Physical modelling of galaxy cluster Sunyaev-Zel'dovich data using Einasto dark matter profiles

ABSTRACT We derive a model for Sunyaev–Zel’dovich data from a galaxy cluster that uses an Einasto profile to model the cluster’s dark matter component. This model is similar to the physical models for clusters previously used by the Arcminute Microkelvin Imager (AMI) consortium, which model the dark matter using a Navarro–Frenk–White (NFW) profile, but the Einasto profile provides an extra degree of freedom. We thus present a comparison between two physical models which differ only in the way they model dark matter: one which uses an NFW profile (PM I) and one that uses an Einasto profile (PM II). We illustrate the differences between the models by plotting physical properties of clusters as a function of cluster radius. We generate AMI simulations of clusters that are created and analysed with both models. From this we find that for 14 of the 16 simulations, the Bayesian evidence gives no preference to either of the models according to the Jeffreys scale, and for the other two simulations, weak preference in favour of the correct model. However, for the mass estimates obtained from the analyses, the values were within 1σ of the input values for 14 out of 16 of the clusters when using the correct model, but only in 6 out of 16 cases when the incorrect model was used to analyse the data. Finally, we apply the models to real data from cluster A611 obtained with AMI, and find the mass estimates to be consistent with one another except in the case of when PM II is applied using an extreme value for the Einasto shape parameter.STFC

AMI SZ observation of galaxy-cluster merger CIZA J2242+5301: Perpendicular flows of gas and dark matter

© 2018 The Author(s). Arcminute Microkelvin Imager observations towards CIZA J2242+5301, in comparison with observations of weak gravitational lensing and X-ray emission from the literature, are used to investigate the behaviour of non-baryonic dark matter (NBDM) and gas during the merger. Analysis of the Sunyaev-Zel'dovich (SZ) signal indicates the presence of high pressure gas elongated perpendicularly to the X-ray and weak-lensing morphologies, which, given the merger-axis constraints in the literature, implies that high pressure gas is pushed out into a linear structure during core passing. Simulations in the literature closely matching the inferred merger scenario show the formation of gas density and temperature structures perpendicular to the merger axis. These SZ observations are challenging for modified gravity theories in which NBDM is not the dominant contributor to galaxy-cluster gravity.STFC (ST/M001172/1) STFC (ST/K00333X/1) STFC (ST/M007065/1) STFC (ST/H008586/1) STFC (ST/J005673/1

AMI observations of 10 CLASH galaxy clusters: SZ and X-ray data used together to determine cluster dynamical states

© 2016 The Authors. Published by Oxford University Press on behalf of The Royal Astronomical Society.Using Arcminute Microkelvin Imager (AMI) Sunyaev-Zel'dovich (SZ) observations towards 10 CLASH (Cluster Lensing and Supernova Survey with Hubble) clusters, we investigate the influence of cluster mergers on observational galaxy cluster studies. Although selected to be largely relaxed, there is disagreement in the literature on the dynamical states of CLASH sample members. We analyse ourAMIdata in a fully Bayesianway to produce estimated cluster parameters and consider the intrinsic correlations in our Navarro, Frenk and White/generalized Navarro, Frenk and White-based model. Varying pressure profile shape parameters, illustrating an influence of mergers on scaling relations, induces small deviations from the canonical selfsimilar predictions - in agreement with simulations of Poole et al. (2007) who found that merger activity causes only small scatter perpendicular to the relations. We demonstrate this effect observationally using the different dependences of SZ and X-ray signals to ne that cause different sensitivities to the shocking and/or fractionation produced by mergers. Plotting YX-Mgas relations (where YX = MgasT) derived from AMI SZ and from Chandra X-ray gives ratios of AMI and Chandra YX and Mgas estimates that indicate movement of clusters along the scaling relation, as predicted by Poole et al. (2007). Clusters that have moved most along the relation have the most discrepant TSZ and TX estimates: all the other clusters (apart from one) have SZ and X-ray estimates of Mgas, T and YX that agree within r500. We use SZ versus X-ray discrepancies in conjunction with Chandra maps and TX profiles, making comparisons with simulated cluster merger maps in Poole et al. (2006) to identify disturbed members of our sample and estimate merger stages.We thank the staff of the Mullard Radio Astronomy Observatory for their invaluable assistance in the commissioning and operation of AMI, which is supported by Cambridge University. WJH and CR are grateful for the support of STFC Studentships. CR also acknowledges the support of Cambridge University. MO and YCP acknowledge support from Research Fellowships from Sidney Sussex College and Trinity College, Cambridge, respectively. We thank Arif Babul for his assistance in accessing the Poole et al. online materials. Much of this work was undertaken on the COSMOS Shared Memory system at DAMTP, Cambridge University, operated on behalf of the STFC DiRAC HPC Facility. This equipment is funded by BIS National E-infrastructure capital grant ST/J005673/1 and STFC grants ST/H008586/1, ST/K00333X/1

