Astrophys Space Sci (2007) 311: 269–273DOI 10.1007/s10509-007-9536-2

O R I G I NA L A RT I C L E

Outflows from AGNs: a brief overview of observations and models

John E. Everett

Received: 30 March 2007 / Accepted: 30 May 2007 / Published online: 21 June 2007© Springer Science+Business Media B.V. 2007

Abstract Recent observations show that outflows from Ac-tive Galactic Nuclei are much more common than previouslythought, with such outflows seen in various wavebands. I re-view the recent advances in observations of wind signaturesand the progress in modelling various wind launching mech-anisms.

Keywords Quasars: absorption lines · Quasars: emissionlines · Galaxies: Seyfert · Hydrodynamics · MHD

1 Introduction

This review aims to help represent the considerable re-cent advances in AGN wind observations and theory. Sec-ond, it stresses that we may now be seeing evidence thatAGN winds are driven by a combination of magnetohy-drodynamic and radiative forces. The interested reader mayalso consult other recent reviews of Active Galactic Nuclei(AGN) outflows (e.g., de Kool 1997; Crenshaw et al. 2003;Hamann and Sabra 2004; Proga 2007).

2 Observational overview

First, I present a quick overview of a few recent obser-vational advances in AGN winds. The main point of this

J.E. Everett (�)Departments of Astronomy and Physics, and Center for MagneticSelf-Organization in Laboratory and Astrophysical Plasmas,University of Wisconsin-Madison, 4302 Chamberlin Hall,1150 University Avenue, Madison, 53706 WI, USAe-mail: everett@physics.wisc.edu

overview is to demonstrate the commonality of AGN out-flows.

Historically, the most dominant examples of AGN out-flows have been found in Broad Absorption Line Quasars(BALQSOs). BALQSOs show absorption troughs in UVand optical lines with velocities up to tens of thousandsof km s−1, almost always blueshifted with respect to theline rest-wavelength. Recent studies with the Sloan DigitalSky Survey (Reichard et al. 2003) have shown that ∼15%of SDSS quasars are BALQSOs. This seems like a smallfraction, but may be simply an effect of the covering frac-tion of the outflowing wind (Murray et al. 1995). In fact,observations of blueshifted C IV emission lines visible inBALQSOs and many other SDSS quasars hint that thesehigh-velocity, high-column winds are even more common(Richards et al. 2002; Leighly 2004).

There are, however, AGN outflows other than those inBALQSOs, and they may be rather common: ∼50% ofSeyfert 1 galaxies surveyed by Crenshaw et al. (1999) hadintrinsic UV absorption features in an outflowing wind(these are narrow absorption features at ∼1000 km s−1,as opposed to broad absorption lines). Detailed modellingof these narrow absorption features leads to very importantconstraints on distances to the absorbers (Gabel et al. 2005).

Outflows have also recently been observed in emissionlines as well, and even resolved. HST/STIS observations ofoutflows in [O III] emission lines in three nearby Seyfertgalaxies show slowly increasing velocities out to ∼100 pc,followed by a slow decrease in velocities at larger distances(e.g., Crenshaw and Kraemer 2000; Das et al. 2005, 2007).This apparent deceleration may be evidence for interactionof a slow AGN wind with the host galaxy’s ISM. It should,however, be noted that there is some debate as to whether the[O III] kinematics could instead be caused by interaction

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with a jet (e.g. Winge et al. 1999) or radiative accelerationon dust (Dopita et al. 2002).

Outflows are also seen in X-ray observations. X-ray ion-ized outflows, called “Warm Absorbers” (e.g., Kriss 2002;Steenbrugge et al. 2005) have been seen in nearly 50% ofnearby Seyfert 1 galaxies, with outflow rates high enoughthat such winds might be important for feedback on the hostgalaxy (Blustin et al. 2005).

Very recently, evidence has been emerging for muchhigher velocity (∼0.1c) outflows (e.g., Chartas et al. 2002,2007; Pounds et al. 2003; O’Brien et al. 2005), seen in ab-sorption in X-rays. In some cases the columns at these largevelocities may be of order 1023 cm−2 or higher. There issome debate about how to interpret these very exciting ob-servations (e.g., Kaspi and Behar 2006; Pounds and Page2006), but such winds could be a very important componentof a “unified” AGN outflow model (Gallagher and Everett2007) and are well worth much attention.

