An X-ray/SDSS sample: II. AGN-driven outflowing gas plasma properties

Aims. Galaxy-scale outflows are currently observed in many active galactic nuclei (AGNs); however, characterisation of them in terms of their (multi-) phase nature, amount of flowing material, and effects on their host galaxy is still unresolved. In particular, ionised gas mass outflow rate and related energetics are still affected by many sources of uncertainty. In this respect, outflowing gas plasma conditions, being largely unknown, play a crucial role. Methods. We have analysed stacked spectra and sub-samples of sources with high signal-to-noise temperature-A nd density-sensitive emission lines to derive the plasma properties of the outflowing ionised gas component. We did this by taking advantage of the spectroscopic analysis results we obtained while studying the X-ray/SDSS sample of 563 AGNs at z < 0.8 presented in our companion paper. For these sources, we also studied in detail various diagnostic diagrams to infer information about outflowing gas ionisation mechanisms. Results. We derive, for the first time, median values for electron temperature and density of outflowing gas from medium-size samples (~ 30 targets) and stacked spectra of AGNs. Evidence of shock excitation are found for outflowing gas. Conclusions. We measure electron temperatures of the order of ~ 1.7 × 104K and densities of ~ 1200 cm-3for faint and moderately luminous AGNs (intrinsic X-ray luminosity 40.5 < log (LX) < 44 in the 2-10 keV band). We note that the electron density that is usually assumed (Ne= 100 cm-3) in ejected material might result in relevant overestimates of flow mass rates and energetics and, as a consequence, of the effects of AGN-driven outflows on the host galaxy.