Uncovering the birth of the Milky Way through accurate stellar ages with Gaia

Abstract

Knowledge of the ages of the stars formed over a galaxy’s lifetime is fundamental to an understanding of its formation and evolution. However, stellar ages are difficult to obtain since they cannot be measured from observations, but require comparison with stellar models1. Alternatively, age distributions can be derived by applying the robust technique of colour–magnitude diagram fitting2, which until now has been used primarily to study nearby galaxies. Accurate distances to individual Milky Way stars now provided by the Gaia spacecraft mission3 have allowed us to derive ages from a thick-disk colour–magnitude diagram and from the two-sequenced colour–magnitude diagram of the kinematically hot local halo4, whose blue sequence has been linked to a major accretion event, Gaia-Enceladus5,6. Because accurate stellar ages were lacking, the time of the merger and its role in our Galaxy’s early evolution remained unclear. Here we show that the stars in both halo sequences share identical age distributions, and are older than most of the thick-disk stars. The sharp halo age distribution cutoff at ten billion years ago can be identified with the time of accretion of Gaia-Enceladus to the Milky Way. Together with state-of-the-art cosmological simulations of galaxy formation7, these robust ages allow us to order the early sequence of events that shaped our Galaxy. We identify the red-sequence stars as the first stars formed within the Milky Way progenitor, and their kinematics indicate that these stars constitute the long-sought in situ halo of the Milky Way.

Access optionsAccess options

Rent or Buy article

Get time limited or full article access on ReadCube.

from$8.99

All prices are NET prices.

Fig. 1: Milky Way halo and thick-disk observed and modelled CMDs.
Fig. 2: Milky Way halo and thick-disk stellar age and iron [Fe/H] distributions.
Fig. 3: Age–metallicity relations for the two halo populations compared to a simulated Milky Way analogue.
Fig. 4: Comparison of the stellar kinematics between observations of Milky Way stars and the simulated Milky Way analogue.

Data availability

All data analysed in this paper are publicly available. They can be retrieved from the DR2 Gaia archive (http://gea.esac.esa.int/archive/) and from the compiled and supplemented GALAH and LAMOST catalogues66, cross-matched as indicated in the Methods. The datasets containing information not available in public catalogues that is necessary to produce the figures can be retrieved from https://zenodo.org/record/3228143#.XRoQs3qH1mM (extended dataset hereafter). A explanatory README file is included. The extended dataset consists of the following files. (1) For Fig. 1a,b, two tables directly retrieved from the Gaia archive as described in the Methods, supplemented by extinction information on a star-by-star basis; the code used to interpolate the three-dimensional extinction maps41 can be retrieved from https://github.com/edober/dust_maps_3d. (2) For Figs. 1c,d, 2 and 3a, two tables with the derived solution CMDs and two files with the data needed to define the boxes used to select stars in the blue and red sequences of the halo CMD. (3) For Fig. 3b, the necessary tables with the age, metallicity and velocity data for the main progenitor and accreted satellite for realization g15784 of the MaGICC program7. The complete information on the final timestep of that simulation, together with scripts to read and plot the data are also included in the extended dataset.

References

  1. 1.

    Soderblom, D. R. The ages of stars. Annu. Rev. Astron. Astrophys. 48, 581–629 (2010).

  2. 2.

    Gallart, C., Zoccali, M. & Aparicio, A. The adequacy of stellar evolution models for the interpretation of the color-magnitude diagrams of resolved stellar populations. Annu. Rev. Astron. Astrophys. 43, 387–434 (2005).

  3. 3.

    Gaia Collaboration Gaia data release 2. Summary of the contents and survey properties. Astron. Astrophys. 616, A1 (2018).

  4. 4.

    Gaia Collaboration et al. Gaia data release 2. Observational Hertzsprung–Russell diagrams. Astron. Astrophys. 616, A10 (2018).

  5. 5.

    Haywood, M. et al. In disguise or out of reach: first clues about in situ and accreted stars in the stellar halo of the Milky Way from Gaia DR2. Astrophys. J. 863, 113 (2018).

