A&A 365, 28-36 (2001)
DOI: 10.1051/0004-6361:20000177
R. Maiolino1 -
A. Marconi1 -
M. Salvati1 -
G. Risaliti2 -
P. Severgnini2 -
E. Oliva1 -
F. La Franca3 -
L. Vanzi 4
Send offprint request: R. Maiolino,
1 - Osservatorio Astrofisico di Arcetri
L.go E. Fermi 5, 50125 Firenze, Italy
2 -
Dipartimento di Astronomia, Università di Firenze,
L.go E. Fermi 5, 50125 Firenze, Italy
3 -
Dipartimento di Fisica, Università degli Studi ``Roma Tre''
Via della Vasca Navale 84, 00146 Roma, Italy
4 -
European Southern Observatory, Alonso de Cordova 3107,
Santiago, Chile
Received 3 July 2000 / Accepted 28 September 2000
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
ratio is
nearly always lower than Galactic by a factor ranging from
3 up to
100. Other observational results indicate that the
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 9.7
m (silicates) and at 2175 Å (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
and
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.
Key words: galaxies: Seyfert - galaxies: nuclei - galaxies: ISM - dust, extinction
Author for correspondance: maiolino@arcetri.astro.it
The properties of the circumnuclear gas are a key issue to understand the physics of Active Galactic Nuclei (AGNs). In particular, gas obscuration has important consequences on the classification of AGNs (Antonucci 1993), on their infrared emission (Granato et al. 1997; Pier & Krolik 1993) and also on the X-ray background (Setti & Woltjer 1989). AGNs are, optically-wise, divided in two main classes: type 1 AGNs, showing broad permitted emission lines, and type 2 AGNs, which only show narrow emission lines. The Unified Model assumes that AGNs of both classes host the same kind of nuclear engine and ascribes their differences solely to orientation effects with respect to an obscuring gaseous medium, possibly arranged in a torus-like geometry. For those lines of sights intercepting the obscuring torus, both the Broad Line Region (BLR, < 1 pc in size) and the nuclear engine are obscured and only the much more extended Narrow Line Region (NLR) can be observed. This model has gained success from a large number of observational tests (see Antonucci 1993 for a review). In particular, X-ray observations have supported the unified scenario by discovering large columns of absorbing gas in type 2 AGNs (e.g. Awaki et al. 1991; Turner et al. 1997; Maiolino et al. 1998; Risaliti et al. 1999). Also, spectroscopic observations in the infrared, where dust absorption is greatly reduced, detected broad permitted lines in several AGNs which are classified as type 2 in the optical.
However, the properties of the absorbing medium result to be
more complex with a more quantitative analysis.
Maccacaro et al. (1982) first noted, in a few AGNs,
that the dust reddening affecting the BLR is significantly lower than
expected from the
measured in the X-rays, assuming a Galactic
standard extinction curve and dust-to-gas ratio. An analogous result
was obtained by Reichert et al. (1985).
Indications for
a low value of
in the obscuring torus
were also found by Granato et al. (1997) by modelling the IR emission
of AGNs; they ascribed the low
ratio to the sublimation of
dust at the inner face of the torus.
The goal of this paper is to observationally verify the
low
and low
phenomena with a higher confidence (Sects. 2 and 3).
We also investigate dust spectral signatures which directly
probe the properties of dust grains in the circumnuclear region of AGNs.
Most of the interpretation of these observational effects is addressed in a
companion paper (Maiolino et al. 2001, Paper II).
Finally, note that in this paper
we will often distinguish the
and the
ratios since, as discussed in Paper II, in the
circumnuclear region of AGNs
dust reddening and obscuration are not necessary tied by the Galactic standard
relation (
AV/EB-V=3.1).
Throughout this paper we will assume a cosmology with H0=65 and q0=0.5.
