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Dust properties in Active Galactic Nuclei
Visfas in Astronomy Vol. 40, No.
I, pp. 127-I 3 1, 1996 Copyright @ 1996 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0083-6656/96 $32.00 + 0.00
0083-6656(95)00114-X
DUSTPROPERTIESINACTIVEGALACTIC NUCLEI ZBIGNIEW LOSKA, ’ BOAENA CZERNY ’ and RYSZARD SZCZERBA 2 ’ N. Copernicus Astronomical Centre, Warsaw, Poland 2 N. Copernicus Astronomical Centre, Torun, Poland
Abstract- The study of the shape of the optical/UV spectrum of an intrinsic nuclear source requires the removal of the effects caused by transmission of the radiation through the dust. There are arguments that the chemical composition of the circumnuclear dust and the distribution of the grain sizes are significantly different from what we know about the interstellar dust. Nonstandard model-dependent extinction curves have to be used. We show the dependence of the new extinction curves on model parameters and discuss the consequences of applying nonstandard extinction for the reproduction of intrinsic spectral slopes of Active Galactic Nuclei. We also call attention to the relatively low X-ray opacity of dust. Copyright @Elsevier Science Ltd.
1. INTRODUCTION Observational data imply that Active Galactic Nuclei are likely to have a substantial amount of dust within some 0.1-100 pc. This dust modifies the observed spectrum of the nucleus by reprocessing a significant fraction of its intrinsic bolometric luminosity. It is also a key element of the currently popular unification schemes of AGN’s (see, e.g. Antonucci, 1993). Polarimetry observations indicating that the narrow lines in Seyfert 2s show broad wings when viewed in polarized light (Antonucci and Miller, 1985) strongly favour the unification scheme, although the inclination angle of an active nucleus may not be the only parameter (Antonucci, 1993). However, there are two major problems with such an interpretation of the Seyfert IlSeyfert 2 classification: Infrared spectroscopy shows the presence of the characteristic absorption or emission features predicted by standard dust models, in particular the 9.7 pm silicate feature, only in a fraction of objects, mainly low luminosity AGN (Roche et al, 1991). (2) There is a strong discrepancy between the estimates of the amount of extinction along the line of sight between the different methods (e.g. Mulchaey et af., 1992).
(1)
127
Z. Loska et al.
128
The discrepancy between the conclusions from polarimetry and spectroscopy data can be removed if we consider two possible effects: (1) The composition of the circumnuclear dust (i.e. chemical composition and the distribution of grain sizes) does not necessarily have to be the same as that of the interstellar dust in our Galaxy because it forms in a different environment (e.g. strong UV and X-ray radiation) (Czerny et al., 1991; Laor and Draine, 1993). (2) the dust along our line of sight does not have to be the same as the dust emitting in the near IR. We address the problem of the IR spectroscopy here by considering the spectrum of NGC 6814, a Seyfert galaxy for which there exists an extensive amount of observational data in the IR, optical and UV The source is bright enough to determine the absorption column in X-rays, but the level of X-ray emission is relatively low, such that neither contamination by IR/X-ray power law nor strong X-ray reprocessing are expected to modify the optical/UV spectrum.
2. ASSUMPTIONS
AND MODEL
We use a model of a central source which consists of an accretion disk surrounding a supermassive black hole and an IR/optical/UV power law. Such a model well represents the overall AGN spectrum in this frequency range. We assume that this central source is surrounded by a spherical optically thin dust cloud and we calculate the temperature distribution of the dust, evaporation radius for each type of grain and grain population at every radius (Loska et al, 1993). The extinction curve is defined as a ratio of the reddening in magnitudes, AA to the difference between the reddening at 0.44 and 0.55 pm, Es-V. X(h) =
rh,eXl T0.44,ext
-
To szi.ext
=- Ah h-v
where h is the wavelength given in microns, T is the optical depth of the medium at a given wavelength. These curves are parametrized by (see Fig. 1): 9 the l the l the l the
relative contribution of silicates to carbon (density ratio SiK), index p of power law size distribution (N(a) - a-p) minimum and maximum grain size amin and amax, gas density profile given by the power law index fi (n - r-b).