  6. 6.

    Helmi, A. et al. The merger that led to the formation of the Milky Way’s inner stellar halo and thick disk. Nature 563, 85–88 (2018).

  7. 7.

    Brook, C. B., Stinson, G., Gibson, B. K., Wadsley, J. & Quinn, T. MaGICC discs: matching observed galaxy relationships over a wide stellar mass range. Mon. Not. R. Astron. Soc. 424, 1275–1283 (2012).

  8. 8.

    Pietrinferni, A., Cassisi, S., Salaris, M. & Castelli, F. A large stellar evolution database for population synthesis studies. II. Stellar models and isochrones for an α-enhanced metal distribution. Astrophys. J. 642, 797–812 (2006).

  9. 9.

    Gilmore, G. & Reid, N. New light on faint stars. III. Galactic structure towards the South Pole and the Galactic thick disc. Mon. Not. R. Astron. Soc. 202, 1025–1047 (1983).

  10. 10.

    Brook, C. B., Kawata, D., Gibson, B. K. & Flynn, C. Galactic halo stars in phase space: a hint of satellite accretion? Astrophys. J. 585, L125–L129 (2003).

  11. 11.

    Nissen, P. E. & Schuster, W. J. Two distinct halo populations in the solar neighborhood. Evidence from stellar abundance ratios and kinematics. Astron. Astrophys. 511, L10 (2010).

  12. 12.

    Schuster, W. J., Moreno, E., Nissen, P. E. & Pichardo, B. Two distinct halo populations in the solar neighborhood. III. Evidence from stellar ages and orbital parameters. Astron. Astrophys. 538, A21 (2012).

  13. 13.

    Belokurov, V., Erkal, D., Evans, N. W., Koposov, S. E. & Deason, A. J. Co-formation of the disc and the stellar halo. Mon. Not. R. Astron. Soc. 478, 611–619 (2018).

  14. 14.

    Hawkins, K., Jofré, P., Gilmore, G. & Masseron, T. On the relative ages of the α-rich and α-poor stellar populations in the Galactic halo. Mon. Not. R. Astron. Soc. 445, 2575–2588 (2014).

  15. 15.

    Ge, Z. S. et al. Ages of 70 dwarfs of three populations in the solar neighborhood: considering O and C abundances in stellar models. Astrophys. J. 833, 161 (2016).

  16. 16.

    Hawkins, K., Jofré, P., Masseron, T. & Gilmore, G. Using chemical tagging to redefine the interface of the Galactic disc and halo. Mon. Not. R. Astron. Soc. 453, 758–774 (2015).

  17. 17.

    Bonaca, A., Conroy, C., Wetzel, A., Hopkins, P. F. & Kereš, D. Gaia reveals a metal-rich, in situ component of the local stellar halo. Astrophys. J. 845, 101 (2017).

  18. 18.

    Hayes, C. R. et al. Disentangling the Galactic halo with APOGEE. I. Chemical and kinematical investigation of distinct metal-poor populations. Astrophys. J. 852, 49 (2018).

  19. 19.

    Fernández-Alvar, E. et al. Disentangling the Galactic halo with APOGEE. II. Chemical and star formation histories for the two distinct populations. Astrophys. J. 852, 50 (2018).

  20. 20.

    Mackereth, J. T. et al. The origin of accreted stellar halo populations in the Milky Way using APOGEE, Gaia, and the EAGLE simulations. Mon. Not. R. Astron. Soc. 482, 3426–3442 (2019).

  21. 21.

    Di Matteo, P. et al. The Milky Way has no in-situ halo but it has a thick disc. Composition of the stellar halo and age-dating the last significant merger with Gaia DR2 and APOGEE. Preprint at https://arxiv.org/abs/1812.08232 (2018).

  22. 22.

    Erb, D. K. et al. The mass–metallicity relation at z > 2. Astrophys. J. 644, 813–828 (2006).

  23. 23.

    Ma, X. et al. The origin and evolution of the galaxy mass-metallicity relation. Mon. Not. R. Astron. Soc. 456, 2140–2156 (2016).

