Unveiling hidden active nuclei in MaNGA star-forming galaxies with HeII$\lambda$4686 line emission

TL;DR

This study uses the HeII$\\lambda$4686 line emission diagnostic to identify hidden active galactic nuclei in star-forming galaxies within the MaNGA survey, revealing a higher AGN fraction than traditional methods and highlighting the importance of alternative diagnostics.

Contribution

First application of spatially-resolved HeII diagnostic in MaNGA data to uncover hidden AGN in star-forming galaxies, expanding the AGN census beyond BPT classifications.

Findings

HeII diagnostic identifies 27 AGN missed by BPT.

Overall AGN fraction increases from 9% to 11% in MaNGA.

HeII-only AGN are in massive, star-forming galaxies.

Abstract

Nebular HeII$\lambda$4686\AA~line emission is useful to unveil active galactic nuclei (AGN) residing in actively star-forming (SF) galaxies, typically missed by the standard BPT classification. Here we adopt the HeII diagnostic to identify hidden AGN in the Local Universe using for the first time spatially-resolved data from the Data Release 15 of the Mapping Nearby Galaxies at APO survey (MaNGA DR15). By combining results from HeII and BPT diagnostics, we overall select 459 AGN host candidates ($\sim$10% in MaNGA DR15), out of which 27 are identified as AGN by the HeII diagram only. The HeII-only AGN population is hosted by massive (M$_*\gtrsim10^{10}$ M$_{\odot}$) SF Main Sequence galaxies, and on average less luminous than the BPT-selected AGN. Given the HeII line faintness, we revisit our census accounting for incompleteness effects due to the HeII sensitivity limit of MaNGA. We thus obtain an overall increased fraction (11%) of AGN in MaNGA compared to the BPT-only census (9%), which further increases to 14% for galaxies more massive than $10^{10}$ M$_{\odot}$; interestingly, on the SF Main Sequence the increase is by about a factor of 2. A substantial number of AGN in SF galaxies points to significant, coeval star formation and black hole accretion, consistently with results from hydrodynamical simulations and with important implications on quenching scenarios. In view of exploring unprecedented high redshifts with JWST and new ground-based facilities, revisiting the standard BPT classification through novel emission-line diagnostics is fundamental to discover AGN in highly SF environments.