Spectropolarimetry of the 3.4 μm Absorption Feature in NGC 1068

Abstract

In order to test the silicate-core/organic-mantle model of galactic interstellar dust, we have performed spectropolarimetry of the 3.4 μm C–H bond stretch that is characteristic of aliphatic hydrocarbons, using the nucleus of the Seyfert 2 galaxy, NGC 1068, as a bright, dusty background source. Polarization calculations show that if the grains in NGC 1068 had the properties assigned by the core-mantle model to dust in the Galactic diffuse interstellar medium (ISM), they would cause a detectable rise in polarization over the 3.4 μm feature. No such increase is observed. We discuss modifications to the basic core-mantle model, such as changes in grain size or the existence of additional nonhydrocarbon aligned grain populations, that could better fit the observational evidence. However, we emphasize that the absence of polarization over the 3.4 μm band in NGC 1068—and, indeed, in every line of sight examined to date—can be readily explained by a population of small, unaligned carbonaceous grains with no physical connection to the silicates.