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We present galaxy velocity dispersions and dynamical mass estimates for 44 galaxy clusters selected via the Sunyaev-Zel'dovich (SZ) effect by the Atacama Cosmology Telescope. Dynamical masses for 18 clusters are reported here for the first time. Using N-body simulations, we model the different observing strategies used to measure the velocity dispersions and account for systematic effects resulting from these strategies. We find that the galaxy velocity distributions may be treated as isotropic, and that an aperture correction of up to 7 per cent in the velocity dispersion is required if the spectroscopic galaxy sample is sufficiently concentrated towards the cluster centre. Accounting for the radial profile of the velocity dispersion in simulations enables consistent dynamical mass estimates regardless of the observing strategy. Cluster masses M200 are in the range (1-15) × 1014M⊙. Comparing with masses estimated from the SZ distortion assuming a gas pressure profile derived from X-ray observations gives a mean SZ-to-dynamical mass ratio of 1.10 ± 0.13, but there is an additional 0.14 systematic uncertainty due to the unknown velocity bias; the statistical uncertainty is dominated by the scatter in the mass-velocity dispersion scaling relation. This ratio is consistent with previous determinations at these mass scales. [ABSTRACT FROM AUTHOR]

Merging galaxy clusters with radio relics provide rare insights to the merger dynamics as the relics are created by the violent merger process. We demonstrate one of the first uses of the properties of the radio relic to reduce the uncertainties of the dynamical variables and determine the three-dimensional (3D) configuration of a cluster merger, ACT-CL J0102-4915, nicknamed El Gordo. From the double radio relic observation and the X-ray observation of a comet-like gas morphology induced by motion of the cool core, it is widely believed that El Gordo is observed shortly after the first core passage of the subclusters. We employ a Monte Carlo simulation to investigate the 3D configuration and dynamics of El Gordo. Using the polarization fraction of the radio relic, we constrain the estimate of the angle between the plane of the sky and the merger axis to be α = 21?±9 11. We find the relative 3D merger speed of El Gordo to be 2400 ±400 200 km s-1 at pericentre. The two possible estimates of the time since pericentre (TSP) are 0.46±0.09 0.16 and 0.91±0.22 0.39 Gyr for the outgoing and returning scenario, respectively. We put our estimates of the TSP into context by showing that if the time-averaged shock velocity is approximately equal to or smaller than the pericentre velocity of the corresponding subcluster in the centre-of-mass frame, the two subclusters are more likely to be moving towards, rather than away, from each other, post apocentre. We compare and contrast the merger scenario of El Gordo with that of the Bullet Cluster, and show that this late-stage merging scenario explains why the south-east (SE) dark matter lensing peak of El Gordo is closer to the merger centre than the SE cool core. [ABSTRACT FROM AUTHOR]

We present Southern African Large Telescope follow-up observations of seven massive clusters detected by the Atacama Cosmology Telescope (ACT) on the celestial equator using the Sunyaev-Zel'dovich (SZ) effect. We conducted multi-object spectroscopic observations with the Robert Stobie Spectrograph in order to measure galaxy redshifts in each cluster field, determine the cluster line-of-sight velocity dispersions, and infer the cluster dynamical masses. We find that the clusters, which span the redshift range 0.3 < z < 0.55, range in mass from (5-20) × 1014 M☉ (M200c). Their masses, given their SZ signals, are similar to those of Southern hemisphere ACT clusters previously observed using Gemini and the VLT. We note that the brightest cluster galaxy in one of the systems studied, ACT-CL J0320.4+0032 at z = 0.38, hosts a type II quasar. Only a handful of such systems are currently known, and therefore ACT-CL J0320.4+0032 may be a rare example of a very massive halo in which quasar-mode feedback is actively taking place. [ABSTRACT FROM AUTHOR]

We present 610 MHz and 2.1 GHz imaging of the massive Sunyaev-Zel'dovich Effect selected z = 0.870 cluster merger ACT-CL J0102–4915 (“El Gordo”), obtained with the Giant Metrewave Radio Telescope and the Australia Telescope Compact Array (ATCA), respectively. We detect two complexes of radio relics separated by 3.′4 (1.6 Mpc) along the system's northwest-to-southeast collision axis that have high integrated polarization fractions (33%) and steep spectral indices (α between 1 and 2; Sν∝ν–α), consistent with creation via Fermi acceleration by shocks in the intracluster medium triggered by the cluster collision. From the spectral index of the relics, we compute a Mach number and shock speed of . With our wide-bandwidth, full-polarization ATCA data, we compute the Faraday depth ϕ across the northwest relic and find a range of values spanning Δϕ = 30 rad m–2, with a mean value of 〈 ϕ〉 = 11 rad m–2 and standard deviation σϕ = 6 rad m–2. With the integrated line-of-sight gas density derived from new Chandra X-ray observations, our Faraday depth measurement implies B∥ ∼ 0.01 μG in the cluster outskirts. The extremely narrow shock widths in the relics (dshock ⩽ 23 kpc), caused by the short synchrotron cooling timescale of relativistic electrons at z = 0.870, prevent us from placing a meaningful constraint on the magnetic field strength B using cooling time arguments. In addition to the relics, we detect a large (rH ≃ 1.1 Mpc radius), powerful (log (L1.4/W Hz–1) = 25.66 ± 0.12) radio halo with a shape similar to El Gordo's “bullet”-like X-ray morphology. The spatially resolved spectral-index map of the halo shows the synchrotron spectrum is flattest near the relics, along the system's collision axis, and in regions of high Tgas, all locations associated with recent energy injection. The spatial and spectral correlation between the halo emission and cluster X-ray properties supports primary-electron processes like turbulent reacceleration as the halo production mechanism. The halo's integrated 610 MHz to 2.1 GHz spectral index is a relatively flat α = 1.2 ± 0.1, consistent with the cluster's high Tgas in view of previously established global scaling relations. El Gordo is the highest-redshift cluster known to host a radio halo and/or radio relics, and provides new constraints on the non-thermal physics in clusters at z > 0.6. [ABSTRACT FROM AUTHOR]