Free-form modelling of galaxy clusters: A Bayesian and data-driven approach

A new method is presented for modelling the physical properties of galaxy clusters. Our technique moves away from the traditional approach of assuming specific parameterized functional forms for the variation of physical quantities within the cluster, and instead allows for a 'freeform' reconstruction, but one for which the level of complexity is determined automatically by the observational data and may depend on position within the cluster. This is achieved by representing each independent cluster property as some interpolating or approximating function that is specified by a set of control points, or 'nodes', for which the number of nodes, together with their positions and amplitudes, are allowed to vary and are inferred in a Bayesian manner from the data. We illustrate our nodal approach in the case of a spherical cluster by modelling the electron pressure profile P e (r) in analyses both of simulated Sunyaev-Zel'dovich (SZ) data from the Arcminute MicroKelvin Imager (AMI) and of real AMI observations of the cluster MACS J0744+3927 in the CLASH sample. We demonstrate that one may indeed determine the complexity supported by the data in the reconstructed P e (r), and that one may constrain two very important quantities in such an analysis: the cluster total volume integrated Comptonization parameter (Y tot ) and the extent of the gas distribution in the cluster (r max ). The approach is also well-suited to detecting clusters in blind SZ surveys, in the case where the population of radio sources is known in advance.This work was performed using both the Darwin Supercomputer of the University of Cambridge High Performance Computing Service (http://www.hpc.cam.ac.uk/), and COSMOS Shared Memory system at DAMTP, University of Cambridge operated on behalf of the STFC DiRAC HPC Facility. Darwin Supercomputer is provided by Dell Inc. using Strategic Research Infrastructure Funding from the Higher Education Funding Council for England and funding from the Science and Technology Facilities Council. COSMOS Shared Memory system is funded by BIS National E-infrastructure capital grant ST/J005673/1 and STFC grants ST/H008586/1, ST/K00333X/1 ... YCP acknowledges support from a Trinity College Junior Research Fellowship

Cosmic microwave background observations from the Cosmic Background Imager and Very Small Array: a comparison of coincident maps and parameter estimation methods

WWe present coincident observations of the cosmic microwave background (CMB) from the Very Small Array (VSA) and Cosmic Background Imager (CBI) telescopes. The consistency of the full data sets is tested in the map plane and the Fourier plane, prior to the usual compression of CMB data into flat bandpowers. Of the three mosaics observed by each group, two are found to be in excellent agreement. In the third mosaic, there is a 2σ discrepancy between the correlation of the data and the level expected from Monte Carlo simulations. This is shown to be consistent with increased phase calibration errors on VSA data during summer observations. We also consider the parameter estimation method of each group. The key difference is the use of the variance window function in place of the bandpower window function, an approximation used by the VSA group. A re-evaluation of the VSA parameter estimates, using bandpower windows, shows that the two methods yield consistent results