  24. 24.

    Behroozi, P. S., Wechsler, R. H. & Conroy, C. The average star formation histories of galaxies in dark matter halos from z = 0–8. Astrophys. J. 770, 57 (2013).

  25. 25.

    Zolotov, A. et al. The dual origin of stellar halos. Astrophys. J. 702, 1058–1067 (2009).

  26. 26.

    Beers, T. C. et al. Population studies. XIII. A new analysis of the Bidelman–MacConnell ‘weak-metal’ stars confirmation of metal-poor stars in the thick disk of the galaxy. Astrophys. J. 794, 58 (2014).

  27. 27.

    Brook, C. B., Kawata, D., Gibson, B. K. & Freeman, K. C. The emergence of the thick disk in a cold dark matter universe. Astrophys. J. 612, 894–899 (2004).

  28. 28.

    Fuhrmann, K. Nearby stars of the Galactic disc and halo—V. Mon. Not. R. Astron. Soc. 414, 2893–2922 (2011).

  29. 29.

    Recio-Blanco, A. et al. The Gaia-ESO survey: the Galactic thick to thin disc transition. Astron. Astrophys. 567, A5 (2014).

  30. 30.

    Haywood, M. et al. When the Milky Way turned off the lights: APOGEE provides evidence of star formation quenching in our Galaxy. Astron. Astrophys. 589, A66 (2016).

  31. 31.

    Schönrich, R., McMillan, P. & Eyer, L. Distances and parallax bias in Gaia DR2. Preprint at https://arxiv.org/abs/1902.02355 (2019).

  32. 32.

    Lindegren, L. et al. Gaia data release 2. The astrometric solution. Astron. Astrophys. 616, A2 (2018).

  33. 33.

    Graczyk, D. et al. Testing systematics of Gaia DR2 parallaxes with empirical surface brightness: color relations applied to eclipsing binaries. Astrophys. J. 872, 85 (2019).

  34. 34.

    Hall, O. J. et al. Testing asteroseismology with Gaia DR2: hierarchical models of the red clump. Mon. Not. R. Astron. Soc. 486, 3569–3585 (2019).

  35. 35.

    Khan, S. et al. New light on the Gaia DR2 parallax zero-point: influence of the asteroseismic approach, in and beyond the Kepler field. Preprint at https://arxiv.org/abs/1904.05676 (2019).

  36. 36.

    Leung, H. W. & Bovy, J. Simultaneous calibration of spectro-photometric distances and the Gaia DR2 parallax zero-point offset with deep learning. Preprint at https://arxiv.org/abs/1902.08634 (2019).

  37. 37.

    Muraveva, T., Delgado, H. E., Clementini, G., Sarro, L. M. & Garofalo, A. RR Lyrae stars as standard candles in the Gaia data release 2 era. Mon. Not. R. Astron. Soc. 481, 1195–1211 (2018).

  38. 38.

    Zinn, J. C., Pinsonneault, M. H., Huber, D. & Stello, D. Confirmation of the Gaia DR2 parallax zero-point offset using asteroseismology and spectroscopy in the Kepler field. Preprint at https://arxiv.org/abs/1805.02650 (2018).

  39. 39.

    Luri, X. et al. Gaia data release 2. Using Gaia parallaxes. Astron. Astrophys. 616, A9 (2018).

  40. 40.

    Bailer-Jones, C. A. L., Rybizki, J., Fouesneau, M., Mantelet, G. & Andrae, R. Estimating distance from parallaxes. IV. Distances to 1.33 billion stars in Gaia data release 2. Astron. J. 156, 58 (2018).

  41. 41.

    Lallement, R. et al. Three-dimensional maps of interstellar dust in the local arm: using Gaia, 2MASS, and APOGEE-DR14. Astron. Astrophys. 616, A132 (2018).

  42. 42.

    Casagrande, L. & VandenBerg, D. A. On the use of Gaia magnitudes and new tables of bolometric corrections. Mon. Not. R. Astron. Soc. 479, L102–L107 (2018).

  43. 43.

