Towards a More Complete Optical Census of Active Galactic Nuclei, Via Spatially-Resolved Spectroscopy

TL;DR

This study demonstrates that spatially resolved spectroscopy significantly increases the detection of active galactic nuclei (AGNs) compared to traditional single-fiber optical spectra, revealing hidden or missed AGNs in galaxy surveys.

Contribution

The paper introduces a method using spatially resolved spectroscopy to identify AGNs that are missed by single-fiber spectra, effectively doubling the known AGN count in the studied sample.

Findings

7-10 out of 10 galaxies host X-ray AGNs despite non-classification in optical spectra.

Many AGNs are obscured, in companion galaxies, or are low luminosity, flickering, or dust-obscured.

Spatially resolved spectroscopy uncovers a more complete AGN census.

Abstract

While emission-line flux ratio diagnostics are the most common technique for identifying active galactic nuclei (AGNs) in optical spectra, applying this approach to single fiber spectra of galaxies can omit entire subpopulations of AGNs. Here, we use spatially resolved spectroscopy from the Mapping Nearby Galaxies at Apache Point Observatory (MaNGA) survey to construct a sample of 10 galaxies where Baldwin-Philips-Terlevich line flux ratio diagnostics classify each galaxy's central $3^{\prime\prime}$ spectrum as LINER or star forming, while $>10\%$ of the spaxels in the galaxy's MaNGA footprint are classified as Seyfert. We obtain Chandra observations of these 10 galaxies with off-nuclear Seyfert regions to determine whether AGNs are actually present in them. Our main result is that 7-10 (depending on strictness of criteria) of the galaxies host one or more X-ray AGNs, even though none of them were classified as AGNs based on their single-fiber optical spectra. We find that these AGNs were not identified in the single-fiber spectra because they are AGNs in the nuclei of companion galaxies, low luminosity AGNs, dust obscured AGNs, and/or flickering AGNs. In summary, we find that off-nuclear AGN signatures may increase the number of known AGNs by a factor of two over what conventional single nuclear fiber spectra identify. Our results show that spatially resolved spectroscopy can be leveraged to reveal a more complete census of AGNs that are traditionally missed by single fiber spectra.