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ISO-SWS spectra of galaxies: Continuum and features *
E. Sturm 1,2,
D. Lutz 1,
D. Tran 1,
H. Feuchtgruber 1,
R. Genzel 1,
D. Kunze 1,
A.F.M. Moorwood 3 and
M.D. Thornley 4
Received 19 October 1999 / Accepted 26 January 2000
We present an inventory of mid-infrared spectral features detected in high resolution (R1500) ISO-SWS 2.4-45µm spectra of the galaxies M 82, NGC 253, Circinus, NGC 1068, and a position in the 30 Doradus region of the Large Magellanic Cloud. We discuss their identifications and highlight possible relations between these features and the physical state of the interstellar medium in galaxies. The spectral features vary considerably from source to source in presence and relative strength. Emission features are largely absent in the intense radiation field close to an AGN. Compared to normal infrared-selected starbursts, they also seem to be weaker in a low metallicity, intensely star forming environment. The large number of features beyond 13µm is remarkable. Some of the features have - to our knowledge - not been reported before in astronomical objects.
In the 5-13µm region, emission from unidentified infrared bands (UIBs), usually ascribed to aromatic molecules, and apparent silicate absorption dominate the spectrum. The density of features makes it difficult to determine the continuum, particularly in ground-based data of limited wavelength coverage. In fact the apparent depth of the 9.7µm silicate absorption may be overestimated in the presence of UIB emission, as we demonstrate by comparing the spectrum of M 82 to the (absorption free) spectrum of the reflection nebula NGC 7023. No strong silicate absorption is present in M 82. The (very small grain) dust continuum under the UIB emission in our starburst templates can be modeled by a simple power law, starting at wavelengths between 8 and 9µm.
We find broad H2O-ice absorption features at 3.0µm in M 82 and NGC 253. Their optical depths (relative to the visual extinction) indicate that the lines of sight towards these galaxies have similar properties as the line of sight towards the Galactic Center. The active galaxy NGC 1068 exhibits a clearly different spectrum of absorption features, indicating different physical conditions in the obscuring regions of this AGN compared to the starburst templates.
The spectra are valuable templates for future mid-infrared missions. We smooth our data to simulate low resolution spectra as obtained with ISOCAM-CVF, ISOPHOT-S, and in the future with the low resolution mode of SIRTF-IRS, and use our high spectral resolution information to highlight possible identification problems at low resolving power that are caused by coincidences of lines and features. The spectra are available in electronic form from the authors.
Key words: galaxies: individual: M 82 galaxies: individual: NGC 253 galaxies: individual: Circinus galaxies: individual: NGC 1068 infrared: ISM: lines and bands infrared: ISM: continuum
* Based on observations with ISO, an ESA project with instruments funded by ESA member states (especially the PI countries: France, Germany, the Netherlands and the United Kingdom) and with participation of ISAS and NASA.
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Online publication: June 8, 2000