    Gallart, C., Freedman, W. L., Aparicio, A., Bertelli, G. & Chiosi, C. The star formation history of the Local Group dwarf galaxy Leo I. Astron. J. 118, 2245–2261 (1999).

  44. 44.

    Hernandez, X., Valls-Gabaud, D. & Gilmore, G. Deriving star formation histories: inverting Hertzsprung–Russell diagrams through a variational calculus maximum likelihood method. Mon. Not. R. Astron. Soc. 304, 705–719 (1999).

  45. 45.

    Holtzman, J. A. et al. Observations and implications of the star formation history of the Large Magellanic Cloud. Astron. J. 118, 2262–2279 (1999).

  46. 46.

    Dolphin, A. E. Numerical methods of star formation history measurement and applications to seven dwarf spheroidals. Mon. Not. R. Astron. Soc. 332, 91–108 (2002).

  47. 47.

    Aparicio, A. & Gallart, C. IAC-STAR: a code for synthetic color-magnitude diagram computation. Astron. J. 128, 1465–1477 (2004).

  48. 48.

    Aparicio, A. & Hidalgo, S. L. IAC-pop: finding the star formation history of resolved galaxies. Astron. J. 138, 558–567 (2009).

  49. 49.

    Monelli, M. et al. The ACS LCID project. III. The star formation history of the Cetus dSph galaxy: a post-reionization fossil. Astrophys. J. 720, 1225–1245 (2010).

  50. 50.

    Cignoni, M. & Tosi, M. Star formation histories of dwarf galaxies from the colour-magnitude diagrams of their resolved stellar populations. Adv. Astron. 2010, 158568 (2010).

  51. 51.

    Ruiz-Lara, T. et al. Integrated-light analyses vs. colour-magnitude diagrams. II. Leo A: an extremely young dwarf in the Local Group. Astron. Astrophys. 617, A18 (2018).

  52. 52.

    Tolstoy, E., Hill, V. & Tosi, M. Star-formation histories, abundances, and kinematics of dwarf galaxies in the Local Group. Annu. Rev. Astron. Astrophys. 47, 371–425 (2009).

  53. 53.

    Benedict, G. F. et al. Hubble Space Telescope fine guidance sensor parallaxes of galactic Cepheid variable stars: period–luminosity relations. Astron. J. 133, 1810–1827 (2007).

  54. 54.

    Beaton, R. L. et al. The Carnegie-Chicago Hubble Program. I. An independent approach to the extragalactic distance scale using only population II distance indicators. Astrophys. J. 832, 210 (2016).

  55. 55.

    Hernandez, X., Valls-Gabaud, D. & Gilmore, G. The recent star formation history of the Hipparcos solar neighbourhood. Mon. Not. R. Astron. Soc. 316, 605–612 (2000).

  56. 56.

    Bertelli, G. & Nasi, E. Star formation history in the solar vicinity. Astron. J. 121, 1013–1023 (2001).

  57. 57.

    Cignoni, M., Degl’Innocenti, S., Prada Moroni, P. G. & Shore, S. N. Recovering the star formation rate in the solar neighborhood. Astron. Astrophys. 459, 783–796 (2006).

  58. 58.

    Bernard, E. J. Reconstructing the star formation history of the solar neighbourhood with Gaia. Proc. Int. Astron. Union 334, 158–161 (2018).

  59. 59.

    Bernard, E. J. et al. The spatially-resolved star formation history of the M31 outer disc. Mon. Not. R. Astron. Soc. 453, L113–L117 (2015).

  60. 60.

    Bernard, E. J. et al. Star formation history of the Galactic bulge from deep HST imaging of low reddening windows. Mon. Not. R. Astron. Soc. 477, 3507–3519 (2018).

  61. 61.

    Pietrinferni, A., Cassisi, S., Salaris, M. & Castelli, F. A large stellar evolution database for population synthesis studies. I. Scaled solar models and isochrones. Astrophys. J. 612, 168–190 (2004).

  62. 62.

    Kroupa, P., Tout, C. A. & Gilmore, G. The distribution of low-mass stars in the Galactic disc. Mon. Not. R. Astron. Soc. 262, 545–587 (1993).

