New Results from a Survey of Galactic Outflows in Nearby Active Galactic Nuclei

8 Pages·0.25 MB·English

by  S. Veilleux

RevMexAA (Serie de Conferencias), 00, 1–8 (2000)
NEW RESULTS FROM A SURVEY OF GALACTIC OUTFLOWS IN
NEARBY ACTIVE GALACTIC NUCLEI
S. Veilleux1, G. Cecil2, J. Bland-Hawthorn3, P. L. Shopbell4
RESUMEN
El resumen ser´a traducido al espan˜ol por los editores. Recent results from a multiwavelength survey
of spatially resolved outflows in nearby active galaxies are presented. Optical Fabry-Perot and long-slit spec-
troscopic data are combined with VLA and ROSAT images, when available, to probe the warm, relativistic
1 and hot gas components involved in the outflow. The emphasis is put on objects which harbor wide-angle
0 galactic-scaleoutflowsbutalsoshowevidenceatradiooropticalwavelengthsforcollimatedjet-likephenomena
0
(e.g., Circinus, NGC 4388, and to a lesser extent NGC 2992). Our results are compared with the predictions
2
from published jet-driven thermal wind models.

n
a ABSTRACT
J
Recent results from a multiwavelength survey of spatially resolved outflows in nearby active galaxies are pre-
5
sented. Optical Fabry-Perot and long-slit spectroscopic data are combined with VLA and ROSAT images,

1 when available,to probe the warm, relativistic andhot gas components involvedin the outflow. The emphasis
v is put on objects which harbor wide-angle galactic-scale outflows but also show evidence at radio or optical
5
wavelengths for collimated jet-like phenomena (e.g., Circinus, NGC 4388, and to a lesser extent NGC 2992).
7
Our results are compared with the predictions from published jet-driven thermal wind models.
0
1 Key Words: GALAXIES:ACTIVE—GALAXIES:JETS—GALAXIES:KINEMATICSANDDYNAM-
0
ICS — GALAXIES: SEYFERT — GALAXIES: STARBURST
1
0
1. INTRODUCTION intotheintergalacticmedium(e.g.,Stickland&
/
h Stevens 2000,and references therein). The out-
Themaintopicatthisconferenceisjet-entrained
p
flow is expected to be wide-angledand oriented
material in Herbig-Haro (HH) objects and in active
-
o galacticnuclei(AGN).However,aswediscussinthis roughlyalongtheminoraxisofthegalacticdisk.
r
t paper, a significant fraction of AGNs harbor poorly
s 2. X-ray–heated torus winds. X-rays emit-
collimated winds which may also be of great dy-
a ted in the inner part of an accretion disk can
: namical significance. A similar wind phenomenon
v Compton-heat the surface of the disk further
is known to take place in a number of HH objects
i out, producing a corona and possibly driving
X (e.g., HH 111; Nagar et al. 1997).
off a wind (e.g., Begelman, McKee, & Shields
r A broad range of processes may be responsible
a 1983;Krolik&Begelman1986,1988;Balsara&
for wide-angle outflows in AGNs:
Krolik1993). ForSeyfertgalaxies,asubstantial
wind with T ≈ 1 × 106 K and V ≈ 200
1. Starburst-driven winds. The deposition of wind wind
- 500 km s−1 is driven off if L/L > 0.08 (e.g.,
a large amount of mechanical energy by a nu- E ∼
Balsara & Krolik 1993). The wind is directed
clear starburstmay create a large-scalegalactic
along the minor axis of the accretion disk and
wind – superwind – which encompasses much
is not necessarily perpendicular to the galactic
of the host galaxy. Depending upon the extent
disk.
of the halo and its density and upon the wind’s
mechanical luminosity and duration, the wind
3. Jet-driven thermal winds. AGN-driven jets
may ultimately blow out through the halo and
entrain and heat gas on kiloparsec scales. The
1Department of Astronomy, University of Maryland, Col- internal energy densities of these loosely colli-
legePark,Maryland mated jets may be dominated by thermal X-
2Department of Physics and Astronomy, University of ray-emitting plasma. The extended soft X-ray
NorthCarolina,ChapelHill,NorthCarolina
emission from these objects should be roughly
3Anglo-AustralianObservatory,Epping,Australia
4Department of Astronomy, California Institute of Tech- cospatial with the large-scaleradio emission, as
nology,Pasadena, California seen in some active galaxies (e.g., Colbert et
1
2 VEILLEUX ET AL.
al. 1996, 1998). The transport of energy and host galaxy (e.g., NGC 4388).
momentumby these “mass-loaded”jets maybe
• The solid angle subtended by these winds,
abletopowerthenarrow-lineregionsofSeyferts
Ω /4π ≈ 0.1 – 0.5.
