Perturbations of Spherical Stellar Systems during Flyby Encounters

We study the internal response of a galaxy to an unbound encounter andpresent a survey of orbital parameters covering typical encounters in differentgalactic environments. Overall, we conclude that relatively weak encounters bylow-mass interloping galaxies can cause observable distortions in theprimaries. The resulting asymmetries may persist long after the interloper isevident. We focus our attention on the production of structure in dark halosand in cluster ellipticals. Any distortion produced in a dark halo can distortthe embedded stellar disk, possibly leading to the formation of lopsided andwarped disks. Features excited in an elliptical are directly observable and wepredict that asymmetries in the morphologies of these systems can be producedby relatively small perturbers. We use these detailed results to predict thedistribution of the $A$ parameter defined by Abraham et al. and the shiftbetween the center of mass of the primary system and the position of the peakof density for a range of environments. We find that high-density, low-velocitydispersion environments are more likely to host galaxies with significantasymmetries. Our distribution for the $A$ parameter is in good agreement withthe range spanned by the observed values for local galaxy clusters and fordistant galaxies in the MDS and in the HDF.Finally, we propose a generalizedasymmetry parameter $A(r)$ which provides detailed information on the radialstructure of the asymmetry produced by the mechanism explored in our work.(abridged)Comment: 24 pages, uses emulateapj.sty, submitted to The Astrophysical Journa