The Differences between High Excitation Radio Galaxies and Low Excitation Radio Galaxies

Radio galaxies can be classified as Fanaroff-Riley (FR) I or FR II sources according to their radio morphology. FR I radio galaxies have an edge-darkened radio structure, while FR II galaxies are defined by edge-brightened radio jets terminating in compact hot spots. Besides the FR dichotomy, radio galaxies can also be separated into high excitation galaxies (HEGs) and low excitation galaxies (LEGs) by their spectroscopic nuclear properties: HEGs have [O III] / Ha > 0.2 and equivalent width of [O III] > 3 Å, while LEGs have relatively weaker [OIII]. The distinction in these two kinds of the sources is still not clear. Someone speculated that the differences may be caused by different accretion modes in them, i.e., LEGs are powered by hot gas while cold material is accreted in HEGs. However, all HEGs have FR II radio morphology, but LEGs involves both FR I and FR II jet. With this in mind, we speculate that besides accretion mode there should be other parameters to determine the classification of RGs, such as the jet interaction with the external environment and the luminosity of host galaxy. (paper)