Name | EB-Va |
![]() |
![]() |
z | Redd. est.d | Log
![]() |
Refs.f | |
(1020 cm-2) | (rel. Gal.) | (ergs-1) | opt/IR | X | ||||
M 81 |
![]() |
9.4 +0.7-0.6 | 4.11 -1.52+1.57 | 0.00037 | H![]() ![]() |
40.52 | 4 | 3 |
NGC 4639 |
![]() |
7.3 +5.6-5.1 | 3.04 -3.02+2.98 | 0.00544 | H![]() ![]() |
40.92 | 1 | 1 |
NGC 5033 |
![]() |
8.7 +1.7-1.7 | 4.84 -1.87+1.92 | 0.00292 | H![]() ![]() |
41.10 | 2 | 3 |
NGC 1365 |
![]() |
2000 +400-500 | 0.022 -0.005+0.005 | 0.005 | Br![]() ![]() |
42.42 | 7 | 3 |
Mk 231 |
![]() |
370 +260-150 | 0.054 -0.041+0.027 | 0.042 | Pa![]() ![]() |
42.66 | 21 | 20 |
IRAS 13197-1627 |
![]() |
3000 +1100-1000 | 0.0092 -0.0044+0.0042 | 0.01654 | H![]() ![]() |
42.66 | 18 | 19 |
SAX J1519+65 |
![]() |
1580 +410-320 | 0.020 -0.012+0.007 | 0.044 | cont. | 42.74 | 5 | 5 |
N5506 |
![]() |
340 +26-12 | 0.27 -0.10+0.10 | 0.00618 | Br![]() ![]() |
42.96 | 8,9 | 3 |
N2992 |
![]() |
90 +3-3 | 0.38 -0.27+0.26 | 0.0077 | Br![]() ![]() |
43.05 | 11 | 11 |
SAX J0045-25 |
![]() |
390 +870-260 | 0.075 -0.173+0.067 | 0.111 | cont. | 43.06 | 5 | 5 |
Mkn6 |
![]() |
333 +29-16 | 0.13 0.02+0.02 | 0.01847 | H![]() ![]() |
43.17 | 14 | 14 |
MCG-5-23-16 |
![]() |
162 +23-21 | 0.220 -0.215+0.214 | 0.00827 | Br![]() ![]() |
43.22 | 6 | 3 |
SAX J1218+29 |
![]() |
1250 +1900-750 | 0.030 -0.047+0.019 | 0.176 | cont. | 43.29 | 5 | 5 |
IRAS 05189-2524 |
![]() |
840 +80-65 | 0.049 -0.025+0.025 | 0.042 | Pa![]() ![]() |
43.44 | 19 | 19 |
NGC 526a |
![]() |
150 +14-14 | 0.39 -0.10+0.10 | 0.01922 | Pa![]() ![]() |
43.55 | 7,22 | 3 |
3C 445 |
![]() |
580 +320-180 | 0.088 -0.053+0.034 | 0.057 | H![]() ![]() |
43.79 | 15 | 16 |
SAX J1353+18 |
![]() |
154 +37-32 | 0.33 -0.10+0.09 | 0.2166 | Pa![]() ![]() |
43.91 | 7,23 | 10 |
AX J0341-44 | <1.59 | 1000 +400-400 | <0.09 | 0.672 | H![]() ![]() |
44.21 | 12 | 13 |
PG 2251+11 |
![]() |
60 +30-22 | 0.33 -0.21+0.17 | 0.3255 | H![]() ![]() |
44.67 | 17 | 16 |
The gaseous
along the line of sight
is derived directly by the photoelectric
cutoff in the X-ray spectrum, provided that the signal-to-noise is high
enough. Table 1 lists the
derived for the sources in our sample
along with the intrinsic 2-10 keV luminosity (i.e. absorption corrected).
As mentioned above we excluded objects whose X-ray spectrum shows
evidence for warm absorption as based on the presence of the absorption edges
of OVII and OVIII at 0.74 and 0.87 keV respectively.
For most of the objects
in Table 1 the X-ray absorption is ``cold'', in the sense that the chi-squared
of the spectral
fit is significantly better with the latter model than
with a warm absorption model (this is discussed in some of the references
reported in Table 1). For some objects
the signal-to-noise of the X-ray spectrum is not high enough to
discard warm absorption; this is certainly the case for
SAX0045-25, SAX1218+29, SAX1519+65, AXJ0341-44, and
Mkn231. Yet, the spectral shape in most of the latter cases is such that a fit
with a warm absorber would require an unrealistically high column of warm gas
(
,
possibly with the exception of Mkn231) thus
favoring the cold absorption model. Finally, even in those cases
for which cold absorption provides a better fit with respect to a warm
absorber this generally does not exclude that both components are present.