  63. 63.

    Evans, D. W. et al. Gaia data release 2. Photometric content and validation. Astron. Astrophys. 616, A4 (2018).

  64. 64.

    Hayden, M. R. et al. Chemical cartography with APOGEE: metallicity distribution functions and the chemical structure of the Milky Way disk. Astrophys. J. 808, 132 (2015).

  65. 65.

    Hidalgo, S. L. et al. The ACS LCID project. V. The star formation history of the dwarf galaxy LGS-3: clues to cosmic reionization and feedback. Astrophys. J. 730, 14 (2011).

  66. 66.

    Sanders, J. L. & Das, P. Isochrone ages for 3 million stars with the second Gaia data release. Mon. Not. R. Astron. Soc. 481, 4093–4110 (2018).

  67. 67.

    Zhao, G., Zhao, Y.-H., Chu, Y.-Q., Jing, Y.-P. & Deng, L.-C. LAMOST spectral survey—an overview. Res. Astron. Astrophys. 12, 723–734 (2012).

  68. 68.

    Buder, S. et al. The GALAH survey: second data release. Mon. Not. R. Astron. Soc. 478, 4513–4552 (2018).

  69. 69.

    Brook, C. B., Kawata, D., Gibson, B. K. & Flynn, C. Stellar halo constraints on simulated late-type galaxies. Mon. Not. R. Astron. Soc. 349, 52–56 (2004).

  70. 70.

    Okamoto, T., Eke, V. R., Frenk, C. S. & Jenkins, A. Effects of feedback on the morphology of galaxy discs. Mon. Not. R. Astron. Soc. 363, 1299–1314 (2005).

  71. 71.

    Font, A. S. et al. Cosmological simulations of the formation of the stellar haloes around disc galaxies. Mon. Not. R. Astron. Soc. 416, 2802–2820 (2011).

  72. 72.

    Pillepich, A. et al. Halo mass and assembly history exposed in the faint outskirts: the stellar and dark matter haloes of Illustris galaxies. Mon. Not. R. Astron. Soc. 444, 237–249 (2014).

  73. 73.

    Rodriguez-Gomez, V. et al. The stellar mass assembly of galaxies in the Illustris simulation: growth by mergers and the spatial distribution of accreted stars. Mon. Not. R. Astron. Soc. 458, 2371–2390 (2016).

  74. 74.

    Tissera, P. B. et al. The central spheroids of Milky Way mass-sized galaxies. Mon. Not. R. Astron. Soc. 473, 1656–1666 (2018).

  75. 75.

    Obreja, A. et al. Introducing galactic structure finder: the multiple stellar kinematic structures of a simulated Milky Way mass galaxy. Mon. Not. R. Astron. Soc. 477, 4915–4930 (2018).

  76. 76.

    Monachesi, A. et al. The Auriga stellar haloes: connecting stellar population properties with accretion and merging history. Mon. Not. R. Astron. Soc. 485, 2589–2616 (2019).

  77. 77.

    Fattahi, A. et al. The origin of galactic metal-rich stellar halo components with highly eccentric orbits. Mon. Not. R. Astron. Soc. 484, 4471–4483 (2019).

  78. 78.

    Wang, L. et al. NIHAO project—I. Reproducing the inefficiency of galaxy formation across cosmic time with a large sample of cosmological hydrodynamical simulations. Mon. Not. R. Astron. Soc. 454, 83–94 (2015).

  79. 79.

    Stinson, G. S. et al. MAGICC haloes: confronting simulations with observations of the circumgalactic medium at z = 0. Mon. Not. R. Astron. Soc. 425, 1270–1277 (2012).

  80. 80.

    Miranda, M. S. et al. Origin of the metallicity distribution in the thick disc. Astron. Astrophys. 587, A10 (2016).

  81. 81.

    Stinson, G. et al. Star formation and feedback in smoothed particle hydrodynamic simulations—I. Isolated galaxies. Mon. Not. R. Astron. Soc. 373, 1074–1090 (2006).

  82. 82.