Context. The recent discovery of a large number of galaxy clusters using the Sunyaev-Zel'dovich (SZ) effect has opened a new era on the study of the most massive clusters in the Universe. Multiwavelength analyses are required to understand the properties of these new sets of clusters, which are a sensitive probe of cosmology. Aims. We aim for a multiwavelength characterization of PLCK G004.5-19.5, one of the most massive X-ray validated SZ effectselected galaxy clusters discovered by the Planck satellite. Methods. We have observed PLCK G004.5-19.5 with GMOS on the 8.1 m-Gemini South Telescope for optical imaging and spectroscopy, and performed a strong lensing analysis. We also searched for associated radio emission in published catalogs. Results. An analysis of the optical images confirms that this is a massive cluster, with a dominant central galaxy and an accompanying red sequence of galaxies, plus a 14"-long strong lensing arc. Longslit spectroscopy of six cluster members shows that the cluster is at z = 0:516±0:002. We also targeted the strongly lensed arc, and found zarc = 1:601. We use LensTool to carry out a strong lensing analysis, from which we measure a median Einstein radius θE(zs = 1:6) ≃ 30" and estimate an enclosed mass ME = 2:45++0:45 -0:47 × 1014 M☉. By extrapolating a Navarro-Frenk-White profile, we find a total mass MSL 500 = 4:0+2:1 ××1:0 1014 M☉. We also include a constraint on the mass from previous X-ray observations, which yields a slightly higher mass, MSL+X 500 = 6:7+2:6 -1:3 × 1014 M☉, consistent with the value from strong lensing alone. Intermediate-resolution radio images from the TIFR GMRT Sky Survey at 150 MHz reveal that PLCK G004.5-19.5 hosts a powerful radio relic on scales .⩽500 kpc. Emission at the same location is also detected in low-resolution images at 843 MHz and 1.4 GHz. This is one of the higher redshift radio relics known to date. [ABSTRACT FROM AUTHOR]

We present the first dynamical mass estimates and scaling relations for a sample of Sunyaev-Zel'dovich effect (SZE) selected galaxy clusters. The sample consists of 16 massive clusters detected with the Atacama Cosmology Telescope (ACT) over a 455 deg2 area of the southern sky. Deep multi-object spectroscopic observations were taken to secure intermediate-resolution (R ~ 700-800) spectra and redshifts for ≈60 member galaxies on average per cluster. The dynamical masses M200c of the clusters have been calculated using simulation-based scaling relations between velocity dispersion and mass. The sample has a median redshift z = 0.50 and a median mass M200c ☉ 12 × 1014 h -1 70 M☉ with a lower limit M200c ≃ 6 × 1014 h -1 70 M☉, consistent with the expectations for the ACT southern sky survey. These masses are compared to the ACT SZE properties of the sample, specifically, the match-filtered central SZE amplitude ~y0, the central Compton parameter ~y0, and the integrated Compton signal Y200c, which we use to derive SZE-mass scaling relations. All SZE estimators correlate with dynamical mass with low intrinsic scatter (≲20%), in agreement with numerical simulations. We explore the effects of various systematic effects on these scaling relations, including the correlation between observables and the influence of dynamically disturbed clusters. Using the three-dimensional information available, we divide the sample into relaxed and disturbed clusters and find that ~50% of the clusters are disturbed. There are hints that disturbed systems might bias the scaling relations, but given the current sample sizes, these differences are not significant; further studies including more clusters are required to assess the impact of these clusters on the scaling relations. [ABSTRACT FROM AUTHOR]

