Plasma diagnostics in the active galactic nuclei environment

Abstract

Active Galactic Nuclei (AGNs) are among the most powerful sources of radiation in the Universe. Their tremendous energy, which can reach values up to 1047 erg s-1, makes them as bright as the entire host galaxy or even brighter, and it is produced within a very small spatial region. According to the widely accepted model of AGNs, the engine hidden in the nuclei of active galaxies is a supermassive black-hole (BH) accreting matter and emitting a complicate thermal and non-thermal continuum able to photoionize the gas filling the interstellar medium. Here we present new results about the physics of plasma in the Broad Line Region (BLR), an ensemble of gaseous clouds believed to orbit the BH at smaller distances than a parsec and to emit bright broad permitted spectral lines. The width of these lines is caused by Doppler broadening because of the clouds motion in the BH gravitational potential. Given its small size, a detailed observation and analysis of the BLR is not possible, and it appears to us as a point-like light source. Therefore, we are forced to use its spectroscopic features to derive its physical parameters. Several indications suggest that BLR clouds are characterized by high values of electron density, higher than 109 cm-3, likely as high as 10 12-1014 cm-3. At such densities it is in principle possible to assume the condition of partial LTE and apply the Boltzmann equation to obtain a rough estimate of the average BLR temperature. We applied this method to the brightest hydrogen emission lines of the Balmer series observed in the optical spectra of 90 type 1 AGNs. The fluxes of Hα, Hβ, Hγ, Hδ and when possible also Hε lines were measured and plotted against the excitation energy featuring the upper level of these transitions to obtain the so-called Boltzmann plot (Popović, 2003). A least-square fit of the points allowed us to successfully determine the temperature of the BLR plasma in most of the targets, obtaining reasonable values typically in the range 10 000-50 000 K. Moreover, we plotted such temperatures against the FWHM of the ionized gas, which is an indication of the clouds kinematics, confirming that, even with a large spread, there is a clear trend for high velocity clouds to be colder. High velocities are observed in quasars, highly luminous AGNs with very massive BHs, while lower velocities are typical of Seyfert galaxies, less luminous and less massive AGNs. Therefore, in the hypothesis of Keplerian motion around the BH, our results suggest that the relation between the average distance of the BLR and the luminosity of the AGN should be featured by a slope higher than 0.5. This strengthens the recent findings of Kaspi et al. (2000), who, fitting observational data of quasars and Seyfert galaxies, obtained a dependence of the BLR size on the AGN luminosity, expressed as RBLR ∝ L0.69 in the optical domain. © 2006 American Institute of Physics.