A Tale of Three Galaxies: Anomalous Dust Properties in IRASF10398+1455,
  IRASF21013-0739 and SDSSJ0808+3948
  On a galactic scale the 9.7um silicate emission is usually only seen in type
1 active galactic nuclei (AGNs). They usually also display a flat emission
continuum at ~5--8um and the absence of polycyclic aromatic hydrocarbon (PAH)
emission bands. In contrast, starburst galaxies, luminous infrared (IR)
galaxies (LIRGs), and ultraluminous IR galaxies (ULIRGs) exhibit a red 5--8um
emission continuum, strong 10um and 18um silicate absorption features, and
strong PAH emission bands. Here we report the detection of anomalous dust
properties by Spitzer/Infrared Spectrograph in three galaxies (IRASF10398+1455,
IRASF21013-0739 and SDSSJ0808+3948) which are characterized by the simultaneous
detection of a red 5--8um emission continuum, the 9.7 and 18um 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.
