An atlas of mid-infrared spectra of galaxy nuclei

Abstract

The spectral energy distributions are presented for 60 galaxies for which 8–13 μm, and in some cases 17–23 μm, spectra have been obtained. These infrared (IR) bright galaxies are discussed in terms of their spectral properties in the mid-IR and the relationship with their far-IR properties. Almost all of the galaxies can be placed in one of three classes on the basis of their 8–13 μm spectra: those dominated by the family of narrow bands between 3–13 μm, those with featureless continua and those that display the silicate absorption band. H II region galaxy nuclei, i.e. those which contain powerful nuclear H II regions and whose mid-IR spectra are dominated by emission in the family of narrow emission bands display little dispersion in their IR properties. The equivalent width of the narrow emission bands, quantified by that of the 11.3- μm feature, the ratios of the intensity of the 11.3- μm feature to the [Ne II] fine-structure emission line and the IRAS 12–100 ,urn flux ratios vary little from object to object and a ‘generic’ 8–100 μm spectrum is proposed for the H II region nuclei. In contrast to the spectral uniformity of H II region nuclei, galaxies containing active nuclei have a marked range in spectral properties, from featureless mid-IR spectra to domination by deep silicate absorption bands, but with little evidence of the narrow emission features. The diversity in the 8–13 μm spectra of active galactic nuclei (AGN) is reflected in the dispersion of their far-IR properties. The silicate absorption band in AGN probably arises in dusty regions relatively close to the central source, but there is no detailed correspondence between the strength of the 10- μm silicate absorption and the far-IR emission. It is possible that the precise geometry of circumnuclear matter and the viewing angle from the Earth are important factors in determining the observed spectra of the active nuclei. The emission from H II region galaxies arises in extended diffuse regions where these factors are of less consequence. The region of the IRAS colour–colour diagrams that lies between the domains of the luminous AGN and the H IIregion galaxies may be populated by galaxies that contain both active nuclei and circumnuclear H II regions. However, the mid-IR spectra do not resemble simple mixtures of the signatures of these components, probably because of the influence of the AGN on the circumnuclear environment. The dramatic differences in the spectral signatures of the AGN and H II region nuclei imply that if starbursts are the precursors of AGN, the latter must become dominant on short time-scales.