3. AN EXAMPLE: NGC 6814 For our analysis we took the data from the low state of the source since in that case any effects of X-ray reprocessing should be relatively insignificant in the optical/UV band. We assume that the radiation from the nucleus undergoes a number of modifications on its way towards an observer:
Active Galactic Nuclei
129
IO
0
40
1
d) 30 !V !I
\\
‘.-.-._
-I
20
IO
0
0 0
2
4
6
8
10
1
0
10
20
30
/G-d
Fig. I, Extinction curves for: (a) Mixture of silicate and carbon grains. Different curves correspond to different relative weight of silicate to carbon dust. (b) Pure carbon dust and three values of the power law index in grain size distribution. (c) Pure carbon dust with different upper limit of grain sizes. Lower limit is at 0.005 pm. (d) Pure carbon dust in a cloud extending up to 300 pc. Curves are parametrized by the power law index fi characterizing the gas density profile inside the dust cloud. Standard Seaton (Seaton 1979) extinction curve (thick solid line) is shown for comparison.
(1) reddening by circumnuclear dust of possibly nonstandard composition (2) contribution from stars of galaxy bulge (3) reddening by cirrus clouds of host galaxy (4) reddening in our Galaxy. Therefore in order to reproduce the intrinsic spectrum we reverse the modification process. We interpret the allowed X-ray absorption column in excess of a Galactic value as a measure of the amount of circumnuclear dust. Such an interpretation is possible due to the fact that the extinction of dust in X-ray range has a similar dependence on photon energy as
130
Z. Loska et al.
-9
/
NGC 6814
%Y iif -10
/
‘. O
-
# ’/ 0 0
-11
-
0
Q 0
Fig. 2. lR/UV part of NGC6814 spectrum. Dashed line, open stars--initial spectrum; dotted line, open circles-spectrum resulting from standard extinction curve: solid line, filled circles-spectrum resulting from non-standard. pure carbon extinction curve; dashed straight line-power law with an energy index I/3.
absorption by a neutral gas; only the effective opacity is by a factor - 3 lower (Czerny et al., 1994; see also Laor and Draine, 1993). As a result we obtain a simple power law with an energy index l/3 (Fv - VI/~), characteristic for Keplerian accretion discs (Fig. 2). The procedure does not necessarily produce a unique solution, since we had to adjust one (almost) free parameter for a given choice of an extinction curve: the normalization of the starlight. However, our choice of the best theoretical extinction curve was based on earlier experience with fitting the IR spectra of four other objects (Loska et al., 1993).
Active Galactic Nuclei
131
4. SUMMARY We show that the assumption about the lack of silicates in the circumnuclear dust in NGC 6814 allows us to reconstruct the spectrum of the intrinsic source which appears to have a simple power law shape in both optical and ultraviolet wavelengths with a slope of 113, as expected for accretion disk without any irradiation, or any additional IRlopticaYUVIX-ray power law. In the case of NGC 6814, the X-ray flux is quite low (the 30 March 1993 1 keV flux density, corrected for Galactic absorption is 0.27 pJy; Madejski er al., 1993), so the lack of contribution from the IR/opticaYW/X-ray power law and negligible effect of irradiation is actually expected. We also suggest that if most of the circumnuclear absorption is rather due to dust than a neutral gas the significant drop of the opacity below the carbon edge at - 0.4 keV may in effect lead to an illusion of weak soft X-ray excess. It has to be kept in mind, however, that the conclusions are based on a set of data which are not simultaneous and on the assumption of a standard shape of the starlight contribution. Further observations of this source are clearly needed, and a careful determination of the starlight contribution, as it was done for several other galaxies (Kotilainen et al., 1993) would be extremely important.
References Antonucci R. (1993) Ann. Rev. Ask Ap. 31,473. Antonucci R. and Miller J.S. (1985) Ap. J 297, 621. Czemy B., Loska Z. and Szczerba R. (1991) Physics of Active Galactic Nuclei (Edited by W.J. Duschl and S.J. Wagner), p. 198. Springer, Berlin. Czerny B., Loska Z., Szczerba R., Madejski G. and Cukierska J. (1994) In preparation. Kotilainen J.K., Ward M.J. and Williger G.M. (1993) M.NR.A.S. 263,655. Laor A. and Draine B.T. (1993) Ap. J. 402,441. Loska Z., Szczerba R. and Czemy B. (1993) M.N. R. A.S. 261,63. Madejski G.M., Done C., Turner T.J., Mushotzky R.F., Serlemitsos P., Fiore F., Sikora M. and Begelman M.C. (1993) Nature 365,626. Mulchaey J.S., Mushotzky R.F. and Weaver K.A. (1992) Ap. J Lett. 390, L69. Roche RF., Aitken D.K, Smith C.H. and Ward M.J. (1991) M.NR.A.S. 248,606. Seaton M.J. (1979) M.NR.A.S. 187,73P.
