4.3 Comparison between sources
4.3.2 non-BAL and mini-BAL
PG 1211+143 is a non-BAL quasar with very strong outflow. According to Table.4.3, we find that the components in PG 1211+143 are very dif-ferent from those in the others. The contribution of components
(ampor-phous olivine, alumina, and periclase) is more uniform, with 38.4%±4.3%
amorphous olivine, 39.2%±0.4% alumina and 20.3%±3.2% periclase. Fig.4.7 and Table.4.3 show the ratio of periclase is higher than in other sources, even though the amorphous olivine is still dominating. On the contrary the amount of the crystalline olivine, forsterite, is still small, but given the small amount of amorphous olivine, the crystalline fraction is higher. The amorphous olivine is completely dominated by Fe-rich amorphous olivine (MgFeSiO4).
PG 1351+640 is a mini-BAL quasar and PG 0050+124 is a non-BAL quasar, but they both present weak or no outflows. They have similar red-shift, 0.0881 and 0.0808 respectively, but as started already there is no phys-ical reason that the redshift has an influence on dust formation. Amorphous olivine still represents the dominant species in these cases, and is even more dominant than the case of BALs. The proportion of amorphous olivine run up to 78.8%±9.5% and 82.8%±0.4% respectively. However, amorphous olivine is more Fe-poor for PG 0050+124, and Fe-rich for PG 1351+640. The amount of forsterite in these quasars with weak outflow is far lower than in the BALs;
With ∼ 0.9% for PG 1351+640, and even absent totally for PG 0050+124.
As a consequence, the crystalline fraction in the quasar with weak outflow is also lower than that in BALs. (See Fig.4.5, Fig.4.6 and Table.4.3)
These three quasars are all non-BALs or mini-BAL, but the amount of alumina in PG 1211+143 with very strong outflow is much higher than other two quasars with weak outflow. Quasars with strong outflow produce warm winds, causing fluctuations in the temperature and density. This may provide a more inhomogeneous density environment for forming alumina. This result
is in agreement with the outflow model, proposed by Elvis et al. (2002). In addition, the high ratio of periclase implies that the temperature in the winds of PG 1211+143 drops rapidly when the BEL clouds expanded after the divergence of the warm outflowing winds taking this system out of pressure balance (Elvis et al., 2002). In the case of the other two quasars with weak outflow, the amount of alumina and periclase is far lower, implying that the weak outflow may not provide suitable conditions to form such species. In fact, the amorphous olivine in PG 1351+640 and PG 0050+124 is just as dominant as in normal galaxies. From our limited sample we can conclude that ratio of highly refractory alumina and less refractory periclase increase with stronger outflows, which provide therefore a good environment for both species to form and coexist. Consequently, our results could support the conclusion in Elvis et al. (2002), and we can say the winds of quasars are necessary for QSOs to form dust, but on the contrary, if the quasar without winds, the dust around quasar may come from AGB stars.
MIE CDE
Figure 4.1: The two plots on left hand side are spectrum of our fitting results, and the left hand side one is the result in Markwick-Kemper et al. (2007).
The power-law index in the Markwick-Kemper et al. (2007) are -0.617±0.004 and -0.674± 0.003 (dashed line: the continuum curve is determined by pho-tometric data, the diamonds. solid line: the continuum curve is determined by spectroscopic data.) The values of power-law index in our results are more slack, and they are 0.555 and 0.559 in the CDE model (left-up) and Mie(left-bottom) individually.
Figure 4.2: Two kinds of method for determining the components of PG 2112+059. Left : We determine the components of PG 2112+095 using our model, including CDE model (top) and Mie (bottom). The different species show in different colors, for instance, Fe-rich amorphous olivine (green), Forsterite (blue), Alumina (orange), Periclase (magenta), PAHs (cyan), and Fe-poor amorphous olivine (violet). Right : Determing the components of PG 2112+095 by the method in Markwick-Kemper et al. (2007) (including two kinds of continuum model: photometric and spectroscopic).
