中国西南天文研究所研究团队揭示银河系径向密度分布特征

SWIFAR research team reveals radial density distribution and size of the Milky Way galaxy

        银河系是我们的家园，我们可以通过观测大量可分辨的恒星（星系的基本构成单元），细致地分析银河系的演化历史。然而，恰恰由于深身处银河系之中以及过去观测能力的限制，我们对银河系的整体结构长期缺乏清晰的认识，恰如“不识庐山真面目，只缘身在此山中”。近几年天文观测技术的进步，特别是大型近红外恒星光谱巡天的开展，为解决这一难题提供了前所未有的机遇。在最新一项由云南大学中国西南天文研究所连建辉副教授主导完成的工作中，研究人员利用APOGEE近红外恒星光谱巡天数据，重构了银河系从内到外完整的恒星径向密度分布，直接测量了银河系的半径，为了解银河系的整体结构及其演化开辟了新的窗口。 

        银河系结构研究的主要难题之一在于，我们的太阳系几乎位于银河系的盘平面上，尘埃消光严重影响了对于银河系主要部分，即内盘和银心方向的观测，使得任何基于光学波段的天文观测都无能为力。由于尘埃消光对波长较长光子的影响快速变小，因此近红外波段的观测可以很大程度缓解这一难题。基于国际首个大型近红外恒星光谱巡天—APOGEE，研究人员对APOGEE巡天观测的不均匀性、尘埃和恒星的特性进行了综合分析，首次系统性地重建了银河系中不同年龄恒星从核球到外盘真实完整的面密度分布。

       该项工作研究结果表明银河系的盘结构在外盘区域（半径R>24000光年，太阳系位于R~26000光年）符合经典的指数分布。然而，在内盘区域（11000<R<24000光年），恒星密度几乎保持平坦，显著偏离外盘的指数分布。这一结果颠覆了过去长期采用的银河系单指数盘的假设，将对银河系许多关键整体物理性质的测量产生重要影响：如过去基于指数盘的假设，人们发现银河系的半径（~10000光年）比同质量星系显著偏小，属于一个致密星系。然而，基于新的恒星面密度分布，我们发现银河系半径几乎是之前估计的两倍（~19000光年），并和近邻同质量星系的半径基本一致，表明银河系在星系大小方面是一个典型的盘星系。

       该项研究延续本团队之前工作的研究思路（https://www.nature.com/articles/s41550-023-01977-z ），开拓了银河系整体物理性质测量、银河系-河外星系交叉对比研究新的维度，研究成果将对银河系相关研究产生重要影响。

       该工作已发表在《自然天文学》（Nature Astronomy）杂志上，题目为《银河系偏离指数的盘径向分布与较大半径》（The broken-exponential profile and larger size of the Milky Way galaxy），文章链接：https://rdcu.be/dL3z5 。云南大学中国西南天文研究所连建辉副教授是文章第一作者和唯一通讯作者，参与该研究的合作者包括Gail Zasowski副教授（犹他大学）、陈丙秋教授（云南大学）、Julie Imig博士（新墨西哥州立大学）、王涛（云南大学）、Nicholas Boardman博士（圣安德鲁斯大学）、刘晓为教授（云南大学）。

图1.左图：银河系河外视角想象图。中图：银河系不同单年龄及所有星族的径向光度面密度分布。右图：银河系在星系质量-半径图中的位置。

Figure 1. Left: Schematic illustration of a face-on view of the Milky Way galaxy. Middle: Radial luminosity surface density distribution of stars of different ages and all stellar populations in the Milky Way Galaxy. Right: Position of the Milky Way galaxy in the mass-radius diagram of galaxies.

    The Milky Way galaxy, our home, allows us to analyze its evolution history by observing a large number of individual stars (the basic building blocks of galaxies). However, due to our position within the Galaxy and past observational limitations, our understanding of the overall structure of the Milky Way has long been incomplete, akin to "not knowing the true face of Mount Lu while being on the mountain." In a recent study led by Associate Professor Jianhui Lian from South-Western Institute For Astronomy Research (SWIFAR), Yunnan University, researchers utilized data from the APOGEE near-infrared stellar spectroscopic survey to reconstruct an unprecedentedly wide radial density distribution of stars from the inner to the outer regions of the Galaxy, directly measuring the Galaxy's radius and opening a new window into understanding its overall structure and evolution.

     One of the primary challenges in studying the Galaxy's structure is that our solar system is nearly located on the Galactic plane, where dust extinction severely hampers observations towards the main parts of the Galaxy, namely the inner disk and the Galactic center, rendering optical wavelength astronomical observations futile. Given that dust extinction diminishes rapidly for longer wavelength photons, observations in the near-infrared regime can largely alleviate this issue. Leveraging the first large-scale near-infrared stellar spectroscopic survey - APOGEE, researchers conducted a comprehensive analysis of the unevenness of APOGEE survey observations, dust effects, and stellar properties, systematically reconstructing the true and comprehensive surface density distribution of stars of different ages in the Galaxy from the bulge to the outer disk.

     The results indicate that the Galactic disk structure in the outer disk region (R > 24,000 light-years, with the Sun located at R ~ 26,000 light-years) conforms to a classical exponential distribution. However, in the inner disk region (11,000 < R < 24,000 light-years), the stellar density remains nearly flat, significantly deviating from the exponential distribution of the outer disk. This finding overturns the long-standing assumption of a single exponential disk for the Galaxy, thus having a significant impact on measuring many key physical properties of the Galaxy as a whole. For instance, based on the assumption of an exponential disk in the past, it was found that the Galaxy's radius (~10,000 light-years) was extraordinarily smaller compared to galaxies of similar mass, classified as a compact galaxy. However, based on the new stellar surface density distribution, it is discovered that the Galaxy's radius is nearly twice the previous estimate (~19,000 light-years), almost in line with the radius of nearby galaxies of similar mass, indicating that the Galaxy is a typical disk galaxy in terms of size.

     This study continues the research of the team's previous work (https://www.nature.com/articles/s41550-023-01977-z ), expanding new dimensions in measuring the overall physical properties of the Milky Way galaxy and comparing the Milky Way galaxy and extragalactic galaxies. The research findings will have a significant impact on related studies of the Milky Way galaxy.

    This work has been published in Nature Astronomy journal under the title "The broken-exponential profile and larger size of the Milky Way galaxy" – Link to the article: https://rdcu.be/dL3z5 . Associate Professor Lian Jianhui from Yunnan University's Southwest China Astronomical Research Institute is the first author and sole corresponding author of the paper, with collaborators including Associate Professor Gail Zasowski (University of Utah), Professor Bingqiu Chen (Yunnan University), Dr. Julie Imig (New Mexico State University), Tao Wang (Yunnan University), Dr. Nicholas Boardman (University of St Andrews), and Professor Xiaowei Liu (Yunnan University).