Sub-kpc radio jets in the brightest central galaxy of the cool-core galaxy cluster RXJ1720.1+2638

ABSTRACT The cool-core galaxy cluster RXJ1720.1+2638 hosts extended radio emission near the cluster core, known as a minihalo. The origin of this emission is still debated and one piece of the puzzle has been the question of whether the supermassive black hole in the brightest central galaxy is actively powering jets. Here, we present high-resolution e-MERLIN observations clearly indicating the presence of sub-kpc jets; this may have implications for the proposed origin of the minihalo emission, providing an ongoing source of relativistic electrons rather than a single burst sometime in the past, as previously assumed in simulations attempting to reproduce observational characteristics of minihalo-hosting systems.</jats:p

Sunyaev-Zel'dovich profile fitting with joint AMI-Planck analysis

We develop a Bayesian method of analysing Sunyaev-Zel'dovich measurements of galaxy clusters obtained from the Arcminute Microkelvin Imager (AMI) radio interferometer system and from the Planck satellite, using a joint likelihood function for the data from both instruments. Our method is applicable to any combination of Planck data with interferometric data from one or more arrays. We apply the analysis to simulated clusters and find that when the cluster pressure profile is known a-priori, the joint dataset provides precise and accurate constraints on the cluster parameters, removing the need for external information to reduce the parameter degeneracy. When the pressure profile deviates from that assumed for the fit, the constraints become biased. Allowing the pressure profile shape parameters to vary in the analysis allows an unbiased recovery of the integrated cluster signal and produces constraints on some shape parameters, depending on the angular size of the cluster. When applied to real data from Planck-detected cluster PSZ2 G063.80+11.42, our method resolves the discrepancy between the AMI and Planck $Y$-estimates and usefully constrains the gas pressure profile shape parameters at intermediate and large radii

A 15.5 GHz detection of the galaxy cluster minihalo in RXJ1720.1+2638

RXJ1720.1+2638 is a cool-core, 'relaxed-appearing' cluster with a minihalo previously detected up to 8.4 GHz, confined by X-ray-detected cold fronts. We present observations of the minihalo at 13 - 18 GHz with the Arcminute Microkelvin Imager telescope, simultaneously modelling the Sunyaev-Zel'dovich signal of the cluster in conjunction with Planck and Chandra data in order to disentangle the non-thermal emission of the minihalo. We show that the previously-reported steepening of the minihalo emission at 8.4 GHz is not supported by the AMI data and that the spectrum is consistent with a single power-law up to 18 GHz. We also show the presence of a larger-scale component of the minihalo extending beyond the cold fronts. Both of these observations could be explained by the 'hadronic' or 'secondary' mechanism for the production of relativistic electrons, rather than the currently-favoured 're-acceleration' mechanism and/or multiple episodes of jet activity from the active galactic nucleus in the brightest cluster galaxy

A digital correlator upgrade for the Arcminute MicroKelvin Imager

The Arcminute Microkelvin Imager (AMI) telescopes located at the Mullard Radio Astronomy Observatory near Cambridge have been significantly enhanced by the implementation of a new digital correlator with 1.2 MHz spectral resolution. This system has replaced a 750-MHz resolution analogue lag-based correlator, and was designed to mitigate the effects of radio frequency interference, particularly from geostationary satellites that contaminate observations at low declinations. The upgraded instrument consists of 18 ROACH2 Field Programmable Gate Array platforms used to implement a pair of real-time FX correlators -- one for each of AMI's two arrays. The new system separates the down-converted RF baseband signal from each AMI receiver into two 2.3 GHz-wide sub-bands which are each digitized at 5-Gsps with 8 bits of precision. These digital data streams are filtered into 2048 frequency channels and cross-correlated using FPGA hardware, with a commercial 10 Gb Ethernet switch providing high-speed data interconnect. Images formed using data from the new digital correlator show over an order of magnitude improvement in dynamic range over the previous system. The ability to observe at low declinations has also been significantly improved