A&A 409, 503-509 (2003)
DOI: 10.1051/0004-6361:20031166
Research Note
D. R. Ballantyne - J. C. Weingartner -
N. Murray
Canadian Institute for Theoretical Astrophysics, McLennan Labs, 60 St. George Street, Toronto, Ontario M5S 3H8, Canada
Received 26 May 2003 / Accepted 23 July 2003
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 I 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 VII 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 VII 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 VII edge (
)
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.
Key words: dust, extinction - galaxies: active - galaxies: Seyfert - galaxies: individual: MCG-6-30-15 - X-rays: galaxies - X-rays: ISM
Soft X-ray absorption by photoionized gas was first used by
Halpern (1984) and Pan et al. (1990) to explain the unusual spectrum of
the quasar MR 2251-178. This "warm absorber'' was characterized by
ROSAT spectra of Seyfert 1s which showed absorption edges due
to O VII and O VIII
(Turner et al. 1993; Fiore et al. 1993; Nandra et al. 1993; Nandra & Pounds 1992). Warm absorber studies matured with
the launch of the more sensitive ASCA observatory, which
allowed more detailed spectral modeling
(e.g., George et al. 1995; Fabian et al. 1994; Reynolds & Fabian 1995). Analysis of a large sample of
Seyfert 1s observed by ASCA showed that approximately 50%
exhibit absorption features from photoionized gas in their soft X-ray
spectra (Reynolds 1997; George et al. 1998). Currently, the study of the warm
absorber is being revolutionized with observations from the dispersion
gratings onboard Chandra and XMM-Newton which have
been able to resolve individual absorption lines from a myriad of
metals and charge states in the warm gas
(e.g., Kaastra et al. 2000; Kaspi et al. 2001,2002; Collinge et al. 2001; Lee et al. 2001). Photoionization
modeling has then shown that, in many cases, more than one ionization
parameter is needed to describe the observed spectrum
(e.g, Kaspi et al. 2001; Kaastra et al. 2002; Morales et al. 2000). Also, the positions of the lines
point toward outflow velocities on the order of a few hundred to a few
thousand km s-1. Thus, this warm absorbing gas seems to be in the
form of an outflowing wind (cf., Elvis 2000; Bottorff et al. 2000). However,
there remains a major uncertainty in the location of the gas, with
many models considering the broad-line region
(e.g., George et al. 1998; Reynolds & Fabian 1995; Netzer 1996) or the putative obscuring torus
(e.g., Krolik & Kriss 1995,2001) as the most likely origin for the warm
absorber.
A possible constraint on the location of at least some of this warm
gas may be provided if it contains any dust, which sublimates at the
radius of the broad-line region (Barvainis 1987) for a typical active
galactic nucleus (AGN). Dusty warm absorbers (hereafter, DWA) were
first considered for the quasar IRAS 13349+2438 (Siebert et al. 1999; Brandt et al. 1996) and the
Seyfert 1 galaxy MCG-6-30-15 (Reynolds et al. 1997). In both of these AGN, the
column of neutral H inferred from the reddening is significantly
larger than that inferred from the neutral absorption in the soft
X-ray band, but is of the same order as the column of ionized gas
inferred from the warm absorber (Reynolds et al. 1997). This suggests that
the dust is similar to Galactic dust and resides within the warm
ionized gas, and may significantly affect the observed soft X-ray
spectrum (Komossa & Fink 1997 and references therein;
Komossa & Bade 1998). Spectroscopic evidence for dust in X-ray warm
absorbers has now been found by Lee et al. (2001) in the Chandra
gratings observation of MCG-6-30-15 (this seems to have been confirmed by
the very recent XMM-Newton data of Turner et al. 2003). This
high resolution spectrum exhibited a sharp drop at
0.7 keV consistent with the L3 absorption edge from neutral
Fe. A similar feature was also found in a Chandra spectrum of
the Galactic X-ray binary Cyg X-1 (Schulz et al. 2002). The column implied by
the depth of the Fe edge in MCG-6-30-15 is of the right order to explain the
observed reddening (
;
Reynolds et al. 1997), assuming Galactic-type dust located within the warm
absorber.