(e.g., Bicknell et al. 1998). W
In the rest of this paper, we aim to determine • The radial extent of the line-emitting material
whichone(s)ofthesemechanismsis(are)responsible involved in the outflow, RW = 1 – 5 kpc.
for the wide-angle outflows seen in AGNs. In §2, we
• The outflow velocity of the line-emitting mate-
describeasurveyourgrouphasbeenconductingover
rial, V = 100 – 1500 km s−1 regardless of the
the yearsonnearbyAGNs. In§3,wesummarizethe W
escapevelocityofthehostgalaxy. Untilrecently
general trends that we see in our data and address
therecordholderwasNGC3079,whereoutflow
theissuesoftheionizationsourceoftheline-emitting
velocities in excess of 1500 km s−1 are directly
material and the energy source of the outflows. In
measured (Veilleux et al. 1994). But the out-
§4,wediscusstherecentresultsonthreeobjectsrep-
flow velocities in NGC 1068 are now known to
resentative of our sample: Circinus, NGC 2992, and
be far larger than this value (see Cecil, these
NGC 4388. We summarize our conclusions and dis-
proceedings, and Cecil et al. 2001b).
cuss future avenues of researchin §5.
• The dynamical time scale, t ≈ R /V =
dyn W W
2. DESCRIPTION OF SURVEY
106−107 years.
Over the past ten years, our group has been
conducting an optical survey of nearby active and • The ionized mass involved in the outflow, M =
starburst galaxies combining Fabry-Perot imaging 105 – 107 M⊙, a relatively small fraction of the
and long-slit spectrophotometry with radio and X- total ISM in the host galaxy.
ray data to track the energy flow of galactic winds
through the various gas phases. The complete
TABLE 1
spatial and kinematic sampling of the Fabry-Perot
data is ideally suited to study the complex and ex- SOME KEY PUBLICATIONS
tended morphology of the warm line-emitting ma-
Galaxy Reference
terial which is associated with the wind flow. The
M 51 Cecil (1988)
radio and X-ray data complement the Fabry-Perot
M 82 Bland & Tully (1988)
data by probing the relativistic and hot gas compo-
nents, respectively. Shopbell & Bland-Hawthorn (1998)
Our sample contains twenty active galaxies with NGC 1068 Cecil et al. (1990)
known galactic-scale outflows. Half of them are Bland-Hawthorn et al. (1991)
Seyfert galaxies and the rest are starburst galax-
Sokolowskiet al. (1991)
ies (not discussedhere; the outflows in these objects
Cecil et al. (2001b)
are starburst-drivenwinds). All of these objects are
NGC 2992 Veilleux et al. (2001)
nearby (z < 0.01 to provide a spatial scale of < 200
pc arcsec−1) and have line-emitting regions that ex- NGC 3079 Veilleux et al. (1994)
tends more than 30′′. So far, the results have been Veilleux et al. (1995)
published for about a dozen of these objects. Table Veilleux et al. (1999a)
1 lists the main papers which have come out from Cecil et al. (2001a)
our survey.
