The radial structure of a galaxy is a fundamental property that reflects its growth and assembly history. Although it is straightforward to measure that of external galaxies, it is challenging for the Milky Way because of our inside perspective. Traditionally, the radial structure of the Milky Way has been assumed to be characterized by a single-exponential disk and a central bulge component. Here we report (1) a measurement of the age-resolved Galactic surface brightness profile in a wide radial range from R = 0 to 17 kpc and (2) the corresponding size of the Milky Way in terms of a half-light radius. We find a broken surface brightness profile with a nearly flat distribution between 3.5 and 7.5 kpc, in contrast to a canonical single-exponential disk. This broken profile results in a half-light radius of 5.75 ± 0.38 kpc, significantly larger than that inferred from a single-exponential disk profile but consistent with that of local disk galaxies of similar mass. We also confirm that the size growth history of the Milky Way is broadly consistent with high-redshift galaxies but with systematically smaller size. Our results suggest that the Milky Way has a more complex radial structure and larger size than previously expected.

星系的径向结构是反映其生长和聚集历史的基本属性。虽然测量外部星系的测量很简单，但由于我们的内部视角，测量银河系具有挑战性。传统上，银河系的径向结构被认为具有单指数盘和中心核球部分的特征。在这里，我们报告（1）在 R = 0 至 17 kpc 的宽径向范围内测量年龄分辨的银河系表面亮度剖面，以及（2）以半光半径表示的银河系的相应大小。我们发现破碎的表面亮度分布在 3.5 到 7.5 kpc 之间几乎平坦分布，与规范的单指数圆盘形成鲜明对比。这种破碎的轮廓导致半光半径为 5.75 ± 0.38 kpc，明显大于从单指数盘轮廓推断的半径，但与质量相似的本地盘星系的半径一致。我们还证实，银河系的尺寸增长历史与高红移星系大致一致，但尺寸系统性较小。我们的结果表明，银河系具有比之前预期更复杂的径向结构和更大的尺寸。