I, pp. 127-I 3 1, 1996 Copyright @ 1996 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0083-6656/96 $32.00 + 0.00
0083-6656(95)00114-X
DUSTPROPERTIESINACTIVEGALACTIC NUCLEI ZBIGNIEW LOSKA, ’ BOAENA CZERNY ’ and RYSZARD SZCZERBA 2 ’ N. Copernicus Astronomical Centre, Warsaw, Poland 2 N. Copernicus Astronomical Centre, Torun, Poland
Abstract- The study of the shape of the optical/UV spectrum of an intrinsic nuclear source requires the removal of the effects caused by transmission of the radiation through the dust. There are arguments that the chemical composition of the circumnuclear dust and the distribution of the grain sizes are significantly different from what we know about the interstellar dust. Nonstandard model-dependent extinction curves have to be used. We show the dependence of the new extinction curves on model parameters and discuss the consequences of applying nonstandard extinction for the reproduction of intrinsic spectral slopes of Active Galactic Nuclei. We also call attention to the relatively low X-ray opacity of dust. Copyright @Elsevier Science Ltd.
1. INTRODUCTION Observational data imply that Active Galactic Nuclei are likely to have a substantial amount of dust within some 0.1-100 pc. This dust modifies the observed spectrum of the nucleus by reprocessing a significant fraction of its intrinsic bolometric luminosity. It is also a key element of the currently popular unification schemes of AGN’s (see, e.g. Antonucci, 1993). Polarimetry observations indicating that the narrow lines in Seyfert 2s show broad wings when viewed in polarized light (Antonucci and Miller, 1985) strongly favour the unification scheme, although the inclination angle of an active nucleus may not be the only parameter (Antonucci, 1993). However, there are two major problems with such an interpretation of the Seyfert IlSeyfert 2 classification: Infrared spectroscopy shows the presence of the characteristic absorption or emission features predicted by standard dust models, in particular the 9.7 pm silicate feature, only in a fraction of objects, mainly low luminosity AGN (Roche et al, 1991). (2) There is a strong discrepancy between the estimates of the amount of extinction along the line of sight between the different methods (e.g. Mulchaey et af., 1992).
(1)
127
Z. Loska et al.
128
The discrepancy between the conclusions from polarimetry and spectroscopy data can be removed if we consider two possible effects: (1) The composition of the circumnuclear dust (i.e. chemical composition and the distribution of grain sizes) does not necessarily have to be the same as that of the interstellar dust in our Galaxy because it forms in a different environment (e.g. strong UV and X-ray radiation) (Czerny et al., 1991; Laor and Draine, 1993). (2) the dust along our line of sight does not have to be the same as the dust emitting in the near IR. We address the problem of the IR spectroscopy here by considering the spectrum of NGC 6814, a Seyfert galaxy for which there exists an extensive amount of observational data in the IR, optical and UV The source is bright enough to determine the absorption column in X-rays, but the level of X-ray emission is relatively low, such that neither contamination by IR/X-ray power law nor strong X-ray reprocessing are expected to modify the optical/UV spectrum.
2. ASSUMPTIONS
AND MODEL
We use a model of a central source which consists of an accretion disk surrounding a supermassive black hole and an IR/optical/UV power law. Such a model well represents the overall AGN spectrum in this frequency range. We assume that this central source is surrounded by a spherical optically thin dust cloud and we calculate the temperature distribution of the dust, evaporation radius for each type of grain and grain population at every radius (Loska et al, 1993). The extinction curve is defined as a ratio of the reddening in magnitudes, AA to the difference between the reddening at 0.44 and 0.55 pm, Es-V. X(h) =
rh,eXl T0.44,ext
-
To szi.ext
=- Ah h-v
where h is the wavelength given in microns, T is the optical depth of the medium at a given wavelength. These curves are parametrized by (see Fig. 1): 9 the l the l the l the
relative contribution of silicates to carbon (density ratio SiK), index p of power law size distribution (N(a) - a-p) minimum and maximum grain size amin and amax, gas density profile given by the power law index fi (n - r-b).