NGC 3516 Veilleux et al. (1993)
NGC 4258 Cecil et al. (1992)
3. GENERAL RESULTS
Cecil et al. (1995)
Evidence for loosely collimated winds are de-
tected in several AGNs. Approximately 75% of Cecil et al. (1995)
the AGNs in our (admittedly biased) sample har- Cecil et al. (2000)
bor wide-angle outflows rather than collimated jets. NGC 4388 Veilleux et al. (1999a)
These outflows typically show the following optical Veilleux et al. (1999b)
properties:
NGC 6240 Bland-Hawthorn et al. (1991)
• The optical winds are often lopsided and some- Circinus Veilleux & Bland-Hawthorn (1997)
timestiltedwithrespecttothepolaraxisofthe
GALACTIC OUTFLOWS IN NEARBY AGNS 3
• The ionized mass outflow rate, dM/dt ≈ Recent maps (Elmouttie et al. 1995, 1998) have re-
M/tdyn = 0.1 – 1 M⊙ yr−1 > dMacc/dt, the solved spectacular radio lobes centered on the nu-
mass accretion rate necessary to fuel the AGN. cleus, and extending more than 90′′ (∼ 2 kpc) on
either side of the galaxy disk (PA ≈ –60◦).
• Thekineticenergyoftheoutflowingionizedma-
terial, Ekin = 1053 – 1055 ergs, taking into ac- 4.2. Morphology and Kinematics of the
countboththebulkand“turbulent”(spectrally Line-Emitting Material
unresolved)motions. This mechanicalenergyis
Our Fabry-Perot data on Circinus (Fig. 1) re-
equivalent to that of ∼ 102 – 104 Type II SNe.
veal a complex of ionized filaments extending radi-
ally from the nucleus out to distances of 1 kpc. The
• Thekineticenergyrateoftheoutflowingionized
most striking [O III] feature extends along position
material, dEkin/dt ≈ Ekin/tdyn = few ×1039− angle∼–50◦ spanningadistanceof∼25–45′′ (500
1042 ergs s−1.
–900pc)fromthenucleus. Thelateralextentofthis
filamentisnearthelimitofourresolution(∼1′.′5af-
• Evidenceforentrainmentof(rotating)diskma-
ter smoothing). This narrow feature is also visible
terial is seen in some objects (e.g., Circinus,
at Hα but with a lower contrast. The gas at this
NGC 2992, NGC 3079).
location is highly ionized with a [O III] λ5007/Hα
flux ratio typically larger than unity. Extrapolation
• The source of ionization of the line-emitting
of this filament to smaller radii comes to within 1′′
material taking part in these outflows is di-
(20pc)oftheinfrarednucleus(Marconietal. 1995),
verse. Pure photoionization by the AGN can
suggestinganuclear(mostlikelyAGN)origintothis
explain the emission-line ratios in NGC 3516
feature.
and NGC 4388. Shock ionization is probably
The most spectacular feature in the Hα data
contributing in Circinus and NGC 2992.
is the hook-shaped filament which extends to 40′′
(800 pc) west of the nucleus (Fig. 2). Such fea-
• Starburst-driven winds are not common among
turesarecommonlyobservedinHHobjectsalthough
AGNs (a possible exception is NGC 3079;
they have never been seen on galactic scales. Addi-
Veilleux et al. 1994; Cecil et al. 2001a). Galac-
tional morphological evidence for outflow exists in
tic winds are roughly aligned with galaxy-scale
the northern portion of our data. The [O III] emis-
(severalkpc) radio emission, sometimes encom-
sionalongPA≈–20◦ formsabroadfilamentary‘fin-
passing what appears to be poorly collimated
ger’ or jet that points back to the nucleus. A knot
radio “jets”. These mass-loaded “jets” are the
is presentatthe tip ofthis ‘finger’,25′′ fromthe nu-
probabledrivingengineforthesewinds. Inmost
cleus. BrightHαemissionisalsovisiblenearthispo-
cases, however, torus-driven winds cannot for-
sition,thesouthernportionofwhichformsawide(∼
mally be ruled out.
8′′ )arcresemblingabowshock. Thearcispointing
in the downstream direction consistent with being
4. CASE STUDIES OF THREE NEARBY AGNS
produced by a collimated jet.
In this section, we discuss the recent results on The kinematics derived from the [O III] Fabry-
Circinus,NGC2992,andNGC4388,threefairlyrep-
Perot data and complementary long-slit spectra
resentative objects of our sample. While the mor- bringcredencetothenuclearoutflowscenario. Non-
phology and kinematics of the optical outflows in gravitational motions are observed throughout the
these objects are very different, they appear funda- [O III] cone, superposed on a large-scale velocity
mentally to be driven by the same process, namely gradient caused by galactic rotation along the ma-
entrainment along poorly collimated “jets”. For jor axis of the galaxy (PA ≈ 30◦; Freeman et al.
maj
moreinformationontheseobjects,thereadershould 1977). Sudden velocity gradients are seen near the
refer to the original papers: Veilleux & Bland- positions of the bright [O III] and Hα knots.
