Infrared Emission of Normal Galaxies from 2.5 to 12 Micron: Infrared Space Observatory Spectra, Near-Infrared Continuum, and Mid-Infrared Emission Features

We present ISOPHOT spectra of the regions 2.5-4.9 μm and 5.8-11.6 μm for a sample of 45 disk galaxies from the US Infrared Space Observatory Key Project on Normal Galaxies. The galaxies were selected to span the range in global properties of normal, star-forming disk galaxies in the local universe. The spectra can be decomposed into three spectral components: (1) continuum emission from stellar photospheres, which dominates the near-infrared (NIR; 2.5-4.9 μm) spectral region; (2) a weak NIR excess continuum, which has a color temperature of ~103 K, carries a luminosity of a few percent of the total far-infrared (FIR) dust luminosity LFIR and most likely arises from the interstellar medium (ISM); and (3) the well-known broad emission features at 6.2, 7.7, 8.6, and 11.3 μm, which are generally attributed to aromatic carbon particles. These aromatic features in emission (AFEs) dominate the mid-infrared (MIR; 5.8-11.6 μm) part of the spectrum and resemble the so-called type A spectra observed in many nonstellar sources and the diffuse ISM in our own Galaxy. The few notable exceptions include NGC 4418, where a dust continuum replaces the AFEs in MIR, and NGC 1569, where the AFEs are weak and the strongest emission feature is [S IV] 10.51 μm. The relative strengths of the AFEs vary by 15%-25% among the galaxies. However, little correlation is seen between these variations and either IRAS 60 μm/100 μm flux density ratio R(60/100) or the FIR/blue luminosity ratio LFIR/LB, two widely used indicators of the current star formation activity, suggesting that the observed variations are not a consequence of the radiation field differences among the galaxies. We demonstrate that the NIR excess continuum and AFE emission are correlated, suggesting that they are produced by similar mechanisms and similar (or the same) material. On the other hand, as the current star formation activity increases, the overall strengths of the AFEs and the NIR excess continuum drop significantly with respect to that of the FIR emission from large dust grains. In particular, the summed luminosity of the AFEs falls from ~0.2 LFIR for the most "IR-quiescent" galaxies to ~0.1 LFIR for the most "IR-active" galaxies. This is likely a consequence of the preferential destruction in intense radiation fields of the small carriers responsible for the NIR/AFE emission.