[Paper Review] Dust in active nuclei. II. Powder or gravel?

R. Maiolino, A. Marconi+1 more|ArXiv.org|Oct 3, 2000

Astrophysics and Star Formation Studies57 references123 citations

TL;DR

This paper proposes that dust in the circumnuclear regions of active galactic nuclei (AGNs) is dominated by large grains formed through coagulation in high-density environments, which explains the observed anomalous extinction properties—such as reduced E_B-V/N_H and A_V/N_H ratios, the absence of the 9.7 μm silicate absorption feature in Sy2s, and the lack of the 2175Å carbon dip in reddened Sy1s. The large-grain scenario produces a flatter, featureless extinction curve that matches multiple observational constraints better than standard Galactic dust models.

ABSTRACT

In a companion paper, Maiolino et al. (2000) presented various observational evidences for "anomalous" dust properties in the circumnuclear region of AGNs and, in particular, the reduced E(B-V)/N_H and Av/N_H ratios, the absence of the silicate absorption feature in mid-IR spectra of Sy2s and the absence of the carbon dip in UV spectra of reddened Sy1s. In this paper we discuss various explanations for these facts. The observational constraints favor a scenario where coagulation, catalyzed by the high densities in the circumnuclear region, yields to the formation of large grains. The resulting extinction curve is featureless, flatter than Galactic and the E(B-V)/N_H and Av/N_H ratios are significantly reduced. These results should warn about an unappropriate use of the standard Galactic extinction curve and Av/N_H ratio when dealing with the extreme gas conditions typical of the circumnuclear clouds of AGNs. We also investigated alternative scenarios for the observed anomalous properties of dust in AGNs. Some of these scenarios might explain some of the observed properties for a few objects, but they generally fail to account for all of the observational constraints obtained for the large sample of AGNs studied in these works.

Motivation & Objective

• To explain the anomalous dust properties observed in AGNs, including reduced E_B-V/N_H and A_V/N_H ratios, absence of silicate absorption in Sy2s, and lack of the 2175Å carbon dip in Sy1s.
• To challenge the assumption that Galactic extinction laws apply to the extreme conditions in AGN circumnuclear regions.
• To evaluate whether alternative explanations—such as high metallicity, scattering geometry, or low dust-to-gas ratios—can account for all observational constraints.
• To demonstrate that dust coagulation in high-density environments naturally produces a flatter, featureless extinction curve consistent with observations.
• To caution against the use of standard Galactic extinction curves in AGN studies, especially for obscured sources and scattering models.

Proposed method

• Modeling dust extinction curves under the assumption of a grain size distribution dominated by large grains formed via coagulation in high-density environments.
• Comparing theoretical extinction curves with observed E_B-V/N_H and A_V/N_H ratios from X-ray and optical data in a sample of 20 AGNs.
• Analyzing mid-IR spectra of Sy2s to assess the strength and presence of the 9.7 μm silicate absorption feature.
• Examining UV spectra of reddened Sy1s for the presence or absence of the 2175Å carbon dip as a tracer of small grain depletion.
• Testing alternative scenarios such as high metallicity, scattering geometry, BLR size effects, and low dust-to-gas ratios using quantitative constraints.
• Using the Cloudy code to simulate ionized gas and dust effects, ensuring consistency with observed emission line fluxes and column densities.

Experimental results

Research questions

• RQ1 Why is the E_B-V/N_H ratio in AGNs significantly lower than the Galactic standard value, especially in high-luminosity Seyfert 2s?
• RQ2 Why is the silicate absorption feature at 9.7 μm absent in the average mid-IR spectrum of Seyfert 2 galaxies despite strong PAH emission indicating high dust column density?
• RQ3 Why is the 2175Å carbon dip missing in the UV spectra of reddened type 1 AGNs, suggesting a depletion of small dust grains?
• RQ4 Can alternative explanations—such as high metallicity, scattering geometry, or low dust-to-gas ratios—account for all observed anomalies simultaneously?
• RQ5 How does a dust grain distribution dominated by large grains affect the shape of the extinction curve and the scattering properties in AGNs?

Key findings

• The reduced E_B-V/N_H ratio in AGNs—ranging from a factor of 3 to 100 below the Galactic standard—can be explained by a dust grain size distribution dominated by large grains formed through coagulation.
• The absence of the 9.7 μm silicate absorption feature in Sy2s is naturally explained by a lack of small grains, as large grains produce a featureless extinction curve.
• The lack of the 2175Å carbon dip in UV spectra of reddened Sy1s indicates a significant depletion of small dust grains, consistent with a large-grain-dominated medium.
• The large-grain scenario produces a flatter, featureless extinction curve that matches all observed constraints better than standard Galactic dust models.
• Coagulation in high-density circumnuclear regions is the most plausible physical mechanism for forming large grains, as it is naturally expected in such environments.
• The scattering efficiency of large grains is reduced by a factor of ~2 to over 100 compared to Galactic dust, meaning that some gray-reflected spectra previously attributed to electron scattering may instead be due to large-grain scattering.