Figure 4.3: Best 5-25 µm fits for BAL quasar, PG 2112+059. The top plots show spectrum with power-law continuum model, and the bottom ones are continuum-divided spectrum. Left : The two plots at left hand side show the compositions determined using CDE (non-spherical grain) model, and we can see the dominant dust components here are corundum (Al2O3; orange line) and Fe-poor amorphous olivine (Mg2SiO4, violet), Fe-rich amorphous olivine (MgFeSiO4; green). Right : The two plots at right hand side represent the component determined using Mie (spherical grain) modle. In this class corun-dum (Al2O3) are still in the great majority, and the percentage of Fe-poor amorphous olivine (Mg2SiO4) and Fe-rich amorphous olivine (MgFeSiO4) are a little similar. Comparing the results of spherical and non-spherical, the per-iclase (MgO; magenta) is much more abundant in the spherical case then in the non-spherical case.
Figure 4.4: Best 5-25 µm fits for BAL quasar, PG 0043+039. The top plots show spectrum with power-law continuum model, and the bottom ones are continuum-divided spectrum. Left : The two plots at left hand side show the compositions are determined using CDE model (non-spherical grain), and dominate here are Fe-rich amorphous olivine (MgFeSiO4; green) and periclase (MgO, magenta), followed by corundum (Al2O3; orange line). Right : the two plots at right hand side represent the component determined using Mie model (spherical grain). The results here are similar with the CDE (non-spherical) results: Fe-rich amorphous olivine (MgFeSiO4; green) and periclase (MgO, magenta) dominate the composition, dust components with the exception of forsterite being more abundant in the spherical grain case.
Figure 4.5: Best 5-25 µm fits for a radio-quiet quasar, PG 1351+640. The top plots show spectrum with power-law continuum model, and the bottom ones are continuum-divided spectrum. Left : The two plots at left hand side show the compositions determined using CDE model (non-spherical grain), and there the dominant component is Fe-rich amorphous olivine (MgFeSiO4; green), followed by alumina (Al2O3; orange line). Right : the two plots at right hand side represent the component determined using the Mie model (spherical grain). The result here are very similar with the CDE (non-spherical) results: Fe-rich amorphous olivine (MgFeSiO4; green) occupy most percentage of compositions.
Figure 4.6: Best 5-25 µm fits for a radio-quiet quasar, PG 0050+124. The top plots show spectrum with power-law continuum model, and the bottom ones are continuum-divided spectrum. Left : The two plots at left hand side show the compositions are determined using CDE model (non-spherical grain). In this case, the dominant component is Fe-poor amorphous olivine (Mg2SiO4, violet). With in addition, some corundum (Al2O3; orange line).
Right : the two plots at right hand side represent the component determined using the Mie model (spherical grain).Results from CDE (non-spherical) and Mie(spherical) are very similar in this case: the Fe-poor amorphous olivine (Mg2SiO4; violet) in spherical is near the percentage in the non-spherical case, and the Fe-rich amorphous olivine (MgFeSiO4; green) are none in the two kinds of cases.
Figure 4.7: Best 5-25 µm fits for a radio-loud quasar, PG 1211+143. The top plots show spectrum with power-law continuum model, and the bottom ones are continuum-divided spectrum. Left : The two plots at left hand side show the compositions determined using Mie model (non-spherical grain).
Right : the two plots at right hand side represent the component are de-termined using the modle (spherical grain). In this case, the non-spherical and spherical models bring very similar results: Fe-rich amorphous olivine (MgFeSiO4; green) and alumina (Al2O3; orange line) dominate the spectrum, while Periclase (MgO) is less abundant.
