Abstract
ABSTRACTUsing 324 numerically modelled galaxy clusters, we investigate the radial and galaxy–halo alignment of dark matter subhaloes and satellite galaxies orbiting within and around them. We find that radial alignment depends on distance to the centre of the galaxy cluster but appears independent of the dynamical state of the central host cluster. Furthermore, we cannot find a relation between radial alignment of the halo or galaxy shape with its own mass. We report that backsplash galaxies, i.e. objects that have already passed through the cluster radius but are now located in the outskirts, show a stronger radial alignment than infalling objects. We further find that there exists a population of well radially aligned objects passing very close to the central cluster’s centre that were found to be on highly radial orbit.
Highlights
Weak gravitational lensing caused by the large-scale structure of the Universe induces correlations in the observed shapes of galaxies
Using the mass-complete sample of 324 numerically modelled galaxy clusters provided by ‘The Three Hundred’ collaboration,11 we investigate how the shape of haloes and galaxies orbiting in and about the central cluster orients itself with respect to position
After applying very conservative selection criteria to the general pool of objects aiming at minimizing numerical artifacts, we find for the shapes that (i) backsplash and infalling haloes have similar shapes that are more spherical than those of subhaloes, and that (ii) backsplash and satellite galaxies have similar shapes that are more spherical than those of infalling galaxies
Summary
Weak gravitational lensing caused by the large-scale structure of the Universe (i.e. the ‘cosmic shear’) induces correlations in the observed shapes of galaxies. For surveys covering a large enough fraction of the sky, such a signal can be used to obtain valuable information about structure formation in the Universe and eventually cosmological parameters This requires a profound and detailed understanding of natural galaxy alignments, i.e. alignments not apparently induced by lensing but caused by other mechanisms such as, for instance, cosmic structure formation itself. We further focus on one particular type of alignment, i.e. the ‘radial alignment’ (sometimes referred to as ‘shape alignment’1) This alignment – as depicted in Fig. 1 below – measures the correlation between the major axis of the elliptical shape of a galaxy (or its dark matter halo) to its position with respect to the centre of the nearest larger object (which most commonly is the galaxy cluster in which the galaxy orbits).
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