Skip to main content

Thank you for visiting nature.com. 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.

  • Letter
  • Published:

Three-dimensional motions in the Sculptor dwarf galaxy as a glimpse of a new era

Abstract

The three-dimensional motions of stars in small galaxies beyond our own are minute, yet they are crucial for understanding the nature of gravity and dark matter1,2. Even for the dwarf galaxy Sculptor—one of the best-studied systems, which is inferred to be strongly dark matter dominated3,4—there are conflicting reports5,6,7 on its mean motion around the Milky Way, and the three-dimensional internal motions of its stars have never been measured. Here, we present precise proper motions of Sculptor’s stars based on data from the Gaia mission8 and Hubble Space Telescope. Our measurements show that Sculptor moves around the Milky Way on a high-inclination elongated orbit that takes it much further out than previously thought. For Sculptor’s internal velocity dispersions, we find σ R = 11.5 ± 4.3 km s−1 and σ T = 8.5 ± 3.2 km s−1 along the projected radial and tangential directions. Thus, the stars in our sample move preferentially on radial orbits as quantified by the anisotropy parameter, which we find to be \({\boldsymbol{\beta }} \sim 0.8{6}_{-0.83}^{+0.12}\) at a location beyond the core radius. Taken at face value, this high radial anisotropy requires abandoning conventional models9 for Sculptor’s mass distribution. Our sample is dominated by metal-rich stars and for these we find \({{\boldsymbol{\beta }}}^{{\rm{M}}R} \sim 0.9{5}_{-0.27}^{+0.04}\)—a value consistent with multi-component spherical models where Sculptor is embedded in a cuspy dark halo10, as might be expected for cold dark matter.

This is a preview of subscription content, access via your institution

Access options

Rent or buy this article

Prices vary by article type

from$1.95

to$39.95

Prices may be subject to local taxes which are calculated during checkout

Fig. 1: Field of view towards the Sculptor dwarf spheroidal galaxy.
Fig. 2: Properties of our sample.
Fig. 3: Comparison with previously published PM estimates for Sculptor.
Fig. 4: Two-dimensional velocity dispersion and orbital anisotropy of Sculptor.

Similar content being viewed by others

References

  1. Lin, D. N. C. & Faber, S. M. Some implications of nonluminous matter in dwarf spheroidal galaxies. Astrophys. J. Lett. 266, 21–25 (1983).

    Article  ADS  Google Scholar 

  2. Strigari, L. E. Galactic searches for dark matter. Phys. Rep. 531, 1–88 (2013).

    Article  ADS  Google Scholar 

  3. Walker, M. G. et al. Velocity dispersion profiles of seven dwarf spheroidal galaxies. Astrophys. J. Lett. 667, 53–56 (2007).

    Article  ADS  Google Scholar 

  4. Battaglia, G. et al. The kinematic status and mass content of the Sculptor dwarf spheroidal galaxy. Astrophys. J. Lett. 681, L13 (2008).

    Article  ADS  Google Scholar 

  5. Schweitzer, A. E., Cudworth, K. M., Majewski, S. R. & Suntzeff, N. B. The absolute proper motion and a membership survey of the Sculptor dwarf spheroidal galaxy. Astron. J. 110, 2747–2757 (1995).

    Article  ADS  Google Scholar 

  6. Piatek, S. et al. Proper motions of dwarf spheroidal galaxies from Hubble Space Telescope imaging. IV. Measurement for Sculptor. Astron. J. 131, 1445–1460 (2006).

    Article  ADS  Google Scholar 

  7. Walker, M. G., Mateo, M. & Olszewski, E. W. Systemic proper motions of Milky Way satellites from stellar redshifts: the Carina, Fornax, Sculptor, and Sextans dwarf spheroidals. Astrophys. J. Lett. 688, L75 (2008).

    Article  ADS  Google Scholar 

  8. Prusti, T. et al. The Gaia mission. Astron. Astrophys. 595, A1 (2016).

    Article  Google Scholar 

  9. Battaglia, G., Helmi, A. & Breddels, M. Internal kinematics and dynamical models of dwarf spheroidal galaxies around the Milky Way. New Astron. Rev. 57, 52–79 (2013).

    Article  ADS  Google Scholar 

  10. Strigari, L. E., Frenk, C. S. & White, S. D. M. Dynamical models for the Sculptor dwarf spheroidal in a CDM universe. Astrophys. J. 838, 123–132 (2017).

    Article  ADS  Google Scholar 

  11. Anderson, J. Variation of the Distortion Solution of the WFC (Space Telescope Science Institute, 2007).

  12. Brown, A. G. A. et al. Gaia Data Release 1. Summary of the astrometric, photometric, and survey properties. Astron. Astrophys. 595, A2 (2016).

