A review of diagnostic emission line ratios in the Narrow Line Region of Active Galactic Nuclei

The availability of new observing facilities both from ground and space suc h as: the Keck 10m telescope and the Hubble Space Telescope is-casting new light on the spectroscopic investigation of emission line galaxies. In part icular, it is now possible to detect spectra with a significantly improved signal to noise ratio in a very wide wavelength range, from the ultraviolet to the near infrared (HST, Keck) and beyond (ISO is unveiling the far infr ared domain). As we move to high redshifts, however, it is more likely that the observed spectra are given by the contribution of different components in the galaxy: in particular, this may be the case if an Active Galactic N ucleus (AGN hereafter) emitting a power-law continuum is surrounded by regi ons with strong star formation activity. The identification of the source w hich ionizes the line emitting regions is then complicated by the fact that Ne are observing the integrated spectrum from regions which are ionized by different sources. In this paper we wish to review which line ratios may b e used in order to discriminate between photoionization by young stars and power-law continuum in the wavelength range from 1200 Angstrom to 100 mu m. To this aim we used the photoionization code Cloudy (Ferland 1996) to pres ent a series of diagnostic diagrams showing the dependence of emission line ratios on the main input parameters of photoionization models in the case of one component models with gaseous clouds ionized by (1) a stellar contin uum typical of an HII region and (2) a power-law continuum typical of an AG N: these line ratios are plotted as isoratio maps for different values of t he hydrogen density, ionization parameter and slope of the power-law contin uum. We then show how the results may be affected by more realistic assumpt ions about the environment of the ionized gas: to this aim, we show the eff ects on selected line ratios of (a) the presence of two populations of clou ds with different densities and (b) art AGN surrounded by regions with stro ng star formation activity.