Active galactic nuclei as seen by the Spitzer Space Telescope

The Spitzer Space Telescope revolutionized studies of active galactic nuclei (AGNs). Its combined sensitivity and mapping speed at mid-infrared wavelengths revealed a substantial population of highly obscured AGNs. This population implies a higher radiative accretion efficiency, and thus possibly a higher spin, for black holes than indicated by surveys at optical or X-ray wavelengths. The unique mid-infrared spectrographic capability of Spitzer gave important insights into the distribution and nature of the dust surrounding AGNs, enabling the separation of AGN and starburst components, the detection of silicate features in emission from hot dust, and the identification of shocked gas associated with AGN activity. Spitzer’s sensitivity allowed almost complete identification of the host galaxies of samples of AGNs selected in the X-ray and radio. As we look forward to the James Webb Space Telescope, the lessons learned from Spitzer studies will inform observational programmes with new and upcoming infrared facilities. Spitzer revolutionized studies of active galactic nuclei through its high sensitivity and mapping speed at mid-infrared wavelengths. This Review summarizes key Spitzer insights on active galactic nuclei, including the properties of their dust and shocked gas.