Different Dependence of Narrow Hα Line Luminosity on Optical Continuum Luminosity between Star-forming Galaxies and Type 2 Active Galactic Nuclei: Globally Negative AGN Feedback in Local Type 2 AGN?

Abstract

In this manuscript, clues are provided to support globally negative active galactic nuclei (AGN) feedback on star formation in the host galaxies of the local low-redshift Sloan Digital Sky Survey (SDSS) Type 2 AGN, based on the different dependence of narrow H α line luminosity L _H _α on optical continuum luminosity λ L _cont between star-forming galaxies and Type 2 AGN. Through the measured L _H _α and λ L _cont in SDSS star-forming galaxies, there is a strong linear correlation between λ L _cont and L _H _α , accepted as a standard correlation without the effects of AGN activity. Meanwhile, considering the apparent contribution of AGN activity to the narrow H α line emissions in Type 2 AGN, the correlation between λ L _cont and L _H _α in the SDSS Type 2 AGN leads to a statistically lower L _H _α in Type 2 AGN than in star-forming galaxies, with a significance level higher than 5 σ , even after considering necessary effects (including effects of host galaxy properties), leading to the accepted conclusion on the globally negative AGN feedback in the local Type 2 AGN. Meanwhile, the properties of Dn(4000) and H δ _A can provide indirect clues to support the globally negative AGN feedback in local Type 2 AGN, due to older stellar ages in Type 2 AGN. Moreover, it is interesting to expect more than 50% narrow H α emissions globally suppressed in the host galaxies of Type 2 AGN relative to the star-forming galaxies. The results not only support globally negative AGN feedback in local Type 2 AGN, but also show further clues on the quantification of suppressions of star formation by the globally negative AGN feedback.

Keywords

• Active galactic nuclei
• Active galaxies
• Emission line galaxies
• AGN host galaxies