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Astron. Astrophys. 340, L51-L54 (1998)

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Letter to the Editor

Molecular gas in the Cartwheel Galaxy *

C. Horellou 1, V. Charmandaris 2, F. Combes 2, P.N. Appleton 3, F. Casoli 2 and I.F. Mirabel 4, 5

1 Onsala Space Observatory, Chalmers University of Technology, S-43992 Onsala, Sweden
2 Observatoire de Paris, DEMIRM, 61 Avenue de l'Observatoire, F-75014 Paris, France
3 Department of Physics and Astronomy, Iowa State University, Ames IA, 50011, USA
4 Service d'Astrophysique, CEA-Saclay, F-91191 Gif-sur-Yvette Cedex, France
5 Instituto de Astronomía y Física del Espacio, cc 67, suc 28, (1428) Buenos Aires, Argentina

Received 28 July 1998 / Accepted 21 October 1998


We present the first detection of molecular gas in the Cartwheel, the prototype of a collisional ring galaxy formed in the head-on encounter of two galaxies. Until now, only very little atomic gas and no CO had been detected in the centre, where gas is theoretically expected to pile up. Using the Swedish ESO Submm Telescope, we detected both 12CO(1-0) and (2-1) line emission towards the central position. The line ratio and the line widths suggest that the CO(2-1) emission is sub-thermal and that the CO(1-0) emission arises within the central 22" (13 kpc); it is probably associated with the inner ring and nucleus. We infer a mass of molecular gas (H2) of 1.5 to 6 [FORMULA] [FORMULA], which is significantly higher than the [FORMULA] [FORMULA] of atomic gas within that region. The low excitation of the gas, whether it is due to a low temperature or a low density, is consistent with the weak star-forming activity observed in the centre of the Cartwheel.

Key words: galaxies: individual: AM 0035-33 – galaxies: individual: The Cartwheel – galaxies: ISM – radio lines: galaxies

* based on observations made with the Swedish-ESO Submillimeter Telescope (SEST) at La Silla, Chile

Send offprint requests to: C. Horellou

© European Southern Observatory (ESO) 1998

Online publication: November 9, 1998