Skip to main content

Thank you for visiting You are using a browser version with limited support for CSS. To obtain the best experience, we recommend you use a more up to date browser (or turn off compatibility mode in Internet Explorer). In the meantime, to ensure continued support, we are displaying the site without styles and JavaScript.

The Andromeda galaxy’s most important merger about 2 billion years ago as M32’s likely progenitor


Although the proximity of the Andromeda galaxy (M31) offers an opportunity to understand how mergers affect galaxies1, uncertainty remains about M31’s most important mergers. Previous studies focused individually on the giant stellar stream2 or the impact of M32 on M31’s disk3,4, thereby suggesting many substantial satellite interactions5. Yet models of M31’s disk heating6 and the similarity between the stellar populations of different tidal substructures in M31’s outskirts7 both suggested a single large merger. M31’s stellar halo (its outer low-surface-brightness regions) is built up from the tidal debris of satellites5 and provides information about its important mergers8. Here we use cosmological models of galaxy formation9,10 to show that M31’s massive11 and metal-rich12 stellar halo, containing intermediate-age stars7, dramatically narrows the range of allowed interactions, requiring a single dominant merger with a large galaxy (with stellar mass about 2.5 × 1010 solar masses, M the third largest member of the Local Group) about 2 billion years (Gyr) ago. This single event explains many observations that were previously considered separately: M31’s compact and metal-rich satellite M3213 is likely to be the stripped core of the disrupted galaxy, its rotating inner stellar halo14 contains most of the merger debris, and the giant stellar stream15 is likely to have been thrown out during the merger. This interaction may explain M31’s global burst of star formation about 2 Gyr ago16 in which approximately a fifth of its stars were formed. Moreover, M31’s disk and bulge were already in place, suggesting that mergers of this magnitude need not dramatically affect galaxy structure.

Access options

Rent or Buy article

Get time limited or full article access on ReadCube.


All prices are NET prices.

Fig. 1: The massive and extensive metal-rich stellar halo of M31 contains a substantial population of young and intermediate-aged stars.
Fig. 2: M31’s large accreted component and high metallicity constrains its dominant merger to have been a single massive, metal-rich galaxy (1010 < MDom/M < 5 × 1010, −0.2 < [M/H]Dom < 0.2) accreted in the last ~5 Gyr.
Fig. 3: The disruption of the most massive progenitor results in a debris field similar to the stellar halo of M31.
Fig. 4: M32p, the most massive progenitor accreted by M31, was the third largest member of the Local Group.


  1. 1.

    Sommerville, R. & Dave, R. Physical models of galaxy formation in a cosmological framework. Ann. Rev. Astron. Astrophys. 53, 51–113 (2015).

    ADS  Article  Google Scholar 

  2. 2.

    Fardal, M. et al. Inferring the Andromeda Galaxy’s mass from its giant southern stream with Bayesian simulation sampling. Mon. Not. R. Astron. Soc. 434, 2779–2802 (2013).

    ADS  Article  Google Scholar 

  3. 3.

    Block, D. et al. An almost head-on collision as the origin of two off-centre rings in the Andromeda galaxy. Nature 443, 832–834 (2006).

    ADS  Article  Google Scholar 

  4. 4.

    Dierickx, M., Blecha, L. & Loeb, A. Signatures of the M31-M32 galactic collision. Astrophys. J. 788, 38–44 (2014).

    ADS  Article  Google Scholar 

  5. 5.

    Bullock, J. & Johnston, K. Tracing galaxy formation with stellar halos. I. Methods. Astrophys. J. 635, 931–949 (2005).

    ADS  Article  Google Scholar 

  6. 6.

    Hammer, F. et al. A 2-3 billion year old major merger paradigm for the Andromeda galaxy and its outskirts. Mon. Not. R. Astron. Soc. 475, 2754–2767 (2018).

    ADS  Google Scholar 

  7. 7.

    Bernard, E. et al. The nature and origin of substructure in the outskirts of M31 – II. Detailed star formation histories. Mon. Not. R. Astron. Soc. 446, 2789–2801 (2015).

    ADS  Article  Google Scholar 

  8. 8.

    D’Souza, R. & Bell, E. The masses and metallicities of stellar haloes reflect galactic merger histories. Mon. Not. R. Astron. Soc. 474, 5300–5318 (2018).

    ADS  Article  Google Scholar 

  9. 9.

    Vogelsberger, M. et al. Properties of galaxies reproduced by a hydrodynamic simulation. Nature 509, 177–182 (2014).

    ADS  Article  Google Scholar 

  10. 10.

    Cooper, A. et al. Galactic accretion and the outer structure of galaxies in the CDM model. Mon. Not. R. Astron. Soc. 434, 3348–3367 (2013).

    ADS  Article  Google Scholar 

  11. 11.

