... M 51

Figure 4 is only available in electronic form at http://www.edpsciences.org

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... center

A description of IRAS HiRes reduction is available at http://irsa.ipac.caltech.edu/IRASdocs/hires_over.html

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... galaxies

Note that TIR corresponds to the bolometric FIR dust continuum emission I(FIR) used in the PDR models of Kaufman et al. (1999).

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... ratios

There are two degrees of freedom since we use four independent line ratios to fit two parameters.

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