Hawthorn(1997),Veilleuxetal. (2001),andVeilleux
et al. (1999a,b), respectively.
4.2.1. Source of Ionization
The motions observed across the ionization
4.1. Circinus
cone are highly supersonic, so high-velocity
The Circinus galaxy is the nearest (∼ 4 Mpc) (V > 100km s−1) shocksare likely to contribute to
s ∼
Seyfert 2 galaxy known, and is therefore ideally the ionization of the line emitting gas. Large varia-
suited for detailed studies of AGN-driven outflows. tionsofthelineratiosaresometimesobservedwithin
4 VEILLEUX ET AL.
a
N knot 1 (south)
knot 2 (south)
W
knot 3
knot 4
b
N
Fig. 3. Binned spectra at four positions along the slit
through the nucleus corresponding to knots 1−4 in Fig.
2. Each spectrum has been offset by 400 counts. Knots
W ′′
1 and 2 are binned along the slit over 6 ; knots 3 and 4
arebinnedover4′′. Notethegreatlyenhanced[NII]/Hα
ratio along thejet, except on thebow shock at knot 4.
a single knot (Fig. 3). The enhanced [N II]/Hα,
[SII]/Hα,and[OIII]/Hβ ratiosinknots1and2fall
near the range produced by the high velocity pho-
Fig. 1. Line flux images of the Circinus galaxy: a, toionizing radiative shocks of Dopita & Sutherland
[OIII]λ5007andb,blueshifted(between–150and0km (1995). Photoionization purely by the active nu-
s−1) Hα. Northis at thetop and west to theright. The
cleus ofa mixture ofionizationandmatter-bounded
position of the infrared nucleus (Marconi et al. 1995)
cloudswhoserelativeproportionsvarywithposition
is indicated in each image by a cross. The spatial scale,
in the galaxy may also provide another explanation
indicatedbyahorizontalbaratthebottomofthe[OIII]
image, is thesame for each image and corresponds to ∼ for the abrupt changesof excitationin the filaments
′′ (e.g., Binette, Wilson, & Storchi-Bergmann1996).
25 or 500 pc for the adopted distance of the Circinus
galaxyof4Mpc. Theminoraxisofthegalaxyrunsalong
PA ≈ –60◦. 4.2.2. Nature of the Outflow
The mass of ionized gas involved in this outflow
a
is fairly modest, ∼ few × 104 X−1 n−e,21 M⊙ where
X is the fraction (< 1) of oxygen which is doubly
1 2 3
ionized and N is the electron density in units of
e,2
102 cm−3. The total kinetic energy (∼ 1053 n−1
e,2
ergs)lies nearthe lowenergyendofthe distribution
b
for wide-angle events observed in nearby galaxies.
The morphology and velocity field of the filaments
1 2 3
suggest that they represent material expelled from
4
the nucleus(possiblyinthe formof“bullets”)oren-
trained in a wide-angle wind roughly aligned with
the radio“jet”andthepolaraxisofthegalaxy. The
Fig. 2. Line flux images of the western hook-shaped
filament: a, [O III] λ5007 and b, blueshifted (between complex morphology of the outflow in the Circinus
–150 and 0 km s−1) Hα. The position of the infrared galaxyisuniqueamongactivegalaxies. Theeventin
nucleus(Marconi et al. 1995) is indicated in each image the Circinus galaxy may represent a relatively com-
bya cross. Theorientation is thesame asin Fig. 1, but monevolutionaryphaseinthelivesofgas-richactive
thehorizontalbaratthebottomofthe[OIII]imagenow galaxiesduring which the dusty cocoon surrounding
corresponds to ∼ 250 pc. the nucleus is expelled by the action of jet or wind
GALACTIC OUTFLOWS IN NEARBY AGNS 5
phenomena.
4.3. NGC 2992
The presence of a galactic-scale outflow in
NGC 2992 has been suspected for several years,
based on the morphology of the radio emission
(Ward et al. 1980; Hummel et al. 1983) and more
recently the X-ray emission (Colbert et al. 1998).