SourceAmorphousolivine ForsteriteCrystallinefractionAl2O3MgO∆χ2 Totalof amorphousolivineFe-richFe-poor PG1351+640...CDE72.0%±4.4%67.2%±2.7%14.8%±3.5%0.38%±0.06%0.53%±0.09%13.9%±0.5%3.8%±0.2%2.21 ...Mie85.5%±5.4%80.7%±3.2%4.8%±4.4%1.43%±0.05%1.65%±7.11%11.09%±0.55%2.0%±0.2%3.58 Average78.8%±9.5%74.0%±9.5%9.8%±7.0%0.91%±0.75%1.1%±0.8%12.5%±2.0%2.9%±1.3% PG0050+124...CDE83.1%±1.1%0%83.1%±1.1%0%0%16.9%±0.7%0%3.58 ...Mie82.5%±1.1%0%82.5%±1.1%0%0%17.5%±0.6%0%2.57 Average82.8%±0.4%0%82.8%±0.4%0%0%17.2%±0.4%0% PG1211+143...CDE41.4%±1.7%41.4%±1.7%0%1.7%±0.1%3.9%±0.3%38.9%±1.1%18.0%±0.5%2.82 ...Mie35.3%±1.5%35.3%±1.5%0%2.8%±0.1%7.4%±0.4%39.4%±1.1%22.5%±0.5%3.58 Average38.4%±4.3%38.4%±4.3%0%2.25%±0.785.7%±2.5%39.2%±0.420.3%±3.2% PG0043+039...CDE54.9%±5.0%54.9%±5.0%0%0.2%±0.4%0.4%±0.7%23.6%±4.1%21.3%±2.5%0.60 ...Mie44.8%±5.0%44.8%±5.0%0%1.8%±0.4%3.9%±0.5%25.2%±4.2%28.2%±3.0%0.76 Average49.9%±7.1%49.9%±7.1%0%1.0%±1.1%2.2%±2.5%24.4%±1.1%24.8%±4.9% PG2112+059...CDE54.0%±3.6%18.5%±2.2%35.5%±2.9%0%0%40.8%±0.9%5.2%±0.5%5.24 ...Mie49.0%±4.8%20.7%±2.9%28.3%±3.8%0.6%±0.06%1.2%±0.2%39.0%±0.9%11.4%±3.8%5.635 Average51.5%±3.5%19.6%±1.6%31.9%±5.1%0.3%±0.4%0.6%±0.8%39.9%±1.27%8.3%±4.4% PG2112+059(2007)56.5±1.4%.........5±3%38±3%5.9±2.6%2 Table4.3:Thebestχ2 fittingresultstoeverysource.Thefirstrowshowsthedustspeciesandfollowingrowsare massfractionsforeverydustspeciesinthedifferentsource.Thefirstcolumnmeansdifferentsource.Allthemass fractionsofeverysourceareobservedwithinthe∆χ2 lessthanafactor2.3,exceptingforPG2112.
Chapter 5 Conclusion
We determined the dust composition of radio-quiet quasars using Spitzer-IRS data: two BAL quasars: PG 2112+059 and PG 0043+039; non-BAL with strong outflow: PG 1211+143; two quasars with weak outflow: PG 1351+640 (mini-BAL) and PG 0050+124 (non-BAL). We developed a new method for deriving the fraction od carious dust species by SED fitting in which the SED consists of a features we adopted power-law continuum and optically thin dust. Six species seen commonly in dust composition, but also providing ideas on the physical processes in action: amorphous olivine, crystalline olivine, forsterite, alumina, periclase, and PAHs, are used in our model fitting, and the spectral features of three of them (amorphous olivine, crystalline olivine, and alumina) are both calculated by CDE model and Mie.
First, we have confirmed that our result is consistent with the result obtained by Markwick-Kemper et al. (2007), who used a different method.
The quasars with strong outflow winds could produce more various envi-ronment, for instance, different degree of temperature as well as density, and
that can help to form more diverse species around quasars. Especially, there is a good condition for forming MgO, because the temperature can cool down rapidly in the quasars with strong winds. Therefore, in our cases, we find the three quasars with strong outflow winds present higher amount of refrac-tory alumina (Al2O3) and periclase (MgO) and smaller amount of crystalline olivine and forsterite than the other two quasars, with weak outflows. The component in those quasars with strong winds display more various species.
On the contrary, in the case of other two quasars with weak outflow winds, amorphous olivine is largely dominant over other species. Their compositions are in fact similar to the composition observed in normal galaxies and AGB stars. Therefore, outflow winds might provide a viable environment for for-mation of diverse dust species coexisting, such as highly refractory alumina, less refractory periclase, amorphous olivine and small amount of crystalline olivine. Our results also imply that the environment of those quasars with winds present inhomogeneous temperature and density, thus can provide suit-able environments for dust formation, conforming to the model developed by Elvis et al. (2002).
