A Tale of Three Galaxies: Anomalous Dust Properties in IRASF10398+1455, IRASF21013-0739 and SDSSJ0808+3948

TL;DR

This paper reports the discovery of three galaxies exhibiting unusual dust emission features that combine characteristics of both AGNs and starburst galaxies, suggesting complex dust compositions and grain properties.

Contribution

It presents the first detection of galaxies with simultaneous silicate emission and PAH features, indicating unique dust properties in young AGN phases.

Findings

Detection of silicate emission in three galaxies with strong PAH bands

Evidence for iron-poor silicate and carbon dust deficit

Implications for dust grain size and temperature in young AGNs

Abstract

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.