![]() | Annu. Rev. Astron. Astrophys. 2005. 43:
769-826 Copyright © 2005 by Annual Reviews. All rights reserved |
The same methods detect outflows in nearby active galaxies and starburst-driven winds, so the same selection effects and observational limitations discussed in Section 4.1 apply. However, contrary to starburst-driven winds, winds associated with AGN need not be perpendicular to the galactic disk. Outflows directed close to the galactic disk are likely to be made very luminous by the high ambient densities. They are more easily observed in near face-on galaxies (e.g., M51: Cecil 1988; NGC 1068: Cecil, Bland, & Tully 1990). In a few systems, the orientation of an inclined AGN disk can be determined independently from maser spots.
As we discuss below, there is ample evidence for outflows in AGN from sub-kpc to galactic scale. But one must be aware of processes that may bias the interpretation of bipolar emission. Several active galaxies display kiloparsec-scale "ionization cones" that align with the radio axes of AGN rather than with the principle axes of the host galaxy (e.g., Wilson & Tsvetanov 1994; Kinney et al. 2000). They arise when ISM is illuminated by hard radiation from the AGN. Gas within the cones is more highly ionized than outside. Because the gas kinematics are unaffected by ionization, one should search for kinematic signatures to evaluate the frequency of occurrence of AGN outflows. Often, both outflows and ionization cones are present, e.g. NGC 1068 (Pogge 1988; Cecil et al. 1990), NGC 1365 (Veilleux et al. 2003), and NGC 4388 (Veilleux et al. 1999), emphasizing the need for high-quality kinematic data.
Circumnuclear starbursts in active galaxies are another complication. Half of all nearby, optically-selected Seyfert 2 galaxies also host a nuclear starburst (see, e.g., Cid Fernandes et al. 2001 and Veilleux 2001 and references therein); the fraction is larger in IR-selected systems. The sustaining conditions for nuclear activity (e.g., deep central potential with a reservoir of gas) also favor starburst activity and perhaps trigger starburst-driven GWs. This symbiotic relation between starbursts and AGN therefore complicates interpretation of GWs in AGN/starburst composites. For example, bipolar ionization cones can also arise in pure starburst galaxies (M82: Fig. 15 of Shopbell & Bland-Hawthorn 1998).
With these caveats, we revisit the detection of winds in active galaxies. Zensus (1997) reviewed the evidence for outflows in powerful radio-loud systems. Here we focus on Seyfert galaxies. Linear radio features suggestive of jet-like ejections on sub-kpc scales have long been known in 20 - 35% of Seyferts (e.g., Ulvestad & Wilson 1989; Morganti et al. 1999 and references therein); this is a lower limit because of limitations in spatial resolution (~ 1"), sensitivity, and de-projection. New high-resolution images with the Very Large Baseline Array (VLBA) have indeed revealed jet-like outflows in previously unresolved sources and in low-luminosity AGN (Ho & Ulvestad 2001). As we describe below, optical studies of several Seyferts with linear radio structure show signs of jet-ISM interaction on a scale of tens of pc. Measurements of proper motion in a few bright sources confirm outwardly moving radio knots (e.g., Middelberg et al. 2004 and references therein). A jet interpretation is sometimes favored even when the radio emission is unresolved at the VLBI scale (e.g., Anderson, Ulvestad, & Ho 2004). Low-power, sub-kpc jets may exist in most Seyfert galaxies. Any thermal wind component cannot be established from the radio observations, yet there are speculations that it can be large (Bicknell et al. 1998), (Section 5.3).
Many active galaxies also show signs of large-scale, galactic
outflows. In an optical study of a distance-limited sample of 22
edge-on Seyfert galaxies,
Colbert et al. (1996a)
found that > 25%
have kinematic signatures of outflows or extraplanar line emission out
to 1 kpc. The
existence of extraplanar radio emission in 60% (6/10) supports this claim
(Colbert et al. 1996b).
Morphology and orientation suggest that this emission comes mainly from
AGN-driven outflows (see
Section 5.2). This detection rate is a lower limit
because of sensitivity and selection effects. For instance,
Baum et al. (1993)
detected kiloparsec-scale extranuclear radio emission in ~ 90% (12/13)
of their objects, a sample of slightly more powerful, face-on
Seyferts. The difference in detection rate may be due to small number
statistics, differences in sample selection, or may indicate that some
of this emission is associated with starburst-driven
winds. Contamination by starburst-driven winds may also explain the
large fraction (11/12
90%) of starburst/Seyfert 2 composites with extended X-ray emission
(Levenson, Weaver,
& Heckman 2001a,
2001b).
