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Preprint G/99

Discovery of molecular hydrogen in a high-velocity cloud of the Galactic halo

P. RICHTER (1), K.S. DE BOER (1), H. WIDMANN (2), N. KAPPELMANN (2), W. GRINGEL (2), M. GREWING (2) & J. BARNSTEDT (2)

(1) Sternwarte, Universität Bonn, Auf dem Hügel 71, D-53121 Bonn, Germany
(2) Institut für Astronomie und Astrophysik, Astronomie, Universität Tübingen, Waldhäuser Str. 64, D-72076 Tübingen, Germany

1999, Nature, 402, 386-387

Abstract. The Galactic halo contains in many directions clouds of neutral hydrogen with high radial velocities. These high­velocity clouds mostly have motions towards the Galactic plane but do not follow the rotational movement of the disc. Since distance information for high­velocity clouds is sparse and the kinematics cannot be fixed from just the radial velocites, a closed theory about the origin of high­velocity clouds is not available today. One idea is that these clouds are the cooling part of a Galactic fountain. Here the gas would predominantly originate in the metal­rich Galactic disc. The other suggests that they represent metal­poor gas falling onto the Milky Way from intergalactic space. The presence of molecular hydrogen, whose formation requires that dust and thus metals are present, might discriminate between the two theories. Here we report the discovery of molecular hydrogen in a Galactic high­velocity cloud in the southern sky. For the same cloud we derive an iron abundance which is half of the solar value. Thus, all evidence points to a Galactic origin for this high­velocity cloud.

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Paper (41k gzip'ed Postscript including figures)

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