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Dynamical history of the Local Group in ΛCDM

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Banik_2016_MNRAS_LocalGroup_FinalPubVersion.pdf (2.724Mb)
Date
21/06/2016
Author
Banik, Indranil
Zhao, Hongsheng
Keywords
Methods: data analysis
Methods: numerical
Galaxy: kinematics and dynamics
Galaxies: groups: local
Local group: cosmological parameters
QB Astronomy
QC Physics
Space and Planetary Science
Astronomy and Astrophysics
NDAS
BDC
R2C
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Abstract
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) measurements of 32 LG galaxies. To do this, we follow the trajectories of many simulated particles starting on a pure Hubble flow at redshift 9. For each observed galaxy, we obtain a trajectory which today is at the same position. Its final velocity is the model prediction for the velocity of that galaxy. Unlike previous simulations based on spherical symmetry, ours are axisymmetric and include gravity from Centaurus A. We find the total LG mass is 4.33+0.37 -0.32 × 1012M⊙, with 0.14 ± 0.07 of this being in the MW. We approximately account for IC 342, M81, the Great Attractor and the Large Magellanic Cloud. No plausible set of initial conditions yields a good match to the RVs of our sample of LG galaxies. Observed RVs systematically exceed those predicted by the best-fitting Lambda Cold Dark Matter (ΛCDM) model, with a typical disagreement of 45.1+7.0 -5.7 km s-1 and a maximum of 110 ± 13 km s-1 for DDO 99. Interactions between LG dwarf galaxies cannot easily explain this. One possibility is a past close flyby of the MW and M31. This arises in some modified gravity theories but not in ΛCDM. Gravitational slingshot encounters of material in the LG with either of these massive fast-moving galaxies could plausibly explain why some non-satellite LG galaxies are moving away from us even faster than a pure Hubble flow.
Citation
Banik , I & Zhao , H 2016 , ' Dynamical history of the Local Group in ΛCDM ' , Monthly Notices of the Royal Astronomical Society , vol. 459 , no. 2 , pp. 2237-2261 . https://doi.org/10.1093/mnras/stw787
Publication
Monthly Notices of the Royal Astronomical Society
Status
Peer reviewed
DOI
https://doi.org/10.1093/mnras/stw787
ISSN
0035-8711
Type
Journal article
Rights
© 2016 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society. This work is made available online in accordance with the publisher’s policies. This is the final published version of the work, which was originally published at: https://dx.doi.org/10.1093/mnras/stw787
Description
IB is supported by a Science and Technology Facilities Council studentship.
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  • University of St Andrews Research
URI
http://hdl.handle.net/10023/9045

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