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Title:
Using velocity dispersion to estimate halo mass: Is the Local Group in tension with LambdaCDM?
Authors:
Elahi, Pascal J.; Power, Chris; Lagos, Claudia del P.; Poulton, Rhys; Robotham, Aaron S. G.
Affiliation:
AA(International Centre for Radio Astronomy Research, University of Western Australia, 35 Stirling Highway, Crawley, WA 6009, Australia; ARC Centre of Excellence for All Sky Astrophysics in 3 Dimensions (ASTRO 3D) 0000-0002-6154-7224), AB(International Centre for Radio Astronomy Research, University of Western Australia, 35 Stirling Highway, Crawley, WA 6009, Australia 0000-0002-4003-0904), AC(International Centre for Radio Astronomy Research, University of Western Australia, 35 Stirling Highway, Crawley, WA 6009, Australia 0000-0003-3021-8564), AD(International Centre for Radio Astronomy Research, University of Western Australia, 35 Stirling Highway, Crawley, WA 6009, Australia), AE(International Centre for Radio Astronomy Research, University of Western Australia, 35 Stirling Highway, Crawley, WA 6009, Australia 0000-0003-0429-3579)
Publication:
Monthly Notices of the Royal Astronomical Society, Volume 477, Issue 1, p.616-623 (MNRAS Homepage)
Publication Date:
06/2018
Origin:
OUP
Astronomy Keywords:
methods: numerical, Galaxy: halo, galaxies: haloes, dark matter
Abstract Copyright:
2018 The Author(s) Published by Oxford University Press on behalf of the Royal Astronomical Society
DOI:
10.1093/mnras/sty590
Bibliographic Code:
2018MNRAS.477..616E

Abstract

Satellite galaxies are commonly used as tracers to measure the line-of-sight (LOS)velocity dispersion (sigmaLOS) of the dark matter halo associated with their central galaxy, and thereby to estimate the halo's mass. Recent observational dispersion estimates of the Local Group, including the Milky Way and M31, suggest sigma ˜50 km s-1, which is surprisingly low when compared to the theoretical expectation of sigma ˜100 km s-1 for systems of their mass. Does this pose a problem for Lambda cold dark matter (LambdaCDM)? We explore this tension using the SURFS suite of N-body simulations, containing over 10000 (sub)haloes with well tracked orbits. We test how well a central galaxy's host halo velocity dispersion can be recovered by sampling sigmaLOS of subhaloes and surrounding haloes. Our results demonstrate that sigmaLOS is biased mass proxy. We define an optimal window in vLOS and projected distance (Dp) - 0.5 ≲ Dp/Rvir ≲ 1.0 and vLOS ≲ 0.5Vesc, where Rvir is the virial radius and Vesc is the escape velocity - such that the scatter in LOS to halo dispersion is minimized - sigmaLOS = (0.5 ± 0.1)sigmav, H. We argue that this window should be used to measure LOS dispersions as a proxy for mass, as it minimises scatter in the sigmaLOS-Mvir relation. This bias also naturally explains the results from McConnachie (2012), who used similar cuts when estimating sigmaLOS, LG, producing a bias of sigmaLG = (0.44 ± 0.14)sigmav, H. We conclude that the Local Group's velocity dispersion does not pose a problem for LambdaCDM and has a mass of log M_{LG, vir}/M_{&sun;}=12.0^{+0.8}_{-2.0}.
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