Different dependence of narrow H$\alpha$ line luminosity on optical continuum luminosity between starforming galaxies and Type-2 AGN: globally negative AGN feedback in local Type-2 AGN?

Abstract

In this manuscript, clues are provided to support globally negative AGN feedback on star formation in the host galaxies of the local low-redshift SDSS Type-2 AGN, based on the different dependence of narrow H$\alpha$ line luminosity $L_{H\alpha}$ on optical continuum luminosity $\lambda L_{cont}$ between the starforming galaxies and the Type-2 AGN. Through the measured $L_{H\alpha}$ and $\lambda L_{cont}$ in SDSS starforming galaxies, there is a strong linear correlation between $\lambda L_{cont}$ and $L_{H\alpha}$, accepted as a standard correlation without effects of AGN activity. Meanwhile, considering apparent contributions of AGN activity to narrow H$\alpha$ line emissions in the Type-2 AGN, the correlation between $\lambda L_{cont}$ and $L_{H\alpha}$ in the SDSS Type-2 AGN leads to statistically lower $L_{H\alpha}$ in the Type-2 AGN than in the starforming galaxies, with significance level higher than 5$\sigma$, even after considering necessary effects (including effects of host galaxy properties), leading to accepted conclusion on the globally negative AGN feedback in the local Type-2 AGN. Meanwhile, properties of Dn(4000) and H$\delta_A$ can provide indirect clues to support the globally negative AGN feedback in the local Type-2 AGN, due to older stellar ages in the Type-2 AGN. Moreover, it is interesting to expect more than 50\% narrow H$\alpha$ emissions globally suppressed in the host galaxies of the Type-2 AGN relative to the starforming galaxies. The results not only support globally negative AGN feedback in the local Type-2 AGN, but also show further clues on the quantification of suppressions of star formation by the globally negative AGN feedback.