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This article has been accepted for publication in Monthly Notices of the Royal Astronomical Society ©: 2012 RAS © The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved, We present a detailed comparison of the substructure properties of a single Milky Way sized dark matter halo from the Aquarius suite at five different resolutions, as identified by a variety of different (sub)halo finders for simulations of cosmic structure formation. These finders span a wide range of techniques and methodologies to extract and quantify substructures within a larger non-homogeneous background density (e.g. a host halo). This includes real-space-, phase-space-, velocity-space- and time-space-based finders, as well as finders employing a Voronoi tessellation, Friends-of-Friends techniques or refined meshes as the starting point for locating substructure. A common post-processing pipeline was used to uniformly analyse the particle lists provided by each finder. We extract quantitative and comparable measures for the subhaloes, primarily focusing on mass and the peak of the rotation curve for this particular study. We find that all of the finders agree extremely well in the presence and location of substructure and even for properties relating to the inner part of the subhalo (e.g. the maximum value of the rotation curve). For properties that rely on particles near the outer edge of the subhalo the agreement is at around the 20per cent level. We find that the basic properties (mass and maximum circular velocity) of a subhalo can be reliably recovered if the subhalo contains more than 100 particles although its presence can be reliably inferred for a lower particle number limit of 20. We finally note that the logarithmic slope of the subhalo cumulative number count is remarkably consistent and, AK is supported by the Spanish Ministerio de Ciencia e Innovación (MICINN) in Spain through the Ramon y Cajal programme as well as the grants AYA 2009-13875-C03-02, AYA2009-12792-C03-03, CSD2009-00064 and CAM S2009/ESP-1496. HL acknowledges a fellowship from the European Commission’s Framework Programme 7, through the Marie Curie Initial Training Network CosmoComp (PITN-GA-2009-238356). MNacknowledges support through Alex Szalay from the Gordon and Betty Moore Foundation. YA is also supported by the Ramon y Cajal programme as well as grant AYA 2010-21887-C04-03. JXH is supported by the European Commissions Framework Programme 7, through the Marie Curie Initial Training Network Cosmo-Comp (PITNGA-2009-238356) and partially supported by NSFC (10878001,11033006, 11121062) and the CAS/SAFEA International Partnership Programme for Creative Research Teams (KJCX2-YW-T23). VS acknowledges partial support by SFB 881 ‘The Milky Way System’ of the DFG. PE acknowledges financial support from the Chinese Academy of Sciences (CAS), from NSFC grants (No. 11121062, 10878001, 11033006), and by the CAS/SAFEA International Partnership Programme for Creative Research Teams (KJCX2-YW-T23).

This article has been accepted for publication in Monthly Notices of the Royal Astronomical Society © 2011 RAS © 2011 The authors Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved, We present a detailed comparison of fundamental dark matter halo properties retrieved by a substantial number of different halo finders. These codes span a wide range of techniques including friends-of-friends, spherical-overdensity and phase-space-based algorithms. We further introduce a robust (and publicly available) suite of test scenarios that allow halo finder developers to compare the performance of their codes against those presented here. This set includes mock haloes containing various levels and distributions of substructure at a range of resolutions as well as a cosmological simulation of the large-scale structure of the universe. All the halo-finding codes tested could successfully recover the spatial location of our mock haloes. They further returned lists of particles (potentially) belonging to the object that led to coinciding values for the maximum of the circular velocity profile and the radius where it is reached. All the finders based in configuration space struggled to recover substructure that was located close to the centre of the host halo, and the radial dependence of the mass recovered varies from finder to finder. Those finders based in phase space could resolve central substructure although they found difficulties in accurately recovering its properties. Through a resolution study we found that most of the finders could not reliably recover substructure containing fewer than 30-40 particles. However, also here the phase-space finders excelled by resolving substructure down to 10-20 particles. By comparing the halo finders using a high-resolution cosmological volume, we found that they agree remarkably well on fundamental properties of astrophysical significance (e.g. mass, position, velocity and peak of the rotation curve). We further suggest to utilize the peak of the rotation curve, vmax, as a proxy for mass, given the arbitrariness in defining a proper halo edge, We are greatly indebted to the ASTROSIM network of the European Science Foundation (Science Meeting 2910) for financially supporting the workshop ‘Haloes going MAD’ held in Miraflores de la Sierra near Madrid in 2010 May where all of this work was initiated.AK is supported by the Spanish Ministerio de Ciencia e Innovación (MICINN) in Spain through the Ramón y Cajal programme as well as the grants AYA 2009-13875-C03-02, AYA2009- 12792-C03-03, CSD2009-00064 and CAM S2009/ESP-1496. SRK acknowledges the support by the MICINN under the Consolider-Ingenio, SyeC project CSD-2007-00050. SP and VQ have also been supported by the MICINN (grants AYA2010-21322-C03-01 and CONSOLIDER2007-00050) and the Generalitat Valenciana (grant PROMETEO-2009-103). MZ is supported by NSF grant AST-0708087. GY acknowledges financial support from MICINN (Spain) under project AYA 2009-13875-C03-02 and the ASTROMADRID project S2009/ESP-1496 financed by Comunidad de Madrid

We present a study of the substructure finder dependence of subhalo clustering in the Aquarius Simulation. We run 11 different subhalo finders on the haloes of the Aquarius Simulation and we study their differences in the density profile, mass fraction and 2-point correlation function of subhaloes in haloes. We also study the mass and vmax dependence of subhalo clustering. As the Aquarius Simulation has been run at different resolutions, we study the convergence with higher resolutions. We find that the agreement between finders is at around the 10% level inside R200 and at intermediate resolutions when a mass threshold is applied, and better than 5% when vmax is restricted instead of mass. However, some discrepancies appear in the highest resolution, underlined by an observed resolution dependence of subhalo clustering. This dependence is stronger for the smallest subhaloes, which are more clustered in the highest resolution, due to the detection of subhaloes within subhaloes (the sub-subhalo term). This effect modifies the mass dependence of clustering in the highest resolutions. We discuss implications of our results for models of subhalo clustering and their relation with galaxy clustering., Comment: 19 pages, 15 figures