Correlation between excitation index and Eddington ratio in radio galaxies

We use a sample of 111 radio galaxies with redshift z < 0.3 to investigate their nuclear properties. The black hole masses of the sources in this sample are estimated with the velocity dispersion/luminosity of the galaxies, or the width of the broad-lines. We find that the excitation index, the relative intensity of low and high excitation lines, is correlated with the Eddington ratio for this sample. The size of the narrow-line region (NLR) was found to vary with ionizing luminosity as R-NLR proportional to L-ion(0.25) (Liu et al. 2013). Using this empirical relation, we find that the correlation between the excitation index and the Eddington ratio can be reproduced by photoionization models. We adopt two sets of spectral energy distributions (SEDs), with or without a big blue bump in ultraviolet as the ionizing continuum, and infer that the modeled correlation between the excitation index and the Eddington ratio is insensitive to the applied SED. This means that the difference between high excitation galaxies and low excitation galaxies is not caused by the different accretion modes in these sources. Instead, it may be caused by the size of the NLR.