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Within the ΛCDM cosmology, dark matter haloes are composed of both a smooth component and a population of smaller gravitationally bound subhaloes. These components are often treated as a single halo when properties, such as density profiles, are extracted from simulations. Recent work has shown that density profiles change substantially when subhalo mass is excluded. In this paper, we expand on this result by analysing three specific host halo properties – concentration (cNFW), spin (λB), and shape (c/a) – when calculated only from the smooth component of the halo. This analysis is performed on both Milky Way-mass haloes and cluster-mass haloes in high-resolution zoom-in N-body simulations. We find that when subhaloes are excluded, the median value of (1) cNFW is enhanced by ≈ 30 ± 11 and ≈ 77 ± 8.1 per cent for Milky Way-mass (1012.1M) and cluster-mass (1014.8M) haloes, respectively, (2) λB is reduced for Milky Way-mass by ≈ 11 ± 4.9 per cent and cluster-mass haloes by ≈ 27 ± 3.5 per cent. Additionally, with the removal of subhaloes, cluster-mass haloes tend to become more spherical as the ratio of minor-to-major axis, c/a, increases by ≈ 11 ± 3.6 per cent, whereas Milky Way-mass haloes remain approximately the same shape with c/a changed by ≈ 1.0 ± 5.8 per cent. Fractional changes of each of these properties depend primarily on the amount of mass in subhaloes and, to a lesser extent, mass accretion history. Our findings demonstrate that the properties of the smooth components of dark matter haloes are biased relative to the total halo mass.

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This article has been accepted for publication in Monthly Notices of the Royal Astronomical Society ©: 2023 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved.

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