There is now irrefutable evidence that powerful AGN-driven outflows
sometimes coexist with starburst-driven winds (e.g., NGC 3079:
Cecil et al. 2001).
Contrary to what we know of starburst-driven winds, AGN outflows are oriented randomly relative to the major axis of the host galaxy (e.g., Ulvestad & Wilson 1984; Kinney et al. 2000). Several well-known active galaxies, including NGC 1068 (Cecil et al. 1990), harbor a wide-angle outflow whose axis is not perpendicular to the galaxy disks and therefore interacts strongly with the disk ISM. Outflows near the disk plane are often well collimated (e.g., NGC 4258: Cecil et al 2000; ESO 428-G14Falcke, Wilson, & Simpson 1998), until they collide with dense disk clouds or until they rise into the halo (e.g., NGC 4258: Wilson, Yang, & Cecil 2001). When the jet is drilling through the disk, detailed correspondence between radio and optical emission-line features indicates strong jet/ISM interactions over tens of pc (e.g., Falcke et al. 1998; Schmitt et al. 2003a, 2003b and references therein). The radio jets compress and shock ambient gas, enhancing line emission that may dominate the morphology of the NLR. The well-known correlation between radio and NLR luminosities supports a dynamical connection between the two (e.g., de Bruyn & Wilson 1978; Wilson & Willis 1980). A few jet deflections by ISM clouds are also seen (e.g., NGC 1068: Gallimore, Baum, & O'Dea 1996b; NGC 4258: Cecil et al. 2000; NGC 4151: Mundell et al. 2003). Jet/ISM interaction has also been mapped in X rays thanks to CXO (e.g., Young, Wilson, & Shopbell 2001; Wilson et al. 2001; Yang, Wilson, & Ferruit 2001).
In many edge-on systems, the radio structure has a linear or elongated morphology on subkiloparsec scale, but beyond the disk becomes more diffuse and wide-angled. The change may arise from the vertical pressure gradient in the surrounding ISM or from momentum loss within the sub-kpc NLR. The morphologies of the warm and hot ionized extraplanar gas are often correlated. They are distributed in a broad cone near the base of the outflow (e.g., NGC 2992: Colbert et al. 1998; Allen et al. 1999; Veilleux, Shopbell, & Miller 2001; Circinus: Veilleux & Bland-Hawthorn 1997; Smith & Wilson 2001) but become filamentary above the disk (e.g., NGC 4388: Veilleux et al. 1999, Fig. 5c; NGC 5506: Wilson, Baldwin, & Ulvestad 1985). Spectacular bow shocks and finger-like structures are seen optically in the Circinus galaxy, perhaps due to the abnormally high gas content of this object (Fig. 5d; Veilleux & Bland-Hawthorn 1997). The correspondence between the extraplanar radio plasma and optical line-emitting material is often not as tight as that seen in the disk. Outflows with optical conical geometries may have very different radio morphologies: e.g., edge-brightened radio bubbles in NGC 2992 (Wehrle & Morris 1988) and Circinus (Elmouttie et al. 1998) but lumpy and filamentary radio structures in NGC 4388 and NGC 5506 (Colbert et al. 1996b). The extraplanar emission at radio wavelengths (where the foreground disk barely influences the emission) is sometimes lop-sided (e.g., NGC 4388) from asymmetric energy injection at the source or from asymmetric ISM on small scales.
Spatially resolved outflows are found on all scales in AGN.
Relativistic outflows on parsec scales in powerful radio-loud sources
are well established
(Zensus 1997;
Worrall &
Birkinshaw 2004
and references therein). Recently, proper motions of radio components
have been measured with VLBI in a few Seyfert galaxies (e.g.,
Middelberg et al. 2004
and references therein). These studies show
that the outward motion of the radio components in these objects is
non-relativistic
( 0.25c) on
pc scales. There is now unambiguous
kinematic evidence that these radio jets transfer momentum and energy
to the ambient gas and drive some of the large-scale outflows seen in
radio-quiet and radio-loud objects.
Gas in the NLR (~ 10 pc - 1 kpc) and extended NLR (ENLR;
1 kpc) is an excellent tracer of this jet/ISM interaction. The good
match between nuclear emission-line widths and bulge gravitational
velocities suggests that the gravitational component dominates in most
Seyferts
(Whittle 1985;
Wilson & Heckman
1985;
Veilleux 1991;
Whittle 1992b;
Nelson & Whittle
1996).
But Seyferts with linear radio structures have long been known to have
emission lines with complex profiles (e.g.,
Veilleux 1991)
and supervirial velocity widths (e.g.,
Whittle 1992a)
that implicate an additional source of KE in the NLR.
