Letter | Published:

Unexpected stellar velocity distribution in the warped Galactic disk

Nature volume 392, pages 471473 (02 April 1998) | Download Citation


It is now over 40 years since radio observations of neutral hydrogen revealed1 the gaseous disk of our Galaxy to be warped. Subsequently, the warp has been detected in the distribution of Galactic dust2, molecular clouds3, and luminous stars4,5. Roughly half of all spiral galaxies have similarly warped disks, which suggests that warps are a common and long-lived phenomenon. However, there is still no consensus as to what induces galactic disks to become warped: intergalactic winds, tidal interactions with satellites, magnetic pressure and massive dark haloes have all been proposed as causative agents. Here we use data from the Hipparcos satellite6 to determine the small stellar motions in the plane of the sky (proper motions) that should accompany the warp, but which are undetectable in the gas. We find that although the spatial distribution of the stars is in line with previous studies of hydrogen, the velocity distribution has the opposite sign to that expected. Finding a plausible explanation of this result may be the key to solving the long-standing puzzle posed by galactic warps.

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  1. 1.

    AMagellanic effect on the galaxy. Astron. J. 62, 93 (1957).

  2. 2.

    DIRBE evidence for a warp in the interstellar dust layer and stellar disk of the Galaxy. Astron. J. 429, L69–L72 (1994).

  3. 3.

    , , & IRAS sources beyond the solar circle. II-Distribution in the Galactic warp. Astron. Astrophys. 230, 21–36 (1990).

  4. 4.

    & The warp of the Galactic stellar disk detected in IRAS source counts. Astrophys. J. 341, L13–L16 (1989).

  5. 5.

    , & An optical warp of the galaxy. Astron. Astrophys. 194, 107–115 (1988).

  6. 6.

    HIPPARCOS — astrometry from space. Nature 340, 111–113 (1989).

  7. 7.

    The Hipparcos and Tycho Catalogues (Tech. Rep. SP-1200, ESA, (1997)).

  8. 8.

    in Proc. HIPPARCOS Venices '97 (ed. Battrick, B.) 45–48 (SP-402, ESA Noordwijk, (1997)).

  9. 9.

    , & in Proc. HIPPARCOS Venice '97 Proc. (ed. Battrick, B.) 715–720 (SP-402, ESA Noordwijk, (1997)).

  10. 10.

    , & in Proc. HIPPARCOS Venice '97 (ed. Battrick, B.) (SP-402, ESA Noordwijk, (1997)).

  11. 11.

    & The Galactic warp in young star proper motions. Astron. Astrophys. 314, 104–107 (1996).

  12. 12.

    The shape and color of the galactic disk. Astron. J. 468, 663–678 (1996).

  13. 13.

    & Neutral hydrogen in the outer galaxy. Astrophys. J. 377, 126–140 (1990).

  14. 14.

    Warps. Annu. Rev. Astron. Astrophys. 30, 51–74 (1992).

  15. 15.

    The vertical distribution and kinematics of HI and mass models of the galactic disk. Astrophys. J. 448, 138–148 (1995).

  16. 16.

    , & Adwarf satellite galaxy in sagittarius. Nature 370, 194–196 (1994).

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  1. *Osservatorio Astronomico di Torino, Pino Torinese, TO 10025, Italy

    • R. L. Smart
    • , R. Drimmel
    •  & M. G. Lattanzi
  2. †Department of Physics, University of Oxford, Oxford OX1 3NP, UK

    • J. J. Binney


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Correspondence to R. L. Smart.

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