Physical properties of the broad line region in active galactic nuclei

We present here the study of the plasma in the broad line region (BLR) of active galactic nuclei (AGN). In order to probe the physical properties of the emitting plasma in the BLR we analyze the fluxes of the following broad emission lines (BELs): the hydrogen Balmer lines (Hα to Hɛ) and the helium lines from two subsequent ionization levels (He II λ4686 and He I λ5876). The BELs are obtained from the spectral synthesis photoionization code CLOUDY. We investigate these BELs in order to find conditions in the BLR where so-called Boltzmann-plot (BP) can be applied, and we found that in a number of modeled spectra it is working. We used these spectra to explore the dependence between plasma parameters (e.g. the averaged temperature, hydrogen density, etc.) and the ratio of He II λ4686 and He I λ5876 lines. In this progress report we present our investigation of the emitting plasma in the BLR using the most intensive broad spectral lines in AGN spectra.