The mid-infrared spectra of normal galaxies

The mid-infrared spectra (2.5-5 and 5.7-11.6 mu m) obtained by ISOPHOT reve al the interstellar medium emission from galaxies powered by star formation to be strongly dominated by the aromatic features at 6.2, 7.7, 8.6, and 11 .3 mu m. Additional emission appears in between the features, and an underl ying continuum is clearly evident at 3-5 mu m. This continuum would contrib ute about a third of the luminosity in the 3-13 mu m range. The features to gether carry 5%-30% of the 40-120 mu m far-infrared (FIR) luminosity. The r elative fluxes in individual features depend very weakly on galaxy paramete rs such as the far-infrared colors, direct evidence that the emitting parti cles are not in thermal equilibrium. The dip at 10 mu m is unlikely to resu lt from silicate absorption since its shape is invariant among galaxies. Th e continuum component has a f(upsilon) proportional to upsilon(+0.65) shape between 3 and 5 mu m and carries 1%-4% of the FIR luminosity; its extrapol ation to longer wavelengths falls well below the spectrum in the 6-12 mu m range. This continuum component is almost certainly of nonstellar origin an d is probably due to fluctuating grains without aromatic features. The spec tra reported here typify the integrated emission from the interstellar medi um of the majority of star-forming galaxies and could thus be used to obtai n redshifts of highly extincted galaxies up to z = 3 with SIRTF.