Also, ionized gas along the line of sight might not be sampled by
the OVII and OVIII absorption edges if the ionization stage is too low or too
high (Kraemer et al. 1999, 2000; Brandt et al.
1996; Reynolds & Fabian 1995).
However, the presence of ionized gas along the line of sight
which is not detected in the X-ray spectra would make the total column
of gas (neutral+ionized) higher than inferred assuming a single cold,
component and, as we shall see, the problem of the reduced
would be even worse.
Ratios between broad components of the hydrogen lines compared
to the intrinsic values give the amount of dust reddening affecting
the BLR. However, radiative transport and collisional excitation effects
in the extreme conditions of the BLR clouds (
)
can affect the standard hydrogen line ratios expected in the case B
recombination. For instance, BLR models expect the H
/H
Balmer
decrement to range
from the ``standard'' ratio of 3.1 up to a factor of 3 higher (Rees et al.
1989; Netzer et al. 1985; Mushotzky & Ferland
1984), this
is mostly due to the large optical thickness of H
whose
de-excitation
transition has a high probability of being split into Pa
and
H
.
Nonetheless, the H
/H
ratio observed in Sy1s and QSOs is often
consistent with the standard case B value. Should the
intrinsic ratio be higher, the observed H
/H
compared to the
case Bvalue provides at least an upper limit to the reddening.
Ratios between infrared broad hydrogen lines
(Pa
,
Pa
and
Br
)
provide a more reliable measure
of the reddening, since they are much less
affected by the radiative transport effects discussed above.
In some cases the broad lines were not measured
simultaneously and therefore variability might have affected the real ratios.
As a consequence of what is discussed above we adopted the following criteria to
select the broad line pairs to be used for the reddening determination.
When more than two
broad lines were available, to avoid problems related to the variability
we chose those lines which
were observed (nearly) simultaneously.
The cases for which the broad lines were measured simultaneously
are marked with a ``*'' in Col. 6
of Table 1 (these cases are the majority). When more than two simultaneous broad
lines were available we used the
ratios involving only near-IR lines rather than
H/H
.
Finally,
Pa
/H
,
Pa
/H
and (in one case) H
/H
were
used only in a few cases.
The line ratio adopted to estimate the reddening for each object is given in
Col. 6 of Table 1.
To determine the reddening EB-V from the broad line ratios an extinction curve must be assumed. We have assumed the ``standard'' Galactic extinction curve (Savage & Mathis 1979). To derive the extinction we also assumed a foreground screen, which is a reasonable assumption given that the BLR is within the sublimation radius (and therefore mostly dust-free) and that the torus is probably more extended than the BLR (Paper II). Column 2 of Table 1 lists the dust reddening toward the BLR derived for each object.
We also include in our study three AGNs (namely SAX J0045-25, SAX J1218+29, SAX J1519+65) whose reddening was not inferred from the ratios of broad hydrogen lines but by a detailed fit to the continuum and of the EW of the H lines. More specifically, the optical spectrum (including shape, stellar features and broad H lines), along with near-IR to U-band spectroscopic measurements, were fitted with stellar population synthesis templates of various ages combined with an AGN template reddened by various degrees of extinction. The three free parameters required to fit the data were the relative contribution of the stellar and of the AGN component, the age of the stellar population and the reddening of the AGNs. For a more detailed description of the method see Maiolino et al. (2000) and Vignali et al. (2000). Although the large number of observational constraints allow a relatively good determination of the three free parameters, the determination of the reddening is not as accurate as for the broad lines ratio method. However, for one object (SAX J1353+18) both methods could be used and gave consistent results (Maiolino et al. 2000; Vignali et al. 2000). The interesting property of these objects is that they were selected (and actually discovered) in the hard X-rays (Fiore et al. 1999) and, therefore, are not affected by some of the biases that might affect the other objects and which will be discussed later.
