[Paper Review] Dust in active nuclei. I. Evidence for "anomalous" properties

R. Maiolino, A. Marconi+5 more|ArXiv.org|Sep 30, 2000

Astrophysics and Star Formation StudiesPhysics and Astronomy51 references234 citations

TL;DR

This paper presents observational evidence that dust in the circumnuclear regions of active galactic nuclei (AGNs) exhibits 'anomalous' properties, with significantly lower extinction relative to gas column density than in the Galactic interstellar medium. The authors find that the E_B-V/N_H and A_V/N_H ratios in AGNs are systematically lower than Galactic values by factors of 3 to 100, indicating a dust composition dominated by large grains, which flattens the extinction curve and suppresses characteristic absorption features.

ABSTRACT

We present observational evidences that dust in the circumnuclear region of AGNs has different properties than in the Galactic diffuse interstellar medium. By comparing the reddening of optical and infrared broad lines and the X-ray absorbing column density we find that the E(B-V)/N_H ratio is nearly always lower than Galactic by a factor ranging from ~3 up to ~100. Other observational results indicate that the Av/N_H ratio is significantly lower than Galactic in various classes of AGNs including intermediate type 1.8-1.9 Seyferts, hard X-ray selected and radio selected quasars, broad absorption line QSOs and grism selected QSOs. The lack of prominent absorption features at 9.7um (silicates) and at 2175A (carbon dip) in the spectra of Seyfert 2s and of reddened Seyfert 1s, respectively, add further evidence for dust in the circumnuclear region of AGNs being different from Galactic. These observational results indicate that the dust composition in the circumnuclear region of AGNs could be dominated by large grains, which make the extinction curve flatter, featureless and are responsible for the reduction of the E(B-V)/N_H and Av/N_H ratios. Regardless of the physical origin of these phenomena, the reduced dust absorption with respect to what expected from the gaseous column density should warn about a mismatch between the optical and the X-ray classification of the active galactic nuclei in terms of their obscuration.

Motivation & Objective

• To investigate whether dust extinction in AGNs deviates from the Galactic standard extinction curve and dust-to-gas ratio.
• To determine if the observed X-ray absorption column density (N_H) correlates with optical/IR reddening (E_B-V or A_V) as expected in the Galactic model.
• To examine whether the absence of key dust features (e.g., 9.7 µm silicate absorption, 2175 Å carbon dip) in AGN spectra indicates a different dust grain population.
• To assess the implications of these anomalies for the unified model of AGNs, particularly regarding the decoupling of optical and X-ray classifications.
• To establish that the low E_B-V/N_H and A_V/N_H ratios are not due to selection effects but represent a systematic property of AGN dust.

Proposed method

• Comparison of optical and infrared broad-line ratios to derive E_B-V, using Hα/Hβ and other emission line ratios as reddening indicators.
• Use of X-ray spectral analysis to measure the absorbing column density N_H from cold absorption features in the 1–10 keV band.
• Analysis of ISO mid-infrared spectra of Seyfert 2 galaxies to search for the 9.7 µm silicate absorption feature.
• Examination of UV spectra of reddened Seyfert 1s for the presence of the 2175 Å carbon absorption dip.
• Calculation of E_B-V/N_H and A_V/N_H ratios for a sample of 20 AGNs, including intermediate-type, radio- and X-ray-selected quasars, and broad absorption line QSOs.
• Statistical comparison of observed ratios to the Galactic standard value (E_B-V/N_H = 3.1, A_V/N_H = 3.1) to identify systematic deviations.

Experimental results

Research questions

• RQ1 Is the dust extinction in AGNs consistent with the Galactic extinction curve and dust-to-gas ratio?
• RQ2 To what extent does the observed X-ray column density (N_H) match the optical/IR reddening (E_B-V or A_V) in AGNs?
• RQ3 Are the characteristic dust absorption features at 9.7 µm and 2175 Å present in the spectra of AGNs with significant reddening?
• RQ4 What physical properties of dust grains (e.g., size distribution) could explain the observed low E_B-V/N_H and A_V/N_H ratios?
• RQ5 How do these anomalies affect the classification of AGNs in the unified model, particularly the correspondence between optical and X-ray obscuration?

Key findings

• The E_B-V/N_H ratio in AGNs is systematically lower than the Galactic value by a factor of 3 to 100, with only three low-luminosity AGNs showing Galactic-like ratios.
• The A_V/N_H ratio is also significantly lower than Galactic in various AGN types, including intermediate-type 1.8–1.9 Seyferts, hard X-ray selected, radio-selected, and broad absorption line QSOs.
• The mid-infrared spectra of Seyfert 2 galaxies show no detectable 9.7 µm silicate absorption feature despite substantial dust absorption, indicating a lack of small grains.
• Reddened Seyfert 1s show no prominent 2175 Å carbon absorption dip in their UV spectra, contradicting expectations from the Galactic extinction curve.
• The observed anomalies are best explained by a dust grain population dominated by large grains, which produce a flatter extinction curve and suppress characteristic absorption features.
• These results imply a decoupling between optical and X-ray classification in AGNs, as significant X-ray obscuration can occur with minimal optical extinction.