    Stinson, G. S. et al. Making galaxies in a cosmological context: the need for early stellar feedback. Mon. Not. R. Astron. Soc. 428, 129–140 (2013).

  83. 83.

    Haardt, F. & Madau, P. Radiative transfer in a clumpy universe. II. The ultraviolet extragalactic background. Astrophys. J. 461, 20 (1996).

  84. 84.

    Shen, S., Wadsley, J. & Stinson, G. The enrichment of the intergalactic medium with adiabatic feedback—I. Metal cooling and metal diffusion. Mon. Not. R. Astron. Soc. 407, 1581–1596 (2010).

  85. 85.

    Ferland, G. J. et al. CLOUDY 90: numerical simulation of plasmas and their spectra. Publ. Astron. Soc. Pac. 110, 761–778 (1998).

Download references

Acknowledgements

C.G., C.B.B., M.M. and T.R.-L. acknowledge financial support through the grants (AEI/FEDER, UE) AYA2017-89076-P, AYA2016-77237-C3-1-P and AYA2015-63810-P, as well as by the Ministerio de Ciencia, Innovación y Universidades (MCIU), through the State Budget and by the Consejer a de Economia, Industria, Comercio y Conocimiento of the Canary Islands Autonomous Community, through the Regional Budget. T.R.-L. is supported by a MCIU Juan de la Cierva - Formación grant (FJCI-2016-30342). C.B.B. is supported by a MCIU Ramón y Cajal Fellowship (RYC 2013-12784). S.C. acknowledges support from Premiale INAF ‘MITIC’ and has been supported by INFN (Iniziativa specifica TAsP). V.H. was supported by the Programme National Cosmology et Galaxies (PNCG) of CNRS/INSU with INP and IN2P3, co-funded by CEA and CNES. We used data from the European Space Agency mission Gaia (http://www.cosmos.esa.int/gaia), processed by the Gaia Data Processing and Analysis Consortium (DPAC; see http://www.cosmos.esa.int/web/gaia/dpac/consortium). Funding for DPAC has been provided by national institutions, in particular the institutions participating in the Gaia Multilateral Agreement. We also used data from the LAMOST and GALAH surveys. Guoshoujing Telescope (the Large Sky Area Multi-Object Fiber Spectroscopic Telescope LAMOST) is a National Major Scientific Project built by the Chinese Academy of Sciences. Funding for the project wasprovided by the National Development and Reform Commission. LAMOST is operated and managed by the National Astronomical Observatories, Chinese Academy of Sciences. The GALAH survey is based on observations made at the Australian Astronomical Observatory, under programmes A/2013B/13, A/2014A/25, A/2015A/19 and A/2017A/18. We acknowledge the traditional owners of the land on which the Australian Astronomical Observatory stands, the Gamilaraay people, and pay our respects to elders past and present.

Author information

All authors critically contributed to different aspects of the data analysis and model calculation, and to the interpretation of the results. The writing of the manuscript, to which all authors contributed, was led by C.G. and C.B.B. Specifically, C.G. selected the Gaia samples and performed the CMD fitting, E.J.B. helped with the data selection, wrote the CMD fitting software (TheStorm) based on earlier work with M.M. and C.G., and performed the calculation of the three-dimensional interstellar reddening. C.B.B. contributed the galaxy formation models, which were key to the interpretation of the results. T.R.-L. participated in the CMD fitting, contributed key software for various steps such as the error simulation in the synthetic CMDs, and created the figures. S.C. contributed the software to calculate the synthetic CMD, including all the necessary libraries of stellar models and bolometric correction tables for the Gaia photometric passbands. V.H. selected the spectroscopic samples, which were analysed together with T.R.-L.

Correspondence to Carme Gallart.

Ethics declarations

Competing interests

The authors declare no competing interests.

Additional information

Peer review information: Nature Astronomy thanks Carine Babusiaux and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

Publisher’s note: Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Supplementary information

Supplementary Information

Supplementary Fig. 1.

Rights and permissions

Reprints and Permissions

About this article

Verify currency and authenticity via CrossMark