Issue |
A&A
Volume 365, Number 2, January 2001
|
|
---|---|---|
Page(s) | 37 - 48 | |
Section | Cosmology (including clusters of galaxies) | |
DOI | https://doi.org/10.1051/0004-6361:20000012 | |
Published online | 15 January 2001 |
Dust in active nuclei
II. Powder or gravel?
Osservatorio Astrofisico di Arcetri L.go E. Fermi 5, 50125 Firenze, Italy
Corresponding author: R. Maiolino, maiolino@arcetri.astro.it
Received:
17
July
2000
Accepted:
28
September
2000
In a companion paper, Maiolino et al. ([CITE]) presented various
observational evidences for
"anomalous"dust properties in the circumnuclear region of AGNs and, in
particular, the reduced and
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
and
ratios are significantly reduced.
These results should warn about an inappropriate
use of the standard Galactic extinction curve and
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.
Key words: galaxies: Seyfert -galaxies: nuclei -galaxies: ISM - dust, extinction
© ESO, 2001
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