Abstract
ABSTRACT If properly calibrated, the shapes of galaxy clusters can be used to investigate many physical processes: from feedback and quenching of star formation, to the nature of dark matter. Theorists frequently measure shapes using moments of inertia of simulated particles’. We instead create mock (optical, X-ray, strong-, and weak-lensing) observations of the 22 most massive ($\sim 10^{14.7}\, \mathrm{ M}_\odot$) relaxed clusters in the BAHAMAS simulations. We find that observable measures of shape are rounder. Even when moments of inertia are projected into 2D and evaluated at matched radius, they overestimate ellipticity by 56 per cent (compared to observable strong lensing) and 430 per cent (compared to observable weak lensing). Therefore, we propose matchable quantities and test them using observations of eight relaxed clusters from the Hubble Space Telescope (HST) and Chandra X-Ray Observatory. We also release our HST data reduction and lensing analysis software to the community. In real clusters, the ellipticity and orientation angle at all radii are strongly correlated. In simulated clusters, the ellipticity of inner (<rvir/20) regions becomes decoupled: for example, with greater misalignment of the central cluster galaxy. This may indicate overly efficient implementation of feedback from active galactic nuclei. Future exploitation of cluster shapes as a function of radii will require better understanding of core baryonic processes. Exploitation of shapes on any scale will require calibration on simulations extended all the way to mock observations.
Highlights
The Lambda cold dark matter ( CDM) concordant model of cosmology assumes that we are living in a Universe dominated by an unknown dark energy, accelerating the expansion of space– time and permeated by a dominant gravitating mass that we do not understand, dark matter (Parkinson et al 2012; Kilbinger et al 2013; Planck Collaboration XVI 2014; Hildebrandt et al 2017; Abbott et al 2018)
The top panel shows the ellipticity from four estimators as a function of mass, they include: (1) Pink, the 2D ellipticity calculated from the projected moment of inertia (MI) measured on all particles within a mean radius at which the mock strong lensing is measured, (2) Cyan, the same ellipticity calculated from the moment of inertia using all particles within the outer most radius at which the mock weak lensing is measured (i.e. r < 0.4rvir), (3) Orange, the ellipticity of the cluster scale halo estimated by mock strong lensing observations, and (4)
We have carried out an investigation into the shape of eight dynamically relaxed galaxy clusters using a combination the Hubble Space Telescope (HST) and the Chandra X-Ray Observatory data (CXO) and compare them to the 22 most massive clusters in the BAHAMAS simulations in a bid to answer two key questions: (1) Is the ellipticity calculated from the projected moment of inertia derived directly from the particle data in simulations a good estimator of the shape derived from strong or weak lensing? and (2) Is there any evidence for a radial dependent ellipticity in galaxy clusters, potentially signalling physics at different scales?
Summary
The Lambda cold dark matter ( CDM) concordant model of cosmology assumes that we are living in a Universe dominated by an unknown dark energy, accelerating the expansion of space– time and permeated by a dominant gravitating mass that we do not understand, dark matter (Parkinson et al 2012; Kilbinger et al 2013; Planck Collaboration XVI 2014; Hildebrandt et al 2017; Abbott et al 2018) In this framework, cosmological simulations predict that structure should form hierarchially, with the smallest haloes forming first (Springel et al 2001; Sanchez et al 2012; Vogelsberger et al 2014; Schaye et al 2015; McCarthy et al 2017; Nelson et al 2018). As extreme peaks in the density field, clusters of galaxies are ideal laboratories to study dark matter (e.g. Kahlhoefer et al 2014; Harvey et al 2017a,b, 2019; Schwinn et al 2017, 2018; Robertson et al 2019; Banerjee et al 2020) and constrain cosmology (e.g. Kratochvil, Haiman & May 2010; Marian et al 2011; Cardone et al 2013)
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