Multifractal magnetic susceptibility distribution models of hydrothermally altered rocks in the Needle Creek Igneous Center of the Absaroka Mountains, Wyoming

International audienceMagnetic susceptibility was measured for 700 samples of drill core from thirteen drill holes in the porphyry copper-molybdenum deposit of the Stinkingwater mining district in the Absaroka Mountains, Wyoming. The magnetic susceptibility measurements, chemical analyses, and alteration class provided a database for study of magnetic susceptibility in these altered rocks. The distribution of the magnetic susceptibilities for all samples is multi-modal, with overlapping peaked distributions for samples in the propylitic and phyllic alteration class, a tail of higher susceptibilities for potassic alteration, and an approximately uniform distribution over a narrow range at the highest susceptibilities for unaltered rocks. Samples from all alteration and mineralization classes show susceptibilities across a wide range of values. Samples with secondary (supergene) alteration due to oxidation or enrichment show lower susceptibilities than primary (hypogene) alteration rock. Observed magnetic susceptibility variations and the monolithological character of the host rock suggest that the variations are due to varying degrees of alteration of blocks of rock between fractures that conducted hydrothermal fluids. Alteration of rock from the fractures inward progressively reduces the bulk magnetic susceptibility of the rock. The model introduced in this paper consists of a simulation of the fracture pattern and a simulation of the alteration of the rock between fractures. A multifractal model generated from multiplicative cascades with unequal ratios produces distributions statistically similar to the observed distributions. The reduction in susceptibility in the altered rocks was modelled as a diffusion process operating on the fracture distribution support. The average magnetic susceptibility was then computed for each block. For the purpose of comparing the model results with observation, the simulated magnetic susceptibilities were then averaged over the same interval as the measured data. Comparisons of the model and data from drillholes show good but not perfect agreement

The evolution of dust-obscured star formation activity in galaxy clusters relative to the field over the last 9 billion years

We compare the star formation (SF) activity in cluster galaxies to the field from z=0.3-1.5 using $Herschel$ SPIRE 250$\mu$m imaging. We utilize 274 clusters from the IRAC Shallow Cluster Survey (ISCS) selected as rest-frame near-infrared overdensities over the 9 square degree Bootes field . This analysis allows us to quantify the evolution of SF in clusters over a long redshift baseline without bias against active cluster systems. Using a stacking analysis, we determine the average star formation rates (SFRs) and specific-SFRs (SSFR=SFR/M$_{\star}$) of stellar mass-limited (M>1.3x10$^{10}$ M$_{\odot}$), statistical samples of cluster and field galaxies, probing both the star forming and quiescent populations. We find a clear indication that the average SF in cluster galaxies is evolving more rapidly than in the field, with field SF levels at z>1.2 in the cluster cores (r<0.5 Mpc), in good agreement with previous ISCS studies. By quantifying the SF in cluster and field galaxies as an exponential function of cosmic time, we determine that cluster galaxies are evolving ~2 times faster than the field. Additionally, we see enhanced SF above the field level at z~1.4 in the cluster outskirts (r>0.5 Mpc). These general trends in the cluster cores and outskirts are driven by the lower mass galaxies in our sample. Blue cluster galaxies have systematically lower SSFRs than blue field galaxies, but otherwise show no strong differential evolution with respect to the field over our redshift range. This suggests that the cluster environment is both suppressing the star formation in blue galaxies on long time-scales and rapidly transitioning some fraction of blue galaxies to the quiescent galaxy population on short time-scales. We argue that our results are consistent with both strangulation and ram pressure stripping acting in these clusters, with merger activity occurring in the cluster outskirts.Comment: 23 pages, 11 figures, 5 tables. Accepted for publication in MNRA

Candidate Clusters of Galaxies at z > 1.3 Identified in the Spitzer South Pole Telescope Deep Field Survey

We present 279 galaxy cluster candidates at z > 1.3 selected from the 94 deg^2 Spitzer South Pole Telescope Deep Field (SSDF) survey. We use a simple algorithm to select candidate high-redshift clusters of galaxies based on Spitzer/IRAC mid-infrared data combined with shallow all-sky optical data. We identify distant cluster candidates adopting an overdensity threshold that results in a high purity (80%) cluster sample based on tests in the Spitzer Deep, Wide-Field Survey of the Boötes field. Our simple algorithm detects all three 1.4 < z ≤ 1.75 X-ray detected clusters in the Boötes field. The uniqueness of the SSDF survey resides not just in its area, one of the largest contiguous extragalactic fields observed with Spitzer, but also in its deep, multi-wavelength coverage by the South Pole Telescope (SPT), Herschel/SPIRE, and XMM-Newton. This rich data set will allow direct or stacked measurements of Sunyaev-Zel'dovich effect decrements or X-ray masses for many of the SSDF clusters presented here, and enable a systematic study of the most distant clusters on an unprecedented scale. We measure the angular correlation function of our sample and find that these candidates show strong clustering. Employing the COSMOS/UltraVista photometric catalog in order to infer the redshift distribution of our cluster selection, we find that these clusters have a comoving number density N_c = (0.7^(+6.3)_(0.6)) x 10^(-7) h^3 Mpc^(-3) and a spatial clustering correlation scale length r_ 0 = (32 ± 7) h^(–1) Mpc. Assuming our sample is comprised of dark matter halos above a characteristic minimum mass, M _(min), we derive that at z = 1.5 these clusters reside in halos larger than M_(min) = 1.5^(+0.9)_(0.7) x 10^(14) h^(-1) M_⊙. We find that the mean mass of our cluster sample is equal to M_(mean) = 1.9^(+1.0)_(0.8) x 10^(14) h^(-1) M_⊙ ; thus, our sample contains the progenitors of present-day massive galaxy clusters

