Genesis of morpho-kinematic lopsidedness in minor merger of galaxies

Abstract

ABSTRACT An m = 1 lopsided asymmetry is common in disc galaxies. Here, we investigate the excitation of an m = 1 lopsidedness in host galaxies during minor mergers while choosing a set of 1:10 merger models (with varying orbital configurations, morphology of the host galaxy) from the GalMer galaxy merger library. We show that a minor merger triggers a prominent m = 1 lopsidedness in stars of the host galaxy. The strength of the m = 1 lopsidedness undergoes a transient amplification phase after each pericenter passage of the satellite, in concordance with past findings of exciting an m = 1 lopsidedness by tidal encounters. However, once the merger happens, and the post-merger remnant readjusts itself, the lopsidedness disappears in short time-scale (∼ 500 − 850 ${{\rm Myr}}$). Furthermore, a delayed merger can drive a prolonged (∼2 ${\rm Gyr}$) lopsidedness in the host galaxy. We demonstrate that the m = 1 lopsidedness rotates with a well-defined pattern speed which is much slower than the m = 2 bar pattern speed, and is retrograde with respect to the bar. This gives rise to a dynamical scenario where the Inner Lindblad resonance of the m = 1 lopsidedness falls in between the corotation and the Outer Lindblad resonance of the m = 2 bar mode. A kinematic lopsidedness also arises in the host galaxy; the resulting temporal variation closely follows that of the density lopsidedness. The minor merger also triggers a transient off-centred stellar disc-dark matter halo configuration due to the tidal encounter with the satellite.