In Col. 4 of Table 1 we report the
ratio relative to the
Galactic standard value of 1.7 10-22 mag cm2 (Bohlin et al. 1978).
Except for a few cases,
is significantly lower than the Galactic
standard value, by a factor ranging from a few to
100.
This is graphically shown in Fig. 1 where the
ratio relative to Galactic is plotted as a function of the intrinsic X-ray
luminosity. Figure 1 does not really show a correlation between the two
quantities, but rather a bimodal behavior:
AGNs with luminosities higher
than
are systematically
characterized by
lower
than Galactic, though they show a large spread, while those few
Low Luminosity AGNs
(LLAGNs) in our sample,
with
ergs-1, are characterized by
consistent with, or even higher, than Galactic. The markedly different
behavior of LLAGNs
with respect to the other AGNs in the sample, might reflect the fact that
the physics of these objects is intrinsically different from the ``classical''
AGNs, as suggested by various authors (e.g. Ho 1999).
In the following we will focus on the other AGNs in our
sample, which have luminosities more typical of classical Seyfert galaxies or
of QSOs.
![]() |
Figure 1:
![]() ![]() |
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Admittedly,
our sample is not very large and is not representative of the
population of obscured AGNs, given that we had to select our objects in a
relatively narrow range of absorptions. In particular, the absorption must
be low enough to
enable us to detect the broad lines. We cannot exclude the existence of
AGNs (in the normal-high luminosity range
)
with
an
consistent with Galactic. For instance, objects with
higher than a few times
and Galactic
would be characterized by an extinction AV so high to
make broad lines undetectable, possibly even in the IR, and therefore would
be excluded from our sample. Nonetheless, we would like to stress the
following results: 1) the existence of a population of AGNs characterized
by a value of
significantly lower than Galactic
is proven beyond any doubt;
2) with the exception of the three LLAGNs discussed above, we could not
find AGNs whose
is consistent with the Galactic
standard value
.
Determining if this is a property common to all AGNs or determining what
fraction of them is characterized by this feature cannot be done with the
current sample alone.
The evidence for a low
discussed above might be
translated into evidence for a low
if a standard
conversion factor
AV/EB-V = 3.1 applies to these objects.
Indeed, such a conversion factor might not apply
and, therefore, a reduced reddening does not necessary
imply a reduced absorption. However, there are other observational
evidences supporting the idea that also the AV/
ratio AGNs
is generally significantly lower than the Galactic standard value.
The intermediate type Seyferts discussed in the former section are only
a small fraction of those available in various Seyfert samples; in
particular the discussion in Sect. 2 is limited to
the sources for which enough information is available to derive both
reddening and gaseous column density. In some of these objects
the weakness of the
broad H
or H
lines is due to intrinsic properties of the BLR
(Goodrich 1995), but more often
the weakness of the broad lines lines is ascribed to dust absorption (Maiolino
& Rieke 1995). In the latter case
the extinction must be about
,
given that usually in these
objects the
faint broad lines have a flux similar to the narrow components, while
in type 1 objects broad lines are about 10 times stronger than the
narrow lines. On the other hand, most of the intermediate type Seyferts
are characterized by an absorbing column density between 1022 cm-2and 1023 cm-2 and, in some cases,
even higher than 1023 cm-2 (Risaliti et al. 1999).
With a Galactic conversion factor such column densities would imply
an extinction of
,
which would obviously make any
broad line undetectable (unless reflected, Paper II). This suggests
that, at least in this class of objects, the AV/
ratio must
be about a factor of 10 lower than Galactic.
Local Seyfert 1 galaxies are generally characterized by an X-ray spectrum with low or no cold absorption, in agreement with the extinction inferred by their optical spectra. However, at higher luminosities, there is evidence for a population of QSOs whose hard X-ray spectrum is characterized by significant cold absorption along the line of sight although their optical spectrum has prominent broad lines and, usually, also a blue continuum typical of unabsorbed AGNs.
In particular, although most of the
optically selected PG QSOs do not show evidence for X-ray
gaseous absorption in excess of the Galactic value, a few of them are
affected by an excess absorption with
cm-2.
