Balmer Lines as Diagnostics of Physical Conditions in Active Galactic Nuclei Broad Emission Line Regions

Abstract

Using a well-known method for laboratory plasma diagnostics, the Boltzmann plot, we discuss the physical properties in the broad-line regions (BLRs) of active galactic nuclei (AGNs). We apply the Boltzmann plot method to Balmer lines on a sample of 14 AGNs and find that it may indicate the existence of case B recombination or partial local thermodynamical equilibrium (PLTE). For the BLR of AGNs, where PLTE exists, we estimated the electron temperature and density of the BLRs. The estimated electron temperatures (T ~ 13,000-37,000 K) are in good agreement with previous estimates. The estimated electron densities depend on the opacity of the emitting plasma in the BLRs. They range from Ne ~ 109 cm-3 for optically thick to Ne ~ 1014 cm-3 for optically thin emission plasma in the BLRs. The estimated electron temperature has been shown to be velocity-dependent, and it decreases for higher velocities. Although the alternative explanation to PLTE indicated by the Boltzmann plot may be considered (e.g., high intrinsic reddening), the method may give a quick estimate of physical conditions in the BLRs before applying the sophisticated methods.