Appendix A
Eddington limit
To deduce the Eddington limit, for simplicity case, we suppose it is a com-pletely ionized hydrogen gas. In order to avoid disintegration, we assume the outward force of radiation pressure is equilibrated with the inward force of gravity. The flux of outward energy at the some distance r from the center is F = L/4πr2.Because the momentum can deduce form the energy (p(momentum)=E(energy)/c(lightspeed)), the outward momentum flux (or pressure) also can know
Prad = F
c = L
4πr2c. (A.1)
L is the luminosity in radiation.
Then the Thomson cross section σT can be used for estimating how much the scattering of low-energy light from an electron
σT = 8π
where e is the electron’s charge ; me is the mass of electron (Peterson, 1997).
Therefore, the momentum transferred to a single electron is
Prad = σT L
4πr2c, (A.3)
and then the momentum per unit time is a force
Frad = σT L
4πr2cˆr. (A.4)
According to above supposing, we assume the outward force of radiation pressure is equilibrated with the inward force of gravity, and then we can suppose that:
|Frad| ≤ |Fgrav|. (A.5)
For the ionized hydrogen case, the gravitational force will act on an electron-proton pair by a M central mass, and the mass of electron-proton is much larger than mass of electron, hence the gravitational force will become Frad =
−GM (mr2p+me)r ≈ −ˆ GM mr2 prˆ
M is the mass of the compact object (central mass) and L is the luminosity
in radiation of the central source. The inward gravitational force acting on the ionized hydrogen gas need taking balance or exceed the outward force, otherwise it will disintegrate. As the result, the L is known as the Eddington limit, and we define the Eddington luminosity from above equation
LE = 4πGcmp
σe M, (A.10)
Which can be estimate the maximum luminosity of a source of mass M,can then we could deduce the minimum of the mass (ME)from Eddington lumi-nosity.
If the case is AGNs,
ME = 8 × 105L44M (A.11)
Where the L44is the luminosity of central source in unit of 1044ergss−1,which is also characteristic of the luminosity Seyfert galaxy. If the case is typical quasar, the luminosity of the central should in unit of 1046ergss−1,therefore, the central mass should be about ∼ 108M .
The method could estimate not only the luminosity, but also could esti-mate the temperature T of a black body of size R. If the luminosity L is a fraction ε of LE,we can get
4πR2σT4 = εLE (A.12)
Here,σ is he Stephan-Boltzmann constant to describe the radiation from a
blackbody. Using the Eddington luminosity and above equations, we get
This estimation could explain what is wave band we can observe for accretion disk around the different mass of black holes. Furthermore, we also obtain the relation between the accretion rate and Eddington luminosity. Because there is conversion of mass to energy in an active nucleus, we could estimate the black hole accretion rate by
L = ξ ˙M c2, (A.15)
Therefore, the mass accretion rate is
M =˙ L
ξc2, (A.16)
where dotM is dMdt ,and ξ is the radiative efficiency.
The equation (A.15) deduce from
E = ξM c2, (A.17)
which shows the energy available from a mass M, and then the rate of energy for the nucleus emitting is L = dE/dt,consequently , we obtain the mass accretion rate as (A.16) showed.
We assume the energy of infalling material and radiation is converted,
therefore, we get the relative equation
L ≈ dU
dt = GM ˙M
r , (A.18)
Form (A.15) and (A.18), we get some information about the efficiency
L ≈ GM ˙M
r = ξ ˙M c2, (A.19)
and then we deduce that ξ ∝ Mr . The ratio ξ ∝ Mr, which characterized relativistic effects in the Sun’s geometry,is only of order 10−6,but the case for the typical neutron stars should be M/R ∼ 0.2 and the relativistic effects for the black holes should be M/R ∼ 0.5.
Bibliography
Antonucci, R. (1993). Unified models for active galactic nuclei and quasars.
