You will be presented with the full text of a science research paper. Please write an abstract for this paper. Your response should include the abstract and no additional text.

Paper text:
dust is common in high redshift ( @xmath0=46 ) quasars ( omont et al . 2001 , priddey & mcmahon 2001 ) , apparently implying that dust is widespread by this epoch .
this presents a puzzle : how can so much dust be formed in the short time available ?
wmap cosmology ( spergel et al . 2003 ) puts the age of the universe at z=6 to be only 0.95  gyr , and the age of first reionization ( and so the first stars ) at an age of @xmath10.2  gyr ( z@xmath120 ) . in our galaxy
the primary source of dust lies in the winds of red giant stars , specifically asymptotic giant branch ( agb ) stars .
only stars with masses greater than 2  @xmath2 can reach the agb in the less than @xmath11  gyr available .
another source of dust is thus required .
normally this extra source is taken to be type  1 ( massive star ) supernovae .
recent scuba results ( dunne et al . 2003 ) show that the 300  year old type  1 supernova remnant ( snr ) cas  a is rich in dust , lending credence to this picture .
however , the possible destruction of snr formed dust in their shocks may not be fully resovled . here
we point out another , seemingly inevitable , path for dust formation up to at least z=6 : _ quasar winds_.    the fate of outflowing broad emission line ( bel ) quasar gas constantly being ejected from quasars had previously not been considered .
we examined that fate ( elvis , marengo & karovska 2002 ) in a manner that applies to any model in which the bel clouds move outward .
we find that dust creation is a natural consequence .
outflowing winds of highly ionized material are common in quasars [ see arav , shlosman & weymann ( eds ) 1997 ] .
the gas emitting the prominent broad emission lines ( bels ) may well participate in these winds .
the bels , which produce e.g. ly@xmath3 , h@xmath3 , h@xmath4 , civ , nv ( e.g. krolik 1999 ) are doppler broadened to a few percent of the speed of light ( @xmath5  km  s@xmath6 ) .
the gas producing these lines lies at temperatures of 10@xmath7  k and at the strikingly high densities of 10@xmath8 - 10@xmath9  @xmath10 ( osterbrock 1989 ) .
these densities are comparable to chromospheric values ( allen 1975 , korista 1999 ) .
the kinematics of this bel gas - infalling , bound , or outflowing - is not well established ( peterson 1997 ) . moreover ,
the issue of how to prevent this high density gas from dispersing has been problematic .
pressure confinement by a hotter surrounding medium would seem straightforward and a 2-phase medium is even predicted for gas irradiated by a hard quasar - like continuum ( krolik , mckee & tarter 1981 ) .
however this model appeared to suffer unsurmountable problems ( mathews 1986 ) .
most obviously the any bel ` clouds ' would be ripped apart by shear forces in less than one orbital time . as a result
this model was abandoned .
a bel wind has advantages .
elvis ( 2000 ) showed how such a wind can include the bel gas as a cool phase in pressure equilibrium the warmer ( 10@xmath11  k ) wind medium . with this model
a large number of other puzzling features of quasar phenomenology seem to fall into place .
a wind in which both bel gas and the hotter confining gas are co - moving solves the cloud survival problem , and if the wind is non - spherical also solves the problem of the confining medium being thomson thick , which can not be the case as rapid x - ray variability is essentially universal in agns .
but a wind requires us to ask what happens to the gas as it flows outward . in any outflow model with the bel region initially in pressure equilibrium with a surrounding warmer medium , the divergence of the outflowing warm wind ( even if only at the sound speed of the warm confining medium , @xmath1100  km  s@xmath6 ) will rapidly take the system out of pressure balance .
the bel clouds will then begin to expand , limited by their sound speed ( initially @xmath110  km  s@xmath6 ) , and will cool to below 1000  k , at which temperatures dust will form _ if _ the pressure is still sufficiently high . could quasar bel gas then be the source of quasar dust at high redshift ?