Rather than place the DWA near the central engine of the AGN, Kraemer et al. (2000) (see also Crenshaw & Kraemer 2001) argued for the existence of a "lukewarm absorber'' outside the narrow-line region. This gas would have sufficient column to explain the observed reddening, and has been ionized to the point where hydrogen is fully stripped, but the metals would only be moderately ionized and would exhibit strong UV absorption lines rather than O VII or O VIII edges. Thus, this model requires an inner warm absorber to account for the highly ionized oxygen features (Kraemer et al. 2000). The lukewarm absorber has been shown to be consistent with the X-ray (Kraemer et al. 2000) and UV (Crenshaw et al. 2001) data of NGC 3227, as well as the UV spectrum of Ark 564 (Crenshaw et al. 2002).
In the case of MCG-6-30-15 (z=0.008,
erg s-1; see Table 1 for a summary of
the absorbing columns), it was clear from early ASCA
variability studies that a multi-zone warm absorber was needed
(e.g., Otani et al. 1996; Morales et al. 2000).
Table 1: A summary of the absorbing columns toward MCG-6-30-15. Reference 1 = Elvis et al. (1989), 2 = Reynolds et al. (1997), 3 = Lee et al. (2002).
In particular, the O VIII edge was found to anticorrelate with the source luminosity while the O VII edge seemed to remain constant (Orr et al. 1997; Otani et al. 1996). The Chandra observations found a strong O VII edge (
A Hubble Space Telescope (HST) image of MCG-6-30-15 shows a distinct dust lane that cuts across the southern part of the
galactic disk (Fig. 1; Malkan et al. 1998; see also Ferruit et al. 2000).
![]() |
Figure 1: Hubble Space Telescope image of MCG-6-30-15 (Malkan et al. 1998). A dust lane is apparent crossing the southern part of the galactic disk. |
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The composition of the dust in the MCG-6-30-15 DWA is also a puzzle. For Cyg X-1,
where the high signal-to-noise allowed a comparison to laboratory data,
Schulz et al. (2002) claimed that pure metallic Fe was the best fit to the
edge structure. This is surprising, since the expected microwave
emission (at 90 GHz) from such grains is not observed in the
Galaxy (Draine & Lazarian 1999). Lee et al. (2001) argued that the Cyg X-1 iron edge was
similar to the one observed in MCG-6-30-15, and thus pure iron grains may be
present in the DWA. In Sect. 3 we discuss the
constraints on a pure iron component in astronomical dust, and
consider if it is a plausible composition for the dust in the MCG-6-30-15 DWA.
The dust lane seen in Fig. 1 passes just south of the
nucleus, so, judging from the Galactic distribution (Dickey & Lockman 1990), our
line of sight likely passes through a column of a few times
1021 cm-2. This is of the same order as that needed to
provide the observed reddening in MCG-6-30-15 assuming Galactic-type grains
(
cm-2;
Reynolds et al. 1997). Further evidence for a low density origin of the DWA is the
recombination timescale argument of Otani et al. (1996). They noted that the
constancy of the O VII edge in the long 1994 ASCA
observation of MCG-6-30-15 implies
in the outer absorber. ISM gas with a fixed hydrogen number
density of
cm-3 will be used as the canonical
absorber for the remainder of the paper.
A simple photoionization argument can be made to constrain the
distance of the O VII absorber. We define the following ionization
parameter
We used Cloudy 96Beta4 (Ferland 2002) to predict the ionization
structure of a
cm-3 cloud of ISM gas at various
distances from an AGN. The neutral hydrogen column density was fixed
at
cm-2, which, for a Galactic
gas-to-dust ratio, is the minimum needed to provide the reddening toward MCG-6-30-15 (
-
cm-2;
Reynolds et al. 1997). The dust (silicate plus graphite) and gas-phase
metal abundances were fixed at the ISM values described by
Ferland (2002). The cloud was illuminated with a
"standard'' AGN continuum (see p. 34 in Ferland 2002) with
and a X-ray power-law photon-index
.
The "big blue bump'' was characterized by a temperature
of
K (the maximum Shakura & Sunyaev (1973) accretion disc
temperature for a 107
black hole accreting at 0.1 of its
Eddington rate) and a UV slope of
.