In the optical, the line emission from the outflow is
severelyblendedwithlineemissionfromthegalactic
disk. The complete two-dimensionalcoverageof our
Fabry-Perot data is therefore critical to disentangle
the material associated with the wind from the gas
in rotation in the galactic disk.
4.3.1. Morphology and Kinematics of the
Outflowing Line-Emitting Material
The distribution of the Hα-emitting gas in the
disk and outflow components is shown in Figure 4.
Fig. 4. Distribution of the Hα and continuum emission
The outflow component (Fig. 4d) is distributed into
in NGC 2992. (a) Continuum emission at rest wave-
two wide cones which extend up to ∼ 18′′ (2.8 kpc) length Hα, (b) total Hα emission, (c) Hα emission from
from the peak in the optical continuum map. Both the disk component, (d) Hα emission from the outflow
cones have similar opening angles of order 125◦ – component. Thepositionoftheradionucleus(“+”)and
135◦. The bisectors of each of the cones coincide theextentofthedustlaneareindicatedoneachofthese
witheachotherandliealongP.A.≈116◦,oralmost panels. Northisatthetopandeasttotheleft. Notethe
exactlyperpendiculartothekinematicmajoraxisof absence of a disk component in the east quadrant.
the inner disk (P.A. ≈ 32◦). This strongly suggests
that the axis of the bicone is perpendicular to the
galactic disk, and that the material in the SE cone
is emerging from under the galaxy disk while the
materialintheNWconeisemergingfromabovethe
disk.
The outflow on the SE side of the nucleus is
made of two distinct kinematic components inter-
preted as the front and back walls of a cone. The
azimuthal velocity gradient in the back-wallcompo-
nent reflects residual rotational motion which indi-
cates either that the outflowing material was lifted
from the disk or that the underlying galactic disk
is contributing slightly to this component. A single
outflow component is detected in the NW cone.
Fig. 5. Contour maps of the X-ray emission (left panel:
ROSAT/HRI data from Colbert et al. 1998) and 6-
4.3.2. Source of the Outflow
cm continuum emission (upper right panel: VLA C-
Abiconicaloutflowmodelwithvelocitiesranging ′′
configuration datafrom Colbert etal. 1996with 3 uni-
from 50 to 200 km s−1 and oriented nearly perpen- form weighting; lower right panel: VLA A-configuration
dicular to the galactic disk can explain most of the datafromUlvestad&Wilson1984)superimposedonthe
data. The broad line profiles and asymmetries in total Hα emission from NGC 2992 (grey-scale). A cross
thevelocityfieldssuggestthatsomeoftheentrained (“+”) in the main figure indicates the position of the
line-emitting material may lie inside the biconical radio nucleus. The arrows in the right panels mark the
′′
structure rather than only on the surface of the bi- direction oftheone-sided90 (13.5kpc)radioextension
along P.A. ∼ 100◦ – 130◦, i.e. close to the axis of the
cone. The mass involved in this outflow is of order
∼ 1 × 107 ne−,21 M⊙, and the bulk and “turbulent” biconical outflow. Same orientation as Fig. 4.
kinematic energies are ∼ 6 × 1053 n−1 ergs and ∼
e,2
6 VEILLEUX ET AL.
3 × 1054 n−1 ergs, respectively. The faint X-ray ex-
e,2
tensiondetected by Colbertetal. (1998)andshown
in Figure 5 lies close to the axis of the outflow bi-
cone,suggestingapossible link betweenthe hotand
warmgasphasesinthe outflow. Thepositionangles
of the optical outflow bisector and the long axis of
the “figure-8” radio structure shown in Fig. 5 differ
by ∼25◦ – 35◦,but the one-sided13.5-kpcradioex-
tensiondetectedbyWardetal. (1980)andHummel
etal. (1983)liesroughlyalongthesamedirectionas
the mid-axis of the SE optical cone (P.A. ∼ 100◦ –
130◦; arrows mark this direction in the right panels
of Fig. 5). The most likely energy source of the op-
tical outflow is a hot bipolar thermal wind powered
onsub-kpcscalebytheAGNanddivertedalongthe
galaxy minor axis by the pressure gradient of the
ISM in the host galaxy. The data are not consistent
withastarburst-drivenwindoracollimatedoutflow
powered by radio jets.