3. AN EXAMPLE: NGC 6814 For our analysis we took the data from the low state of the source since in that case any effects of X-ray reprocessing should be relatively insignificant in the optical/UV band. We assume that the radiation from the nucleus undergoes a number of modifications on its way towards an observer:
Active Galactic Nuclei
129
IO
0
40
1
d) 30 !V !I
\\
‘.-.-._
-I
20
IO
0
0 0
2
4
6
8
10
1
0
10
20
30
/G-d
Fig. I, Extinction curves for: (a) Mixture of silicate and carbon grains. Different curves correspond to different relative weight of silicate to carbon dust. (b) Pure carbon dust and three values of the power law index in grain size distribution. (c) Pure carbon dust with different upper limit of grain sizes. Lower limit is at 0.005 pm. (d) Pure carbon dust in a cloud extending up to 300 pc. Curves are parametrized by the power law index fi characterizing the gas density profile inside the dust cloud. Standard Seaton (Seaton 1979) extinction curve (thick solid line) is shown for comparison.
(1) reddening by circumnuclear dust of possibly nonstandard composition (2) contribution from stars of galaxy bulge (3) reddening by cirrus clouds of host galaxy (4) reddening in our Galaxy. Therefore in order to reproduce the intrinsic spectrum we reverse the modification process. We interpret the allowed X-ray absorption column in excess of a Galactic value as a measure of the amount of circumnuclear dust. Such an interpretation is possible due to the fact that the extinction of dust in X-ray range has a similar dependence on photon energy as
130
Z. Loska et al.
-9
/
NGC 6814
%Y iif -10
/
‘. O
-
# ’/ 0 0
-11
-
0
Q 0
Fig. 2. lR/UV part of NGC6814 spectrum. Dashed line, open stars--initial spectrum; dotted line, open circles-spectrum resulting from standard extinction curve: solid line, filled circles-spectrum resulting from non-standard. pure carbon extinction curve; dashed straight line-power law with an energy index I/3.
absorption by a neutral gas; only the effective opacity is by a factor - 3 lower (Czerny et al., 1994; see also Laor and Draine, 1993). As a result we obtain a simple power law with an energy index l/3 (Fv - VI/~), characteristic for Keplerian accretion discs (Fig. 2). The procedure does not necessarily produce a unique solution, since we had to adjust one (almost) free parameter for a given choice of an extinction curve: the normalization of the starlight. However, our choice of the best theoretical extinction curve was based on earlier experience with fitting the IR spectra of four other objects (Loska et al., 1993).
Active Galactic Nuclei
131
4. SUMMARY We show that the assumption about the lack of silicates in the circumnuclear dust in NGC 6814 allows us to reconstruct the spectrum of the intrinsic source which appears to have a simple power law shape in both optical and ultraviolet wavelengths with a slope of 113, as expected for accretion disk without any irradiation, or any additional IRlopticaYUVIX-ray power law. In the case of NGC 6814, the X-ray flux is quite low (the 30 March 1993 1 keV flux density, corrected for Galactic absorption is 0.27 pJy; Madejski er al., 1993), so the lack of contribution from the IR/opticaYW/X-ray power law and negligible effect of irradiation is actually expected. We also suggest that if most of the circumnuclear absorption is rather due to dust than a neutral gas the significant drop of the opacity below the carbon edge at - 0.4 keV may in effect lead to an illusion of weak soft X-ray excess. It has to be kept in mind, however, that the conclusions are based on a set of data which are not simultaneous and on the assumption of a standard shape of the starlight contribution. Further observations of this source are clearly needed, and a careful determination of the starlight contribution, as it was done for several other galaxies (Kotilainen et al., 1993) would be extremely important.
References Antonucci R. (1993) Ann. Rev. Ask Ap. 31,473. Antonucci R. and Miller J.S. (1985) Ap. J 297, 621. Czemy B., Loska Z. and Szczerba R. (1991) Physics of Active Galactic Nuclei (Edited by W.J. Duschl and S.J. Wagner), p. 198. Springer, Berlin. Czerny B., Loska Z., Szczerba R., Madejski G. and Cukierska J. (1994) In preparation. Kotilainen J.K., Ward M.J. and Williger G.M. (1993) M.NR.A.S. 263,655. Laor A. and Draine B.T. (1993) Ap. J. 402,441. Loska Z., Szczerba R. and Czemy B. (1993) M.N. R. A.S. 261,63. Madejski G.M., Done C., Turner T.J., Mushotzky R.F., Serlemitsos P., Fiore F., Sikora M. and Begelman M.C. (1993) Nature 365,626. Mulchaey J.S., Mushotzky R.F. and Weaver K.A. (1992) Ap. J Lett. 390, L69. Roche RF., Aitken D.K, Smith C.H. and Ward M.J. (1991) M.NR.A.S. 248,606. Seaton M.J. (1979) M.NR.A.S. 187,73P.