Towards a complete census of AGN in nearby Galaxies: a large population of optically unidentified AGN

2009/06/22 by Andy Goulding, Andy D. Goulding, D. M. Alexander +1 · 228 citations
Physics and Astronomy · #Active galactic nucleus #Astronomy #Astrophysics #Astrophysics and Star Formation Studies #Extinction (optical mineralogy) #Galaxies: Formation, Evolution, Phenomena #Galaxy #Luminosity #Luminous infrared galaxy #Physics #Population #Star formation #Stellar, planetary, and galactic studies #astro-ph.CO #astro-ph.HE

paper · pdf · doi:10.1111/j.1365-2966.2009.15194.x

published in Monthly Notices of the Royal Astronomical Society 398(3), 1165-1193 (Oxford University Press) · 33 pages, 11 figures, accepted for publication in MNRAS, revised after received feedback

arxiv created 2009/06/22 · openalex publication_date 2009/08/04 · arxiv updated 2015/05/13 · openalex created_date 2016/06/24 · openalex updated_date 2026/08/05

Abstract

Using Spitzer-IRS spectroscopy, we investigate the ubiquity of Active Galactic Nuclei (AGN) in a complete (~94%), volume-limited sample of the most bolometrically-luminous galaxies (L_IR > (0.3-20) x 10¹0 L_sun) to D < 15 Mpc. Our analyses are based on the detection of the high-excitation emission line [NeV](λ 14.32 um; 97.1 eV) to unambiguously identify AGN activity. We find that 17 of the 64 IR-bright galaxies in our sample host AGN activity (~27⁺⁸_-6%), >50% of which are not identified as AGNs using optical spectroscopy. The large AGN fraction indicates a tighter connection between AGN activity and IR luminosity for galaxies in the local Universe than previously found, potentially indicating a close association between AGN activity and star formation. The optically unidentified AGNs span a wide range of galaxy type (S0-Ir) and are typically starburst-dominated systems hosting modest-luminosity AGN activity (L_[NeV] ~ 10³7 - 10³9 erg s^-1). The non-identification of optical AGN signatures in the majority of these galaxies appears to be due to extinction towards the AGN, rather than intrinsically low-luminosity AGN activity. Examination of optical images shows that the optically unidentified AGNs with evidence for extinction are hosted in either highly inclined galaxies or galaxies with dust lanes, indicating that obscuration of the AGN is not necessarily due to an obscuring torus. We therefore conclude that optical spectroscopic surveys miss approximately half of the AGN population simply due to extinction through the host galaxy.

Citations

• Detection of Nuclear X-Ray Sources in Nearby Galaxies with [ITAL]Chandra[/ITAL]
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• The Spitzer Space Telescope Mission
• A Relationship between Nuclear Black Hole Mass and Galaxy Velocity Dispersion
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• On the X-ray, optical emission line and black hole mass properties of local Seyfert galaxies

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• The highly obscured Seyfert 2 nucleus in NGC 1448 observed with MUSE
• ACCESS - II. A complete census of star formation in the Shapley supercluster - UV and IR luminosity functions
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