Letter to the Editor

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

¹ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany e-mail: shoenig@mpifr-bonn.mpg.de
² European Southern Observatory, Casilla 19001, Santiago 19, Chile
³ Institut für Theoretische Physik und Astrophysik, Christian-Albrechts-Universität zu Kiel, Leibnizstr. 15, 24098 Kiel, Germany
⁴ Zentrum für Astronomie, ITA, Universität Heidelberg, Albert-Ueberle-Str. 2, 69120 Heidelberg, Germany
⁵ RIKEN Cosmic Radiation Lab, 2-1 Hirosawa, Wakoshi Saitama 351-0198, Japan

Received: 5 May 2008
Accepted: 14 May 2008

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, [AR III](λ8.99 μm), [S IV](λ10.51 μm), and [Ne II](λ12.81 μm). The observations were carried out using the VLT/VISIR imager and spectrograph at the ESO/Paranal observatory. The bulk of the observed line emission originates in the innermost region within a diameter of 04, which corresponds to spatial scales of less than 100 pc within the targeted galaxies. The correlation index is approximately -0.6 and does not vary significantly for all lines studied. To date, this is the strongest anticorrelation that has been measured between line equivalent width and continuum luminosity. In the case of Circinus, we show that, despite using mid-infrared lines, obscuration by either the host galaxy or the circumnuclear dust torus might affect the measurement. Given the small observed spatial scales from which most of the line emission emanates, it is unclear how well these observations agree with the favored “disappearing NLR” scenario for the narrow-line Baldwin effect.