a full treatment of dust formation requires coupling the dust forming medium hydrodynamics with the full set of dust condensation chemistry equations ( sedlmayr 1997 ) .
such an exercise is limited by our knowledge of the highly non - linear dust condensation chemical paths , and by the uncertainties related to the role of non - equilibrium chemistry .
we therefore use a simple comparison between agn and cool star atmospheres , to derive reasonable estimates for the conditions of dust formation in the bel clouds .    the general scenario for dust formation
is based on the concept of the `` dust formation window '' .
effective dust condensation seems to take place whenever a chemically enriched medium has a sufficiently low temperature , and a large enough density , to allow dust grain condensation .
the amount of dust produced , the chemical composition and the final size of the grains , depend on the length of time over which the conditions remain favorable .
figure  1 shows the dust formation window in pressure - temperature space , for the chemical species that lead to the formation of dust in an o - rich cool star circumstellar envelope .
the thin solid lines mark the phase transition region below which the most important dust precursor molecules are formed .
the hatched area is the dust forming region in the circumstellar envelope of a cool giant star .
the region is limited on the right by the thermodynamical path of a static outflowing wind typical for an agb star .
the left side is obtained by increasing the maximum pressure in the envelope by a factor 100 , as during the propagation of pulsational shocks in the atmospheres of long period variables .
dust formation in the envelopes of evolved giants occurs in the region between the two tracks , below the phase transition lines for each chemical species .
a closely similar diagram can be drawn for c - rich gas .
figure  1 shows adiabats for bel gas , assuming @xmath12 .
clearly they pass through the dust formation window for plausible initial densities .
the bel gas enters the dust formation window after expanding by a factor of three .
this process resembles that which makes smoke in terrestrial settings .
quasars are then dusty because they themselves create dust which , since carbon may be overabundant in quasars , may resemble soot .
hence we called them ` smoking quasars ' .
if quasar winds are driven by line radiation then the characteristic instability of such winds suggests that shocks may be common . in stars with ` p  cygni ' profiles ( ob stars and wolf - rayet stars ) such winds produce highly velocity - structured opacity ( e.g. stahl et al .
, 1993 , owocki 2001 ) .
quasars show similar structures ( turnshek 1988 ) , and are suggested by hydrodynamic modeling ( proga 2001 ) . in agb star
winds these intermittent pressure enhancements boost dust production ( figure  1 ) , starting a nonlinear `` avalanche '' effect , as observed in miras and other long period variables ( hffner 1999 ) .
this suggests that pressure fluctuations could greatly enhance the quasar dust creation rate .
can the dust survive the radiation field from the quasar bh ?
surprisingly a quasar wind provides a _ more benign _ environment for dust than an agb star wind , despite the quasar luminosity being much higher than the @xmath13  l@xmath14 luminosity of a typical giant star .
the large flux of energetic photons from the quasar continuum might have overheated the newborn dust grains above their sublimation temperature , delaying the occurence of dust condensation , perhaps until it becomes impossible due to the ever decreasing gas density .
however , due to the much larger geometrical dilution in quasars , the radiative flux reaching the bel clouds interior is actually lower than the stellar flux in the dust forming region of the giant s wind . for a quasar of luminosity @xmath15
erg s@xmath6 the flux density 3  pc from the quasar center is @xmath16  erg  @xmath17  s@xmath6 .
this is at least one order of magnitude less than the @xmath18  erg  @xmath17  s@xmath6 in the stellar case .
so the dust formation window of the bel clouds is determined by the polytropic expansion of the clouds gas , as we have assumed , and not by radiative transfer , as in the case of circumstellar envelopes ( ivezi & elitzur 1997 ) .
other destruction mechanisms , such as dust sputtering by electrons and ions , or chemical sputtering , are not very effective at the rather low ( @xmath19  k ) kinetic temperature of the cloud medium .
