Lopsidedness of self-consistent galaxies caused by the external field effect of clusters

Funding: An early stage of this work has been performed under the Project HPC-EUROPA (211437), with the support of the European Community - Research Infrastructure Action under the FP8 “Structuring the European Research Area” Programme. XW thanks the support from “Hundred Talents Project of Anhui Province”. Part of this project was carried out during the PhD programme of XW. XW, MF, and HZ acknowledge partial support from the Scottish Universities Physics Alliance (SUPA). YGW acknowledges support of the 973 Program (No. 2014CB845703), and the NSFC grants 11390372, 1163004. The research of MF was supported by the I-CORE Program of the Planning and Budgeting Committee, THE ISRAEL SCIENCE FOUNDATION (grants No. 1829/12 and No. 203/09), and the Asher Space Research Institute. MF acknowledges support through a fellowship from the Minerva FoundationAdopting Schwarzschild's orbit-superposition technique, we construct a series of self-consistent galaxy models, embedded in the external field of galaxy clusters in the framework of Milgrom's MOdified Newtonian Dynamics (MOND). These models represent relatively massive ellipticals with a Hernquist radial profile at various distances from the cluster center. Using N-body simulations, we perform a first analysis of these models and their evolution. We find that self-gravitating axisymmetric density models, even under a weak external field, lose their symmetry by instability and generally evolve to triaxial configurations. A kinematic analysis suggests that the instability originates from both box and nonclassified orbits with low angular momentum. We also consider a self-consistent isolated system that is then placed in a strong external field and allowed to evolve freely. This model, just like the corresponding equilibrium model in the same external field, eventually settles to a triaxial equilibrium as well, but has a higher velocity radial anisotropy and is rounder. The presence of an external field in the MOND universe generically predicts some lopsidedness of galaxy shapes.PostprintPeer reviewe