    Article  Google Scholar 

  13. Massari, D., Posti, L., Helmi, A., Fiorentino, G. & Tolstoy, E. The power of teaming up HST and Gaia: the first proper motion measurement of the distant cluster NGC 2419. Astron. Astrophys. 598, L9 (2017).

    Article  ADS  Google Scholar 

  14. Dinescu, D. I., Girard, T. M. & van Altena, W. F. Space velocities of globular clusters. III. Cluster orbits and halo substructure. Astron. J. 117, 1792–1815 (1999).

    Article  ADS  Google Scholar 

  15. Sohn, S. T., Anderson, J. & van der Marel, R. P. The M31 velocity vector. I. Hubble Space Telescope proper-motion measurements. Astrophys. J. 753, 7 (2012).

    Article  ADS  Google Scholar 

  16. Lindegren, L. et al. Gaia Data Release 1. Astrometry: one billion positions, two million proper motions and parallaxes. Astron. Astrophys. 595, A4 (2016).

    Article  Google Scholar 

  17. Martnez-Vázquez, C. E. et al. Variable stars in local group galaxies. I. Tracing the early chemical enrichment and radial gradients in the Sculptor dSph with RR Lyrae stars. Mon. Not. R. Astron. Soc. 454, 1509–1516 (2015).

    Article  ADS  Google Scholar 

  18. Piffl, T. et al. Constraining the Galaxy’s dark halo with RAVE stars. Mon. Not. R. Astron. Soc. 445, 3133–3151 (2014).

    Article  ADS  Google Scholar 

  19. Mayer, L. et al. The metamorphosis of tidally stirred dwarf galaxies. Astrophys. J. 559, 754–784 (2001).

    Article  ADS  Google Scholar 

  20. Michalik, D., Lindegren, L., Hobbs, D. & Butkevich, A. G. Gaia astrometry for stars with too few observations. A Bayesian approach. Astron. Astrophys. 583, A68 (2015).

    Article  ADS  Google Scholar 

  21. Foreman-Mackey, D., Hogg, D. W., Lang, D. & Goodman, J. emcee: the MCMC hammer. Public. Astron. Soc. Pac. 125, 306–312 (2013).

    Article  ADS  Google Scholar 

  22. Strigari, L. E., Bullock, J. S. & Kaplinghat, M. Determining the nature of dark matter with astrometry. Astrophys. J. Lett. 657, 1–4 (2007).

    Article  ADS  Google Scholar 

  23. Walker, M. G., Mateo, M. & Olszewski, E. W. Stellar velocities in the Carina, Fornax, Sculptor, and Sextans dSph galaxies: data from the Magellan/MMFS Survey. Astron. J. 137, 3100–3108 (2009).

    Article  ADS  Google Scholar 

  24. Walker, M. G. & Peñarrubia, J. A method for measuring (slopes of) the mass profiles of dwarf spheroidal galaxies. Astrophys. J. 742, 20 (2011).

    Article  ADS  Google Scholar 

  25. Amorisco, N. C. & Evans, N. W. Dark matter cores and cusps: the case of multiple stellar populations in dwarf spheroidals. Mon. Not. R. Astron. Soc. 419, 184–196 (2012).

    Article  ADS  Google Scholar 

  26. Breddels, M. A. & Helmi, A. Model comparison of the dark matter profiles of Fornax, Sculptor, Carina and Sextans. Astron. Astrophys. 558, A35 (2013).

    Article  ADS  Google Scholar 

  27. Navarro, J. F., Frenk, C. S. & White, S. D. M. The structure of cold dark matter halos. Astrophys. J. 462, 563–575 (1996).

    Article  ADS  Google Scholar 

  28. An, J. H. & Evans, N. W. A cusp slope-central anisotropy theorem. Astrophys. J. 642, 752–758 (2006).

    Article  ADS  Google Scholar 

  29. Angus, G. W. Dwarf spheroidals in MOND. Mon. Not. R. Astron. Soc. 387, 1481–1488 (2008).

    Article  ADS  Google Scholar 

  30. Anderson, J. & Bedin, L. R. An empirical pixel-based correction for imperfect CTE. I. HST’s advanced camera for surveys. Public. Astron. Soc. Pac. 122, 1035–1064 (2010).

    Article  ADS  Google Scholar 

  31. Ubeda, L. & Anderson, J. Study of the Evolution of the ACS/WFC Charge Transfer Efficiency (Space Telescope Science Institute, 2012).

