The spatial variation of the 3-μm dust features in Circinus

Abstract

We report spatially resolved variations in the 3.4-μm hydrocarbon absorption feature and the 3.3-μm polycyclic aromatic hydrocarbon (PAH) emission band in the Circinus galaxy over the central few arcsec. The absorption is measured towards warm emitting dust associated with Coronal line regions to the east and west of the nucleus. There is an absorption optical depth τ 3.4μm~0.1 in the core which decreases to the west and increases to the east. This is consistent with increased extinction out to ~40pc east of the core, supported by the Coronal emission line intensities which are significantly lower to the east than the west. PAH emission is measured to be symmetrically distributed out to ±4 arcsec, outside the differential extinction region. The asymmetry in the 3.4-μm absorption band reflects that seen in the 9.7-μm silicate absorption band reported by Roche et al., and the ratio of the two absorption depths remains approximately constant across the central regions, with τ 3.4μm/τ 9.7μm~0.06 ± 0.01. This indicates wellmixed hydrocarbon and silicate dust populations, with no evidence for significant changes near the nucleus. © 2009 The Authors. Journal compilation © 2009 RAS.