A study of [O III]/[O II] lines ratio in type 1 active galactic nucleus: Influence of radio jets and Eddington ratio to narrow line region emission

Abstract

AbstractWe analyze emission line properties and their correlations for 18,043 type 1 active galactic nucleus (AGN) in the range of 0.02 ≤ z ≤ 0.8, based on Sloan Digital Sky Survey Data Release 14 data. We complement the data with photometric measurements from ROSAT, GALEX, 2MASS, and FIRST. We find the following: (a) The average correlation between luminosity of [O III] and AGN luminosity is stronger than those for [O II] line. This is consistent with the stratified model of narrow line region (NLR). Furthermore, we also find a weak dependence of ionization degree with AGN luminosity. (b) The narrow lines luminosity and radio luminosity correlation is stronger for radio quiet AGN and weaker for radio loud (RL) AGN, most likely due to radio jets. A possible explanation is as the jets penetrate the NLR, the shock will ionize its gas, increase the narrow lines emission, and cause a bigger dispersion in the correlation for RL AGN. (c) The investigated narrow lines, [O II], [O III], Hβ, Hα, [N II], and [S II], show a significant trend of decreasing luminosity ratio relative to the broad Hβ luminosity where the slopes in the range of −0.26 to −0.50. We did not find any correlation of the trend slopes with ionization potential or critical density of the lines. We suggest that this correlation is due to decrease in covering factor of NLR as AGN luminosity increases. (d) The EV1 correlation is found in our data and represents change in Eddington ratio. By using L[O III]/L[O II] ratio, we found that the ionization degree decreases as Eddington ratio increases. We also confirm that objects with higher Eddington ratio tend to have narrower Hβ, stronger Fe II emission, and weaker [O III] strength. In contrast to that, objects with lower Eddington ratio tend to have broader Hβ, weaker Fe II emission, and higher [O III] strength.