天文学进展2011，Vol.29Issue(3)：291-308,18.
银盘外区的翘曲结构
Warp of the Outer Region of the Galactic Disc
摘要
Abstract
It has been over 50 years since radio observations of neutral hydrogen first revealed the gaseous disk of the Milky Way Galaxy to be warped and flaring. Subsequently, such a structure has also been observed in the distribution of various components of the Galaxy in different wavebands, such as population I objects, molecular clouds, interstellar dust and older stellar population.Radio observations of neutral hydrogen were firstly used to find the warp and the flare of the outer gaseous disc of the Galaxy. The general warping of the H I layer above the Galactic plane in the north and below the plane in the south is clearly shown. According to the results obtained from some researches, for the north, the warping increases rapidly with galactocentric distance R, from the average distance above the plane z ~ 0.1 kpc at R ~ 13 kpc to z=4-4.5 kpc at R ~ 24-30 kpc, whereas in the south, it reaches an extreme value near R ~ 17 kpc and then returns to the plane z=0. At R > 24 kpc, the H I layer exhibits a "scalloping" up and down with respect to distances above the plane z= 0 as the azimuth is changed. The azimuth range of maximum warp directions is 90°- 110°. And the characteristic thickness, which can be used to describe the flaring outer disc, increases rapidly with increasing R, from 1 kpc at R ~ 14 kpc to 3.5-4 kpc at R ~ 24-30 kpc.The population I tracers, which can be used to detected the warp and flare structure of the outer stellar disc of the Galaxy, include OB stars, WR stars, classical cepheids, pulsars, open clusters and even supernova remnants, among which many studies have been done for OB stars since 1970. Prom some samples of such young stars, it has been found that the inclination of the plane denned by OB stars exterior to the solar circle is some 3° and the nodal line of the inclined plane with the Galactic plane is quite close to the Galactic center-Sun-anticenter line, which is in accordance with the global structure of the warped H I layer.In addition to the population I objects, molecular clouds and interstellar dust have also been observed to study the large scale structure of the outer Galaxy, and some interesting conclusions have been obtained. For example, the ensemble of clouds shows the same warped shape and flaring thickness as shown by the outer Galaxy HI gas layer.Interstellar extinction remains a serious obstacle for observing stars near the Galactic plane. Since the extinction suffered in the near infrared is about an order of magnitudelower than in visible, the data provided recently by large scale infrared surveys, including IRAS, DENIS, DIRBE/COBE and 2MASS, have been used to investigate the warp and flare structure of our Galaxy. It is discovered from some studies that a feature of infrared point source counts can be interpreted as a signature of the warp in the Galactic older stellar disc. The warp is less obvious in stars than in the gas and it also shows different shapes of the disc at negative and positive longitudes.Up to date, it is not completely clear how the warped disc of the outer Galaxy was formed and whether the warp is a persistent feature of the Galaxy, or a transient phenomenon. Most recent work has focused on one of the following 3 possible mechanisms: (1) a satellite dwarf orbiting the Galaxy, such as the Large Magellanic Cloud or the Sagittarius dwarf galaxy, causes a warp by tidel interaction with the Galactic disc; (2) the warp is caused by infall and accretion of intergalactic medium onto the disc; (3) the warp can be formed by interaction between the outer Galactic disc and a triaxial or an oblate massive dark matter halo, misaligned with the disc. Anyway, further studies are needed to finally solve the puzzle of the formation mechanism of the Galactic warp.关键词
银河系/银盘/翘曲/近边增厚Key words
Galaxy/ Galactic disc/ warp/ flare分类
天文与地球科学引用本文复制引用
赵君亮..银盘外区的翘曲结构[J].天文学进展,2011,29(3):291-308,18.基金项目
国家自然科学基金(10773020、10778003、 10833005) （10773020、10778003、 10833005）
