Free access article

A&A 402, 499-507 (2003)
DOI: 10.1051/0004-6361:20030290

Detection of strongly processed ice in the central starburst of NGC 4945

H. W. W. Spoon¹, A. F. M. Moorwood², K. M. Pontoppidan³, J. Cami⁴, M. Kregel¹, D. Lutz⁵ and A. G. G. M. Tielens<sup>1, 6

1</sup>  Kapteyn Institute, PO Box 800, 9700 AV Groningen, The Netherlands
²  European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching, Germany
³  Leiden Observatory, PO Box 9513, 2300 RA Leiden, The Netherlands
⁴  NASA-Ames Research Center, Mail Stop 245-6, Moffett Field, CA 94035, USA
⁵  Max-Planck-Institut für Extraterrestrische Physik (MPE), PO Box 1312, 85741 Garching, Germany
⁶  SRON, PO Box 800, 9700 AV Groningen, The Netherlands

(Received 7 February 2003 / Accepted 20 February 2003 )

Abstract
The composition of ice grains provides an important tool for the study of the molecular environment of star forming regions. Using ISAAC at the VLT to obtain spectra around 4.65  m we have detected for the first time "XCN" and CO ice in an extragalactic environment: the nuclear region of the nearby dusty starburst/AGN galaxy NGC 4945. The profile of the solid CO band reveals the importance of thermal processing of the ice while the prominence of the XCN band attests to the importance of energetic processing of the ice by FUV radiation and/or energetic particles. In analogy to the processing of ices by embedded protostars in our Galaxy, we attribute the processing of the ices in the center of NGC 4945 to ongoing massive star formation. Our M-band spectrum also shows strong HI Pf and H ₂ 0-0 S(9) line emission and gas phase CO absorption lines. The HI, H ₂, PAH, gas phase CO and the ices seem to be embedded in a rotating molecular disk which is undergoing vigorous star formation. Recently, strong OCN ^- absorption has been detected in the spectrum of the Galactic center star GC: IRS 19. The most likely environment for the OCN ^- absorption is the strongly UV-exposed GC molecular ring. The presence of processed ice in the center of NGC 4945 and our Galactic center leads us to believe that processed ice may be a common characteristic of dense molecular material in star forming galactic nuclei.

Key words: galaxies: individual: NGC 4945 -- galaxies: ISM -- galaxies: nuclei -- galaxies: starburst -- infrared: galaxies

Offprint request: H. W. W. Spoon, spoon@astro.rug.nl

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© ESO 2003

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