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mass outflows from active galactic nuclei ( agn ) can profoundly affect the evolution of the central engine @xcite , the host galaxy and its interstellar medium @xcite and also the surrounding intergalactic medium ( igm ) @xcite .
winds from the high metal - abundance nuclear regions may be a significant source for enriching the igm @xcite .
absorption by the outflow can also collimate the ionizing radiation @xcite and thereby influence the ionization structure of the host galaxy and the surrounding igm .
the outflowing gas in agn is sometimes visible as extended , bi - conical emission at visible or x - ray wavelengths ( e.g. , ngc 4151 , @xcite , @xcite ; ngc 1068 , @xcite ) , but it most frequently manifests itself as blue - shifted absorption features in their uv and x - ray spectra . about half of all low - redshift agn show x - ray absorption by highly ionized gas @xcite , and a similar fraction show associated uv absorption in ionized species such as c  iv @xcite and o  vi @xcite . in more luminous quasars ,
the fraction of objects in the sloan digital sky survey that shows broad c  iv absorption troughs rises steeply as the troughs become narrower @xcite , comprising over 30% of the quasar population at widths narrower than 1000 . for agn that have been observed in both the x - ray and the uv , there is a one - to - one correspondence between objects showing x - ray and uv absorption , implying that the phenomena are related in some way ( crenshaw et al .
the high frequency of occurrence of uv and x - ray absorption suggests that the absorbing gas has a high covering fraction , and that it is present in all agn .
the gas has a total mass exceeding @xmath0 ( greater than the broad - line region , or blr ) , and is outflowing at a rate @xmath1 ( @xmath2 the accretion rate in some objects ) @xcite .
key questions for understanding the outflowing , absorbing gas in agn are :     -0pt    @xmath3 what are the column densities and ionic abundances in the absorbing gas ?
+ even such a basic question is uncertain since the absorption line profiles are complex .
doublet ratios show that the absorbers can be optically thick , but they are not black at line center . thus column densities are frequently underestimated , sometimes by as much as an order of magnitude @xcite .
@xmath3 where is the absorbing gas located ?
+ the location of the absorbers is a vital clue to the process producing the outflow .
winds arising from an accretion disk @xcite will have material at a broad range of radii reaching from near the disk itself to beyond the blr .
thermally driven winds arising from the obscuring torus @xcite will lie at much larger radii , typically at distances of @xmath4 pc .
the radial location also determines the mass flux  the larger the distance , the higher the total mass and the mass flux in the wind .
@xmath3 do variations reflect an ionization response , or are they due to bulk motion ? + in the limited number of monitoring campaigns carried out thus far , examples of both have been seen .
the neutral hydrogen and the c  iii absorption in ngc  4151 responds quite clearly to continuum variations @xcite .
some absorbing clouds in ngc  3783 have shown changes consistent with an ionization response , while others appear to be due to bulk motion @xcite .
@xmath3 how are the x - ray and uv absorption related ?
+ in some cases , uv absorbing gas may be directly associated with the x - ray warm absorber ( 3c351 : @xcite ; ngc  5548 : @xcite ; ngc 3516 : @xcite ) . in the extensive recent _ chandra / fuse / hst _
campaign on ngc 3783 @xcite , the kinematics of the x - ray absorbing gas are also a good match to the uv - absorbing gas .
in other cases , however , the uv gas appears to be in an even lower ionization state , and there is no direct relation between the x - ray absorption and the multiple kinematic components seen in the uv ( ngc  4151 : @xcite , @xcite ; ngc  3516 : @xcite ; ngc  5548 : @xcite , @xcite ; mrk  509 : @xcite , @xcite ; ngc  7469 : @xcite ) .
the x - ray absorbing gas itself contains material spanning a large range of ionization parameters @xcite , and it is likely that this broad range of physical conditions can also include the uv - absorbing ions .
this is a natural prediction of the thermally driven wind model of @xcite , and would also be likely in disk - driven winds .    to address some of these questions ,
we have been conducting a survey of the @xmath5 brightest agn using the _ far ultraviolet spectroscopic explorer ( fuse)_. the short wavelength response ( 9121187  ) of _ fuse _ @xcite enables us to make high - resolution spectral measurements ( @xmath6 ) of the high - ionization ion o  vi and the high - order lyman lines of neutral hydrogen . as of november 1 , 2002 , we have observed a total of 87 agn . of these ,
57 have @xmath7 , so that the o  vi doublet is visible in the _ fuse _ band .
the o  vi  doublet is a crucial link between the higher ionization absorption edges seen in the x - ray and the lower ionization absorption lines seen in earlier uv observations .
the high - order lyman lines provide a better constraint on the total neutral hydrogen column density than ly@xmath8 alone .
lower ionization species such as c  iii and n  iii also have strong resonance lines in the  band , and these often are useful for setting constraints on the ionization level of any detected absorption .
the lyman and werner bands of molecular hydrogen also fall in the  band , and we have searched for ( but not found ) intrinsic @xmath9 absorption that may be associated with the obscuring torus .
as shown in the left panel of figure 1 , absorption is common at all luminosities , and over 50% ( 30 of 53 ) of the low - redshift type 1 agn observed using  show detectable o  vi  absorption , comparable to those seyferts that show longer - wavelength uv @xcite or x - ray @xcite absorption .
none show intrinsic @xmath9 absorption .
we see three basic morphologies for o  vi  absorption lines : ( 1 ) * single * : 13 of 30 objects exhibit single , narrow , isolated o  vi  absorption lines , as illustrated by the spectrum of ton  s180 @xcite .
pg0804 + 761 , shown in the top panel of figure 2 , is another example .
