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Abstract: While the best tracer of the molecular component and its dynamics in galaxiesis the CO molecule, which excitation is revealed by its isotopic and2-1-1-0 ratios, the denser gas is revealed by molecules such as HCN, HNC,HCO+ or CN, which are now widely used to probe star formation regions, or toquantify the impact of the nuclear activity on the interstellar medium.This paper reviews recent observations in nearby galaxies, where thesemolecular line ratios serve as diagnostic tools of the physical conditions ofthe gas and also of its chemical properties. Those differ significantlyaccording to the proximity of an AGN or of a starburst. The origin of thedifferences is not yet well known and could be due to different densities,temperatures, chemical abundances or non-collisional excitation of the gase.g. Aalto et al 2007, Krips et al 2007.HCN or HNC line enhancements can be caused not only by higher gasdensities-temperatures, but also UV-X-ray radiation, and global IR pumping. Thechemistry can be dominated by PDR regions near a starburst, or X-ray dominatedin a molecular torus surrounding an AGN XDR regions. The molecular lineratios expected in those regions vary according to the different modelsMeijerink et al. 2007.

Autor: F. Combes LERMA, Obs-Paris


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