银河系内区的棒结构

Barlike Structure in the Inner Galaxy

最近十几年来,银河系已被确认为是一个棒旋星系,而不是无棒结构的普通旋涡星系。人们利用各类示踪源,如气体的(l,v)图、红外面测光、红团簇星、恒星计数、微引力透镜效应、恒星运动学等,来探测银河系内区的棒结构和它的结构参数,该文对此做了简要评述.

The presence of a large scale bar in the inner Galaxy has been well established by various tracers since 1990s, such as near infrared radiation （NIR） light, red clump giants, AGB stars and IRAS sources, and by different methods, such as kinematics of Galactic Center gas, bulge surface brightness, NIR star counts and kinematics of stars. Therefore, our Milky Way should be morphologically catalogued as a barred spiral galaxy instead of a normal spiral one. In earlier years the（l, v） diagrams of the 21cm emission of neutral hydrogen and/or the 2.6 mm emission of CO were used to detect the bar-like structure in the inner Galaxy. Based on the noncircular radial motions seen in the HI in the inner suggested the Galaxy is likely to be a barred spiral region of our Galaxy, de Because of the presence the（l, v） diagram in the inner Galaxy differs remarkably from what it would be on circular orbits in an axisymmetric potential. Vaucouleurs first of a triaxial bar, if the gas was all All-sky surveys in the infrared bands are powerful tools for exploring the structure at the Galactic center since the dust absorption is greatly reduced in the IR. In 1991, the first direct evidence for a triaxial stellar bar at the galactic center was detected in the 2.4μm. Since the bar is oriented obliquely to the Sun-Galactic center line, for stars at the closer end of the bar their heliocentric distances should be smaller relatively to stars at the farther end, and hence their surface brightness should be relatively brighter. In this respect, the low angular resolution maps of the Galactic bulge at infrared bands obtained by COBE have provided very useful observational data. The bulge red clump stars can be used as quite good tracers to investigate the barlike structure in the inner Galaxy, From the distribution in the CM diagram of red clump stars and stellar evolution theory it is expected that these stars are relatively bright and have a narrow luminosity distribution, so they can be adopted as a standard candle to estimate their distances. Observations show that the visual magnitudes of such kind of stars in the fields with positive Galactic longitudes are significantly brighter than those with negative longitudes. This means that there is a triaxial structure, or a bar in the center, which is inclined to the Sun-center line. Another approach for studies of the Galactic bar is star counts. The principle of the approach is simple： if there is a bar in the inner Galaxy which is tilted to the Sun-center line, the star number density in the region of the longitude l should be systematically increased or decreased with l changing. Based on star number density distribution from the TMGS, the presence of the barlike structure in the inner Galaxy is confirmed, the length and the position angle of the bar are estimated. Since 1990s, the microlensing effect has been used to explore the barlike structure in the inner Galaxy. Information about the mass distribution in the inner Galaxy such as a bar can be drawn from the mierolensing probability （optical depth） towards the Galactic center. It has been found that the optical depth deduced from observed mierolensing events is obviously larger than that of expected, which can be well interpreted by a barlike structure. The position angle and the axis ratio of the bar can be estimated by detailed analyses of the differences between the observed and expected optical depths. Finally, some main results of the structure parameters of the triaxial bar determined from various methods by authors are listed, and the possible double-barred structure in the inner Galaxy is briefly described.