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the torus was introduced by @xcite to explain the revelation from spectropolarimetry that type  2 ( narrow - line ) seyfert galaxies can look like type  1 ( broad - line ) seyferts along different lines of sight . since that discovery , the torus primary purpose has been to obscure the broad - line region , but it also serves to explain the near- and mid - infrared ( ir ) emission observed in the vast majority of quasars whose spectral energy distributions ( seds ) indicate dust heated to near sublimation temperatures , presumably by the accretion - disk continuum .    typically , the torus has been modeled as a static , axisymmetric structure ( e.g. , * ? ? ?
* ) that may have condensations ( clumps or clouds ) within it ( e.g. , * ? ? ?
a notable exception to this paradigm is the dusty wind model of @xcite , who started with a magneto - hydrodynamic ( mhd ) wind , and added in radiation driving on grains beyond the dust sublimation radius .
they were motivated in part by the large scale - height of the obscurer required by the observed seyfert  1 to seyfert  2 ratio , the inevitability of radiation driving in the presence of such a strong source of uv photons , and the apparent luminosity - dependent covering fraction of the torus @xcite . since then others ( e.g. , * ? ? ?
* ) have also incorporated some wind component into their models , and/or the effects of ir radiation - pressure in inflating the dusty medium ( e.g. , * ? ? ?
the questions remains : how to test for the existence of this dusty wind ?    in the uv ,
wind signatures are clearly evident in the broad resonance lines of high ionization species such as c  iv . while most apparent in classic p - cygni - like broad absorption lines ( e.g. , * ? ? ?
* ) , the asymmetries in the same broad emission lines are also evidence for winds ( e.g. , * ? ? ?
* ; * ? ? ?
the most natural mechanism for accounting for the high velocities ( @xmath1  km  s@xmath2 ) of the outflows is resonance - line driving by uv continuum photons .    unlike the clear wind signatures in the uv ,
quasar ir spectra are characterized by broad features , such as the silicate emission bumps from the vibrational stretching and bending modes of the grains at 10 and 18  @xmath0 m , respectively ( see figure  [ fig : gall_one ] ) .
the widths and peaks of the silicate bumps are affected by the grain size distribution , and so extracting information on the velocity of the emitting grains is impossible given the uncertainties in the inherent shape of the bumps .
narrow atomic features in the ir are from forbidden lines such as [ o  iv ] and [ ne  v ] generated in low density , photoionized gas in the inner parts of the narrow - line region .
clear spectral signatures of a dusty wind are therefore elusive .
luminous sdss quasars from the _ spitzer _ irs archive .
prominent emission features are labeled ; the horizontal bars indicate the approximate location of the broad silicate emission bumps . from hill
( in preparation ) . ]
in the uv , the maximum possible terminal velocity of outflows seen in absorption appears to be set by the uv luminosity @xcite .
this makes sense for a radiation - driven uv outflow ; a high uv photon density has the ability to transfer more momentum to the ionized wind .
while this particular mechanism is not relevant in the ir , spectral features in this regime whose strength depends on luminosity are potentially fruitful for looking for evidence of the wind from the dust .
one such feature is the 35  @xmath0 m bump , which can be successfully modeled as a blackbody with a temperature of 12001500  k ( e.g. , * ? ? ?
* ; * ? ? ?
these temperatures are interesting because they are near the grain sublimation temperature of graphites , but higher than the grain sublimation temperature of silicates .
@xcite first noted that this feature becomes stronger at higher luminosities .
@xcite confirmed this trend with a much larger sample of sdss quasars with _
irac + mips photometry by searching for convex spectra between 1 and 8  @xmath0 m ; the amount of spectral curvature was significantly correlated with luminosity .
the increasing prominence of the 35  @xmath0 m bump can be qualitatively understood in a wind paradigm , because more luminous quasars will have more radial ir outflows .
assuming an accretion disk is the source of the optical through x - ray continuum , the dust - driving continuum will always be interior to ( and significantly more compact than ) the dusty wind , and thus act in the radial direction . in this scenario ,
the hottest dust is visible from a larger range of inclination angles in luminous objects . in the uv , an ionized wind launched from the accretion disk at small radii ( @xmath3  cm ) could have a significant vertical component to its acceleration .
this geometric effect can naturally account for the larger observed fraction of bal quasars ( seen when looking through the uv wind ) compared to type 2 quasars ( seen when the broad - line region is blocked by thick , dusty material ) .
to investigate the interplay of radiation driving and the ir sed , we have developed a dynamical model of the torus as a dusty wind launched by mhd forces and by radiation pressure from the accretion disk continuum . the dusty wind generated in this manner can cover a large fraction of the sky as seen from the central black hole .
this model has the benefit of being more self - consistent than static torus models and including the important physics of motion around the black hole and radiative acceleration .
ultimately , we wish to use this model to understand how the physical properties of dusty winds in quasars correlate with their observable spectral signatures in the ir .    the model and its corresponding code has been expanded from its original form ( described in * ? ? ?