Detailed long-slit spectra from the ground and
from HST have presented evidence for a dynamical connection between
the NLR and radio components in many of these galaxies (e.g.,
Whittle & Wilson
2004
and references therein). The complete
spatio-kinematic coverage afforded by Fabry-Perot and integral-field
spectrometers has constrained efficiently the intrinsic, 3D velocity
fields of the outflowing ionized gas (e.g.,
Ferruit et al. 2004;
Veilleux et al. 2002
a
and references therein). The signatures of jet-driven kinematics seen in
some Seyfert galaxies are also detected in several powerful radio sources,
particularly in compact steep-spectrum radio galaxies and quasars
(e.g.,
Baum, Heckman, & van
Breugel 1992;
Gelderman & Whittle
1994;
McCarthy, Baum, &
Spinrad 1996;
Best, Röttgering,
& Longair 2000;
Solórzano-Iñarrea, Tadhunter, & Axon
2001;
O'Dea et al. 2002).
This large data set indicates that expanding radio lobes (Pedlar, Dyson, & Unger 1985) or bow shocks/cocoons driven by radio jets (Taylor, Dyson, & Axon 1992; Ferruit et al. 1997; Steffen et al. 1997a, 1997b) accelerate some of the line-emitting gas to ~ 100 - 1000 km s-1. The fate of the gas clouds - whether undisturbed, destroyed, or accelerated - depends on factors such as the cloud mass, jet energy flux, and interaction geometry. Dense, molecular clouds in Seyfert galaxies can deflect radio jets by a significant angle without experiencing significant damage (e.g., NGC 1068: Gallimore et al. 1996a, 1996b; NGC 4258: Cecil et al. 2000). Jet-cloud interactions can be used to deduce key properties of the jet. In their analysis of the jet-molecular cloud interaction in the NLR of NGC 1068, Bicknell et al. (1998) argued that this jet (and perhaps those in other Seyferts) is heavily loaded with thermal gas and has low bulk velocities (~ 0.06c), contrary to jets in powerful radio galaxies. Not surprisingly, the jets in Seyfert galaxies often deposit an important fraction of their KE well within ~ 1 kpc of the nucleus. The radial velocities of the emission-line knots in the NLRs of Seyfert galaxies show a slight tendency to increase out to ~ 100 pc from the nucleus then decrease beyond, whereas the line widths of the knots decrease monotonically with increasing distance from the nucleus (e.g., Crenshaw & Kraemer 2000; Ruiz et al. 2005). Deceleration beyond ~ 100 pc is likely due to drag from ambient gas.
Ram pressure from radio jets/lobes may not always dominate the acceleration of line-emitting gas in Seyfert galaxies. High-resolution studies with HST sometimes fail to find a one-to-one correspondence between the NLR cloud kinematics and the positions of the radio knots. Ram pressure from a diffuse, highly ionized wind or radiation pressure by the AGN radiation field has been suggested as the possible culprit in these cases (Kaiser et al. 2000; Ruiz et al. 2001). The high-velocity (~ 3000 km s-1) line-emitting knots detected in NGC 1068 (Cecil et al. 2002b; Groves et al. 2004) may be explained by radiation pressure acting on dust grains in the clouds (Dopita et al. 2002). These knots may correspond to the well-known absorbers seen projected on the UV continua of some AGN (Crenshaw et al. 2003). Bright nuclear emission-line knots detected in several other nearby Seyfert galaxies may also contribute to the population of intrinsic UV absorbers (Crenshaw & Kraemer 2005; Ruiz et al. 2005).
Wind ram pressure or radiation pressure may also be responsible for blueshifted (~ 100 - 1000 km s-1) neutral material detected in several AGN-dominated ULIRGs (Rupke, Veilleux, & Sanders 2005c), because few of them show jet-like radio structures. There is some evidence for higher average and maximum velocities in Seyfert-2 ULIRGs than in starburst-dominated ULIRGs, although the evidence for a strong influence of the AGN on these outflows is inconclusive. The situation is quite different among Seyfert-1 ULIRGs, where the outflows are driven mostly or solely by the AGN (Rupke, Veilleux, & Sanders 2005c; Fig. 6a). Similarly, nuclear activity is almost certainly responsible for the broad, blueshifted H I absorption wings detected in a growing number of compact radio galaxies. But here, jet-driven acceleration is implied (e.g., Morganti et al. 2003).
