给银河系称体重：中国西南天文研究所团队发现银河系比以往认为的更"轻"
Weighing the Milky Way: SWIFAR Team Finds the Galaxy is "Lighter" Than Previously Thought
银河系是我们生活的家园星系，它由数千亿颗恒星、气体和暗物质构成。银河系的恒星总质量，不仅决定着它的整体“体重”，还影响着气体转化成恒星的速度、化学元素含量以及中心黑洞的规模等多方面特性。对于遥远的其他星系，科学家可以通过拍摄整个星系的多波段图像和光谱来比较准确地测量其恒星质量。但由于我们身处银河系内部，无法从“外面”整体观测，再加上银河系中心区域恒星密集且尘埃严重遮挡，导致长期以来对银河系恒星质量的测量并不精确。过去的研究往往只能根据太阳系以外的区域情况来推算中心区域，因此难免存在偏差。
近日，云南大学中国西南天文研究所（SWIFAR）连建辉研究员团队联合国家天文台、北京大学科研人员，利用欧洲空间局的 Gaia 卫星 和 APOGEE 光谱巡天提供的最新观测数据，对银河系恒星的分布和总质量进行了全面分析。研究人员利用 Gaia 的精确数据，仔细修正了观测中可能存在的偏差和尘埃影响，绘制了太阳附近恒星在垂直银盘方向上的分布情况，从而精确测量了这一地区的恒星质量密度。同时，他们利用 APOGEE 的大量恒星光谱数据，重建了银河系从中心到外围的恒星分布。两者结合，使科学家们能够在统一的框架下同时评估银河系局部和整体的恒星质量。
研究结果显示，在太阳所在区域，每平方秒差距（大约 3.26 光年的正方形范围）内的恒星总质量为 31.6 个太阳质量。其中大部分（约 25 个太阳质量）来自正在发光的恒星，其余（约 6.5 个太阳质量）来自白矮星、中子星和黑洞等恒星“遗迹”。这一结果与以往的估计基本一致，但精度更高。同时，研究还发现，在距离银河盘面 1 千秒差距以上的高度，恒星分布存在明显的南北不对称。这表明银河系的结构比传统的简单模型更复杂。
进一步的分析表明，银河系的恒星总质量为 (2.6 ± 0.35) × 10¹⁰ 个太阳质量，只有过去普遍认为的约 6 × 10¹⁰ 个太阳质量的一半左右。这一发现意味着银河系比我们原先想象的“更轻”。新的结果使银河系在中心黑洞大小、整体金属元素含量以及星系大小等方面更符合“典型星系”的特征，也说明暗物质在银河系结构中所占的比例比过去认为的更大。这项研究为我们了解银河系的恒星质量提供了迄今最为精确的测量结果，也让我们更清楚地认识到银河系在宇宙星系大家庭中所处的位置。
这项研究工作已在国际知名天文期刊《天体物理学杂志通讯》（ApJL）上发表：https://doi.org/10.3847/2041-8213/adfc73 。云南大学连建辉研究员是文章第一与通讯作者，合作者包括云南大学王涛博士、北京大学研究生冯齐康、中国科学院大学黄样副教授、云南大学郭贺龙博士。研究工作得到国家重点研发计划、国家自然科学基金面上项目、云南省顶尖团队等基金支持。
图1.（左）银河系艺术家想象图；（中）太阳系邻域垂向密度轮廓；（右）银河系从核球到外盘的质量面密度轮廓。
Figure 1. (Left) An artist's impression of the Milky Way; (Middle) Vertical density profile of the solar neighborhood; (Right) Radial mass surface density profile of the Milky Way from the bulge to the outer disk.
The Milky Way is the home galaxy where we live. It is composed of hundreds of billions of stars, gas, and dark matter. The total stellar mass of the Milky Way not only determines its overall "weight" but also affects various characteristics such as the rate of gas conversion into stars, the abundance of chemical elements, and the scale of the central black hole. For distant galaxies, scientists can relatively accurately measure their stellar mass by taking multi - band images and spectra of the entire galaxy. However, since we are inside the Milky Way, we cannot observe it as a whole from the "outside". Moreover, the central region of the Milky Way has a high density of stars and is severely blocked by dust, resulting in inaccurate measurements of the Milky Way's stellar mass for a long time. Past studies often had to estimate the central region based on the situation outside the solar system, so there were inevitably biases.
Recently, the research team led by Researcher Lian Jianhui from the Southwest Institute for Astronomy Research (SWIFAR) at Yunnan University, in collaboration with researchers from the National Astronomical Observatories of China and Peking University, comprehensively analyzed the distribution and total mass of stars in the Milky Way using the latest observational data provided by the Gaia satellite of the European Space Agency and the APOGEE spectroscopic survey. The researchers used the precise data of Gaia to carefully correct for possible biases and the effects of dust in the observations, and mapped the distribution of stars near the Sun in the direction perpendicular to the galactic disk, thus accurately measuring the stellar mass density in this region. At the same time, they used the large amount of stellar spectral data from APOGEE to reconstruct the stellar distribution from the center to the outskirts of the Milky Way. The combination of these two methods enabled scientists to simultaneously evaluate the local and overall stellar mass of the Milky Way within a unified framework.
The research results show that in the region where the Sun is located, the total stellar mass per square parsec (a square area of approximately 3.26 light - years) is 31.6 solar masses. Most of this (about 25 solar masses) comes from luminous stars, and the rest (about 6.5 solar masses) comes from stellar "remnants" such as white dwarfs, neutron stars, and black holes. This result is basically consistent with previous estimates but is more accurate. At the same time, the research also found that at heights more than 1 kiloparsec above the galactic disk, there is a significant north - south asymmetry in the stellar distribution. This indicates that the structure of the Milky Way is more complex than the traditional simple models.
Further analysis shows that the total stellar mass of the Milky Way is (2.6 ± 0.35) × 10¹⁰ solar masses, which is only about half of the previously commonly believed value of about 6 × 10¹⁰ solar masses. This discovery means that the Milky Way is "lighter" than we originally thought. The new results make the Milky Way more in line with the characteristics of a "typical galaxy" in terms of the size of the central black hole, the overall metallicity, and the size of the galaxy. It also indicates that the proportion of dark matter in the structure of the Milky Way is larger than previously thought. This research provides the most accurate measurement result of the Milky Way's stellar mass to date and also gives us a clearer understanding of the position of the Milky Way in the cosmic galaxy family.
This research work has been published in the internationally renowned astronomical journal《The Astrophysical Journal Letters》(ApJL): https://doi.org/10.3847/2041 - 8213/adfc73. Researcher Lian Jianhui from Yunnan University is the first and corresponding author of the article. The co - authors include Dr. Wang Tao from Yunnan University, graduate student Feng Qikang from Peking University, Associate Professor Huang Yang from the University of Chinese Academy of Sciences, and Dr. Guo Helong from Yunnan University. The research work was supported by funds such as the National Key Research and Development Program of China, the General Program of the National Natural Science Foundation of China, and the Top - level Team of Yunnan Province.
