The Spatial Variation of the 3micron Dust Features in Circinus

TL;DR

This study maps the spatial variations of hydrocarbon and PAH features in the Circinus galaxy, revealing dust distribution and extinction patterns near the galaxy's core.

Contribution

It provides the first spatially-resolved analysis of 3.4 micron hydrocarbon absorption and PAH emission in Circinus, linking dust properties to galaxy structure.

Findings

Hydrocarbon absorption decreases westward and increases eastward from the core.

PAH emission is symmetrically distributed beyond the extinction region.

The ratio of hydrocarbon to silicate absorption remains constant across the central region.

Abstract

We report spatially-resolved variations in the 3.4micron hydrocarbon absorption feature and the 3.3micron 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 tau(3.4um) ~0.1 in the core which decreases to the west and increases to the east. This is consistent with increased extinction out to ~40 pc 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 3.4micron absorption band reflects that seen in the 9.7micron silicate absorption band reported by Roche et al. (2006) and the ratio of the two absorption depths remains approximately constant across the central regions, with tau(3.4um) / tau(9.7um) ~ 0.06 +/-0.01. This indicates well-mixed hydrocarbon and silicate dust populations, with no evidence for significant changes near the nucleus.