2007ApJ...668..918B

In this paper we report on the detection of mid-infrared radiation extending to 5 kpc from the midplane of the nearby, edge-on galaxy NGC 891. The observations cover the range from 2 to 20 kpc above the galaxy midplane on both sides of the disk. The detection is made with the 16 and 22 µm peak-up imaging modes of the Infrared Spectrograph aboard the Spitzer Space Telescope. The measured surface brightness profiles at these two wavelengths are the same, within the uncertainties of about 30%. The surface brightness at 22 µm decreases exponentially with distance from the galaxy with a scale height of 1.3±0.3 kpc. The size of the inferred thick disk is therefore in agreement with findings at visible wavelengths. The 16 µm:22 µm brightness ratio is consistent with the mid-infrared spectral energy distribution of asymptotic giant branch stars, whose density was recently found to follow a similar spatial distribution. Diffuse interstellar dust, however, emits most of the observed mid-infrared radiation.