In Seyferts where the energy injection rate of the jet or wind
suffices to eject radio/thermal plasma from the host galaxy, warm
line-emitting gas is entrained into wide-angle outflows. The
intrinsic, 3D velocity field of the line-emitting gas over
1 kpc
indicates roughly conical radial outflow with opening angles
2
60°- 135°
and hollow (NGC 1365:
Hjelm & Lindblad
1996;
NGC 2992:
Veilleux et al. 2001),
partially filled (NGC 5506:
Wilson et al. 1985),
or filamentary geometry (Circinus:
Veilleux &
Bland-Hawthorn 1997;
NGC 4388:
Veilleux et al. 1999).
Deprojected
velocities are ~ 100 - 500 km s-1 in Seyferts, sometimes
larger in radio galaxies. The geometry of the kinematic structures on
large scales depends not only on the energy source but also on the
galactic and intergalactic environment (e.g., the abnormally large gas
content of Circinus may explain the peculiar filamentary
morphology of its outflow). Rotation is sometimes detected in the
velocity field of the outflowing gas (e.g., Circinus:
Veilleux &
Bland-Hawthorn 1997;
NGC 2992:
Veilleux et al. 2001),
confirming that most of the line-emitting material comes from the disk ISM.
5.4. Mass Outflow Rates & Energetics
Kinematic deprojection is an essential prerequisite to meaningful
dynamical analysis of AGN-driven winds. Data with complete
spatio-kinematic coverage should be used to derive their masses and
KE. We therefore discuss nearby (z
0.1) objects for
which spatially resolved kinematic data are available. The broad range of
scales and velocities discussed in
Sections 5.2 and 5.3 implies dynamical
times for the entrained line-emitting material of ~ 104 -
106 years in the NLR (including the 3000 km s-1
knots in
NGC 1068) and ~ 106 - 107
years in the ENLR.
Deriving ionized masses for AGN outflows follows the same steps as for
starburst-driven winds, and therefore again favors high-density
material (Section 4.5).
Given uncertain volume filling factors, the warm ionized gas masses
derived from optical data on Seyferts, ~ 105 - 107
M, are probably
accurate to no better than about ± 0.5
dex. The corresponding mass outflow rates, M /
tdyn
0.1
- 10 M
yr-1, generally exceed tenfold the mass accretion
rates necessary to fuel the AGN, indicating strong mass loading of the
outflow by the galaxy ISM.
The KE of the warm ionized component in Seyfert galaxies is ~
1053 - 1056 erg, including both bulk and "turbulent"
(spatially unresolved) motions. This range is similar to that in
starburst-driven winds. Dynamical timescales yield KE outflow rates of
Ekin / tdyn
1040 -
1043 erg s-1.
The power radiated by Seyferts in line emission is 10 times larger
and ~ 102 - 104 times larger than in the radio
(e.g., Fig. 1 of
Wilson, Ward, &
Haniff 1988).
Simple jet-driven models
(Blandford &
Königl 1979a,
1979b;
Wilson 1981)
reproduce these derived values
using reasonable efficiency factors to relate jet and radio powers.
If the efficiency of energy/momentum transfer to the ambient material
is nearly constant, both the entrained mass and KE scale with AGN
luminosity (e.g.,
Baum & McCarthy
2000).
Powerful quasars and radio
galaxies with Lbol ~ 1045 -
1047 erg s-1 (not
discussed so far) seem to have KE outflow rates that exceed those of
Seyferts by several orders of magnitude. The KE of some outflows is
comparable to the gravitational binding energy of gas in the host
(1058 - 1060 erg). Comparable energy is stored in
the large radio lobes of radio-loud objects:
/ (
- 1)
P V ~
1058 - 1061 erg, where P and V are
the pressure and volume of the radio lobes, respectively, and
= 4/3
is the mean adiabatic index of the relativistic fluid.
Much of the energy dissipated by the AGN accretion disk appears to
find its way into jet mechanical luminosity: powerful radio sources
are Eddington-tuned engines with jet powers Q ~ 0.1
LEdd
(Willott et al. 1999;
see also
Falcke, Malkan, &
Biermann 1995).
Outflowing neutral gas detected in absorption in H I and
Na I D in
many AGN may add significant energy. Unfortunately, the mass of
entrained neutral gas in these objects is poorly constrained because
its location is unknown. The mass-conserving wind model of Rupke et al.
(2005b;
Router = 5 kpc) applied to the Na I D absorption
data on Seyfert 2 ULIRGs yields neutral mass outflow rates similar to
those in starburst-driven winds, i.e. ~ 10 - 1000
M yr-1.
