APEX mapping of H3O+ in the Sgr B2 region

Context: The cosmic-ray ionization rate zeta(CR) of dense molecular clouds is a key parameter for their dynamics and chemistry. Aims. Variations of zeta(CR) are well established, but it is unclear if these are related to source column density or to Galactic location. Methods. Using the APEX telescope, we have mapped the 364 GHz line of H3O+ in the Sgr B2 region and observed the 307 GHz line at selected positions. With the IRAM 30-m telescope we have observed the (H2O)-O-18 203 GHz line at the same positions. Results. Strong H3O+ emission is detected over a similar to 3 x 2 pc region, indicating an H3O+ column density of 10(15)-10(16) cm(-2) in an 18 beam. The H3O+ abundance of similar to 3 x 10(-9) and H3O+/H2O ratio of similar to 1/50 in the Sgr B2 envelope are consistent with models with zeta(CR) similar to 4 x 10(-16) s(-1), 3 x lower than derived from H-3(+) observations toward Sgr A, but 10 x that of local dense clouds. Conclusions. The ionization rates of interstellar clouds thus seem to be to first order determined by the ambient cosmic-ray flux, while propagation effects cause a factor of similar to 3 decrease from diffuse to dense clouds.