The 'cosmic ray gradient' and far infrared observations as a tracer of molecular material in the Galaxy

The derivation of the spatial distribution of cosmic rays from an analysis of cosmic gamma rays requires a knowledge of the distribution of gas in the Inner Galaxy, most notably that of molecular hydrogen. There has been considerable argument about the amount of this gas, an argument manifested by disagreement over the magnitude of the conversion factor, X, used in converting the strength of the 12CO line to the column density of molecular hydrogen. A promising technique has been to study the far infrared radiation from dust associated with gas but here, too, there have been differences. In the present work we analyse the situation in detail and conclude that virtually all the differences can be understood. It follows that our earlier claims for a significant gradient of cosmic ray intensity in the Galaxy (more cosmic rays towards the galactic centre) - and thus a galactic origin for the particles - is still valid. (author)