With this assumption, neutral-gas KEs and rates of ~ 1056 -
1060 erg and Ekin / tdyn
1041 -
1045 erg s-1 are derived for Seyfert-2 ULIRGs,
suggesting that this material may be a very important dynamical
component of AGN-driven outflows.
An upper limit to the size of the energy injection zone of AGN-driven
winds can be derived from the geometry of the optical outflow.
Injection zones are often small enough to exclude a starburst origin
for the wind. A good example is the Circinus galaxy
(Fig. 5d), where
the base of the outflow cone extends over
20 pc, 10% of the
size of the starburst (e.g.,
Smith & Wilson 2001
and references therein). The same technique has
been used to measure the size of the primary energy source for the
outflows in NGC 1068
(
50 pc; e.g.,
Cecil et al. 1990;
Arribas, Mediavilla,
& Garcia-Lorenzo 1996;
Garcia-Lorenzo,
Mediavilla, & Arribas 1999),
NGC 2992
(
100 pc;
Veilleux et al. 2001;
Garcia-Lorenzo,
Arribas, & Mediavilla 2001),
and in several other jet-induced outflows (e.g.,
Whittle & Wilson
2004).
More accurate assessment of the energy source in AGN-driven winds
often relies on detailed VLA, MERLIN, and VLBA radio maps of the
central regions of AGN, down to scales below tens of parsecs. Radio
emission generally resolves into a few compact sources, occasionally
accompanied by elongated, jet-like features. On VLA scales, approximately
half of all Seyfert galaxies contain at least one component with a
flat or inverted radio spectrum (i.e. spectral slope
-0.20; e.g.,
Ulvestad & Ho
2001).
Radio sizes are uncorrelated with
Seyfert type, as expected from the unified scheme of AGN. On VLBI
scales,
80% of
sources are flat-spectrum
(Middelberg et
al. 2004;
Anderson, Ulvestad,
& Ho 2004),
are often unresolved or
only slightly resolved (~ 1 pc), and have brightness temperatures
> 107 K. Synchrotron self-absorption from the base of the
jet is the
usual explanation for compact and flat- or inverted-spectrum radio
emission from the central engines of radio galaxies. The same scenario
is likely in some Seyfert galaxies and low-luminosity AGN. Free-free
absorption by the nuclear torus or NLR may also occur. In NGC 1068,
the flat-spectrum source has modest brightness temperature (~
106 K) and extends over ~ 1 pc perpendicular to the jet axis;
this emission is believed to be optically thin, free-free emission
from the ionized inner edge of the torus
(Gallimore, Baum, &
O'Dea 1997).
Regardless of the exact origin of the emission, the compact
flat-spectrum component is thought to be the main energy source of the
outflow, because of frequent alignment of the various radio components
and from direct measurements of proper motions in a few objects
(Middelberg et al. 2004
and references therein; see Section 5.3 above).
It is now generally agreed that the gas in Seyferts is photoionized by the nucleus, although whether spatial variations in line ratios result from a "classical" range of ionization parameters (e.g. Davidson & Netzer 1979), a range in the numbers of illuminated ionization- and matter-bounded clouds (Binette, Wilson, & Storchi-Bergmann 1996), or a dusty plasma at high ionization parameter (Dopita et al. 2002) are still discussed. The detection of X-ray cones coincident with the emission-line ionization cones in several Seyferts rules out photoionizing shocks as the primary source of ionization (e.g., Kinkhabwala et al. 2002; Iwasawa et al. 2003; Yang et al. 2001), although the relative importance of shocks almost certainly increases with jet power and the degree of interaction with the ambient medium (e.g., compact radio galaxies; Tadhunter 2002).
Dust is present in AGN outflows. Its reddening is evident in the
narrow-line spectrum of radio-quiet and radio-loud galaxies (e.g.,
Osterbrock 1989
and references therein)
and in the broad absorption line clouds of BAL QSOs (e.g.,
Crenshaw et al. 2003
and references therein). Dust has often been invoked to
explain the extended blue wings in the narrow-line profiles (e.g,
Veilleux 1991
and references therein), and in the IR continuum emission of several AGN
(Barvainis 1987;
Sanders et al. 1989;
Pier & Krolik
1992.
But more relevant to outflowing dust is that the
correlation between color excess, E(B - V), and the
strength of the
Na I D line noted for starburst galaxies is even stronger for AGN
(Veilleux et
al. 1995b).
Dust outflow rates of ~ 0.1 - 10
M
yr-1 are inferred from the neutral-gas mass outflow
rates discussed in Section 5.4, assuming the Galactic
gas-to-dust ratio. These are comparable to the dust outflow rates in
starburst galaxies (Section 4.10).