Unidentified infrared bands in the interstellar medium across the Galaxy

We present a set of 6-12  (mu)m ISOPHOT-S spectra of the general interstellar medium of the Milky Way. This part of the spectrum is dominated by a series of strong, wide emission features commonly called the Unidentified Infrared Bands. The sampled area covers the inner Milky Way from l =  -60(degree) to +60(degree) with a ten-degree step in longitude and nominal latitudes b = 0(degree), +- 1(degree). For each grid position the actual observed direction was selected from IRAS 100  (mu)m maps to minimize contamination by point sources and molecular clouds. All spectra were found to display the same spectral features. Band ratios are independent of band strength and Galactic coordinates. A comparison of total observed flux in band features and IRAS 100  (mu)m emission, a tracer for large interstellar dust grains, shows high correlation at large as well as small (1´) scales. This implies a strong connection between large dust grains and the elusive band carriers; the evolutionary history and heating energy source of these populations must be strongly linked. The average mid-infrared spectrum of the Milky Way is found to be GROUP the average spectrum of spiral galaxy NGC 891 GROUP The common spectrum can therefore be used as a template for the 6-12  (mu)m emission of late-type spiral galaxies. Finally, we show that interstellar extinction only weakly influences the observed features even at (lambda)= 10(mu)m, where the silicate absorption feature is strongest.