Pattern Speed of Lopsidedness in Galactic Disks

TL;DR

This paper reviews the phenomenon of lopsidedness in galactic disks, discusses potential origins, and emphasizes the importance of measuring pattern speeds to understand its physical mechanisms.

Contribution

It provides a comprehensive overview of disk lopsidedness, summarizes proposed formation mechanisms, and highlights the need for measuring pattern speeds to distinguish these origins.

Findings

Lopsidedness amplitude is about 10% in spiral galaxies.

Higher lopsidedness observed in group environments.

Theoretical models suggest long-lived slow modes for m=1 asymmetries.

Abstract

The disks of spiral galaxies commonly show a lopsided mass distribution, with a typical fractional amplitude of 10 % for the Fourier component m=1. This is seen in both stars and gas, and the amplitude is higher by a factor of two for galaxies in a group. The study of lopsidedness is a new topic, in contrast to the extensively studied bars and two-armed spirals (m=2). Here, first a brief overview of the observations of disk lopsidedness is given, followed by a summary of the various mechanisms that have been proposed to explain its physical origin. These include tidal interactions, gas accretion, and a global instability. The pattern speed of lopsidedness in a real galaxy has not been measured so far, the various issues involved will be discussed. Theoretical studies have shown that the m=1 slow modes are long-lived, while the modes with a moderate pattern speed as triggered in interactions, last for only about a Gyr. Thus a measurement of the pattern speed of lopsided distribution will help identify the mechanism for its origin.