In Physics Reports, Volume 471, Issue 2, p. 75-111, 2009.
astro-ph/0810.2534

For a PDF version of the article, click here.

Abstract. The light distribution in the disks of many 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 near-IR observations show that lopsidedness is common. The stellar disks in nearly 30% of galaxies have significant lopsidedness of > 10% measured as the Fourier amplitude of the m = 1 component normalized to the average value. This asymmetry is traced particularly well by the atomic hydrogen gas distribution lying in the outer parts. The lopsidedness also occurs in the nuclear regions, where the nucleus is offset with respect to the outer isophotes. The galaxies in a group environment show higher lopsidedness. The origin of lopsidedness could be due to the disk response to a tidally distorted halo, or via gas accretion. An m = 1 perturbation in a disk leads to a shift in the center of mass in the disk, and this then acts as an indirect force on the original center of the disk. The disk is inherently supportive of an m = 1 mode, which is a particular feature only of lopsided modes, and which makes their dynamical study interesting and challenging.

The lopsidedness has a large impact on the dynamics of the galaxy, its evolution, the star formation in it, and on the growth of the central black hole and on the nuclear fueling, merging of binary black holes etc. The disk lopsidedness can be used as a diagnostic to study the halo asymmetry. This is an emerging area in galactic structure and dynamics. In this review, the observations to measure the lopsided distribution, as well as the theoretical progress made so far to understand its origin and properties, and the related open problems will be discussed.

Keywords galaxies : kinematics and dynamics - galaxies : ISM - galaxies : spiral - galaxies : structure - galaxies : individual: M101

Table of Contents

• INTRODUCTION
• OBSERVATIONS OF LOPSIDEDNESS IN GALACTIC DISKS
• Morphological lopsidedness
• Kinematical lopsidedness
• Phase of the disk lopsidedness
• Observations of off-centered nuclear disks
• THEORETICAL MODELS FOR THE ORIGIN OF LOPSIDEDNESS
• Kinematical model for the origin of lopsidedness
• Dynamical models for the origin of lopsidedness
• Comparison between origin of m = 1 and m = 2, stars and gas
• A summary of the various mechanisms
• LOPSIDEDNESS IN THE CENTRAL REGION
• Stability of central nuclear disks
• Double nuclei due to infalling bodies
• Core wandering
• Other mechanisms
• LOPSIDEDNESS IN GALAXIES IN GROUPS, CLUSTERS, AND MERGERS
• Lopsidedness in galaxies in groups
• Lopsidedness in galaxies in clusters
• Lopsidedness in centers of advanced mergers
• RELATED TOPICS
• Relative strengths of lopsidedness (m = 1) and spiral arms/bars (m = 2)
• Asymmetry in the dark matter halo
• Comparison with warps
• Implications for high redshift galaxies
• EFFECT OF LOPSIDEDNESS ON GALAXY EVOLUTION
• SUMMARY & FUTURE DIRECTIONS
• REFERENCES

Corresponding author: Chanda J. Jog (email: cjjog@physics.iisc.ernet.in)