kinetic sputtering by the surrounding medium becomes effective ( draine & salpeter 1979 ) only for @xmath20  k. by the time the bel clouds start forming dust , they will be surrounded by a warm medium having a temperature @xmath21  k , which is already low enough to prevent their immediate destruction by sputtering .
the total amount of dust that is formed , however , depends on the time spent by the clouds in the region favorable to grain condensation and growth .
this is given by the cloud expansion time - scale @xmath22  yr , where @xmath23  km s@xmath6 is the initial sound speed of the cloud and @xmath24  cm ( peterson 1997 ) their initial average size .
this timescale is comparable with the time spent by circumstellar grains in the region where dust growth is most active , suggesting that the efficiency in dust production of bel region and late type giant winds may be similar .
the most luminous quasars , in which the highest dust masses are found ( omont et al .
2001 ) , have luminosities over 10@xmath25erg  s@xmath6 and so may have mass loss rates @xmath2610  m@xmath27  yr@xmath6 .
assuming the same dust fraction as in agb stars gives @xmath110@xmath28  m@xmath27 of dust over a nominal 10@xmath29  yr lifetime .
this approaches the amounts detected but is still short by about an order - of - magnitude .
however , super - solar abundances are common in high redshift quasars , certainly in carbon ( hamann & ferland 1999 ) , providing a higher density of raw material for the formation of precursor molecules .
dust condensation is highly nonlinear ( frenklach & feigelson 1989 ) , the amount of dust formed in any particular quasar is hard to predict , but the dependence on abundance is likely to be an @xmath30 process initially , allowing for more dust to be produced than in our simple estimate . as a result
the infrared emission of quasars may not require the normally assumed large associated burst of star formation ( sanders et al . 1988 ) .
quasar winds provide an economical explanation for high z dust .
dust at z@xmath14 is observed primarily from observations of quasars and of sub - mm sources .
if dust is everywhere and quasars illuminate this pre - existing dust then the total dust mass at z=4 is large .
if the only dust at high z is manufactured in quasars , then far less dust is implied than if quasars simply illuminate pre - existing dust , which would then need to be distributed among all high  z galaxies , active or not .
the metals in the dust must , of course , have been created by an earlier generation of massive stars , that go supernova quickly . to establish high quasar bel abundances ,
the star formation must be local to the quasar nucleus , again suggesting a more limited amount of high z star formation .
high  z counterparts to sub - mm scuba sources seem to imply enormous starbursts , but conclusively distinguishing between a starburst and a quasar as the root power source heating the dust seen by scuba is extraordinarily difficult , leaving open the quasar option .    in this picture dust can be formed as soon as quasars form .
quasar winds ( @xmath311000  km  s@xmath6 ) readily exceed the escape velocity from a galaxy ( v@xmath32200  km  s@xmath6 ) , or even a rich cluster of galaxies ( v@xmath321000  km  s@xmath6 ) . hence the dust they produce will be ejected into the intergalactic medium .
dust is an important catalyst of star formation , as dust provides both shielding from ambient ultraviolet radiation and an efficient cooling path for the surrounding medium .
the early creation of dust by quasars may then be important for seeding star formation at slightly later times .
in summary , dust will inevitably be created from the free expansion of quasar broad emission line clouds in an outflowing wind .
the association of dust with quasars is not then _ necessarily _ linked with intense star formation around quasars , but may be a consequence of the quasar activity itself .
the creation of dust in quasar winds may solve the puzzle of where the very first dust comes from , and in doing so suggests an unexpected role for quasars in cosmology .
winds from agn and quasars will form large amounts of dust , as the cool gas in these winds passes through the ( pressure , temperature ) region where dust is formed in agb stars . 
 conditions in the gas are benign to dust at these radii . as a result 
 quasar winds may be a major source of dust at high redshifts , obviating a difficulty with current observations , and requiring far less dust to exist at early epochs .    
 # 1_#1 _ # 1_#1 _ =    # 1 1.25 in .125 in .25 in