The Growth of Cool Cores and Evolution of Cooling Properties in a Sample of 83 Galaxy Clusters at 0.3 < z < 1.2 Selected from the SPT-SZ Survey

We present first results on the cooling properties derived from Chandra X-ray observations of 83 high-redshift (0.3 < z < 1.2) massive galaxy clusters selected by their Sunyaev-Zel'dovich signature in the South Pole Telescope data. We measure each cluster's central cooling time, central entropy, and mass deposition rate, and compare to local cluster samples. We find no significant evolution from z~0 to z~1 in the distribution of these properties, suggesting that cooling in cluster cores is stable over long periods of time. We also find that the average cool core entropy profile in the inner ~100 kpc has not changed dramatically since z ~ 1, implying that feedback must be providing nearly constant energy injection to maintain the observed "entropy floor" at ~10 keV cm^2. While the cooling properties appear roughly constant over long periods of time, we observe strong evolution in the gas density profile, with the normalized central density (rho_0/rho_crit) increasing by an order of magnitude from z ~ 1 to z ~ 0. When using metrics defined by the inner surface brightness profile of clusters, we find an apparent lack of classical, cuspy, cool-core clusters at z > 0.75, consistent with earlier reports for clusters at z > 0.5 using similar definitions. Our measurements indicate that cool cores have been steadily growing over the 8 Gyr spanned by our sample, consistent with a constant, ~150 Msun/yr cooling flow that is unable to cool below entropies of 10 keV cm^2 and, instead, accumulates in the cluster center. We estimate that cool cores began to assemble in these massive systems at z ~ 1, which represents the first constraints on the onset of cooling in galaxy cluster cores. We investigate several potential biases which could conspire to mimic this cool core evolution and are unable to find a bias that has a similar redshift dependence and a substantial amplitude.Comment: 17 pages with 15 figures, plus appendix. Published in Ap

The effects of surface pretreatment on the cyclic-fatigue characteristics of bonded aluminium-alloy joints

Accepted versio

SPT-CL J0205-5829: A z = 1.32 Evolved Massive Galaxy Cluster in the South Pole Telescope Sunyaev-Zel'dovich Effect Survey

The galaxy cluster SPT-CL J0205-5829 currently has the highest spectroscopically-confirmed redshift, z=1.322, in the South Pole Telescope Sunyaev-Zel'dovich (SPT-SZ) survey. XMM-Newton observations measure a core-excluded temperature of Tx=8.7keV producing a mass estimate that is consistent with the Sunyaev-Zel'dovich derived mass. The combined SZ and X-ray mass estimate of M500=(4.9+/-0.8)e14 h_{70}^{-1} Msun makes it the most massive known SZ-selected galaxy cluster at z>1.2 and the second most massive at z>1. Using optical and infrared observations, we find that the brightest galaxies in SPT-CL J0205-5829 are already well evolved by the time the universe was <5 Gyr old, with stellar population ages >3 Gyr, and low rates of star formation (<0.5Msun/yr). We find that, despite the high redshift and mass, the existence of SPT-CL J0205-5829 is not surprising given a flat LambdaCDM cosmology with Gaussian initial perturbations. The a priori chance of finding a cluster of similar rarity (or rarer) in a survey the size of the 2500 deg^2 SPT-SZ survey is 69%.Comment: 11 pages, 5 figures, submitted to Ap

THE MASSIVE AND DISTANT CLUSTERS OF WISE SURVEY. III. SUNYAEV-ZEL'DOVICH MASSES OF GALAXY CLUSTERS AT z ~ 1

We present CARMA 30 GHz Sunyaev–Zel'dovich (SZ) observations of five high-redshift (z [> over ~] 1), infrared-selected galaxy clusters discovered as part of the all-sky Massive and Distant Clusters of WISE Survey (MaDCoWS). The SZ decrements measured toward these clusters demonstrate that the MaDCoWS selection is discovering evolved, massive galaxy clusters with hot intracluster gas. Using the SZ scaling relation calibrated with South Pole Telescope clusters at similar masses and redshifts, we find these MaDCoWS clusters have masses in the range M[subscript 200] ≈ 2-6 X 10[superscript 14] M[subscript ʘ. Three of these are among the most massive clusters found to date at z [> over ~] 1, demonstrating that MaDCoWS is sensitive to the most massive clusters to at least z = 1.3. The added depth of the AllWISE data release will allow all-sky infrared cluster detection to z ≈ 1.5 and beyond