ARA&A, 31:473–521.
Barvainis, R., Lonsdale, C., and Antonucci, R. (1996). Radio Spectra of Radio Quiet Quasars. Astron. J., 111:1431.
Beckmann, V. and Shrader, C. R. (2013). The AGN phenomenon: open issues. ArXiv e-prints.
Blommaert, J. A. D. L., Groenewegen, M. A. T., Okumura, K., Ganesh, S., Omont, A., Cami, J., Glass, I. S., Habing, H. J., Schultheis, M., Simon, G., and van Loon, J. T. (2006). ISO mid-infrared spectroscopy of Galactic Bulge AGB stars. Astron. Astrophys., 460:555–563.
Bonning, E. W., Cheng, L., Shields, G. A., Salviander, S., and Gebhardt, K. (2007). Accretion Disk Temperatures and Continuum Colors in QSOs.
Astrophys. J., 659:211–217.
Bouwman, J., Lawson, W. A., Dominik, C., Feigelson, E. D., Henning, T., Tielens, A. G. G. M., and Waters, L. B. F. M. (2006). Binarity as a Key
Factor in Protoplanetary Disk Evolution: Spitzer Disk Census of the η Chamaeleontis Cluster. Astrophys. J., 653:L57–L60.
Bradley W. Carroll, D. A. O. (2006). An Introduction to Modern Astrophysics (2nd Edition). Addison-Wesley.
Clavel, J., Wamsteker, W., and Glass, I. S. (1989). Hot dust on the outskirts of the broad-line region in Fairall 9. Astrophys. J., 337:236–250.
Demyk, K., Jones, A. P., Dartois, E., Cox, P., and D’Hendecourt, L. (1999).
The chemical composition of the silicate dust around RAFGL7009S and IRAS 19110+1045. Astron. Astrophys., 349:267–275.
Dhanda, N., Baldwin, J. A., Bentz, M. C., and Osmer, P. S. (2007). Quasars with Super-Metal-rich Emission-Line Regions. Astrophys. J., 658:804–814.
Elitzur, M. (2005). IR Emission from AGNs. ArXiv Astrophysics e-prints.
Elitzur, M. (2007). Unification Issues and the AGN Torus. In Ho, L. C.
and Wang, J.-W., editors, The Central Engine of Active Galactic Nuclei, volume 373 of Astronomical Society of the Pacific Conference Series, page 415.
Elitzur, M. and Shlosman, I. (2006). The AGN-obscuring Torus: The End of the “Doughnut” Paradigm? Astrophys. J., 648:L101–L104.
Elvis, M., Marengo, M., and Karovska, M. (2002). Smoking Quasars: A New Source for Cosmic Dust. Astrophys. J., 567:L107–L110.
Evans, A. (1993). The Dusty Universe. Ellis Horwood.
Fan, X., Narayanan, V. K., Lupton, R. H., Strauss, M. A., Knapp, G. R., Becker, R. H., White, R. L., Pentericci, L., Leggett, S. K., Haiman, Z., Gunn, J. E., Ivezi´c, ˇZ., Schneider, D. P., Anderson, S. F., Brinkmann, J., Bahcall, N. A., Connolly, A. J., Csabai, I., Doi, M., Fukugita, M., Geballe, T., Grebel, E. K., Harbeck, D., Hennessy, G., Lamb, D. Q., Miknaitis, G., Munn, J. A., Nichol, R., Okamura, S., Pier, J. R., Prada, F., Richards, G. T., Szalay, A., and York, D. G. (2001). A Survey of z > 5.8 Quasars in the Sloan Digital Sky Survey. I. Discovery of Three New Quasars and the Spatial Density of Luminous Quasars at z˜6. Astron. J., 122:2833–2849.
Fanidakis, N., Baugh, C. M., Benson, A. J., Bower, R. G., Cole, S., Done, C., and Frenk, C. S. (2011). Grand unification of AGN activity in the ΛCDM cosmology. MNRAS, 410:53–74.
Ferrarese, L. and Ford, H. (2005). Supermassive Black Holes in Galactic Nuclei: Past, Present and Future Research. Space Sci. Rev., 116:523–624.