( 2 ) * blend * : multiple o  vi  absorption components that are blended together .
10 of 30 objects fall in this class , and the spectrum of mrk  279 is typical @xcite .
the middle panel of figure 2 shows mrk 478 as another example .
( 3 ) * smooth * : the 7 objects here are an extreme expression of the  blend " class , where the o  vi  absorption is so broad and blended that individual o  vi  components can not be identified .
ngc  4151 typifies this class @xcite .
the mini - bal qso pg1411 + 442 is another example of this class , shown in the bottom panel of figure 2 .
individual o  vi  absorption components in our spectra have fwhm of 50750 , with most objects having fwhm @xmath10 .
the multiple components that are typically present are almost always blue shifted , and they span a velocity range of 2004000 ; half the objects span a range of @xmath11 .
as shown in the right panel of figure 1 , the maximum outflow velocities show a tendency to increase with source luminosity , perhaps indicating that radiative acceleration plays some role in the dynamics .
note also that there is a population of low - velocity absorbers present at all luminosities .
the multiple kinematic components frequently seen in the uv absorption spectra of agn clearly show that the absorbing medium is complex , with separate uv and x - ray dominant zones . in some cases
, the uv absorption component corresponding to the x - ray warm absorber can be clearly identified ( e.g. , mrk  509 , @xcite ) . in others , however , _ no _ uv absorption component shows physical conditions characteristic of those seen in the x - ray absorber ( ngc  3516 , @xcite ; ngc  5548 , @xcite ) .
one potential geometry for this complex absorbing structure is high - density , low - column uv - absorbing clouds embedded in a low - density , high - ionization medium that dominates the x - ray absorption .
disk - driven winds are a possible explanation for some cases of agn outflows . by analogy to stellar winds , one would expect the terminal velocity of an agn outflow to reflect the gravity of its origin .
disk - driven winds should therefore have velocities in the range of several thousand .
objects with broad , smooth profiles might fall in this category .
the geometry proposed by @xcite suggests that these objects should have only modest inclinations .
however , two prime examples of seyferts with broad smooth absorption troughs , ngc 3516 @xcite and ngc 4151 @xcite , are likely the highest inclination sources in our sample given their extended , bi - conical narrow emission - line region morphologies @xcite and their opaque lyman limits @xcite .
the lower velocities we observe in objects like ngc 3783 and ngc 5548 are more compatible with thermally driven winds from the obscuring torus @xcite . in these thermally driven winds ,
photoionized evaporation in the presence of a copious mass source ( the torus ) locks the ratio of ionizing intensity to total gas pressure ( the ionization parameter @xmath12 ) at a critical value . for agn
spectral energy distributions lacking a strong extreme ultraviolet bump , such as the composite spectra of quasars assembled by @xcite , @xcite , and @xcite , the ionization equilibrium curve exhibits an extensive vertical branch .
thus , at the critical ionization parameter for evaporation , there is a broad range of temperatures that can coexist in equilibrium at nearly constant pressure .
for this reason , the flow is expected to be strongly inhomogeneous .
outflow velocities are typical of the sound speed in the heated gas , or several hundred , comparable to the velocities seen in many agn .    in summary , we find that o  vi  absorption is common in low - redshift ( @xmath7 ) agn .
30 of 53 type 1 agn with @xmath7 observed using  show multiple , blended o  vi  absorption lines with typical widths of @xmath13 that are blueshifted over a velocity range of @xmath14 1000 .
those galaxies in our sample with existing x - ray or longer wavelength uv observations also show c  iv absorption and evidence of a soft x - ray warm absorber .
in some cases , a uv absorption component has physical properties similar to the x - ray absorbing gas , but in others there is no clear physical correspondence between the uv and x - ray absorbing components .
more than half of all low - redshift agn exhibit uv and x - ray absorption by highly ionized gas . 
 the observed uv and x - ray absorption lines are almost always blue - shifted at velocities of hundreds of km / s , indicating that the absorbing gas is outflowing from the active nucleus . in some cases the inferred mass flux rivals 
 the eddington limit of the central black hole , an indication that these outflows are intimately related to the mass accretion and energy generation mechanism in agn . 
 the ejected material can also have an affect on the interstellar medium of the host galaxy and the surrounding intergalactic medium . over the past several years , coordinated uv and x - ray observations of several bright agn at high spectral resolution using _ hst _ , _ fuse _ , _ chandra _ , and _ xmm - newton _ have contributed greatly to our understanding of these outflows . 
 i will give an overview of these recent observations , summarize our _ fuse _ survey of low - redshift agn , and interpret the results in the context of models of winds from accretion disks and thermally driven winds from the obscuring torus .