We present ISO-PHOT spectra of the regions 2.5–4.9 \mu { m } and 5.8–11.6 \mu { m } for a sample of 45 disk galaxies from the U.S . ISO 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 ( 2.5–4.9 \mu { m } ; NIR ) spectral region ; ( 2 ) a weak NIR excess continuum , which has a color temperature of \sim 10 ^ { 3 } K , carries a luminosity of a few percent of the total far-infrared dust luminosity L _ { \mathrm { FIR } } , and most likely arises from the ISM ; and ( 3 ) the well-known broad emission features at 6.2 , 7.7 , 8.6 and 11.3 \mu { m } , which are generally attributed to aromatic carbon particles .
These aromatic features in emission ( AFEs ) dominate the mid-infrared ( 5.8–11.6 \mu { m } ; MIR ) part of the spectrum , and resemble the so-called Type-A spectra observed in many non-stellar 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 \mu { m } .

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 \mu { m } -to-100 \mu { m } flux density ratio R ( 60 / 100 ) or the far-infrared-to-blue luminosity ratio L _ { \mathrm { FIR } } / L _ { \mathrm { B } } , 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 far-infrared emission from large dust grains .
In particular , the summed luminosity of the AFEs falls from \sim 0.2 \mbox { $L _ { \mathrm { FIR } } $ } for the most “ IR-quiescent ” galaxies to \sim 0.1 \mbox { $L _ { \mathrm { FIR } } $ } 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 .