星系翘曲盘及其统计研究

河外旋涡星系外区普遍存在翘曲结构,其特征可用干翘曲参数来描述,包括翘曲角、翘曲半径、不对称度等。一些翘曲星系表已相继发表,并用于相关的统计分析。关于翘曲盘的形成已提出多种理论机制,如星系间的潮汐相互作用、星系际介质的吸积、盘与暗晕的角动量错向以及星系际磁场的作用等。

Warped Discs of External Galaxies and their Statistical Studies

It has been known from both radio and optical measurements that the disk warps in the outer regions of spiral galaxies are a very common phenomenon,the feature of which can be described by some parameters,such as warp radius,warp angle,warp amplitude, warp asymmetry etc.During the past two decades,various shapes of warped disks have been found,including S-shaped(or integral sign shaped),U-shaped and L-shaped warps, among which U means that the two warps are not asymmetric with respect to the galactic center,and L means that only one of the two sides of the galaxy is warped. Since the end of the 20 century,catalogues of warps in spirals identified from edgeon galaxies have been published,some of which provide a lot of information on structure parameters of warped disks of more than 500 galaxies.On the basis of these data many statistical analysis and discussions on warped disks have been made in order to investigate the observational frequencies of warps with different types and different environment,and to find possible intrinsic relationships among the warp parameters,which could provide some useful diagnostic indicators of the origin of warps. Generally speaking,observational frequencies of warped galaxies found from both radio and optical measurements are up to 60%-70%or even higher for all the warp samples presently available.This observational fact shows that the warped disk is a permanent structure,or a transient phenomenon but excited frequently.In a statistical study it was found that the relative fraction of warps among galaxies without nearby companions is the lowest,the fraction in interacting systems is the highest,and galaxies with close companions but without obvious signs of interaction are intermediate between the above two subgroups. Furthermore,the frequency and the shape of warps seem to be independent of galaxy morphology. Many warps are asymmetric,as they show up in only one side of the disk or exhibit large differences in the two sides of the galaxy. So far as the warp parameters are concerned,it is found by some authors that warp angles,the warp radii and the warp asymmetries don’t depend on galaxy morphology.A quite tight anticorrelation is observed between warp radius and warp amplitude,and a positive correlation is found between warp amplitude and warp asymmetry.An another interesting statistical result is that asymmetries are more pronounced in rich environments with denser galaxy distribution. A number of the formation mechanisms have been proposed by different authors to explain the origin of warp disks:interaction with nearby companions or satellites,intergalactic medium infall into the dark matter halo or directly into the disk,non-spherical dark halo misaligned with the disk,and intergalactic magnetic fields.It cannot be ruled out that warps may be originated by different mechanisms from galaxy to galaxy,and even perhaps more than one mechanism may play different roles in the formation of a warp.