This is shown in the top panel of Fig. 2, where the distribution of
cold absorbing columns, as inferred from the hard X-rays, is
reported for the PG QSOs.
![]() |
Figure 2: Distribution of gaseous absorbing column density, as inferred from hard X-ray observations, for various samples of type 1, broad line AGNs, more specifically: a subsample of the PG QSOs (top), a sample of hard X-ray selected QSO (middle) and a sample of radio selected AGNs (bottom) |
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Obviously, even a small amount of dust associated with the gaseous column
along the line of sight can redden the QSO continuum enough to be missed
by U-B color selection criterion of the PG survey. Indeed,
surveys of type 1 QSOs selected at wavelength less sensitive to
dust obscuration and reddening
have found a larger fraction of objects which show
evidence for significant gaseous absorption. In particular, the lower
two panels of Fig. 2 show the distribution of
for the
type 1 QSOs in
the ASCA survey presented by Akiyama et al. (2000)
and in the radio-selected type 1
AGNs presented by Sambruna et al. (1999). These distributions clearly
show a larger fraction of type 1 AGNs with
even in excess of
.
On the other hand, the prominent broad lines and,
often, the blue continuum (although not as ``blue'' as for the PG QSOs)
suggest that the optical absorption and reddening cannot be large.
In particular, Akiyama et al. (2000)
show that, although significant X-ray
absorption is observed in their ASCA selected objects, the ratio between
optical and hard X-ray emission
is not different from soft X-ray
selected QSOs (i.e. not affected by significant
absorption). This indicates
that optical absorption must be lower than about 1 mag
and, together with the
inferred from the X-rays,
indicates an
about a factor of ten lower than Galactic.
The HELLAS BeppoSAX survey (Fiore et al. 1999; La Franca et al.
in prep)
has also
found some type 1 blue QSOs with flat X-ray spectra suggesting large columns of
gas along our line of sight. Although statistically less relevant than the
surveys mentioned above, we could study some of these blue X-ray-absorbed
QSO more in
detail (e.g. Maiolino et al. 2000): we found an EB-V which is a factor of
30 lower than expected from the X-ray absorbing
and a Galactic
.
Within this context it is puzzling that, at variance with the ASCA and
BeppoSAX results,
Chandra has not found a large number of objects of this class
so far (Fiore et al. 2000; Brandt et al. 2000b;
Mushotzky et al. 2000):
only a few blue type 1 QSOs with significant
were discovered by Chandra's
surveys. Possibly this is due to the sensitivity of Chandra which peaks below 2
keV and, therefore, probably biases the results in favor of
soft, little absorbed X-ray sources. XMM, whose sensitivity is much more
uniform up to
7 keV, should tackle this issue.
Probably the most extreme case of type 1 AGNs with low
are the Broad Absorption Line (BAL) QSOs. Brandt et al. (2000a) and
Gallagher et al. (1999)
found that these objects are extremely weak both in soft X-rays and
hard X-rays with respect to the optical.
They suggest that this property is not due their intrinsic SED,
but to a large column of absorbing gas along the line of sight, probably related
to the same medium seen in UV resonant absorption lines.
The X-ray emission, heavily suppressed even in the
2-10 keV band, suggests that the gaseous absorbing column must be relatively
high:
1023 cm-2 or higher. Yet, the optical to UV spectrum is rather
typical of normal, unabsorbed QSOs (except for the presence of the broad
resonant absorption lines), implying little or no dust absorption.
Combined with the large absorbing column inferred from the X-rays, this
implies that these objects are characterized by an
ratio
which is nearly two orders of magnitude lower than Galactic.
As mentioned above, even a small amount of dust might redden the QSO
spectra enough to exclude them from surveys based on color selection
criteria such as the Palomar Green survey.
On the other hand, grism QSO surveys are mostly
based on the detection of broad lines whose flux might be little
affected by extinction in the case of a low
,
even if a substantial amount of gas were present
along the line of sight. Therefore, grism-selected QSOs are among
the best suited objects
to search for effects of a low
ratio. Indeed,
by comparing the optical and X-ray emission of grism selected QSOs,
Risaliti et al. (2000) find evidence
for a significant population of QSO characterized by a low
ratio.