3 Theory overview

3.1 Radiative acceleration

Some of the first models of BALQSO outflows and AGNemission lines concentrated on radiative acceleration (e.g.,Scargle et al. 1970; Shields 1977; Shlosman et al. 1985).This is very reasonable, of course, as quasars are notori-ous for their high luminosities: judging simply from the mo-mentum in the radiation field and the momentum in the out-flow, there appears to be enough momentum to drive a wind(Mv∞ ∼ L/c). And in fact, simply by observing the largeabsorption troughs in BALQSOs, we are observing that mo-mentum must be removed from the radiation field, and somust be contributing to wind acceleration, at least at somelevel.

The above simple analysis is not meant in any way, how-ever, to underestimate the task for researchers who modelthis process. Accurately modelling both the dynamics andphotoionization of such flows simultaneously is very de-manding. Researchers have overcome this problem with var-ious approximations: see, for instance, Arav et al. (1994),Murray et al. (1995), Proga et al. (1998), Chelouche andNetzer (2001, 2003b), Proga and Kallman (2004). Many ofthese wind models have been largely developed (at least ini-tially) to understand BALQSO absorption columns and ve-locities (e.g., Proga 2003).

Another success with this model has been the rela-tively straightforward explanation of single-peaked broademission lines in AGN (Murray and Chiang 1998): in aradiatively-accelerated wind, the (Sobolev) dependence ofthe wind opacity on the velocity gradient means that pho-tons emitted radially outward in the outflow will see a lower

opacity (because they are moving along the direction ofwind acceleration) than photons emitted from other portionsof the wind. In this way, the double-peaked “wings” charac-teristic of accretion disks are suppressed by the wind opac-ity, yielding single-peaked emission lines. These models arealso successful in reproducing emission line variability. Itis important to note, therefore, that in this model, single-peaked emission lines mean that a wind is being launchedfrom the disk. Like the work of Kuncic and Bicknell (seethese proceedings), this exemplifies the importance of con-sidering the accretion disk and wind system together in theprediction of observables.

Another important component of this model has been theidea (Murray et al. 1995) that the outflow is continuous,and not composed of discrete “clouds.” This idea was bol-stered by the work of Arav et al. (1998), who determinedthat smooth AGN emission lines are most likely not the re-sult of clouds. However, it should be noted that cloud mod-els have been successful at explaining emission line ratiosand smooth line profiles using magnetohydrodynamic waves(see Bottorff and Ferland 2000, and references therein).

Another important constraint on radiative acceleration isthe presence of line-locking. Some evidence for this hasbeen found and modelled in BALQSO N V absorptiontroughs (Arav 1996; Chelouche and Netzer 2003a). Inter-estingly, there are also claims of line-locking in narrow-absorption line systems (e.g., Vilkoviskij and Irwin 2001).However, in this case, is not clear how single narrow-lineabsorption features could dominate the radiative accelera-tion and produce such locking; radiative acceleration usuallydepends on an ensemble of thousands of lines.

On the whole, radiatively-accelerated disk-wind modelsare fairly successful in explaining high-velocity BALQSOoutflows as well as emission line profiles. There are stillsome open questions, however. One difficulty with the ra-diative driving in AGNs is the high flux of ionizing X-rayphotons. Without some “shielding gas” in the wind model,the X-ray bright AGN continuum ionizes the gas, elimi-nating the UV transitions that radiative driving depends on(see, e.g., Gallagher and Everett 2007). This problem hasbeen known and studied for some time, with solutions rang-ing from a inner “failed wind” like that in Murray et al.(1995), or shielding from the disk atmosphere (Proga andKallman 2004). Observationally, it is known that BALQSOsare X-ray weak because of absorption (Gallagher et al. 2006)probably situated at very small radii (Gallagher et al. 2004),so BALQSOs do appear to supply the required X-ray ab-sorber.

3.2 Magnetocentrifugal winds

The magnetocentrifugal wind model was originally devel-oped by Blandford and Payne (1982) to explain the launch-ing and collimation of radio jets. Magnetocentrifugal winds

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work via the centrifugal force applied to matter by large-scale, ordered magnetic fields anchored to a rotating accre-tion disk. The magnetic fields not only help accelerate thegas, but the back-reaction of the matter on the fields farther“downstream” creates a helical magnetic field which helpscollimate the outflow. Finally, this large scale magnetic fieldcan also help remove angular momentum from the accretiondisk, as required for accretion to occur.

