2015A&A...578A..49G

M 82 is one of the nearest and brightest starburst galaxies. It has been extensively studied in the past decade and by now is considered the prototypical extragalactic photon-dominated region (PDR) and a reference for studying star formation feedback. Our aim is to characterize the molecular chemistry in M 82 at spatial scales of giant molecular clouds (GMCs), ∼100pc, to investigate the feedback effects of the star formation activity. We present interferometric observations of the CN 1->0 (113.491GHz), N₂H⁺ 1->0 (93.173GHz), H(41)α (92.034GHz), CH₃CN (91.987GHz), CS 3->2 (146.969GHz), c-C₃H₂ 3_1,2->2_2,1 (145.089GHz), H₂CO 2_0,2->1_0,1 (145.603GHz), and HC₃N 16->15 (145.601GHz) lines carried out with the IRAM Plateau de Bure Interferometer (PdBI). PDR chemical modeling was used to interpret these observations. Our results show that the abundances of N₂H⁺, CS and H¹³CO⁺ remain quite constant across the galaxy, confirming that these species are excellent tracers of the dense molecular gas. In contrast, the abundance of CN increases by a factor of ∼3 in the inner x2 bar orbits. The [CN]/[N₂H⁺] ratio is well correlated with the H(41)α emission at all spatial scales down to ∼100 pc. Chemical modeling shows that the variations in the [CN]/[N₂H⁺] ratio can be explained as the consequence of differences in the local intestellar UV field and in the average cloud sizes within the nucleus of the galaxy. Our high spatial resolution imaging of the starburst galaxy M 82 shows that the star formation activity has a strong impact on the chemistry of the molecular gas. In particular, the entire nucleus behaves as a giant PDR whose chemistry is determined by the local UV flux. The detection of N₂H⁺ shows the existence of a population of clouds with A_v>20mag all across the galaxy plane. These clouds constitute the molecular gas reservoir for the formation of new stars and, although it is distributed throughout the nucleus, the highest concentration occurs in the outer x1 bar orbits (R∼280pc).