    Ibata, R. et al. The large-scale structure of the halo of the Andromeda galaxy. I. Global stellar density, morphology and metallicity properties. Astrophys. J. 780, 128–148 (2014).

    ADS  Article  Google Scholar 

  12. 12.

    Gilbert, K. et al. Global properties of M31’s stellar halo from the SPLASH survey. II. Metallicity profile. Astrophys. J. 796, 76–96 (2014).

    ADS  Article  Google Scholar 

  13. 13.

    Monachesi, A. et al. The star formation history of M32. Astrophys. J. 745, 97–117 (2012).

    ADS  Article  Google Scholar 

  14. 14.

    Ibata, R. et al. On the accretion origin of a vast extended stellar disk around the Andromeda galaxy. Astrophys. J. 634, 287–313 (2005).

    ADS  Article  Google Scholar 

  15. 15.

    Ibata, R. et al. A giant stream of metal-rich stars in the halo of the galaxy M31. Nature 412, 49–52 (2001).

    ADS  Article  Google Scholar 

  16. 16.

    Williams, B. et al. A global star-forming episode in M31 2-4 Gyr ago. Astrophys. J. 806, 48–57 (2015).

    ADS  Article  Google Scholar 

  17. 17.

    van der Marel, R. et al. Improved evidence for a black hole in M32 from HST/FOS spectra. I. Observations. Astrophys. J. 488, 119–135 (1997).

    ADS  Article  Google Scholar 

  18. 18.

    Haring, N. & Rix, H.-W. On the black hole mass-bulge mass. Astrophys. J. 604, 89–92 (2004).

    ADS  Article  Google Scholar 

  19. 19.

    Irwin, M. et al. A minor axis surface brightness profile of M31. Astron. J. 628, 105–108 (2005).

    Article  Google Scholar 

  20. 20.

    Gilbert, K. et al. Global properties of M31’s stellar halo from the SPLASH survey I. Surface brightness. Astrophys. J. 760, 76–97 (2012).

    ADS  Article  Google Scholar 

  21. 21.

    Bekki, K. et al. A new formation model for M32: a threshed early-type spiral galaxy? Astrophys. J. 557, 39–42 (2001).

    ADS  Article  Google Scholar 

  22. 22.

    Burkert, A. On the formation of compact ellipticals. Mon. Not. R. Astron. Soc. 266, 877–885 (1994).

    ADS  Article  Google Scholar 

  23. 23.

    Choi, P. et al. Tidal interaction of M32 and NGC205 with M31: surface photometry and numerical simulations. Astron. J. 124, 310–331 (2002).

    ADS  Article  Google Scholar 

  24. 24.

    Kormendy, J. in Astrophysics and Space Science Library Vol 418 Galactic Bulges 431–477 (Springer, Berlin, 2016).

  25. 25.

    Howley, K. Internal stellar kinematics of M32 from the SPLASH survey: dark halo constraints. Astrophys. J. 765, 65–87 (2013).

    ADS  Article  Google Scholar 

  26. 26.

    Read, J. et al. The tidal stripping of satellites. Mon. Not. R. Astron. Soc. 366, 429–437 (2006).

    ADS  Article  Google Scholar 

  27. 27.

    Dalcanton, J. et al. The Panchromatic Hubble Andromeda Treasury VIII. A wide-field, high-resolution map of dust extinction in M31. Astrophys. J. 814, 3–50 (2015).

    ADS  Article  Google Scholar 

  28. 28.

    Olsen, K. et al. The star formation histories of the bulge and disk of M31 from resolved stars in the near-infrared. Astron. J. 132, 271–289 (2006).

    ADS  Article  Google Scholar 

  29. 29.

    Genel, S. et al. Introducing the Illustris project: the evolution of galaxy populations across cosmic time. Mon. Not. R. Astron. Soc. 445, 175–200 (2014).

    ADS  Article  Google Scholar 

  30. 30.

    Vogelsberger, M. et al. Introducing the Illustris Project: simulating the coevolution of dark and visible matter in the Universe. Mon. Not. R. Astron. Soc. 444, 1518–1547 (2014).

    ADS  Article  Google Scholar 

  31. 31.

    Guo, Q. et al. From dwarf spheroidals to cD galaxies: simulating the galaxy population in a ΛCDM cosmology. Mon. Not. R. Astron. Soc. 413, 101–131 (2011).

    ADS  Article  Google Scholar 

  32. 32.

    Penarrubia, J. et al. A timing constraint on the (total) mass of the Large Magellanic Cloud. Mon. Not. R. Astron. Soc. 456, 54–58 (2016).

    ADS  Article  Google Scholar 

  33. 33.

    Tamm, A. et al. Stellar mass map and dark matter distribution in M31. Astron. Astrophys. 546, 4–15 (2013).

    Article  Google Scholar 

  34. 34.