This
spectral energy distribution (SED) is not intended to be a realistic
model of the MCG-6-30-15 continuum (which is unknown because of the large
reddening), but rather representative of a generic AGN. The
normalization of the SED was set by defining the 2-10 keV luminosity to be 1043 erg s-1, typical of many Seyfert 1s
(Reynolds 1997).
Models were calculated with the inner edge of the gas cloud at various distances rfrom the continuum source, and the computed O VII column density was
compared with the result from the Chandra observation of MCG-6-30-15,
cm-2(Lee et al. 2001). The maximum O VII column found in the Cloudy runs
was
cm-2 for a cloud distance of
150-175 pc. Models with the gas closer in were too ionized, and if
the cloud was further out, it was not ionized enough. Of course,
lowering the density by a factor f would allow a distance
larger. However, if the distance exceeds
200-250 pc,
then, even with
,
the physical length
of the column exceeds the distance to the AGN.
A similar calculation with a power-law SED of energy index -1.17between 0.013 and 100 keV resulted in
cm-2 at r=150 pc. While these models cannot
account for the entire O VII column in MCG-6-30-15 (it is likely that some
fraction of the O VII edge originates within the inner warm
absorber), we have shown that this dusty ISM cloud will have an
detectable impact on the observed spectrum (see also Komossa & Bade 1998).
To illustrate the extent of this impact, the incident and transmitted
continua for the AGN models with r=100, 175, and 250 pc are
shown in Fig. 2.
![]() |
Figure 2:
The results of photoionization models of a DWA shown over the
X-ray band. Constant density (
![]() ![]() ![]() |
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The fact that there is a significant column of He II (
cm-2) in the models shows that the gas is very close to
recombining to a more neutral configuration. This is in large part due
to the presence of dust which, for a Galactic size distribution,
dominates the continuum opacity in the extreme UV (EUV) and soft
X-ray bands (see Fig. 3). Indeed, our Cloudy models show that
neutral oxygen begins to be present in the gas when r=175 pc. Thus,
our results seem to be fine-tuned in the sense that the greatest
O VII column is found when the gas is on the verge of
recombining. This argues that it may be difficult, in general, for ISM
gas to contribute to the warm absorber.
In this section we have argued that ISM material associated with dust
lanes in the host galaxy of an AGN is a natural contributor to the
observed DWAs. Cloudy models suggest that this gas, when illuminated
by a toy AGN SED, can produce significant warm absorption (including a
significant O VII edge) when placed 150 pc from the AGN. This
may be particularly relevant to MCG-6-30-15, where a dust lane does pass just
below the nucleus. Using Fig. 1 and the plate scale from
Malkan et al. (1998), we can estimate the inner edge of the dust lane from
the HST image. Interestingly, this is also of the order of 150-450 pc, assuming H0=50 km s-1 Mpc-1.
It may be difficult applying this idea to other sources which harbor DWAs (e.g., IRAS 13349+2438). The dust lane in MCG-6-30-15 lies nearly along our line of sight and may be a relatively unusual alignment. Nevertheless, the strong ionizing power of an AGN will have a great impact on its local ISM. The observational consequences of these effects will depend both on the viewing angle into the source (Crenshaw & Kraemer 2001) and on the SED of the AGN. Indeed, if our line-of-sight into MCG-6-30-15 were slightly different and passed through the dust lane, rather than just above it, the obscuration could be high enough that the AGN would appear as a Seyfert 2 (see, e.g., Matt 2000).
As the EUV/soft X-ray opacity in the DWA is dominated by dust, the
absorption properties depend on the grain composition. The top panel
of Fig. 3 shows the transmitted spectrum for the
r=150 pc model with Galactic silicate and graphite dust abundances
(Mathis et al. 1977).
