On the location and composition of the dust in the MCG–6-30-15 warm absorber

Abstract

The warm absorber observed in the Seyfert 1 galaxy MCG-6-30-15 is known to consist of at least two zones and very likely contains dust. Hubble Space Telescope images of MCG-6-30-15 show a dust lane crossing the galaxy just below the nucleus. In this paper, we argue that this dust lane is responsible for the observed reddening of the nuclear emission and the Fe  edge hinted at in the Chandra spectrum of MCG-6-30-15. We further suggest that the gas within the dust lane can comprise much of the low ionization component (i.e., the one contributing the O  edge) of the observed warm absorber. Moreover, placing the warm absorbing material at such distances (hundreds of pc) can account for the small outflow velocities of the low ionization absorption lines as well as the constancy of the O  edge. Photoionization models of a dusty interstellar gas cloud (with a column appropriate for the reddening toward MCG-6-30-15) using a toy Seyfert 1 spectral energy distribution show that it is possible to obtain a significant O  edge (τ ∼ 0.2) if the material is ∼150 pc from the ionizing source. For MCG-6-30-15, such a distance is consistent with the observed dust lane. We emphasize the point first made by Kraemer et al.: dusty interstellar material will likely contribute to the warm absorber, and should be included in spectral modeling. The current data on MCG-6-30-15 is unable to constrain the dust composition within the warm absorber. Astronomical silicate is a viable candidate, but there are indications of a very low O abundance in the dust, which is inconsistent with a silicate origin. If true, this may indicate that there were repeated cycles of grain destruction and growth from shocks in the interstellar medium of MCG-6-30-15. Pure iron grains are an unlikely dust constituent due to the limit on their abundance in the Galaxy, yet they cannot be ruled out. The high column densities inferred from the highly ionized zone of the warm absorber implies that this gas is dust-free.