Infrared emission of normal galaxies from 3 to 12 mu m

We report infrared spectra over wavelength ranges from 2.5 to 5 mu m and from 6 to 11.6 mu m on a sample of 45 disk galaxies whose bolometric luminosities are derived predominately from stellar sources. The vast majority of the sample spectra look "alike" and can be modelled by a set of emission features (EFs or UIR bands) including those at 6.2, 7.7, 8.6 and 11.3 mu m and their associated emission plateaux, superposed on a low-level continuum that possibly extends to and mixes with a photospheric continuum at near-infrared (IR) wavelengths. These continua, which dominate the near-IR part of the spectrum, and the EFs, which dominate the mid-IR part, contribute about equally to the total spectral energy between 3 and 12 mu m that in turn carries a luminosity equivalent to about 10% to 40% of the far-IR luminosity for a normal galaxy. From the most quiescent to the most active galaxies in the sample, we found (i) little systematic trend in the relative strengths of the mid-IR features, except for the 11.3 mu m feature that could be depressed relative to the other features by up to a factor of 2 or so; and (ii) the ratio of the EF flux to the far-IR flux drops by about 60% on average.