Mid-Infrared Fine Structure Line Ratios in Active Galactic Nuclei Observed with Spitzer IRS: Evidence for Extinction by the Torus

摘要

We present the first systematic investigation of the [NeV] (14um/24um) and[SIII] (18um/33um) infrared line flux ratios, traditionally used to estimatethe density of the ionized gas, in a sample of 41 Type 1 and Type 2 activegalactic nuclei (AGNs) observed with the Infrared Spectrograph on boardSpitzer. The majority of galaxies with both [NeV] lines detected have observed[NeV] line flux ratios consistent with or below the theoretical low densitylimit, based on calculations using currently available collision strengths andignoring absorption and stimulated emission. We find that Type 2 AGNs havelower line flux ratios than Type 1 AGNs and that all of the galaxies with lineflux ratios below the low density limit are Type 2 AGNs. We argue thatdifferential infrared extinction to the [NeV] emitting region due to dust inthe obscuring torus is responsible for the ratios below the low density limitand we suggest that the ratio may be a tracer of the inclination angle of thetorus to our line of sight. Because the temperature of the gas, the amount ofextinction, and the effect of absorption and stimulated emission on the lineratios are all unknown, we are not able to determine the electron densitiesassociated with the [NeV] line flux ratios for the objects in our sample. Wealso find that the [SIII] emission from the galaxies in our sample is extendedand originates primarily in star forming regions. Since the emission fromlow-ionization species is extended, any analysis using line flux ratios fromsuch species obtained from slits of different sizes is invalid for most nearbygalaxies.