![]() |
Figure 3: The effect of grains on the transmitted spectrum (see also Komossa & Bade 1998). Both panels show the results of Cloudy models where the gas cloud was 150 pc from the AGN. The dashed curve shows the incident spectrum and the solid curve shows the transmitted spectrum. The positions of the C I, O I, Fe I, and O VII edges are indicated. Solar abundances were used for the dust-free model, except that the oxygen abundance was set to the F and G star value from Sofia & Meyer (2001). |
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Unfortunately, the High Energy Transmission Gratings (HETG) Chandra spectra only extend to an energy of 0.45 keV, so we have no X-ray evidence for or against the presence of graphite; however the Low Energy Transmission Gratings (LETG) can reach these low energies although the instrumental C edge will need to be well calibrated to measure the intrinsic edge due to graphite. The Reflection Grating Spectrometer (RGS) onboard XMM-Newton also does not go below 0.3 keV, although the low-resolution European Photon Imaging Cameras (EPIC) are able to do so.
As mentioned in Sect. 1, the strength of the measured
Fe I edge from the Chandra observation of MCG-6-30-15 implies
,
consistent
with the observed reddening for Galactic-type dust. If the Fe really
is incorporated into silicate grains, then we expect
,
since a silicate structural unit contains more O
atoms than Fe atoms. For example, the Cloudy models discussed above
assume a MgFeSiO4 stoichiometry and predict an O I edge that is
roughly the same depth as the Fe I edge. Lee et al. (2002) claimed
that silicates with an O:Fe number ratio of 2:1 rather than 4:1 were
consistent with the Chandra data, but also mentioned that the
data were limited by statistics at the position of the O I
edge. Therefore, if the silicate dust had an O:Fe ratio of 4:1, it
could have been detected by Chandra. However, Sako et al. (2003)
quote an upper limit of
cm-2 from their
XMM-Newton RGS observation of MCG-6-30-15, which has higher
signal-to-noise, but lower resolution, than the Chandra
spectrum. These authors also find
cm-2 for the column of neutral iron, which implies a
V-band optical depth of 0.07 (assuming a silicate grain radius of
0.1
m). Therefore, this column could not account for the observed
reddening of MCG-6-30-15, unless it is provided predominantly by iron-poor
grains.
Further constraints on the mineralogical structure of the dust can, in
principle, be provided by the detailed structure of the Fe I edge.
Lee et al. (2001) found that the edge structure in MCG-6-30-15 matches very well
that found in Cyg X-1. Schulz et al. (2002) showed that the Cyg X-1 edge
could be fairly accurately reproduced if the Fe resides in pure Fe grains (taking experimental data from Kortright & Kim 2000), but not if the
Fe is in oxides (taking data from Crocombette et al. 1995). A measurement of
the oxygen column in the dust could not be made for Cyg X-1, as the
large O I edge ()
was dominated by neutral oxygen in the
ISM along the line-of-sight. Therefore, Schulz et al. (2002) concluded that
pure iron grains were most consistent with the data, and Lee et al. (2001)
present this as a possibility for the dust in MCG-6-30-15. However, for
Cyg X-1 metallic Fe dust is probably ruled out since it would produce
excessive thermal magnetic dipole emission at 90 GHz
(Draine & Lazarian 1999). Draine & Lazarian suggest that at most
5% of the
Galactic interstellar Fe can be in metallic iron grains. Thus, it
seems unlikely that there is a significant column of metallic iron
grains toward Cyg X-1. Although we have not performed a detailed
spectral fitting, it appears that the observed edge structure in Cyg
X-1 could be due to Fe in silicates, based on the spectrum of an
olivine sample from Gloter et al. (2000).
The above argument cannot be directly applied to MCG-6-30-15 as there are no
microwave observations of this galaxy, so the key to uncovering the
dust composition in the DWA is an accurate measurement of other edges
such as O I, C I, or Si I at 1.85 keV (
in the Cloudy models). The
Chandra data may imply wüstite (FeO) or hematite
(
Fe2O3) dust, but the XMM-Newton limit by
Sako et al. (2003) points to little or no oxygen in the grains.
Perhaps the lack of O I can be explained by selective destruction
of O-bearing grains (e.g., silicates). Depletion patterns in the
Galaxy suggest that Fe and Si are incorporated into different dust
populations, with the Fe-bearing component significantly longer-lived
than the Si-bearing component (e.g., Tielens 1998). If the dust in
the DWA in MCG-6-30-15 were subjected to repeated episodes of destruction
(in shocks) and re-growth, then the Si-bearing dust component could be
largely removed while the Fe-bearing component remains largely intact.