  32. Anderson, J. & King, I. PSFs, Photometry, and Astronomy for the ACS/WFC (Space Telescope Science Institute, 2006).

  33. Bellini, A. et al. Hubble Space Telescope PROper MOtion (HSTPROMO) catalogs of galactic globular clusters. I. Sample selection, data reduction, and NGC 7078 results. Astrophys. J. 797, 115 (2014).

    Article  ADS  Google Scholar 

  34. Anderson, J. Empirical PSFs and distortion in the WFC camera. In 2005 HST Calibration Workshop: Hubble After the Transition to Two-Gyro Mode (eds Koekemoer, A. M., Goudfrooij, P. & Dressel, L. L.) (2006).

  35. Arenou, F. et al. Gaia Data Release 1. Catalogue validation. Astron. Astrophys. 599, A50 (2017).

    Article  Google Scholar 

  36. Trippe, S. et al. High-precision astrometry with MICADO at the European Extremely Large Telescope. Mon. Not. R. Astron. Soc. 402, 1126–1140 (2010).

    Article  ADS  Google Scholar 

  37. Chiaberge, M., Riess, A., Mutchler, M., Sirianni, M. & Mack, J. ACS charge transfer efficiency. Results from internal and external tests. 2005 HST Calibration Workshop: Hubble After the Transition to Two-Gyro Mode (eds Koekemoer, A. M., Goudfrooij, P. & Dressel, L. L.) (2006).

  38. Schönrich, R., Binney, J. & Dehnen, W. Local kinematics and the local standard of rest. Mon. Not. R. Astron. Soc. 403, 1829–1833 (2010).

    Article  ADS  Google Scholar 

  39. Binney, J. & Tremaine, S. Galactic Dynamics 2nd edn (eds Binney, J. & Tremaine, S.) (Princeton Univ. Press, Princeton, NJ, 2008).

  40. Tolstoy, E. et al. Two distinct ancient components in the Sculptor dwarf spheroidal galaxy: first results from the Dwarf Abundances and Radial velocities Team. Astrophys. J. Lett. 617, 119–122 (2004).

    Article  ADS  Google Scholar 

  41. Zinn, R. & West, M. J. The globular cluster system of the galaxy. III—Measurements of radial velocity and metallicity for 60 clusters and a compilation of metallicities for 121 clusters. Astrophys. J. Suppl. 55, 45–66 (1984).

    Article  ADS  Google Scholar 

  42. Irwin, M. & Hatzidimitriou, D. Structural parameters for the galactic dwarf spheroidals. Mon. Not. R. Astron. Soc. 277, 1354–1378 (1995).

    Article  ADS  Google Scholar 

Download references

Acknowledgements

We made use of data from the European Space Agency mission Gaia (http://www.cosmos.esa.int/gaia), processed by the Gaia Data Processing and Analysis Consortium (http://www.cosmos.esa.int/web/gaia/dpac/consortium). Funding for the Data Processing and Analysis Consortium was provided by national institutions—in particular, the institutions participating in the Gaia Multilateral Agreement. This work was also based on observations made with the National Aeronautics and Space Administration/European Space Agency HST, obtained from the Data Archive at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy under National Aeronautics and Space Administration contract NAS 5-26555. A.H. and L.P. acknowledge financial support from a Vici grant from the Netherlands Organisation for Scientific Research. M.A.B. and A.H. are grateful to Nederlandse Onderzoekschool Voor Astronomie for financial support.

Author information

Authors and Affiliations

Authors

Contributions

D.M. performed the data analysis and the PM measurements. M.A.B. developed the statistical tools. A.H. derived the relations between observables and orbital anisotropy, coordinated the work and led the scientific interpretation. L.P. performed the orbit computation. A.G.A.B. and E.T. contributed to the presentation of the paper. All authors critically contributed to the work presented here.

Corresponding authors

Correspondence to D. Massari or A. Helmi.

Ethics declarations

Competing interests

The authors declare no competing financial interests.

Additional information

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

Electronic supplementary material

Supplementary Information

Supplementary Figures 1–4

Rights and permissions

Reprints and permissions

About this article

Check for updates. Verify currency and authenticity via CrossMark

Cite this article

Massari, D., Breddels, M.A., Helmi, A. et al. Three-dimensional motions in the Sculptor dwarf galaxy as a glimpse of a new era. Nat Astron 2, 156–161 (2018). https://doi.org/10.1038/s41550-017-0322-y

Download citation

  • Received:

  • Accepted:

  • Published:

  • Issue Date:

  • DOI: https://doi.org/10.1038/s41550-017-0322-y

This article is cited by

Search

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