Singular value decomposition: A tool to separate elementary contributions in ISOCAM spectral maps

Singular value decomposition (SVD) is applied to a mid-infrared ISOCAM spec tral map of NGC 7023. As a first result, this decomposition provides a math ematical analysis of the map in terms of a linear combination of elementary spectra. After further processing, it is shown that the spectrum observed on each pixel can be described as the physical superposition of four compon ents. Three of them can be attributed to the intrinsic spectra of polycycli c aromatic hydrocarbons (PAH), very small grains and larger dust grains res pectively. The fourth component is a differential spectrum that could trace the ionisation state of PAHs.