Lopsided spiral galaxies

Abstract

The light distribution in disks of many spiral galaxies is non-axisymmetric or 'lopsided' with a spatial extent much larger along one half of a galaxy than the other, as in M101. Recent observations show that 30% of spiral galaxies have significant lopsidedness in both old stars and gas, indicating asymmetry in the disk mass distribution. This is a new and exciting area in galactic structure and dynamics. Here, first a brief review of observations will be given, and then the results from our theoretical work will be presented. We have studied the dynamics of particles in closed orbits, the isophotes, and the local disk stability in an axisymmetric galactic disk perturbed by a lopsided halo potential. The results obtained are shown to be valid for both stars and gas in the same region of the disk. Our results naturally explain a number of observations of these galaxies, including the non-axisymmetric (m = 1) surface density distribution, the isophotal elongation, and the azimuthally asymmetric formation of HII regions. From the observed disk lopsidedness, the degree of halo lopsidedness is obtained to be ~ 5%. Our calculation of the self-consistent disk response shows that the resulting disk lopsidedness is important only beyond 1.4 disk scale-lengths and its magnitude increases with radius - this is exactly in agreement with the recent near-IR observations in the literature.