMPLEX DYNAMICS OF THE CORE OF THE MASSIVE INTERMEDIATE-REDSHIFT CLUSTER MACSJ0717.5+3745

Using CHANDRA, we investigate the spatial temperature distribution of the intracluster medium (ICM) within 700 kpc of the center of the massive merging cluster MACSJ0717.5+3745 at z = 0.55. Combining the X-ray evidence with information about the distribution and velocities of the cluster galaxies near the core provides us with a snapshot of the three-dimensional geometry and dynamics of one of the most complex clusters studied to date. We find MACSJ0717.5+3745 to be an active triple merger with ICM temperatures exceeding 20 keV. Although radial velocity information and X-ray/optical offsets indicate that all three mergers proceed along distinctly different directions, the partial alignment of the merger axes points to a common origin in the large-scale filament south-east of the cluster core. Clear decrements in the ICM temperature observed near two of these subclusters identify the respective X-ray surface brightness peaks as remnants of cool cores; the compactness and low temperature of 5.7 keV of one of these features suggest that the respective merger, a high-velocity collision at 3,000 km s–1, is still in its very early stages. Looking beyond the triple merger, we find the large-scale filament to not only provide a spatial as well as temporal arrow for the interpretation of the dynamics of the merger events near the cluster core, but we also find tantalizing, if circumstantial, evidence for direct, large-scale heating of the ICM by contiguous infall of low-density gas from the filament.

A spectacular giant arc in the massive cluster lens MACS J1206.2-0847

We discuss the X-ray and optical properties of the massive galaxy cluster MACS J1206.2−0847 (z = 0.4385), discovered in the Massive Cluster Survey (MACS). Our Chandra observation of the system yields a total X-ray luminosity of 2.4 × 10 45 erg s −1 (0.1–2.4 keV) and a global gas temperature of 11.6 ± 0.7 keV, very high values typical of MACS clusters. In both optical and X-ray images, MACS J1206.2−0847 appears close to relaxed in projection, with a pronounced X-ray peak at the location of the brightest cluster galaxy (BCG); we interpret this feature as the remnant of a cold core. A spectacular giant gravitational arc, 15 arcsec in length, bright (V ∼ 21) and unusually red (R − K = 4.3), is seen 20 arcsec west of the BCG; we measure a redshift of z = 1.036 for the lensed galaxy. From our Hubble Space Telescope image of the cluster, we identify the giant arc and its counter image as a sevenfold imaged system. An excess of X-ray emission in the direction of the arc coincides with a mild galaxy overdensity and could be the remnant of a minor merger with a group of galaxies. We derive estimates of the total cluster mass as well as of the mass of the cluster core using X-ray, dynamical and gravitational-lensing techniques. For the mass enclosed by the giant arc ( r< 119 kpc), our strong-lensing analysis based on Hubble Space Telescope imaging yields a very high value of 1.1 × 10 14 M� , inconsistent with the much lower X-ray estimate − − − − − − ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ,

MACS J1423.8+2404 : gravitational lensing by a massive, relaxed cluster of galaxies at z = 0.54.

We present results of a gravitational lensing and optical study of MACS J1423.8+2404 (z=0.545, MACS J1423), the most relaxed cluster in the high-redshift subsample of clusters discovered in the MAssive Cluster Survey (MACS). Our analysis uses high-resolution images taken with the Hubble Space Telescope in the F555W and F814W passbands, ground-based imaging in eight optical and near-infrared filters obtained with Subaru and Canada–France–Hawaii Telescope, as well as extensive spectroscopic data gathered with the Keck telescopes. At optical wavelengths, the cluster exhibits no sign of substructure and is dominated by a cD galaxy that is 2.1 mag (K band) brighter than the second brightest cluster member, suggesting that MACS J1423 is close to be fully virialized. Analysis of the redshift distribution of 140 cluster members reveals a Gaussian distribution, mildly disturbed by the presence of a loose galaxy group that may be falling into the cluster along the line of sight. Combining stronglensing constraints from two spectroscopically confirmed multiple-image systems near the cluster core with a weak-lensing measurement of the gravitational shear on larger scales, we derive a parametric mass model for the mass distribution. All constraints can be satisfied by a unimodal mass distribution centred on the cD galaxy and exhibiting very little substructure. The derived projected mass of M[<65 arcsec (415 kpc)] = (4.3 ± 0.6) × 10^(14) M_☉ is about 30 per cent higher than the one derived from X-ray analyses assuming spherical symmetry, suggesting a slightly prolate mass distribution consistent with the optical indication of residual line-of-sight structure. The similarity in shape and excellent alignment of the centroids of the total mass, K-band light and intracluster gas distributions add to the picture of a highly evolved system. The existence of a massive cluster like MACS J1423, nearly fully virialized only ~7 Gyr after the big bang, may have important implications for models of structure formation and evolution on cosmological time scales.

SPECTROSCOPIC REDSHIFTS OF GALAXIES WITHIN THE FRONTIER FIELDS

We present a catalog of 1921 spectroscopic redshifts measured in the fields of the massive galaxy clusters MACSJ0416.1–2403 (z = 0.397), MACSJ0717.5+3745 (z = 0.546), and MACSJ1149.5+2223 (z = 0.544), i.e., three of the four clusters selected by Space Telescope Science Institute as the targets of the Frontier Fields (FFs) initiative for studies of the distant Universe via gravitational lensing. Compiled in the course of the Massive Cluster Survey project (MACS) that detected the FF clusters, this catalog is provided to the community for three purposes: (1) to allow the identification of cluster members for studies of the galaxy population of these extreme systems, (2) to facilitate the removal of unlensed galaxies and thus reduce shear dilution in weak-lensing analyses, and (3) to improve the calibration of photometric redshifts based on both ground- and spacebased observations of the FF clusters.

Probing the large-scale structure around the most distant galaxy clusters from the massive cluster survey

We present maps of the cosmic large-scale structure around the 12 most distant galaxy clusters from the Massive Cluster Survey (MACS) as traced by the projected surface density of galaxies on the cluster red sequence. Taken with the SuprimeCam wide-field camera on the Subaru telescope, the images used in this study cover a 27 x 27 arcmin 2 area around each cluster, corresponding to 10 x 10 Mpc 2 at the median redshift of z = 0.55 of our sample. We directly detect satellite clusters and filaments extending over the full size of our imaging data in the majority of the clusters studied, supporting the picture of mass accretion via infall along filaments suggested by numerical simulations of the growth of clusters and the evolution of large-scale structure. A comparison of the galaxy distribution near the cluster cores with the X-ray surface brightness as observed with Chandra reveals, in several cases, significant offsets between the gas and galaxy distribution, indicative of ongoing merger events. The respective systems are ideally suited for studies of the dynamical properties of gas, galaxies and dark matter. In addition, the large-scale filaments viewed at high contrast in these MACS clusters are prime targets for the direct detection and study of the warm-hot intergalactic medium (WHIM).