Astron. Astrophys. 349, 424-434 (1999)

Giant molecular clouds in the dwarf galaxy NGC 1569

C.L. Taylor ¹, S. Hüttemeister ², U. Klein ² and A. Greve <sup>3

1</sup> Ruhr-Universität Bochum, Astronomisches Institut, Universitätsstrasse 150, D-44780 Bochum, Germany
² Universität Bonn, Radioastronomisches Institut, Auf dem Hügel 71, D-53121 Bonn, Germany
³ IRAM, 300 Rue de la Piscine, F-38406 St. Martin d'Hères, France

Received 17 February 1999 / Accepted 18 June 1999

Abstract

We present CO 10 and 21 observations of the dwarf starburst galaxy NGC 1569 with the IRAM interferometer on Plateau de Bure. We find the CO emission is not spatially associated with the two super star clusters in the galaxy, but rather is found in the vicinity of an HII region. With the resolution of our data, we can resolve the CO emission into five distinct giant molecular clouds, four are detected at both transitions. In the 10 transition the sizes and linewidths are similar to those of GMCs in the Milky Way Galaxy and other nearby systems, with diameters ranging from 40 to 50 pc and linewidths from 4 to 9 km s^-1. The (2-1)/(1-0) line ratios range from 0.64 0.30 to 1.31 0.60 in the different clouds. The lower line ratios are similar to those seen in typical Galactic GMCs, while values higher than unity are often seen in interacting or starburst galaxies. We use the virial theorem to derive the CO-H₂ conversion factor for three of the clouds, and we adopt an average value of 6.6 1.5 times the Galactic conversion factor for NGC 1569 in general. We discuss the role of the molecular gas in NGC 1569, and its relationship to the hot component of the ISM. Finally, we compare our observations with blue compact dwarf galaxies which have been mapped in CO.

Key words: ISM: molecules galaxies: individual: NGC 1569 galaxies: ISM galaxies: starburst radio lines: galaxies

Send offprint requests to: C.L. Taylor

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Contents

• 1. Introduction
• 2. Observations and data reduction
• 3. Results
  • 3.1. Cloud diameters, line widths and line ratios
  • 3.2. The CO-H₂ conversion factor
• 4. Discussion
  • 4.1. The physical conditions of the molecular gas
  • 4.2. The relationship between the super star clusters and the GMCs
  • 4.3. The hot and cold phases of the ISM
  • 4.4. Comparision with other galaxies
    • 4.4.1. dIrrs
    • 4.4.2. BCDs
• 5. Summary and conclusions
• Acknowledgements
• References

© European Southern Observatory (ESO) 1999

Online publication: September 2, 1999
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