The Messy Nature of Fiber Spectra: Star-Quasar Pairs Masquerading as Dual Type 1 AGNs

TL;DR

This study reveals that apparent dual Type 1 AGNs in minor mergers are often false positives caused by fiber spillover effects and foreground stars, highlighting challenges in optical spectroscopic identification of dual AGNs.

Contribution

It demonstrates that fiber spillover effects can mimic dual AGN signatures in optical spectra, emphasizing the need for spatially-resolved spectroscopy to confirm true dual AGNs.

Findings

Fiber spillover effects can produce false dual AGN signatures.

Foreground stars can be mistaken for secondary AGN nuclei.

Spatially-resolved spectroscopy is essential for accurate dual AGN identification.

Abstract

Theoretical studies predict that the most significant growth of supermassive black holes occurs in late-stage mergers, coinciding with the manifestation of dual active galactic nuclei (AGNs), and both major and minor mergers are expected to be important for dual AGN growth. In fact, dual AGNs in minor mergers should be signposts for efficient minor merger-induced SMBH growth for both the more and less massive progenitor. We identified two candidate dual AGNs residing in apparent minor mergers with mass ratios of $\sim$1:7 and $\sim$1:30. SDSS fiber spectra show broad and narrow emission lines in the primary nuclei of each merger while only a narrow [O III] emission line and a broad and prominent H$\alpha$/[N II] complex is observed in the secondary nuclei. The FWHMs of the broad H$\alpha$ lines in the primary and secondary nuclei are inconsistent in each merger, suggesting that each nucleus in each merger hosts a Type 1 AGN. However, spatially-resolved LBT optical spectroscopy reveal rest-frame stellar absorption features, indicating the secondary sources are foreground stars and that the previously detected broad lines are likely the result of fiber spillover effects induced by the atmospheric seeing at the time of the SDSS observations. This study demonstrates for the first time that optical spectroscopic searches for Type 1/Type 1 pairs similarly suffer from fiber spillover effects as has been observed previously for Seyfert 2 dual AGN candidates. The presence of foreground stars may not have been clear if an instrument with more limited wavelength range or limited sensitivity had been used.