An analysis of galaxy cluster mis-centring using cosmological hydrodynamic simulations

Abstract

The location of a galaxy cluster's centroid is typically derived from observations of the galactic and/or gas component of the cluster, but these typically deviate from the true centre. This can produce bias when observations are combined to study average cluster properties. Using data from the BAHAMAS cosmological hydrodynamic simulations we study this bias in both two and three dimensions for 2000 clusters over the 10 13 − 10 15 M mass range. We quantify and model the offset distributions between observationally-motivated centres and the 'true' centre of the cluster, which is taken to be the most gravitationally bound particle measured in the simulation. We fit the cumulative distribution function of offsets with an exponential distribution and a Gamma distribution fit well with most of the centroid definitions. The galaxy-based centres can be seen to be divided into a mis-centred group and a well-centred group, with the well-centred group making up about 60% of all the clusters. Gas-based centres are overall less scattered than galaxy-based centres. We also find a cluster-mass dependence of the offset distribution of gas-based centres, with generally larger offsets for smaller mass clusters. We then measure cluster density profiles centred at each choice of the centres and fit them with empirical models. Stacked, mis-centred density profiles fit to the Navarro-Frenk-White dark-matter profile and Komatsu-Seljak gas profile show that recovered shape and size parameters can significantly deviate from the true values. For the galaxy-based centres, this can lead to cluster masses being underestimated by up to 10%. of galaxy cluster science is the construction of cluster catalogues, which involves cluster identification and the measurement of basic features such as size, profile, redshift, morphology and mass. A crucial element of these measurements that is not often discussed is the definition of cluster centres.Deconstruction of the cluster weak-lensing signal is a well developed and often used technique that allows us to study these clusters, most notably to recover their mass. However, in the process of this measurement, several assumptions about the shape and structure of the cluster have to be made. One of these is the choice of selecting a centre for the cluster. Traditionally, the definition for the 'true' centre of a galaxy cluster is taken to be the deepest point of the gravitational potential well, i.e. where a test particle is most bounded to the system. However, calculating this requires knowing the mass distribution for the cluster in at least 2D (and ideally 3D including redshift) in the first place, which is difficult to do using the lensing signal alone. The work-around that has been employed

A hydrodynamical halo model for weak-lensing cross correlations

“…The KS profile is the correct model for undisturbed gas that is in hydrostatic equilibrium in an NFW halo, but this state will clearly be violated in the presence of AGN feedback. It was shown in Yan et al (2020) that the KS model could provide a reasonable match to gas-density profiles in the bahamas simulations, but it is likely that the polytropic link between density and pressure is not respected in the presence of feedback (Battaglia et al 2012a,b). It is probable that a better model could be generated by either changing the gas profile, for example by explicitly including heated gas, or by weakening the links between the density and pressure modelling.…”

“…However, this would necessitate having reliable measurements of individual halo masses and would require modelling effects such as halo mis-centring (e.g. Yan et al 2020). It would also need to be ascertained whether the halo model can be trusted for calculations that are binned in halo mass and how problems with these calculations compare to problems in the standard when integrated over all halo masses.…”

Thermal energy census with the Sunyaev-Zel'dovich effect of DESI galaxy clusters/groups and its implication on the weak lensing power spectrum

“…In appendix.C, we discuss how the parameter 𝜂 mc would influence the results and find 𝜂 mc = 0.2 is an optimal choice. Further analysis may adopt more complicated and more realistic description of mis-centering (e.g Yan et al (2020)). With higher resolution CMB experiments such as ACT, SPT, and CMB-S4, the tSZ data alone will have constraining power for both 𝑐 and the mis-centering effect.…”

Galaxy cluster mass estimation with deep learning and hydrodynamical simulations

“…By comparing best-fitting mass with the true cluster mass, they find mass biases of M/M true = −8.9 +0.3 −0.2 per cent, and −6.4 +0.3 −0.2 per cent for the NFW and Einasto profile, respectively. Yan et al (2020) analyse the same BAHAMAS catalogue by fitting an NFW model to the density profile of all particles in a cluster and found a mean mass bias of −10 per cent. These studies are based on weak lensing profiles, which is an unbiased tracer of the cluster masses, while the present study uses biased tracers like galaxy or gas.…”

“…However, even with these simulations, the complexities of substructure, morphology, and small-scale physical processes, such as active galactic nuclei (AGN) feedback (Gitti, Brighenti & McNamara 2012) and gas clumping (Nagai & Lau 2011), hinder the accuracy of many cluster mass estimates. Yan et al (2020) used hydrodynamical simulations to assess the effect of cluster miscentring on mass determinations.…”