# Using velocity dispersion to estimate halo mass: Is the Local Group in tension with ΛCDM?

Research output: Contribution to journalArticle

### Abstract

Satellite galaxies are commonly used as tracers to measure the line-of-sight (LOS)velocity dispersion (σLOS) 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 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 σLOS of subhaloes and surrounding haloes. Our results demonstrate that σLOS 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 – σLOS = (0.5 ± 0.1)σv, H. We argue that this window should be used to measure LOS dispersions as a proxy for mass, as it minimises scatter in the σLOS–Mvir relation. This bias also naturally explains the results from McConnachie (2012), who used similar cuts when estimating σLOS, LG, producing a bias of σLG = (0.44 ± 0.14)σv, H. We conclude that the Local Group's velocity dispersion does not pose a problem for ΛCDM and has a mass of $$\log M_{\rm LG, vir}/\mathrm{M}_{\odot }=12.0^{+0.8}_{-2.0}$$.
Language English 616-623 8 Monthly Notices of the Royal Astronomical Society 477 1 10.1093/mnras/sty590 Published - 11 Jun 2018

halos
estimates
line of sight
galaxies
dark matter
escape velocity
group velocity
tracers
estimating
tracer
sampling
orbits
simulation

### Cite this

@article{7229f7078aa44e2785fedd347b7a35fa,
title = "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 line-of-sight (LOS)velocity dispersion (σLOS) 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 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 σLOS of subhaloes and surrounding haloes. Our results demonstrate that σLOS 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 – σLOS = (0.5 ± 0.1)σv, H. We argue that this window should be used to measure LOS dispersions as a proxy for mass, as it minimises scatter in the σLOS–Mvir relation. This bias also naturally explains the results from McConnachie (2012), who used similar cuts when estimating σLOS, LG, producing a bias of σLG = (0.44 ± 0.14)σv, H. We conclude that the Local Group's velocity dispersion does not pose a problem for ΛCDM and has a mass of $$\log M_{\rm LG, vir}/\mathrm{M}_{\odot }=12.0^{+0.8}_{-2.0}$$.",
author = "P.J. Elahi and C. Power and C. Lagos and R. Poulton and A. Robotham",
note = "10.1093/mnras/sty590",
year = "2018",
month = "6",
day = "11",
doi = "10.1093/mnras/sty590",
language = "English",
volume = "477",
pages = "616--623",
journal = "Monthly Notices of the Royal Astronomical Society",
issn = "0035-8711",
publisher = "OXFORD UNIV PRESS UNITED KINGDOM",
number = "1",

}

In: Monthly Notices of the Royal Astronomical Society, Vol. 477, No. 1, 11.06.2018, p. 616-623.

Research output: Contribution to journalArticle

TY - JOUR

T1 - Using velocity dispersion to estimate halo mass: Is the Local Group in tension with ΛCDM?

AU - Elahi,P.J.

AU - Power,C.

AU - Lagos,C.

AU - Poulton,R.

AU - Robotham,A.

N1 - 10.1093/mnras/sty590

PY - 2018/6/11

Y1 - 2018/6/11

N2 - Satellite galaxies are commonly used as tracers to measure the line-of-sight (LOS)velocity dispersion (σLOS) 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 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 σLOS of subhaloes and surrounding haloes. Our results demonstrate that σLOS 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 – σLOS = (0.5 ± 0.1)σv, H. We argue that this window should be used to measure LOS dispersions as a proxy for mass, as it minimises scatter in the σLOS–Mvir relation. This bias also naturally explains the results from McConnachie (2012), who used similar cuts when estimating σLOS, LG, producing a bias of σLG = (0.44 ± 0.14)σv, H. We conclude that the Local Group's velocity dispersion does not pose a problem for ΛCDM and has a mass of $$\log M_{\rm LG, vir}/\mathrm{M}_{\odot }=12.0^{+0.8}_{-2.0}$$.

AB - Satellite galaxies are commonly used as tracers to measure the line-of-sight (LOS)velocity dispersion (σLOS) 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 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 σLOS of subhaloes and surrounding haloes. Our results demonstrate that σLOS 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 – σLOS = (0.5 ± 0.1)σv, H. We argue that this window should be used to measure LOS dispersions as a proxy for mass, as it minimises scatter in the σLOS–Mvir relation. This bias also naturally explains the results from McConnachie (2012), who used similar cuts when estimating σLOS, LG, producing a bias of σLG = (0.44 ± 0.14)σv, H. We conclude that the Local Group's velocity dispersion does not pose a problem for ΛCDM and has a mass of $$\log M_{\rm LG, vir}/\mathrm{M}_{\odot }=12.0^{+0.8}_{-2.0}$$.

U2 - 10.1093/mnras/sty590

DO - 10.1093/mnras/sty590

M3 - Article

VL - 477

SP - 616

EP - 623

JO - Monthly Notices of the Royal Astronomical Society

T2 - Monthly Notices of the Royal Astronomical Society

JF - Monthly Notices of the Royal Astronomical Society

SN - 0035-8711

IS - 1

ER -