Such a process would lead to a reduced
.
However,
the metallicity at
pc from the nucleus of MCG-6-30-15 is
probably higher than at the location of the Sun in the Galaxy, so that
in the DWA in this scenario could be comparable to
its value in the local ISM of the Galaxy. One potential difficulty
for this scenario (and for silicate dust models in general) is that
the destruction timescale inferred by Tielens (1998) for the Si-bearing
dust is substantially shorter than predicted for silicate grains
(Jones et al. 1996). Counter-intuitively, Jones et al. also find the
lifetime of pure Fe dust to be shorter than the lifetimes of graphite
or silicate dust, because Fe grains are accelerated to higher speeds
in the shock. Weingartner & Draine (1999) suggested the Fe may largely be
incorporated into the carbonaceous dust population rather than the
silicate component. In this picture, gas-phase Fe atoms rapidly
accrete onto polycylic aromatic hydrocarbon molecules, forming
organometallic "sandwich'' molecules, which may ultimately coagulate
to form larger grains. Laboratory Fe L-edge spectra of such compounds
would be very useful.
The Chandra data also place a constraint on the inner warm
absorbing gas which is responsible for the O VIII
edge in the
ASCA data. From the depth of the high-ionization absorption
lines, Lee et al. (2002) found
for this material. If the Fe I edge observation were not
available one might argue that the inner
absorber contains large grains that do not redden the optical nuclear
emission. However, since all of the neutral Fe can be accounted for by the
outer absorber, no such Fe-bearing grains exist in the inner absorber,
suggesting that the inner absorber is dust-free. This, in turn,
suggests than the inner absorber is located within the dust
sublimation radius, unless an alternative mechanism can be found for
depleting the gas of dust at larger distances.
Indeed, if grains are to be found close to the central engine of an
AGN, they must be shielded from the outpouring radiation. The
radiation pressure opacity of dust is
times that of
electrons (Laor & Draine 1993), so within a certain
radius the dust will see a significantly super-Eddington source. The
gravitational and radiation forces will balance when the dust particle
is outside a mass
,
where
is the black hole mass of the AGN. As an example, the
inner velocity profiles of S0 galaxies are approximately linear
(Loyer et al. 1998; Seifert & Scorza 1996), implying that
.
Thus, if at
r=1 pc
,
then the balancing radius for the
dust will be
10 pc, independent of the black hole
mass. Therefore, if a DWA is at 150 pc it will not feel a
significant radiation force, and should not have a large outflow
velocity. In the case of MCG-6-30-15, this is consistent with the preliminary results from the
Chandra data (Lee et al. 2002,2001), and the low velocity
component inferred by Sako et al. (2003) from the XMM-Newton spectrum.
Motivated by the the dust lane seen in the HST image of MCG-6-30-15, we have argued in this paper that DWAs may reside in the ISM of the host galaxy. Photoionization modeling shows that a detectable O VII edge can be produced by such material if it is placed 100-200 pc away from the AGN and therefore can contribute to the warm absorption features. Furthermore, the dust contained within this gas is dynamically and thermally stable and can account for any observed reddening. Supporting evidence for this interpretation for MCG-6-30-15 is given by the very small outflow velocity of the low ionization absorption lines, the observed constancy of the O VII edge, and the additional absorption required at low energies.
The composition of the dust in DWAs may be constrained by the relative strength of the Fe I and O I edges, or by the detailed structure around the Fe I edge, as was done in Cyg X-1. The present MCG-6-30-15 data is consistent with silicate dust having a O:Fe ratio of 2:1 or less, which could result from a process of repeated grain destruction and growth in the ISM. The presence of pure iron grains seems unlikely given the limit on their abundance from our own Galaxy. High signal-to-noise data around the O I edge, or a measurement of the C I edge, is needed to fully exploit the observations in determining the dust composition in distant AGN.
Acknowledgements
We thank Gary Ferland for helpful discussions regarding the Cloudy models, and Julia Lee for comments on a draft of the manuscript. This research was supported by the Natural Sciences and Engineering Research Council of Canada and by the Canada Research Chair Program. This research has made use of the NASA/IPAC Extragalactic Database (NED) which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration.