Dependence of 4000\AA~ break strength on narrow emission line properties in local Type-2 AGN

Abstract

In this manuscript, we report evidence to support the dependence of D{\it n}4000 (4000\AA~ break strength to trace stellar ages) on central AGN activity traced by narrow emission line properties in local Type-2 AGN in SDSS DR16. Based on the measured D{\it n}4000 and flux ratios of [O~{\sc iii}] to narrow H$\beta$ (O3HB) and [N~{\sc ii}] to narrow H$\alpha$ (N2HA) and narrow H$\alpha$ line luminosity $L_{H\alpha}$, linear dependence of the D{\it n}4000 on the O3HB, N2HA and $L_{H\alpha}$ in the local Type-2 AGN can provide clues to support the dependence of D{\it n}4000 on properties of narrow emission lines. Linear correlations between the D{\it n}4000 and the O3HB and N2HA can be found in the local Type-2 AGN, with Spearman rank correlations about -0.39 and 0.53. Meanwhile, stronger dependence of the D{\it n}4000 on the $L_{H\alpha}$ can be confirmed in Type-2 AGN, with Spearman rank correlation coefficient about -0.7. Moreover, combining the $L_{H\alpha}$ and the N2HA, a more robust and stronger linear correlation can be confirmed between the D{\it n}4000 and the new parameter $LR=0.2\log(L_{H\alpha})-0.5\log(\rm N2HA)$, with Spearman rank correlation coefficient about -0.76 and with smaller RMS scatters. After considering necessary effects, the dependence of D{\it n}4000 on $LR$ is stable and robust enough for the local Type-2 AGN, indicating the $LR$ on the narrow emission lines can be treated as a better indicator to statistically trace stellar ages of samples of more luminous AGN with weaker host galaxy absorption features.