    Sanchez-Blazquez, P. et al. Stellar population gradients in galaxy disks from the CALIFA survey. Astron. Astrophys. 570, 6–85 (2016).

    Article  Google Scholar 

  35. 35.

    Amorisco, N. Contributions to the accreted stellar halo: an atlas of stellar deposition. Mon. Not. R. Astron. Soc. 464, 2882–2895 (2017).

    ADS  Article  Google Scholar 

  36. 36.

    Brown, T. et al. The detailed star formation history in the spheroid, outer disk, and tidal stream of the Andromeda galaxy. Astrophys. J. 652, 323–353 (2006).

    ADS  Article  Google Scholar 

  37. 37.

    Fardal, M. et al. Investigating the Andromeda stream – II. Orbital fits and properties of the progenitor. Astrophys. J. 366, 1012–1028 (2006).

    Google Scholar 

  38. 38.

    Dorman, C. et al. The SPLASH survey: kinematics of Andromeda’s inner spheroid. Astrophys. J. 752, 147–166 (2012).

    ADS  Article  Google Scholar 

  39. 39.

    Gilbert, K. et al. Stellar kinematics in the complicated inner spheroid of M31: discovery of substructure along the southeastern minor axis and its relationship to the giant southern stream. Astrophys. J. 668, 245–267 (2007).

    ADS  Article  Google Scholar 

  40. 40.

    Gilbert, K. et al. Global properties of M31’s stellar halo from the SPLASH survey III: measuring the stellar velocity dispersion profile. Astrophys. J. 852, 128–149 (2017).

    ADS  Article  Google Scholar 

  41. 41.

    Brown, T. et al. The extended star formation history of the Andromeda spheroid at 21 kpc on the minor axis. Astrophys. J. 658, 95–98 (2007).

    ADS  Article  Google Scholar 

  42. 42.

    Brown, T. et al. The extended star formation history of the Andromeda spheroid at 35 kpc on the minor axis. Astrophys. J. Lett. 685, 121 (2008).

    ADS  Article  Google Scholar 

  43. 43.

    Sheth, K. et al. The Spitzer Survey of stellar structures in galaxies. Proc. Astron. Soc. Pacif. 122, 1379–1414 (2010).

    ADS  Google Scholar 

  44. 44.

    Muñoz-Mateos, J. et al. The Spitzer Survey of stellar structure in galaxies (S4G): stellar masses, sizes, and radial profiles for 2352 nearby galaxies. Astrophys. J. Suppl. Ser. 219, 2–31 (2015).

    ADS  Article  Google Scholar 

  45. 45.

    Ferrarese, L. et al. The ACS Virgo Cluster Survey. VI. Isophotal analysis and the structure of early-type galaxies. Astrophys. J. Suppl. Ser. 164, 334–434 (2006).

  46. 46.

    Rodriguez-Gomez, V. et al. The merger rate of galaxies in the Illustris simulation: a comparison with observations and semi-empirical models. Mon. Not. R. Astron. Soc. 449, 49–64 (2015).

    ADS  Article  Google Scholar 

  47. 47.

    Kirby, E. et al. The universal stellar mass-stellar metallicity relation for dwarf galaxies. Astrophys. J. 779, 102–123 (2013).

    ADS  Article  Google Scholar 

  48. 48.

    Gallazzi, A. et al. The ages and metallicities of galaxies in the local universe. Mon. Not. R. Astron. Soc. 362, 41–58 (2005).

    ADS  Article  Google Scholar 

Download references


We thank J. Dalcanton, A. Cooper, A. Monachesi, S. Trager, M. Valluri, K. Hattori, K. Johnston, P. Guhathakurta, B. Devour, S. Vegetti, J. Runnoe, M. Reiter, P. Mueller, J. Wagner and R. Macke for comments on the draft. We thank A. Cooper for access to his particle-tagging simulations. We thank R. Ibata and K. Gilbert for permission to use their figures in this publication.

Author information




R.D'S. led the project. Both authors contributed equally to writing the Letter. R.D'S. prepared the figures.

Corresponding author

Correspondence to Richard D’Souza.

Ethics declarations

Competing interests

The authors declare no competing interests.

Additional information

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

Supplementary information

Supplementary Information

Supplementary Figures 1−13, Supplementary References 1−33

Rights and permissions

Reprints and Permissions

About this article

Verify currency and authenticity via CrossMark

Cite this article

D’Souza, R., Bell, E.F. The Andromeda galaxy’s most important merger about 2 billion years ago as M32’s likely progenitor. Nat Astron 2, 737–743 (2018).

Download citation

Further reading


Quick links

Nature Briefing

Sign up for the Nature Briefing newsletter — what matters in science, free to your inbox daily.

Get the most important science stories of the day, free in your inbox. Sign up for Nature Briefing