We present a Subaru weak lensing measurement of ACT-CL J0022.2−0036, one of the most luminous, high-redshift (z = 0.81) Sunyaev–Zel'dovich (SZ) clusters discovered in the 268 deg2 equatorial region survey of the Atacama Cosmology Telescope that overlaps with Sloan Digital Sky Survey (SDSS) Stripe 82 field. Ours is the first weak lensing study with Subaru at such high redshifts. For the weak lensing analysis using i′-band images, we use a model-fitting (Gauss–Laguerre shapelet) method to measure shapes of galaxy images, where we fit galaxy images in different exposures simultaneously to obtain best-fitting ellipticities taking into account the different point spread functions (PSFs) in each exposure. We also take into account the astrometric distortion effect on galaxy images by performing the model fitting in the world coordinate system. To select background galaxies behind the cluster at z = 0.81, we use photometric redshift estimates for every galaxy derived from the co-added images of multi-passband Br′i′z′Y, with PSF matching/homogenization. After a photometric redshift cut for background galaxy selection, we detect the tangential weak lensing distortion signal with a total signal-to-noise ratio of about 3.7. By fitting a Navarro–Frenk–White model to the measured shear profile, we find the cluster mass to be $M_{200\bar{\rho }_{\rm m}} = [7.5^{+3.2}_{-2.8}({\rm stat.})^{+1.3}_{-0.6}({\rm sys.})]\times 10^{14} \,\mathrm{M}_{\odot }\,h^{-1}$. The weak lensing-derived mass is consistent with previous mass estimates based on the SZ observation, with assumptions of hydrostatic equilibrium and virial theorem, as well as with scaling relations between SZ signal and mass derived from weak lensing, X-ray and velocity dispersion, within the measurement errors. We also show that the existence of ACT-CL J0022.2−0036 at z = 0.81 is consistent with the cluster abundance prediction of the Λ-dominated cold dark matter structure formation model. We thus demonstrate the capability of Subaru-type ground-based images for studying weak lensing of high-redshift clusters. [ABSTRACT FROM AUTHOR]

We present follow-up observations with the Sunyaev-Zel'dovich Array (SZA) of optically confirmed galaxy clusters found in the equatorial survey region of the Atacama Cosmology Telescope (ACT): ACT-CL J0022-0036, ACT-CL J2051+0057, and ACT-CL J2337+0016. ACT-CL J0022-0036 is a newly discovered, massive (≃ 1015M☼), high-redshift (z = 0.81) cluster revealed by ACT through the Sunyaev-Zel'dovich effect (SZE). Deep, targeted observations with the SZA allow us to probe a broader range of cluster spatial scales, better disentangle cluster decrements from radio point-source emission, and derive more robust integrated SZE flux and mass estimates than we can with ACT data alone. For the two clusters we detect with the SZA we compute integrated SZE signal and derive masses from the SZA data only. ACT-CL J2337+0016, also known as A2631, has archival Chandra data that allow an additional X-ray-based mass estimate. Optical richness is also used to estimate cluster masses and shows good agreement with the SZE and X-ray-based estimates. Based on the point sources detected by the SZA in these three cluster fields and an extrapolation to ACT's frequency, we estimate that point sources could be contaminating the SZE decrement at the ≲ 20% level for some fraction of clusters. [ABSTRACT FROM AUTHOR]

We present a detailed analysis from new multi-wavelength observations of the exceptional galaxy cluster ACT-CL J0102–4915, likely the most massive, hottest, most X-ray luminous and brightest Sunyaev-Zel'dovich (SZ) effect cluster known at redshifts greater than 0.6. The Atacama Cosmology Telescope (ACT) collaboration discovered ACT-CL J0102–4915 as the most significant SZ decrement in a sky survey area of 755 deg2. Our Very Large Telescope (VLT)/FORS2 spectra of 89 member galaxies yield a cluster redshift, z = 0.870, and velocity dispersion, σgal = 1321 ± 106 km s–1. Our Chandra observations reveal a hot and X-ray luminous system with an integrated temperature of TX = 14.5 ± 0.1 keV and 0.5-2.0 keV band luminosity of LX = (2.19 ± 0.11) × 1045h–270 erg s–1. We obtain several statistically consistent cluster mass estimates; using empirical mass scaling relations with velocity dispersion, X-ray YX, and integrated SZ distortion, we estimate a cluster mass of M200a = (2.16 ± 0.32) × 1015h–170M☼. We constrain the stellar content of the cluster to be less than 1% of the total mass, using Spitzer IRAC and optical imaging. The Chandra and VLT/FORS2 optical data also reveal that ACT-CL J0102–4915 is undergoing a major merger between components with a mass ratio of approximately 2 to 1. The X-ray data show significant temperature variations from a low of 6.6 ± 0.7 keV at the merging low-entropy, high-metallicity, cool core to a high of 22 ± 6 keV. We also see a wake in the X-ray surface brightness and deprojected gas density caused by the passage of one cluster through the other. Archival radio data at 843 MHz reveal diffuse radio emission that, if associated with the cluster, indicates the presence of an intense double radio relic, hosted by the highest redshift cluster yet. ACT-CL J0102–4915 is possibly a high-redshift analog of the famous Bullet cluster. Such a massive cluster at this redshift is rare, although consistent with the standard ΛCDM cosmology in the lower part of its allowed mass range. Massive, high-redshift mergers like ACT-CL J0102–4915 are unlikely to be reproduced in the current generation of numerical N-body cosmological simulations. [ABSTRACT FROM AUTHOR]