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Intrinsic alignments of galaxies in the EAGLE and cosmo-OWLS simulations.

Velliscig, M. and Cacciato, M. and Schaye, J. and Hoekstra, H. and Bower, R. G. and Crain, R. A. and van Daalen, M. P. and Furlong, M. and McCarthy, I. G. and Schaller, M. and Theuns, T. (2015) 'Intrinsic alignments of galaxies in the EAGLE and cosmo-OWLS simulations.', Monthly notices of the Royal Astronomical Society., 454 (3). pp. 3328-3340.


We report results for the alignments of galaxies in the EAGLE and cosmo-OWLS hydrodynamical cosmological simulations as a function of galaxy separation (−1 ≤ log10(r/[ h−1 Mpc]) ≤ 2) and halo mass (10.7 ≤ log10(M200/[h−1 M⊙]) ≤ 15). We focus on two classes of alignments: the orientations of galaxies with respect to either the directions to, or the orientations of, surrounding galaxies. We find that the strength of the alignment is a strongly decreasing function of the distance between galaxies. For galaxies hosted by the most massive haloes in our simulations the alignment can remain significant up to ∼100 Mpc. Galaxies hosted by more massive haloes show stronger alignment. At a fixed halo mass, more aspherical or prolate galaxies exhibit stronger alignments. The spatial distribution of satellites is anisotropic and significantly aligned with the major axis of the main host halo. The major axes of satellite galaxies, when all stars are considered, are preferentially aligned towards the centre of the main host halo. The predicted projected direction–orientation alignment, ϵg+(rp), is in broad agreement with recent observations. We find that the orientation–orientation alignment is weaker than the orientation–direction alignment on all scales. Overall, the strength of galaxy alignments depends strongly on the subset of stars that are used to measure the orientations of galaxies and it is always weaker than the alignment of dark matter haloes. Thus, alignment models that use halo orientation as a direct proxy for galaxy orientation overestimate the impact of intrinsic galaxy alignments.

Item Type:Article
Keywords:Galaxies: formation, Galaxies: haloes, Cosmology: theory, Large-scale structure of Universe.
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Publisher statement:This article has been accepted for publication in Monthly notices of the Royal Astronomical Society ©: 2015 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved.
Date accepted:21 September 2015
Date deposited:18 February 2016
Date of first online publication:December 2015
Date first made open access:No date available

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