Milky Way and Andromeda past-encounters in different gravity models: the impact on the estimated Local Group mass. (arXiv:1904.03153v1 [astro-ph.GA])
<a href="http://arxiv.org/find/astro-ph/1/au:+Benisty_D/0/1/0/all/0/1">David Benisty</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Guendelman_E/0/1/0/all/0/1">Eduardo I. Guendelman</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Lahav_O/0/1/0/all/0/1">Ofer Lahav</a>
The two-body problem of $M31$ and the Milky Way (MW) galaxies with a
cosmological constant background is studied, with emphasis on the possibility
that they experienced past encounters. By implementing the initial conditions
of the big bang and the last measured relative distance and velocities (i.e.
the Timing Argument), it is shown that if $M{31}$ and the MW had more than one
encounter then the mass of the Local Group (LG) would be a few times higher
than if there had been no encounters. Past encounters are possible only for
non-zero transverse velocity, and their viability is subject to observations of
the imprints of such near collisions. While it has been previously shown that
the presence of the cosmological constant requires a higher mass for the LG,
here, using a recent $Gaia$ – based measurement of the transverse velocity the
derived LG mass is $3.36^{+1.14}_{-0.7} cdot 10^{12} M_{odot}$ with no
cosmological constant or $4.54^{+1.2}_{-0.75} cdot 10^{12} M_{odot}$ with a
cosmological constant background. If the LG has had one past encounter, LG mass
is $9.70^{+2.19}_{-1.55}cdot 10^{12} M_{odot}$ or $9.99^{+2.22}_{-1.58}cdot
10^{12} M_{odot}$ with a cosmological constant background. Modified Newtonian
Dynamics (MOND) is also studied, as the accelerations of the Local Group are
fully in the deep MOND regime. MOND yields the order of magnitude for the
expected baryonic mass only after one past encounter, assuming MOND does not
include dark matter. While we only consider the LG as two point masses, our
calculations provide a benchmark for future work with simulations to test the
effect of the finite size of galaxies and tidal fields due to the neighbouring
structures. This model can be also used to test screening mechanisms and
alternative theories of gravity.
The two-body problem of $M31$ and the Milky Way (MW) galaxies with a
cosmological constant background is studied, with emphasis on the possibility
that they experienced past encounters. By implementing the initial conditions
of the big bang and the last measured relative distance and velocities (i.e.
the Timing Argument), it is shown that if $M{31}$ and the MW had more than one
encounter then the mass of the Local Group (LG) would be a few times higher
than if there had been no encounters. Past encounters are possible only for
non-zero transverse velocity, and their viability is subject to observations of
the imprints of such near collisions. While it has been previously shown that
the presence of the cosmological constant requires a higher mass for the LG,
here, using a recent $Gaia$ – based measurement of the transverse velocity the
derived LG mass is $3.36^{+1.14}_{-0.7} cdot 10^{12} M_{odot}$ with no
cosmological constant or $4.54^{+1.2}_{-0.75} cdot 10^{12} M_{odot}$ with a
cosmological constant background. If the LG has had one past encounter, LG mass
is $9.70^{+2.19}_{-1.55}cdot 10^{12} M_{odot}$ or $9.99^{+2.22}_{-1.58}cdot
10^{12} M_{odot}$ with a cosmological constant background. Modified Newtonian
Dynamics (MOND) is also studied, as the accelerations of the Local Group are
fully in the deep MOND regime. MOND yields the order of magnitude for the
expected baryonic mass only after one past encounter, assuming MOND does not
include dark matter. While we only consider the LG as two point masses, our
calculations provide a benchmark for future work with simulations to test the
effect of the finite size of galaxies and tidal fields due to the neighbouring
structures. This model can be also used to test screening mechanisms and
alternative theories of gravity.
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