Ferrarotti, A. S. and Gail, H.-P. (2003). Mineral formation in stellar winds.
IV. Formation of magnesiow¨ustite. Astron. Astrophys., 398:1029–1039.
Frenklach, M., Carmer, C. S., and Feigelson, E. D. (1989). Silicon carbide and the origin of interstellar carbon grains. Nature, 339:196–198.
Frenklach, M. and Feigelson, E. D. (1989). Formation of polycyclic aromatic hydrocarbons in circumstellar envelopes. Astrophys. J., 341:372–384.
Gail, H.-P. and Sedlmayr, E. (1998). Dust Formation in M Stars. The Molec-ular Astrophysics of Stars and Galaxies, edited by Thomas W. Hartquist and David A. Williams. Clarendon Press, Oxford, 1998., p.285, 4:285.
Gallagher, S. C., Abado, M. M., Everett, J. E., Keating, S., and Deo, R. P.
(2013). Why a Windy Torus? ArXiv e-prints.
Gallagher, S. C., Brandt, W. N., Wills, B. J., Charlton, J. C., Chartas, G., and Laor, A. (2004). Dramatic X-Ray Spectral Variability of the Broad Absorption Line Quasar PG 2112+059. Astrophys. J., 603:425–435.
Gaskell, C. M. (2009). What broad emission lines tell us about how active galactic nuclei work. New Astron.Rev., 53:140–148.
Ghez, A. M., Salim, S., Hornstein, S. D., Tanner, A., Lu, J. R., Morris, M., Becklin, E. E., and Duchˆene, G. (2005). Stellar Orbits around the Galactic Center Black Hole. Astrophys. J., 620:744–757.
Granato, G. L., Danese, L., and Franceschini, A. (1997). Thick Tori around Active Galactic Nuclei: The Case for Extended Tori and Consequences for Their X-Ray and Infrared Emission. Astrophys. J., 486:147.
Hao, L., Spoon, H. W. W., Sloan, G. C., Marshall, J. A., Armus, L., Tielens, A. G. G. M., Sargent, B., van Bemmel, I. M., Charmandaris, V., Weedman, D. W., and Houck, J. R. (2005). The Detection of Silicate Emission from Quasars at 10 and 18 Microns. Astrophys. J., 625:L75–L78.
Hewett, P. C. and Foltz, C. B. (2003). The Frequency and Radio Properties of Broad Absorption Line Quasars. Astron. J., 125:1784–1794.
Houck, J. R., Roellig, T. L., van Cleve, J., Forrest, W. J., Herter, T., Lawrence, C. R., Matthews, K., Reitsema, H. J., Soifer, B. T., Watson,
D. M., Weedman, D., Huisjen, M., Troeltzsch, J., Barry, D. J., Bernard-Salas, J., Blacken, C. E., Brandl, B. R., Charmandaris, V., Devost, D., Gull, G. E., Hall, P., Henderson, C. P., Higdon, S. J. U., Pirger, B. E., Schoenwald, J., Sloan, G. C., Uchida, K. I., Appleton, P. N., Armus, L., Burgdorf, M. J., Fajardo-Acosta, S. B., Grillmair, C. J., Ingalls, J. G., Morris, P. W., and Teplitz, H. I. (2004). The Infrared Spectrograph (IRS) on the Spitzer Space Telescope. Astrophys. J. Suppl., 154:18–24.
Imanishi, M. and Ueno, S. (2000). The 9.7 Micron Silicate Dust Absorption toward the Cygnus A Nucleus and the Inferred Location of the Obscuring Dust. Astrophys. J., 535:626–631.
Ivezic, Z. and Elitzur, M. (1997). Self-similarity and scaling behaviour of in-frared emission from radiatively heated dust - I. Theory. MNRAS, 287:799–
811.
