Using velocity dispersion to estimate halo mass: Is the Local Group in tension with ΛCDM?
Abstract
Satellite galaxies are commonly used as tracers to measure the lineofsight (LOS)velocity dispersion (σ_{LOS}) 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 σ ∼50 km s^{1}, which is surprisingly low when compared to the theoretical expectation of σ ∼100 km s^{1} for systems of their mass. Does this pose a problem for Lambda cold dark matter (ΛCDM)? We explore this tension using the SURFS suite of Nbody 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 σ_{LOS} of subhaloes and surrounding haloes. Our results demonstrate that σ_{LOS} is biased mass proxy. We define an optimal window in v_{LOS} and projected distance (D_{p})  0.5 ≲ D_{p}/R_{vir} ≲ 1.0 and v_{LOS} ≲ 0.5V_{esc}, where R_{vir} is the virial radius and V_{esc} is the escape velocity  such that the scatter in LOS to halo dispersion is minimized  σ_{LOS} = (0.5 ± 0.1)σ_{v, H}. We argue that this window should be used to measure LOS dispersions as a proxy for mass, as it minimises scatter in the σ_{LOS}M_{vir} relation. This bias also naturally explains the results from McConnachie (2012), who used similar cuts when estimating σ_{LOS, LG}, producing a bias of σ_{LG} = (0.44 ± 0.14)σ_{v, H}. We conclude that the Local Group's velocity dispersion does not pose a problem for ΛCDM and has a mass of log M_{LG, vir}/M_{⊙}=12.0^{+0.8}_{2.0}.
 Publication:

Monthly Notices of the Royal Astronomical Society
 Pub Date:
 June 2018
 DOI:
 10.1093/mnras/sty590
 arXiv:
 arXiv:1712.01989
 Bibcode:
 2018MNRAS.477..616E
 Keywords:

 methods: numerical;
 Galaxy: halo;
 galaxies: haloes;
 dark matter;
 Astrophysics  Astrophysics of Galaxies;
 Astrophysics  Cosmology and Nongalactic Astrophysics
 EPrint:
 8 pages, 7 figures, accepted for publication