In AGNs, magnetocentrifugal winds (MCWs) have beenapplied to explain many of the same observations that radia-tive acceleration has been applied to. Like the radiatively-accelerated wind model, MCW models have been appliedto BALQSO outflows, using pure magnetic accelerationof clouds (Emmering et al. 1992), and including radiativedriving as well (de Kool and Begelman 1995). MCWs havealso been used to explain both emission line profiles andtheir variability (Bottorff et al. 1997).

Another question that MCWs have addressed is the na-ture of the “dusty torus” that is central to the AGN Unifica-tion model. The Unification model posits an equatorial ob-scuring medium that extends to high latitudes to explain theobserved differences between Type I (unobscured) and TypeII (obscured) AGNs. It has been quite difficult, however, toexplain the large global covering fraction and stability ofthis obscuring medium (cf., Krolik and Begelman 1988). Toaddress this problem, Konigl and Kartje (1994) proposed alarge-scale MCW: beyond the dust sublimation radius, thiswind lofts dust to high latitudes. This MCW model thus ex-plained the vertical extent of the obscuring medium, and, inaddition, explained the change in the obscuration with lumi-nosity: as the luminosity increases, radiative acceleration onthe dust “flattens” the wind, lowering the fraction of the skycovered by the obscuring medium, as observed (see, e.g.,Ballantyne et al. 2006). Much recent work has been done onthis in the context of a clumpy outflow (Elitzur 2006)

MCWs are most likely dominated by radiative acceler-ation at relatively high luminosities (L � 0.1LEdd, but notethis cutoff is very approximate). At lower luminosities, how-ever, it is not clear that radiative acceleration can success-fully drive winds, and there are AGNs with outflows at lowluminosity. Kraemer et al. (2005) find that for outflowing ab-sorbers in the Seyfert galaxy NGC 4151, radiative accelera-tion cannot explain the velocities of the absorbers seen. Justas important, they find evidence for significant transversevelocities, which would be naturally explained in a MCWmodel (Blandford and Payne 1982). So, this work finds thatmagnetocentrifugal winds may perhaps be dominant in thislow-luminosity AGN.

In addition, MCWs may also play a role in higher lumi-nosity systems, helping to make radiative acceleration pos-sible. Everett (2005) showed that an highly-ionized MCWcan act as “shielding gas”, absorbing X-rays and prevent-ing the over-ionization of a larger-scale wind, thus allowing

radiative acceleration to be more efficient. This model hasbeen used to successfully model observed columns in oneBALQSO (Everett et al. 2002), whose shielding column waslater observationally verified (Brotherton et al. 2005).

However, MCWs, like radiatively-driven winds, still re-quire more work. For instance, large-scale magnetic fieldsare crucial to these outflows, but it is not yet clear howthose fields arise. Perhaps they are generated via shearingof corona-like field loops above the accretion disk. In addi-tion, MCW models cannot easily predict the mass outflowrate from first principles, unlike radiative acceleration mod-els, where the outflow rate is set relatively simply by the(UV) luminosity.

4 Thermally driven winds

Recent work on the dynamics and photoionization of theX-ray Warm Absorber (Chelouche and Netzer 2005), haverevived the possibility of thermal winds on large-scales inAGNs. This model is quite impressive for the detailed com-parison between model and observations, and the accuratehandling of both photoionization and dynamics in the mod-elling.

The only difficulty with thermal wind models is drivingthe wind at large scales, and specifically, maintaining thetemperature of the wind against adiabatic cooling. This wasaddressed in Chelouche and Netzer (2005) via turbulentheating added to the outflow. A similar issue was foundin Everett and Murray (2007), where a large-scale thermalwind was used to try to explain the outflow observations ofDas et al. (2005): again, the wind cooled too quickly in thelarge-scale galactic potential to explain the observed dynam-ics.

5 Future directions

From the observations and modelling work described so far,I would propose that the simplest model to explain the ob-servations is a magnetically-launched wind which is drivenequatorially by radiative acceleration at high luminosities.Thermal winds most likely also play a role at very largedistances from the central black hole. Much work remainsto be done on all of the above models to make them fullyself-consistent, to compare them to observations, and makefurther predictions.

Acknowledgements This work was supported by NSF AST-0507367 and NSF PHY-0215581 (to the Center for Magnetic Self-Organization in Laboratory and Astrophysical Plasmas).

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