2014ApJ...794L..19X

On a galactic scale, the 9.7 µm silicate emission is usually only seen in type 1 active galactic nuclei (AGNs). They usually also display a flat emission continuum at ∼5-8 µm and the absence of polycyclic aromatic hydrocarbon (PAH) emission bands. In contrast, starburst galaxies, luminous infrared (IR) galaxies, and ultraluminous IR galaxies exhibit a red 5-8 µm emission continuum, strong 9.7 µm and 18 µm silicate absorption features, and strong PAH emission bands. Here, we report the detection of anomalous dust properties by the Spitzer/Infrared Spectrograph in three galaxies (IRAS F10398+1455, IRAS F21013-0739, and SDSS J0808+3948) which are characterized by the simultaneous detection of a red 5-8 µm emission continuum, the 9.7 and 18 µm silicate emission features, as well as strong PAH emission bands. These apparently contradictory dust IR emission properties are discussed in terms of iron-poor silicate composition, carbon dust deficit, small grain size, and low dust temperature in the young AGN phase of these three galaxies.