Dependence of 4000 Å 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 n 4000 (4000 Å break strength to trace stellar ages) on central active galactic nuclei (AGN) activity traced by narrow emission line properties in local Type-2 AGN in SDSS DR16. Based on the measured D n 4000 and flux ratios of [O iii ] to narrow H β (O3HB) and [N ii ] to narrow H α (N2HA) and narrow H α line luminosity L H α , linear dependence of the D n 4000 on the O3HB, N2HA and L H α in the local Type-2 AGN can provide clues to support the dependence of D n 4000 on properties with narrow emission lines. Linear correlations between the D n 4000 and the O3HB and N2HA can be found in the local Type-2 AGN, with Spearman rank correlations of about −0.39 and 0.53. Meanwhile, stronger dependence of the D n 4000 on the L H α can be confirmed in Type-2 AGN, with a Spearman rank correlation coefficient of about −0.7. Moreover, combining the L H α and the N2HA, a more robust and stronger linear correlation can be confirmed between the D n 4000 and the new parameter <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>LR</mml:mi> <mml:mo>=</mml:mo> <mml:mn>0.2</mml:mn> <mml:mi>log</mml:mi> <mml:mo stretchy="false">(</mml:mo> <mml:msub> <mml:mi>L</mml:mi> <mml:mrow> <mml:mi mathvariant="normal">H</mml:mi> <mml:mi>α</mml:mi> </mml:mrow> </mml:msub> <mml:mo stretchy="false">)</mml:mo> <mml:mo>−</mml:mo> <mml:mn>0.5</mml:mn> <mml:mi>log</mml:mi> <mml:mo stretchy="false">(</mml:mo> <mml:mi mathvariant="normal">N</mml:mi> <mml:mn>2</mml:mn> <mml:mi>HA</mml:mi> <mml:mo stretchy="false">)</mml:mo> </mml:math> , with a Spearman rank correlation coefficient of about −0.76 and with smaller rms scatters. After considering necessary effects, the dependence of D n 4000 on LR is stable and robust enough for the local Type-2 AGN, indicating that 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.