河外星系恒星潮汐流的探测

Stellar Tidal Streams Around External Galaxies

不少河外星系的周边发现有不同形态的恒星潮汐流,它们大多源自主星系潮汐力场对伴星系的作用,少数来自球状星团,而对它们进行探测有着重要的天体物理意义。对几个代表性河外星系的潮汐流、星系团环境中的星流结构、潮汐流探测的统计性质和球状星团潮汐尾的探测现状及若干相关问题作了简要介绍。

Many stellar tidal streams around external galaxies have been found since 1998.Most of such streams that can be observationally detected are formed from striped matter of nearby dwarf satellites, and a few ones from old globular clusters, which are usually with relatively small sizes and can also be called as tidal tails of clusters. A lot of observational and theoretical studies on the tidal streams have been done during recent more than 10 years,including tidal streams of individual field galaxies and in environment of galaxy clusters,statistics of tidal streams, tidal tails of globular clusters etc.Andromeda galaxy（M31） is the best one which can be studied in detail so as to detect its halo substructure such as tidal streams. In the early 20 th century, a great southern stellar stream in M31 was found, which is 100 kpc further away along the line of sight than M31 and some 30 kpc in front of the galaxy. Then, the complex structure of the M31 stellar halo was also detected, which contains five stellar streams at least. In addition to M31, tidal streams of many other external galaxies were found and studied during much the same period, including NGC 891, NGC 1097, NGC 4013, NGC 4449, NGC 5128, NGC 5055,NGC 7331 etc.In order to discuss the effects of environment on formation of stellar streams, tidal features of cluster galaxies are studied, they being in environments different from field galaxies.Two great tidal streams have been discovered in the center of Hydra I cluster, emerging from a dwarf galaxy and a S0 galaxy respectively, and the stream around the S0 galaxy extends over at least ～110 kpc. Recently, it is shown from a study based on a sample of 54 clusters with 3551 galaxies that a deficit of tidal disturbed galaxies with decreasing clustercenter radius, and no evidence of a relationship between local density and the incidence of tidal features has been found.Besides detailed studies on the tidal streams of individual galaxies, attentions are also paid to looking for statistical properties of streams for big samples of galaxies from the beginning of this century. Using the SDSS DR7 archive, a significant number of stellar streams around galaxies were found, including relatively short streams, medium sized streams and even a stream which seems to have already completed two orbits around NGC 7711, that means they may correspond to different stages of accretion. On the other side, some 700 galaxies with tidal tails were detected from HST data, and a statistical law that the observed space densities of galaxies with tidal structure increase with z as（1 ＋ z）2.6was found.