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Astron. Astrophys. 352, 371-382 (1999)

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7. Conclusions

We have constructed a sample of 102 nearby galaxies detected by IRAS at 60 µm and for which UV observations at 0.2 µm are available down to [FORMULA]. Only five galaxies have no UV detection implying an extinction larger than 2-3 mag for these objects which are also very faint in FIR.

The FIR and UV properties of our sample have been compared to the mean properties of the local Universe deduced from the luminosity functions and densities at both wavelengths. As the galaxies become brighter in FIR their FIR to UV flux ratio, i.e. their extinction increases: [FORMULA] which translates to an increase of [FORMULA] mag for the dust extinction in UV per decade of FIR luminosity.

The ratio of the FIR to UV local luminosity densities is much lower than that found in individual galaxies. It is also true for other samples of nearby galaxies usually considered as low redshift templates like the IUE sample of Calzetti Kinney and collaborators. Such a difference is likely to be due to the large contribution of low UV luminosity galaxies to the UV luminosity density: these galaxies are deficient in any survey. At FIR wavelengths such faint galaxies do not significantly contribute and our sample is more representative of the galaxy population in terms of its contribution to the FIR luminosity density.

As a consequence, much caution must be taken to correct large samples of galaxies for extinction. In particular a uniform correction deduced from the study of some individual cases cannot be valid.

Star formation rates can be estimated by accounting for both the FIR and UV emissions: each one is translated to a quantitative SFR; then, the two SFRs are summed. The SFRs such deduced are consistent with those calculated from the UV emission corrected for extinction.

A local volume-average star formation rate is calculated from the FIR and UV luminosity density: [FORMULA]. This is consistent with a global extinction of [FORMULA] mag at 0.2 µm.

Endly, we have compared the FIR and UV properties of our sample of galaxies to those of nearby and high redshift UltraLuminous Infrared Galaxies observed both at UV and FIR rest frame wavelengths and to the ISOCAM survey of a CFRS field. All these objects extend toward the large luminosities the trend found for the nearby galaxies of a larger FIR to UV ratio for brighter galaxies. The ULIGs are very extreme with UV extinctions reaching 8-11 mag. Although more moderate the extinctions we find for the ISOCAM/CFRS objects not classified as Seyfert are comprised between 2 and 5.5 mag. These calculations are only tentative due to the large uncertainties about the FIR emission of these objects.

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© European Southern Observatory (ESO) 1999

Online publication: December 2, 1999