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The broken-exponential radial structure and larger size of the Milky Way galaxy

Nature Astronomy (2024)Cite this article

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Abstract

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.

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Fig. 1: Luminosity surface density profiles of the integrated and mono-age stellar populations of the Milky Way.
Fig. 2: The half-light radius of the Milky Way in comparison with that of local disk galaxies.
Fig. 3: The evolution of the Milky Way’s half-light radius with age.

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Data availability

All data presented in this work are available from a public repository https://github.com/lianjianhui/Source-data-for-MW-size-paper.git.

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Acknowledgements

J.L. acknowledges support from Yunnan Province Science and Technology Department (Grant Nos. 202005AB160002, 202105AE160021 and 202305AT350002) and the Start-up Fund of Yunnan University (Grant No. CY22623101). B.C. is supported by the National Natural Science Foundation of China (Grant Nos.12173034 and 12322304). N.F.B. acknowledges support from the Science and Technologies Facilities Council (Grant No. ST/V000861/1). Funding for the Sloan Digital Sky Survey IV has been provided by the Alfred P. Sloan Foundation, the US Department of Energy Office of Science and the participating institutions. SDSS-IV acknowledges support and resources from the Center for High-Performance Computing at the University of Utah. The SDSS web site is www.sdss.org. SDSS-IV is managed by the Astrophysical Research Consortium for the participating institutions of the SDSS Collaboration including the Brazilian Participation Group, the Carnegie Institution for Science, Carnegie Mellon University, the Chilean Participation Group, the French Participation Group, Harvard-Smithsonian Center for Astrophysics, Instituto de Astrofísica de Canarias, Johns Hopkins University, the Kavli Institute for the Physics and Mathematics at the University of Tokyo, the Korean Participation Group, Lawrence Berkeley National Laboratory, the Leibniz Institute for Astrophysics Potsdam, the Max Planck Institute for Astronomy at Heidelberg, the Max Planck Institute for Astrophysics at Garching, the Max Planck Institute for Extraterrestrial Physics, the National Astronomical Observatories of China, New Mexico State University, New York University, University of Notre Dame, National Observatory of the Ministry of Science, Technology and Innovation, Ohio State University, Pennsylvania State University, Shanghai Astronomical Observatory, the United Kingdom Participation Group, the National Autonomous University of Mexico, University of Arizona, University of Colorado Boulder, University of Oxford, University of Portsmouth, University of Utah, University of Virginia, University of Washington, University of Wisconsin, Vanderbilt University and Yale University.

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Authors and Affiliations

  1. South-Western Institute for Astronomy Research, Yunnan University, Kunming, People’s Republic of China

    Jianhui Lian, Bingqiu Chen, Tao Wang & Xiaowei Liu

  2. Department of Physics & Astronomy, University of Utah, Salt Lake City, UT, USA

    Gail Zasowski

  3. Department of Astronomy, New Mexico State University, Las Cruces, NM, USA

    Julie Imig

  4. School of Physics and Astronomy, University of St Andrews, St Andrews, UK

    Nicholas Boardman

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Contributions

J.L. developed the initial idea. J.L. and G.Z. prepared the APOGEE data and calculated the APOGEE survey selection function. J.L. conducted the data analysis. B.C. provided the alternative dust extinction map towards the inner Galaxy. T.W. prepared the Gaia data for confirmation of the inner flattened profile. J.L., G.Z., B.C., J.I., N.B. and X.L. wrote the manuscript.

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Correspondence to Jianhui Lian.

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Lian, J., Zasowski, G., Chen, B. et al. The broken-exponential radial structure and larger size of the Milky Way galaxy. Nat Astron (2024). https://doi.org/10.1038/s41550-024-02315-7

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