4.4. NGC 4388
NGC 4388 was the first Seyfert galaxy discov-
eredin the Virgocluster (Phillips & Malin 1982). It
isplungingedge-onat1500kms−1 throughthecen- Fig. 6. The distributions of the line emission in NGC
4388. (top) Hα; (bottom) [O III]λ5007. North is at the
ter of the Virgo cluster and experiencing the effects
top and East to the left. The spatial scale, indicated by
of ram-pressure stripping by the densest portions of
ahorizontalbaratthebottomoftheimage,corresponds
the intracluster medium (ICM; e.g., Cayatte et al. ′′
to 12 , or 1 kpc for the adopted distance of 16.7 Mpc.
1994). Nuclear activity has been detected at nearly
Theopticalcontinuumnucleusisindicatedineachpanel
all wavelengths. The morphology of the nuclear ra- by a cross.
diosourcesuggestsacollimatedAGN-drivenoutflow
(Stoneetal. 1988;Falcke,Wilson,&Simpson1998).
At optical wavelengths, NGC 4388 has been known gas associated with star formation is also present in
forsometimetopresentextendedlineemission(e.g., the disk. Evidence for bar streaming is detected in
Pogge 1988; Corbin, Baldwin, & Wilson 1988 and the disk component and is discussed in Veilleux et
references therein). A rich complex of ionized gas al. (1999a,b). Non-rotational blueshifted velocities
extends both along the disk of the galaxy and up to of 50− 250km s−1 aremeasuredinthe extraplanar
50′′ (4 kpc) above that plane. The extraplanar gas gas north-east of the nucleus. The brighter features
component has considerably higher ionization than in this complex tend to have more blueshifted ve-
the disk gas, and appears roughly distributed into locities. A redshifted cloud is also detected 2 kpc
two opposed radiation cones that emanate from the south-west of the nucleus. The total mass and ki-
nucleus (Pogge 1988; Falcke et al. 1998). Detailed netic energy of the extraplanar gas are ∼ 4 × 105
spectroscopicstudiesstronglysuggestthatthiscom- M⊙ and Ekin = Ebulk + Eturb ∼> 1 × 1053 ergs, re-
ponentismainlyionizedbyphotonsfromthenuclear spectively.
continuum (Pogge 1988; Colina 1992; Petitjean &
Duret 1993). 4.4.2. Geometry of the Outflow
The velocity field of the extraplanar gas of
4.4.1. Morphology and Kinematics of the NGC 4388 appears to be unaffected by the inferred
Extraplanar Line-Emitting Material supersonic (Mach number M ≈ 3) motion of this
Figure 6 shows the distribution of the line- galaxy through the ICM of the Virgo cluster. This
emitting material in this galaxy derived from our is because the galaxy and the high-|z| gas lie behind
Fabry-Perot data. We confirm the existence of a a Mach cone with opening angle ∼ 80◦ (see Fig. 7).
rich complex of highly ionized gas that extends ∼ The shocked ICM that flows near the galaxy has a
4 kpc above the disk of this galaxy. Low-ionization velocity of ∼ 500km s−1 andexerts insufficient ram
GALACTIC OUTFLOWS IN NEARBY AGNS 7
Pre-shock ICM Post-Shocked ICM veys of high-redshift sources.
V ,n V ,n
ICM ICM PS PS
S.V.isgratefulforpartialsupportofthisresearch
by a Cottrell Scholarship, NASA/LTSA grant NAG
NE Complex 56547,and NSF/CAREER grant AST-9874973.
Bow RH I 90 o - | i | REFERENCES
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S. Veilleux: Department of Astronomy, University of Maryland, College Park, MD 20742
([email protected])
G. Cecil: Department of Physics and Astronomy, University of North Carolina, Chapel Hill, NC 27599-3255.
([email protected])
J. Bland-Hawthorn: Anglo-Australian Observatory, P.O. Box 296 Epping, NSW 2121 Australia.
([email protected])
P. L. Shopbell: Department of Astronomy, California Institute of Technology, Pasadena, CA 91125.
([email protected])