Based on observations with ISO, an ESA project with instruments funded by ESA member states (especially the PI countries: France, Germany, The Netherlands, and the UK), and with participation of ISAS and NASA.

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Present address: Leiden Observatory, PO Box 9513, 2300 RA Leiden, The Netherlands.

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... Paris

Chercheur Associé.

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... luminosities

As computed from the IRAS fluxes following the prescription of Sanders & Mirabel (1996).

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... (CIA)

CIA is a joint development by the ESA Astrophysics Division and the ISOCAM consortium led by the ISOCAM P.I. C. Cesarsky, Direction des Sciences de la Matière, C.E.A. Saclay, France.

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... nucleus

As defined by the peak of the near-infrared 2 $~\mu$ m emission; see Table 2 for coordinates and references.

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... magnitude

Additional though indirect support for our assumption is provided by the relationship between the 7 $~\mu$ m flux measured through the ISOCAM LW2 filter (5- $8.5~{\rm\mu m}$ ) and the far-infrared (FIR, 43- $123~{\rm\mu m}$ ) flux obtained by Roussel et al. (2001b) for galactic disks, with $F_{\rm 7~\mu m} \propto (F_{\rm FIR})^{1.2}$ . Furthermore, Dale et al. (2001) derived a typical value of $\approx$ $15\%$ for the 5- $20~{\rm\mu m}$ to FIR luminosity ratio in normal galaxies while Charmandaris et al. (2002) found this ratio to be $\sim$ $5\%$ for three ULIRGs.

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