Jaffe, W., Meisenheimer, K., R¨ottgering, H. J. A., Leinert, C., Richichi, A., Chesneau, O., Fraix-Burnet, D., Glazenborg-Kluttig, A., Granato, G.-L., Graser, U., Heijligers, B., K¨ohler, R., Malbet, F., Miley, G. K., Paresce, F., Pel, J.-W., Perrin, G., Przygodda, F., Schoeller, M., Sol, H., Waters, L. B. F. M., Weigelt, G., Woillez, J., and de Zeeuw, P. T. (2004). The central dusty torus in the active nucleus of NGC 1068. Nature, 429:47–49.
J¨ager, C., Dorschner, J., Mutschke, H., Posch, T., and Henning, T. (2003).
Steps toward interstellar silicate mineralogy. VII. Spectral properties and crystallization behaviour of magnesium silicates produced by the sol-gel method. Astron. Astrophys., 408:193–204.
Kemper, F., Vriend, W. J., and Tielens, A. G. G. M. (2004). The Absence of Crystalline Silicates in the Diffuse Interstellar Medium. Astrophys. J., 609:826–837.
Kemper, F., Vriend, W. J., and Tielens, A. G. G. M. (2005). Erratum:
“The Absence of Crystalline Silicates in the Diffuse Interstellar Medium”
(¡A href=”/abs/2004ApJ...609..826K”¿ApJ, 609, 826 [2004]¡/A¿). Astro-phys. J., 633:534–534.
Knacke, R. F. and Thomson, R. K. (1973). Infrared Extinction Cross Sections of Silicate Grains. Publ. Astron. Soc. Pacific, 85:341.
Krawczynski, H. and Treister, E. (2013). Active Galactic Nuclei - the Physics of Individual Sources and the Cosmic History of Formation and Evolution.
ArXiv e-prints.
Krolik, J. H. and Begelman, M. C. (1988). Molecular tori in Seyfert galaxies - Feeding the monster and hiding it. Astrophys. J., 329:702–711.
Laor, A. and Draine, B. T. (1993). Spectroscopic constraints on the proper-ties of dust in active galactic nuclei. Astrophys. J., 402:441–468.
Lebouteiller, V., Barry, D. J., Spoon, H. W. W., Bernard-Salas, J., Sloan, G. C., Houck, J. R., and Weedman, D. W. (2011). CASSIS: The Cor-nell Atlas of Spitzer/Infrared Spectrograph Sources. Astrophys. J. Suppl., 196:8.
Lodders, K. and Fegley, Jr., B. (1999). Condensation Chemistry of
Circum-stellar Grains. In Le Bertre, T., Lebre, A., and Waelkens, C., editors, Asymptotic Giant Branch Stars, volume 191 of IAU Symposium, page 279.
Madejski, G. (2003). Black holes in active galactic nuclei. pages 36–45.
Markwardt, C. B. (2009). Non-linear Least-squares Fitting in IDL with MPFIT. In Bohlender, D. A., Durand, D., and Dowler, P., editors, nomical Data Analysis Software and Systems XVIII, volume 411 of Astro-nomical Society of the Pacific Conference Series, page 251.
Markwick-Kemper, F., Gallagher, S. C., Hines, D. C., and Bouwman, J.
(2007). Dust in the Wind: Crystalline Silicates, Corundum, and Periclase in PG 2112+059. Astrophys. J., 668:L107–L110.
Molster, F. J., Waters, L. B. F. M., and Kemper, F. (2010). The Mineralogy of Interstellar and Circumstellar Dust in Galaxies. In Henning, T., editor, Lecture Notes in Physics, Berlin Springer Verlag, volume 815 of Lecture Notes in Physics, Berlin Springer Verlag, pages 143–201.
Molster, F. J., Waters, L. B. F. M., Tielens, A. G. G. M., Koike, C., and Chihara, H. (2002). Crystalline silicate dust around evolved stars. III.
A correlations study of crystalline silicate features. Astron. Astrophys., 382:241–255.
Nenkova, M., Sirocky, M. M., Nikutta, R., Ivezi´c, ˇZ., and Elitzur, M. (2008).
AGN Dusty Tori. II. Observational Implications of Clumpiness. Astro-phys. J., 685:160–180.
Novak, G. S., Ostriker, J. P., and Ciotti, L. (2012). Radiative transfer and