Infrared spectroscopy in the mid- and far-infrared provides powerful
diagnostics for studying the emission regions in active galaxies. The large
variety of ionic fine structure lines can probe gas conditions in a variety of
physical conditions, from highly ionized gas excited by photons originated by
black hole accretion to gas photoionized by young stellar systems. The critical
density and the ionization potential of these transitions allow to fully cover
the density-ionization parameter space. Some examples of line ratios diagrams
using both mid-infrared and far-infrared ionic fine structure lines are
presented. The upcoming space observatory Herschel will be able to observe the
far-infrared spectra of large samples of local active galaxies. Based on the
observed near-to-far infrared emission line spectrum of the template galaxy
NGC1068, are presented the predictions for the line fluxes expected for
galaxies at high redshift. To observe spectroscopically large samples of
distant galaxies, we will have to wait fot the future space missions, like
SPICA and, ultimately, FIRI.Comment: Proc. of the Winter School "Astronomy in the submillimeter and far
infrared domains with the Herschel Space Observatory", Centre de Physique des
Houches 23 april - 04 may 200
