Perturbations of spherical stellar systems during fly-by encounters

Abstract

We study the internal response of a galaxy to an unbound encounter and present a survey of orbital parameters covering typical encounters in different galactic environments. Overall, we conclude that relatively weak encounters by low-mass interloping galaxies can cause observable distortions in the primaries. The resulting asymmetries may persist long after the interloper is evident. We focus our attention on the production of structure in dark halos and in cluster ellipticals. Any distortion produced in a dark halo can distort the embedded stellar disk, possibly leading to the formation of lopsided and warped disks. Features excited in an elliptical are directly observable and we predict that asymmetries in the morphologies of these systems can be produced by relatively small perturbers. We use these detailed results to predict the distribution of the $A$ parameter defined by Abraham et al. and the shift between the center of mass of the primary system and the position of the peak of density for a range of environments. We find that high-density, low-velocity dispersion environments are more likely to host galaxies with significant asymmetries. Our distribution for the $A$ parameter is in good agreement with the range spanned by the observed values for local galaxy clusters and for distant galaxies in the MDS and in the HDF.Finally, we propose a generalized asymmetry parameter $A(r)$ which provides detailed information on the radial structure of the asymmetry produced by the mechanism explored in our work. (abridged)