The reader is addressed to that paper for an exhaustive discussion.
In the former two sections we have discussed observational evidences
on a reduced dust reddening and absorption towards active nuclei
with respect to what would be
expected from the gaseous ,
assuming a Galactic
gas-to-dust ratio and extinction curve.
This discrepancy can be ascribed to various effects,
as discussed in detail in Paper II. One
possibility is that the properties of dust grains in the circumnuclear region of
AGNs are different from the diffuse ISM of our Galaxy. In the following we
present two observational evidences supporting this scenario.
The silicate feature at 9.7 m observed in the mid-IR spectra of
many Galactic sources is commonly ascribed to silicate grains with
sizes smaller than
3
m.
Ground-based mid-IR studies of type 2 Seyferts claimed
the detection of a deep silicate absorption feature
in the spectra of many
Sy2s, which was regarded as an evidence supporting the unified model.
However, recent ISO spectra have shown that many of such detections were
probably spoilt by the narrow bandwidth of the data, limited
by the atmospheric transmission. Indeed, the presence of strong PAH
features on both sides of the silicate dip prevented a reliable determination
of the continuum in ground-based observations. The average ISO spectrum
of a sample of Sy2s obtained by Clavel et al. (2000)
does not show evidence
for any silicate dip, thus questioning the case for a significant
absorption feature claimed in former studies. In particular the
very conservative upper limits on the EW of the silicate feature given by
Clavel and collaborators for the average spectrum of Sy2s (
m)
implies
.
On the other hand the mid-IR continuum produced
by the active nucleus is
certainly suppressed in Sy2s with respect to Sy1s. This is apparent, as
discussed by Clavel et al. (2000),
in the much larger equivalent width of the PAH
features of Sy2s with respect to Sy1s. In particular, Clavel et al. estimate
an average dust extinction in the mid-IR band and, more specifically,
at 7.7 m of about 1.83 mag with a dispersion
of
0.74 mag. For a Galactic dust composition this would give
,
i.e. one order of magnitude
higher than the upper limit of 0.6 derived from the constraints on the silicate
absorption feature, as discussed above.
Note that the absorptions derived above assume a uniform
foreground screen model. In the reality the emitting region is extended (10 pc) and mixed with the absorber, though most of the emission comes from the
hot, inner region. As a consequence, the optical depths derived above actually
gives an ``equivalent'' optical depth for screen absorption.
Nonetheless, the relation between
and
does not change much with the geometry of the absorber,
since we are comparing two different forms of absorption of the
same continuum at nearly the same wavelength.
In particular, the relation remains unchanged in the case of partial
covering of the absorber (eg. patchy extinction) since the optical depth
at 7.7
m and at 9.7
m are measured by means of the equivalent width of
two features at nearly the same wavelength. In the case that the emitting
region is mixed with the absorber the expected depth of the silicate feature
is lower. If the emitting
region is completely mixed with the absorber (which is an extreme case
since the colder absorbing gas must be located in the outer
parts with respect to the mid-IR nuclear emitting region)
the ``equivalent'' optical depth in the silicate feature inferred
from the (equivalent)
should be
which is still much higher than the upper limit given by the non-detection
of the silicate feature in absorption.
Therefore, although the meaning of the optical depth derived at 7.7
m and
at 9.7
m depends on the geometry of the circumnuclear emitting/absorbing
matter, the discrepancy between these two quantities remains significant and is
not to be ascribed to geometrical effects.
The only concern to this regard, is that
the inner hot region, which is absorbed along our line of sight by the outer
colder regions, might be characterized by the silicate feature in emission, as
it is observed in Sy1s (Clavel et al. 2000).
Since the upper limit on
was based on the upper limit on the equivalent width of the 9.7
m
absorption feature assuming an intrinsic featureless continuum, the possible
presence of the emission feature implies a higher upper limit on
.
However, even in Sy1s the 9.7
m silicate emission is relatively
faint (
m) and, in the worst case, accounting for this
emission feature would only
increase the upper limit on
by 0.35,
i.e.
,
which is still much lower than what was derived
from the featureless absorption at 7.7
m (
).
