Discovery of a strong Baldwin effect in mid-infrared AGN lines

TL;DR

This study reports the discovery of a strong Baldwin effect in mid-infrared emission lines of nearby Seyfert galaxies, revealing an anti-correlation between line strength and luminosity at small spatial scales.

Contribution

It presents the first detection of a Baldwin effect in mid-infrared AGN lines, using high-resolution observations from VLT/VISIR, highlighting a strong anti-correlation at sub-100 pc scales.

Findings

Strong anti-correlation with a slope of approximately -0.6.

Most line emission originates from within 100 pc of the nucleus.

Obscuration effects may influence the observed Baldwin effect in some cases.

Abstract

We present the discovery of a Baldwin effect in 8 nearby Seyfert galaxies for the three most prominent mid-infrared forbidden emission lines observable from the ground that are commonly found in AGN, [ArIII](8.99 micron), [SIV](10.51 micron), and [NeII](12.81 micron). The observations were carried out using the VLT/VISIR imager and spectroraph at the ESO/Paranal observatory. The bulk of the observed line emission comes from the inner <0.4 arcsec which corresponds to spatial scales <100 pc in our object sample. The correlation index is approximately -0.6 without significant difference among the lines. This is the strongest anti-correlation between line equivalent width and continuum luminosity found so far. In the case of Circinus, we show that despite the use of mid-infrared lines, obscuration by either the host galaxy or the circumnuclear dust torus might affect the equivalent widths. Given the small observed spatial scales from which most of the line emission emanates, it is unclear how these observations fit into the favored "disappearing NLR'' scenario for the narrow-line Baldwin effect.