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A confirmed location in the Galactic halo for the high-velocity cloud ‘chain A’

Nature volume 400, pages 138–141 (1999)Cite this article

Abstract

The high-velocity clouds of atomic hydrogen, discovered about 35 years ago1,2, have velocities inconsistent with simple Galactic rotation models that generally fit the stars and gas in the Milky Way disk. Their origins and role in Galactic evolution remain poorly understood3, largely for lack of information on their distances. The high-velocity clouds might result from gas blown from the Milky Way disk into the halo by supernovae4,5, in which case they would enrich the Galaxy with heavy elements as they fall back onto the disk. Alternatively, they may consist of metal-poor gas—remnants of the era of galaxy formation2,6,7,8, accreted by the Galaxy and reducing its metal abundance. Or they might be truly extragalactic objects in the Local Group of galaxies7,8,9. Here we report a firm distance bracket for a large high-velocity cloud, chain A, which places it in the Milky Way halo (2.5 to 7 kiloparsecs above the Galactic plane), rather than at an extragalactic distance, and constrains its gas mass to between 105 and 2×106 solar masses.

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Figure 1: Partial lightcurve of the RR Lyr star AD UMa, measured in yellow light.
Figure 3: Spectrum of AD UMa near the Ca IIK line (λ0 = 3,933.663 å), taken during maximum phase on 18 January 1997.
Figure 4: The predicted profile of the stellar Fe Iline (with λ0 = 3,930.299 å) compared with the observed spectrum.
Figure 2: H Iemission (top panel) and Ca IIabsorption measured towards AD UMa, which lies at 10 kpc distance, projected on HVC chain A.

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Acknowledgements

The William Herschel Telescope (WHT) is operated by the Royal Greenwich Observatory, in the Observatorio del Roque de los Muchachos of the Instituto de Astrofisica de Canarias, with financial support from PPARC (UK) and NWO (NL). We thank the NFRA La Palma Programme Committee, and in particular H. Henrichs, for their support for our programme. The Effelsberg Telescope belongs to the Max Planck Institute for Radio Astronomy in Bonn. B.P.W. was partly supported by NASA through STScI, which is operated by AURA, Inc, and also thanks B. Savage for financial support and useful discussions.

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Authors and Affiliations

  1. Kapteyn Institute, Postbus 800, AV Groningen, 9700, The Netherlands

    Hugo van Woerden, Ulrich J. Schwarz & Reynier F. Peletier

  2. Department of Physics, University of Durham, South Road, Durham, DH1 3LE, UK

    Reynier F. Peletier

  3. Department of Astronomy, University of Wisconsin, Madison, 53706, Wisconsin, USA

    Bart P. Wakker

  4. Radio-astronomisches Institut, Universität Bonn, Bonn, 53121, Germany

    Peter M. W. Kalberla

Authors
  1. Hugo van Woerden
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  4. Bart P. Wakker
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  5. Peter M. W. Kalberla
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Correspondence to Hugo van Woerden.

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van Woerden, H., Schwarz, U., Peletier, R. et al. A confirmed location in the Galactic halo for the high-velocity cloud ‘chain A’. Nature 400, 138–141 (1999). https://doi.org/10.1038/22061

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