* ) by @xcite , where a comprehensive account of the model s components and key equations are described .
we advance on @xcite by adding the continuum opacity of ism dust grains , as specified by the ism dust model @xcite in ` cloudy ` @xcite .
the radiation pressure on the dust from the central source is very strong  even for cases where l / l@xmath4 , it is approximately 10 times the force required to unbind dust from the gravitational potential @xcite .
dust grains absorb radially streaming photons originating from the accretion disk , and then cool by radiating isotropically .
the force , felt by the dust particles due to conservation of momentum , feeds back on the wind structure by bending the magnetic wind from a vertical to more radial structure .
the radiative force works in conjunction with the magneto - centrifugal forces to accelerate the wind flow and therefore modifies the structure of the outflow .
after iterating the mhd+radiation pressure calculations to set up the structure of the wind , we use the monte carlo radiative - transfer code mc3d @xcite to send rays through the wind and predict the observed ir seds .
the input parameters for the wind such as eddington ratio ( @xmath5 ) , black hole mass ( @xmath6 ) , and illuminating continuum ( the empirical optical - to - x - ray composite of @xcite , hereafter r06 ) are varied to investigate their effects on the output sed .
m bump emission .
_ right : _ the output ir seds generated by the wind models ; different line styles are used to distinguish the observed inclination angle , @xmath7 , with respect to the normal to the accretion disk .
the power and shape of the sed is not particularly sensitive to the angle , indicating that the wind is optically thin to ir radiation for @xmath8  @xmath0 m .
the short wavelength emission increases with smaller inclination angle . this effect could contribute to relatively stronger 35 @xmath0 m emission in more luminous objects because their more radial dusty winds reveal the hottest dust in a larger range of inclination angles .
figures from keating et al .
( 2012).,title="fig : " ] + m bump emission .
_ right : _ the output ir seds generated by the wind models ; different line styles are used to distinguish the observed inclination angle , @xmath7 , with respect to the normal to the accretion disk .
the power and shape of the sed is not particularly sensitive to the angle , indicating that the wind is optically thin to ir radiation for @xmath8  @xmath0 m . the short wavelength emission increases with smaller inclination angle .
this effect could contribute to relatively stronger 35 @xmath0 m emission in more luminous objects because their more radial dusty winds reveal the hottest dust in a larger range of inclination angles .
figures from keating et al .
( 2012).,title="fig : " ]
with input parameters appropriate for luminous quasars ( luminosity of the incident continuum : @xmath9  erg  s@xmath2 ; @xmath10 ; @xmath6 = 10@xmath11  m@xmath12 ; column density at the base of the wind : @xmath13 @xmath14 ; and observer s inclination angle from the disk normal : @xmath15 ) , we can produce reasonable ir seds with approximately the right shape and luminosity ( @xmath16 erg s@xmath2 ) as expected from the r06 composite of optically luminous sdss quasars ( see  figure  [ fig : gall_two ] ) .
this is a promising result for a relatively simple model given that we have not attempted any fitting .
a benefit to our model is that we are able to see directly the effects of various physical parameters on the final ir sed , which ultimately will allow us to understand the physical properties of the torus itself . by determining which physical parameters have an observable effect on the ir seds , and narrowing them down so that we generally reproduce the power expected , we have established a reasonable starting point from which we can expand and further refine our model .
in the near future , we plan to implement a more sophisticated treatment of dust grains that takes into account the different sublimation temperatures of graphite and silicate .
the luminosity dependence of emission from the hottest grains  graphites  is promising as a means of demonstrating the connection between radiative driving of grains , wind geometry , and the observed sed .
we expect that radiation pressure must be important , because otherwise the luminosity dependence of the torus covering fraction and of the 35  @xmath0 m emission is hard to understand .
our ultimate goal is to generate a library of realistic , ir seds that can be compared with _ spitzer _
irs observations of quasars .
we aim to determine ( 1 ) if reasonable input properties ( e.g. , the input continuum shape , @xmath5 , and @xmath6 ) result in seds that match in detail those observed , and more ambitiously , ( 2 ) how to use the empirical ir seds to independently constrain the unknown physical properties of quasars .                    ,
j.  e. , gallagher , s.  c. , & keating , s.  k. 2009 , in the monster s fiery breath : feedback in galaxies , groups , and clusters , edited by s.  heinz & e.  wilcots , vol .
1201 of american institute of physics conference proceedings , 56                            , g.  t. , lacy , m. , storrie - lombardi , l.  j. , hall , p.  b. , gallagher , s.  c. , hines , d.  c. , fan , x. , papovich , c. , vanden berk , d.  e. , trammell , g.  b. , schneider , d.  p. , vestergaard , m. , york , d.  g. , jester , s. , anderson , s.  f. , budavri , t. , & szalay , a.  s. 2006 , apjs , 166 , 470 .
the blue - shifted broad emission lines and/or broad absorption lines seen in many luminous quasars are striking evidence for a broad line region in which radiation driving plays an important role . 
 we consider the case for a similar role for radiation driving beyond the dust sublimation radius by focussing on the infrared regime where the relationship between luminosity and the prominence of the 35  @xmath0 m bump may be key . to investigate this further 
 , we apply the 3d hydrodynamic wind model of @xcite to predict the infrared spectral energy distributions of quasars . 
 the presence of the 35  @xmath0 m bump and strong , broad silicate features can be reproduced with this dynamical wind model when radiation driving on dust is taken into account .