Interloper treatment in dynamical modelling of galaxy clusters

Wojtak, Radosław; Łokas, Ewa L.; Mamon, G. A.; Gottlöber, Stefan; Prada, Francisco; Moles, M.

doi:10.1051/0004-6361:20066813

Cited by 82 publications

(117 citation statements)

References 43 publications

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“…These fractions are consistent with the values ofκ one can read off of Fig. 3 of Wojtak et al (2007) that illustrates the same velocity cut model (den Hartog & Katgert 1996). We then considered a cone of projected size 1.5/0.93 = 1.6 r 200 .…”

Section: Summary and Discussionsupporting

confidence: 79%

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The universal distribution of halo interlopers in projected phase space

Mamon

Biviano

Murante

2010

A&A

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106

149

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When clusters of galaxies are viewed in projection, one cannot avoid picking up a fraction of foreground/background interlopers, that lie within the virial cone, but outside the virial sphere. Structural and kinematic deprojection equations are known for the academic case of a static Universe, but not for the real case of an expanding Universe, where the Hubble flow (HF) stretches the line-of-sight distribution of velocities. Using 93 mock relaxed clusters, built from the dark matter (DM) particles of a hydrodynamical cosmological simulation, we quantify the distribution of interlopers in projected phase space (PPS), as well as the biases in the radial and kinematical structure of clusters produced by the HF. The stacked mock clusters are well fit by an m = 5 Einasto DM density profile (but only out to 1.5 virial radii), with velocity anisotropy (VA) close to the Mamon-Łokas model with characteristic radius equal to that of density slope −2. The surface density of interlopers is nearly flat out to the virial radius, while their velocity distribution shows a dominant Gaussian cluster-outskirts component and a flat field component. This distribution of interlopers in PPS is nearly universal, showing only small trends with cluster mass, and is quantified. A local κ = 2.7 sigma velocity cut is found to return the line-ofsight velocity dispersion profile (LOSVDP) expected from the NFW density and VA profiles measured in three dimensions. The HF causes a shallower outer LOSVDP that cannot be well matched by the Einasto model for any value of κ. After this velocity cut, which removes 1 interloper out of 6, interlopers still account for 23 ± 1% of all DM particles with projected radii within the virial radius (surprisingly very similar to the observed fraction of cluster galaxies lying off the Red Sequence) and over 60% between 0.8 and 1 virial radius. The HF causes the best-fit projected NFW or m = 5 Einasto model to the stacked cluster to underestimate the true concentration measured in 3D by 6 ± 6% (16 ± 7%) after (before) the velocity cut. These biases in concentration are reduced by over a factor two once a constant background is included in the fit. The VA profile recovered from the measured LOSVDP by assuming the correct mass profile recovers fairly well the VA measured in 3D, with a slight, marginally significant, bias towards more radial orbits in the outer regions. These small biases in the concentration and VA of the galaxy system are overshadowed by important cluster-to-cluster fluctuations caused by cosmic variance and by the strong inefficiency caused by the limited numbers of observed galaxies in clusters. An appendix provides an analytical approximation to the surface density, projected mass and tangential shear profiles of the Einasto model. Another derives the expressions for the surface density and mass profiles of the NFW model projected on the sphere (for future kinematic modeling).

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Section: Summary and Discussionsupporting

confidence: 79%

“…One can add interlopers beyond the virial radius as a separate component to the kinematical modeling (van der Marel et al 2000;Wojtak et al 2007). Unfortunately, we have no knowledge of the distribution of interlopers in projected phase space (projected distance to the halo center and line-of-sight velocity).…”

Section: Introductionmentioning

confidence: 99%

The universal distribution of halo interlopers in projected phase space

Mamon

Biviano

Murante

2010

A&A

Self Cite

106

149

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“…Wojtak et al 2007). The algorithm is similar to the one proposed by Fadda et al (1996) and used by SG1 and SG2.…”

Section: Num (Mamon; Phase Space Richness)mentioning

confidence: 99%

“…Though these techniques generate an impression of which galaxies are associated with a cluster, deducing which galaxies are true members of the cluster is often problematic due to interloping galaxies. These interlopers are close to but not gravitationally bound to the cluster and their inclusion can lead to strong bias in velocity dispersion based mass estimates (e.g., Lucey 1983;Borgani et al 1997;Cen 1997; Biviano et al 2006;Wojtak et al 2007; Mamon et al 2010). …”

Section: Introductionmentioning

confidence: 99%

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Galaxy Cluster Mass Reconstruction Project – II. Quantifying scatter and bias using contrasting mock catalogues

Old

Wojtak

Mamon

et al. 2015

Monthly Notices of the Royal Astronomical Society

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This article is the second in a series in which we perform an extensive comparison of various galaxy-based cluster mass estimation techniques that utilise the positions, velocities and colours of galaxies. Our aim is to quantify the scatter, systematic bias and completeness of cluster masses derived from a diverse set of 25 galaxy-based methods using two contrasting mock galaxy catalogues based on a sophisticated halo occupation model and a semi-analytic model. Analysing 968 clusters, we find a wide range in the RMS errors in log M 200c delivered by the different methods (0.18 to 1.08 dex, i.e., a factor of ∼1.5 to 12), with abundance matching and richness methods providing the best results, irrespective of the input model assumptions. In addition, certain methods produce a significant number of catastrophic cases where the mass is under-or over-estimated by a factor greater than 10. Given the steeply falling high-mass end of the cluster mass function, we recommend that richness or abundance matching-based methods are used in conjunction with these methods as a sanity check for studies selecting high mass clusters. We see a stronger correlation of the recovered to input number of galaxies for both catalogues in comparison with the group/cluster mass, however, this does not guarantee that the correct member galaxies are being selected. We do not observe significantly higher scatter for either mock galaxy catalogues. Our results have implications for cosmological analyses that utilise the masses, richnesses, or abundances of clusters, which have different uncertainties when different methods are used.

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The ART of Cosmological Simulations

Gottlöber

Klypin

High Performance Computing in Science and Engineering, Garching/Munich 2007

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We describe the basic ideas of MPI parallelization of the N-body Adaptive Refinement Tree (ART) code. The code uses self-adaptive domain decomposition where boundaries of the domains (parallelepipeds) constantly move -with many degrees of freedom -in the search of the minimum of CPU time. The actual CPU time spent by each MPI task on a previous time-step is used to adjust boundaries for the next time-step. For a typical decomposition of 5 3 domains, the number of possible changes in boundaries is 3 84 ≈ 10 40 . We describe two algorithms of finding minimum of CPU time for configurations with a large number of domains. Each MPI task in our code solves the N-body problem where the large-scale distribution of matter outside of the boundaries of a domain is represented by relatively few temporary large particles created by other domains. At the beginning of a zerolevel time-step, domains create and exchange large particles. Then each domain advances all its particles for many small time-steps. At the end of the large step, the domains decide where to place new boundaries and re-distribute particles. The scheme requires little communications between processors and is very efficient for large cosmological simulations.

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Interloper treatment in dynamical modelling of galaxy clusters

Cited by 82 publications

References 43 publications

The universal distribution of halo interlopers in projected phase space

The universal distribution of halo interlopers in projected phase space

Galaxy Cluster Mass Reconstruction Project – II. Quantifying scatter and bias using contrasting mock catalogues

The ART of Cosmological Simulations

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