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High accuracy proper motions (PMs) of M31 and other Local Group satellites have now been provided by the {\it Gaia} satellite. We revisit the Timing Argument to compute the total mass $M$ of the Local Group from the orbit of the Milky Way…

Astrophysics of Galaxies · Physics 2022-03-23 David Benisty , Eugene Vasiliev , N. Wyn Evans , Anne-Christine Davis , Odelia V. Hartl , Louis E. Strigari

The classic model of the Local Group (LG) is that of two dominant constituents, the Milky Way and M31, first separating and then detaching from the Hubble flow, leading to a nearly radial approaching orbit. This simple model has been…

Cosmology and Nongalactic Astrophysics · Physics 2025-08-26 Louis E. Strigari

We use updated data on distances and velocities of galaxies in the proximity of the Local Group (LG) in order to establish properties of the local Hubble flow. For 30 neighbouring galaxies with distances 0.7 < D_LG < 3.0 Mpc, the Local flow…

Astrophysics · Physics 2009-11-13 I. D. Karachentsev , O. G. Kashibadze , D. I. Makarov , R. B. Tully

The positions and velocities of galaxies in the Local Group (LG) measure the gravitational field within it. This is mostly due to the Milky Way (MW) and Andromeda (M31). We constrain their masses using distance and radial velocity (RV)…

Astrophysics of Galaxies · Physics 2026-05-12 Indranil Banik , Hongsheng Zhao

The total mass of the Local Group (LG) and the masses of its primary constituents, the Milky Way and M31, are important anchors for several cosmological questions. In recent years, independent measurements have consistently yielded halo…

Astrophysics of Galaxies · Physics 2023-03-29 Till Sawala , Meri Teeriaho , Peter H. Johansson

The Local Group (LG) consists of two giant spiral galaxies, the Milky Way (MW) and Andromeda (M31), and several smaller galaxies. The MW and M31 are approaching each other at a radial velocity of about $-109\,$km\,s$^{-1}$. Observational…

Astrophysics of Galaxies · Physics 2023-06-30 Luis Biaus , Sebastián E. Nuza , Cecilia Scannapieco

We analyze the velocity field of peripheral members of the Local Group. The Hubble flow at distances from 400 to 1400~kpc, formed by 7 of 11 nearby galaxies, is characterized by an extremely small line-of-sight velocity dispersion of 15…

Astrophysics of Galaxies · Physics 2025-06-11 Danila Makarov , Dmitry Makarov , Lidia Makarova , Noam Libeskind

The Local Group (LG) is {an appropriate} test system for Modified Newtonian Dynamics, since the acceleration of M31 galaxy is fully in the deep MOND regime $a \ll a_0$. We model the LG as a two body problem of $M31$ and the Milky Way (MW)…

Cosmology and Nongalactic Astrophysics · Physics 2020-08-25 David Benisty , Eduardo I. Guendelman

In the rest frame of the Local Group (LG), the total momentum of the Milky Way (MW) and Andromeda (M31) should balance to zero. We use this fact to constrain new solutions for the solar motion with respect to the LG centre-of-mass, the…

Astrophysics of Galaxies · Physics 2015-06-19 Jonathan D. Diaz , Sergey E. Koposov , Mike Irwin , Vasily Belokurov , Wyn Evans

We use recent proper motion measurements of the tangential velocity of M31, along with its radial velocity and distance, to derive the likelihood of the sum of halo masses of the Milky Way and M31. This is done using a sample halo pairs in…

Cosmology and Nongalactic Astrophysics · Physics 2015-06-18 Roberto E. Gonzalez , Andrey V. Kravtsov , Nickolay Y. Gnedin

The kinematics of the Milky Way (MW) and M31, the dominant galaxies in the Local Group (LG), can be used to estimate the LG total mass. New results on the M31 proper motion have recently been used to improve that estimate. Those results are…

Cosmology and Nongalactic Astrophysics · Physics 2022-09-21 J. E. Forero-Romero , D. Sierra-Porta

We present a novel approach based on Bayesian field-level inference capable of resolving individual galaxies within the Local Group (LG), enabling detailed studies of its structure and formation via posterior simulations. We extend the…

Astrophysics of Galaxies · Physics 2024-11-27 Ewoud Wempe , Guilhem Lavaux , Simon D. M. White , Amina Helmi , Jens Jasche , Stephen Stopyra

This dynamical model for the 28 galaxies with distances less than 1.5 Mpc, and not apparently tight satellites, is constrained by the initial condition that peculiar velocities at high redshift are small and growing in accordance with the…

Cosmology and Nongalactic Astrophysics · Physics 2011-05-30 P. J. E. Peebles , R. Brent Tully , Edward J. Shaya

This paper explores the effect of the LMC on the mass estimates obtained from the timing argument. We show that accounting for the presence of the LMC systematically lowers the Local Group mass ($M_{\rm LG}$) derived from the relative…

Astrophysics of Galaxies · Physics 2015-12-02 Jorge Peñarrubia , Facundo A. Gómez , Gurtina Besla , Denis Erkal , Yin-Zhe Ma

I discuss Local Group galaxies from the perspective of external galaxies that define benchmark scaling relations. Making use of this information leads to a model for the Milky Way that includes bumps and wiggles due to spiral arms. This…

Astrophysics of Galaxies · Physics 2023-05-02 Stacy McGaugh

The total mass of the Local Group (LG) is a fundamental quantity that enables interpreting the orbits of its constituent galaxies and placing the LG in a cosmological context. One of the few methods that allows inferring the total mass…

We determine the velocity vector of M31 with respect to the Milky Way and use this to constrain the mass of the Local Group, based on HST proper-motion measurements presented in Paper I. We construct N-body models for M31 to correct the…

We study the dynamics of the Magellanic Clouds in a model for the Local Group whose mass is constrained using the timing argument/two-body limit of the action principle. The goal is to evaluate the role of M31 in generating the high angular…

Astrophysics of Galaxies · Physics 2009-07-22 Nitya Kallivayalil , Gurtina Besla , Robyn Sanderson , Charles Alcock

We attempt to fit the observed radial velocities (RVs) of ${\,{\sim}\,}30$ Local Group (LG) galaxies using a 3D dynamical model of it and its immediate environment within the context of the standard cosmological paradigm, $\Lambda$CDM. This…

Astrophysics of Galaxies · Physics 2019-02-05 Indranil Banik , Hongsheng Zhao

We present Hubble Space Telescope/WFPC2 images of eighteen galaxies situated in the vicinity of the Local Group (LG) as part of an ongoing snapshot survey of nearby galaxies. Their distances derived from the magnitude of the tip of the red…

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