This result is a very convincing
indication that although dust must be absorbing the mid-IR nuclear radiation,
such dust must have properties different from the Galactic
diffuse interstellar medium.
As discussed in Paper II the most likely explanation is that dust in the
circumnuclear region is
predominantly composed of grains with size larger than
3 m, which do not contribute to the feature at 9.7
m.
![]() |
Figure 3: UV spectra of four type 1 AGN whose broad lines ratios and continuum suggest dust absorption along the line of sight. The thin dashed line is the average spectrum of type 1 AGNs (mostly QSOs) reddened with a standard Galactic extinction curve with an EB-V consistent with that inferred from the broad lines ratio and adapted (within the uncertainties of the EB-Vmeasured with the broad lines) to match the shape of the continuum in those regions not affected by the carbon dip around 2175 Å. Note that the Galactic extinction curve always predicts a significant absorption by this feature which is not observed |
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The absorption feature
at 2175 Å observed in the diffuse interstellar medium was
commonly ascribed to small graphite grains with radii 100-200 Å.
More recent studies ascribe most of the profile of
this absorption feature to even smaller dust
particles (PAHs, Weingartner & Draine 2000).
Observing this feature in absorbed AGNs is really difficult since even a small
amount of dust generally suppresses nearly completely the UV emission. On the
other hand unabsorbed AGNs are generally free of any dust absorption.
Yet there are a few type 1 Seyferts and
QSOs whose optical and near-IR broad line ratios
suggest the presence of some dust reddening along the line of sight (Lacy et
al. 1982; McAlary et al. 1986; Puetter et al. 1981).
Some of these objects are still relatively bright in the UV and have been
observed with the HST spectrometers (FOS and STIS); these are therefore among
the best suited objects to look for the carbon dip feature at 2175 Å.
In Fig. 3 we show the UV spectra of five of these slightly reddened type 1
AGNs for which we could retrive HST archival spectra. Since we are
mostly interested in the continuum shape and in the broad carbon absorption
feature the spectra
were smoothed to a resolution of about 1000 kms-1. The thin
dashed line shows the template of type 1 AGNs obtained by Francis
et al. (1991) reddened with the standard Galactic extinction curve by EB-V consistent both
with the reddening of the broad lines and with the
shape of the UV continuum
outside the carbon dip.
The most important result is that the Galactic
extinction curve systematically predicts a deep feature around 2175 Å which
is undetected or much weaker in the observed
spectra
.
As discussed in Paper II, this observational evidence supports the idea that small grains are depleted in the dusty medium responsible for the reddening.
Finally, we shall discuss whether there is any evidence in other previous
studies for an
higher than Galactic in any AGNs,
besides the three LLAGNs discussed in Sect. 2.
Veilleux et al. (1997) found a few
AGNs whose
is higher than Galactic (some of which are also
included in our sample). For most of these objects a high lower limit to AV was obtained by comparing a broad Pa
with an upper limit on the
broad component of H
.
Paradoxically, as acknowledged by the same
authors, for the same objects the comparison between broad Pa
and broad
Br
gives little or no reddening. We believe that this result
reflects problems with estimating the upper limits for the broad
component of H
which, most probably, was underestimated.
The only object in their paper for which
seems
larger than Galactic and for which they do not use upper limits on the broad
line components is NGC 2992. However,
the much higher quality optical and near-IR
(simultaneous) spectra obtained by Gilli et al. (2000)
clearly indicate an
absorption significantly lower than inferred by Veilleux et al.
(1997).
A significant fraction of Sy1s shows evidence for highly ionized gas along our
line of sight, identified through the presence of absorption edges of OVII and
OVIII in the soft X-rays,
referred to as ``warm absorber'' (Reynolds 1997;
George et al. 1998). In
some cases the X-ray spectra also show evidence for neutral gas in addition to
the highly ionized gas. For some of these Sy1s with warm absorbers there is
also evidence for dust reddening (Komossa & Fink 1997a,
1997b; Leighly et al. 1997;
Komossa & Bade 1998; Reynolds & Fabian 1995).
