The Mid-Infrared Narrow Line Baldwin Effect Revealed by Spitzer

Abstract

We present our discovery of a narrow-line Baldwin effect, an anti-correlation between the equivalent width (EW) of a line and the flux of the associated continuum, in 5-20$\mu$m mid-infared lines from a sample of 68 Active Galactic Nuclei (AGN), located at z$<$0.5, observed with the Infrared Spectrograph on the {\it Spitzer Space Telescope}. Our analysis reveals a clear anti-correlation between the EW of the [SIV] 10.51$\mu$m, [NeII] 12.81$\mu$m, and [NeIII] 15.56$\mu$m lines and their mid-IR continuum luminosities, while the Baldwin effect for [NeV] 14.32$\mu$m is not as obvious. We suggest that this anti-correlation is driven by the central AGN and not circumnuclear star formation in the host galaxy. We also find that the slope of the narrow-line Baldwin effect in the mid-infrared does not appear to steepen with increasing ionization potential. Examining the dependence of the EW to the Eddington Ratio ($L/L_{Edd}$) we find no strong relationship for mid-IR lines. Our study indicates that the narrow-line mid-infrared Baldwin Effect is quite different from the broad-line optical/UV Baldwin effect and it is possible that the two effects are unrelated. The discovered anti-correlations open new possibilities in the understanding the physics of the ionizing region and the continuum reprocessing by dust.