The comparison
between the EB-V and the column of neutral gas
suggesting an
higher than
Galactic. However, we note that there is no reason for the highly ionized gas
to be dust-free if it is located at a distance larger than the sublimation
radius. Indeed, the columns inferred for the highly ionized component are in
agreement (or higher) with the dust reddening for a Galactic dust-to-gas
ratio. Secondly, as pointed out by various authors, the ionized gas probably
has multiple components at different ionization stages, which are not sampled
by the OVII and OVIII edges, but which are characterized by columns high enough
to account for the observed reddening (Kraemer et al. 1999,
2000; Brandt et al. 1996;
Reynolds & Fabian 1995).
We have reported various observational evidences indicating that the dust reddening and absorption of the nuclear region of AGNs is generally much lower than the values expected from the gaseous column density measured in the X-rays, if a standard Galactic dust-to-gas ratio and extinction curve are assumed.
Quantitatively, the most convincing argument supporting a low
ratio is the comparison between the reddening inferred from
the optical and infrared broad line ratios and the
derived from the
hard X-rays for a sample of 19 objects.
With the
exception of three low luminosity AGNs (
ergs-1),
whose physics might differ from Seyfert- and QSO-like luminosity systems,
all the objects appear characterized by an
ratio
sistematically lower than the Galactic standard value by a factor ranging from
3 up to
100.
Also, we have presented
additional evidences suggesting a reduced
ratio in AGNs.
The presence of substantial gaseous absorbing
columns (
a few times 1022) in intermediate
type 1.8-1.9 Seyferts
contrasts with their optical appearance, but can be
reconciled by a low
ratio. A more extreme case of this
effect is observed in a number of type 1 AGNs (mostly QSOs) whose X-ray
spectrum shows evidence for substantial gaseous absorption despite their
optical unabsorbed appearance. Two classes of type 1 AGNs which seems to be
nearly systematically affected by this phenomenon
(i.e. significant X-ray absorption
but little, or no, optical absorption) are the
Broad Absorption Line QSOs and the grism
selected QSOs.
The samples used are probably affected by selection
effects, which are discussed in the body of the paper, and therefore cannot
be considered as representative of the whole population of AGNs.
However, we can certainly state that at least
a sub-population of the AGNs is
characterized by a low
or
with respect to
the Galactic value. Also, we could not find evidence for a Galactic
standard
or
ratio in nearly any object.
We presented additional evidences indicating that the properties of dust grains
in the circumnuclear region of AGNs are different with respect to the Galactic
diffuse interstellar medium:
- although type 2 Seyfert nuclei appear significantly absorbed by dust in the
mid-IR, their average ISO spectrum does not show evidence for the
silicate absorption feature at 9.7 m which, instead, is expected to be very
deep in case of heavy absorption;
- some type 1 Seyferts which appear affected
by some reddening do not show evidence for the carbon dip at 2175 Å,
while according to the Galactic standard extinction curve this absorption
feature should be prominent in their UV spectra.
Both these observational evidences suggest that dust in the circumnuclear
region of AGNs is depleted of the small grains (m and 100-200 Å respectively) which are responsible for these absorption features.
A dust grain distribution biased in favor of large grains would also make the
extinction curve flatter (Laor & Draine 1993).
If the bias for large grains is
due to coagulation this would also explain the reduced
and
(Kim & Martin 1996).
A more detailed discussion on the interpretation of these results is given in a
companion paper (Maiolino et al. 2001, Paper II).
Regardless of the intepretation of these observational phenomena, these results have important consequences on the unified theories of AGNs. The finding that the dust absorption in generally significanlty lower with respect to what expected from the gaseous column density implies that for several AGNs the optical classification might be de-coupled from the X-ray classification. In particular, AGNs which appear obscured (type 2) in the X-rays might appear as relatively unobscured (type 1) in the optical.
Acknowledgements
We are grateful to B. Draine for enlightening discussions during the early stages of this work. This paper also benefits of useful comments from A. Natta, M. Walmsley and from the referee R. Antonucci. This work was partially supported by the Italian Space Agency (ASI) under grant ARS-99-15 and by the Italian Ministry for University and Research (MURST) under grant Cofin98-02-32.