Jet driven motions in the Narrow Line region of NGC1068

11 Pages·0.35 MB·English

by  D. J. Axon

Jet driven motions in the
Narrow Line Region of NGC 1068 1
D.J. Axon2,3, A. Marconi4,5
Space Telescope Science Institute
3700 San Martin Drive
8
9 Baltimore, MD 21218
9
1

n A. Capetti
a
J Osservatorio Astronomico di Torino

6
Strada Osservatorio 25
2
40025, Pino Torinese, Italy

1
v
3 2
F.D. Macchetto , E. Schreier
5
2 Space Telescope Science Institute
1
0
8
9 A. Robinson
/
h
Division of Physics and Astronomy,
p
Department of Physical Sciences,
-
o
University of Hertfordshire,
r
t College Lane, Hatfield, Herts AL10 9AB,UK
s
a
:
v
i
X
r
a ABSTRACT
We have obtained HST FOC f/48 long–slit spectroscopy of the inner 4 arcseconds of
the Narrow Line Region of NGC 1068 between 3500-5400˚A with a spectral resolution
of 1.78˚A/pixel. At a spatial scale of 0′.′0287 per pixel these data provide an order of
magnitude improvement in resolution over previous ground based spectra and allow
us to trace the interaction between the radio jet and the gas in the NLR. Our results
show that, within ±0′.′5 of the radio-jet the emission lines are kinematically disturbed
1Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute,
which is operated by AURA,Inc., underNASAcontract NAS 5-26555 and bySTScI grant GO-3594.01-91A
2Affiliated to theAstrophysics Division, Space Science Department,ESA
3On leave from the Universityof Manchester
4Dipartimento diAstronomia e Scienza dello Spazio, Universit`adi Firenze, Largo E. Fermi5, I–50125, Italy
5Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, I–50125, Italy
1
−1
and split into two components whose velocity separation is 1500 km s . The filaments
associated with the radio lobe also show a redshifted kinematic disturbance of the order
−1
of 300 km s which probably is a consequence of the expansion of the radio plasma.
Furthermore, the material enveloping the radio-jet is in a much higher ionization
state than that of the surrounding NLR gas. The highest excitation is coincident with
the jet axis where emission in the coronal line of [FeVII] λ3769˚A is detected and the
HeIIλ4686 ˚A is strong but where [OII] λ3727 ˚A is depressed. This large localized
increase in ionization on the jet axis is accompanied by the presence of an excess con-
tinuum. Because the electron density is substantially larger in the jet compared to the
surrounding NLR, these results can only be explained if there is a more intense ionizing
continuum associated with the jet. This can be accomplished in a variety of ways which
include an intrinsically anisotropic nuclear radiation field, a reduced gas covering factor
or the presence of a local ionization source.
The morphology, kinematics and, possibly, the ionization structure of the NLR in
the vicinity of the jet of NGC 1068 are a direct consequence of the interaction with the
radio outflow.
Subject headings: Galaxies-individual (NGC1068); Galaxies-Seyfert; Galaxies-active
2
1. Introduction emittedbyundisturbedgasinthedisk. Atlarger
radii this triple structure disappears.
Extensive HST emission line imagery of the
The complexity of the velocity field observed
Narrow Line Regions (NLR) of Seyfert galaxies
in the inner region is almost certainly due to the
withlinearradiosourceshasshownthatthemor-
spatialconfusioncausedbyprojectionandseeing
phology of the NLR is directly related to that
effects which mixes kinematic components from
of the radio emission. Seyfert galaxies with a
different regions along the line of sight. In par-
lobe-like radio morphology (e.g. Mrk 573, Mrk
ticular the velocity structure created by the ex-
78, NGC 3393, IRAS 0421+045 and IRAS 1105-
pansion of the hot gas associated with thelobe is
035)havebow-shockshapedemissionlineregions
projected onto theregion interacting with thejet
(Bower et al. 1994, Axon et al. 1996, Capetti et
whichcanbespatiallyresolvedonlywithHST.In
al. 1996), while those with a jet-like radio struc-
order to obtain a more detailed picture of the re-
ture (e.g. Mrk 3, Mrk 348, Mrk 6, Mrk1066)
lationship between the velocity field and the ion-
have jet-like emission line structures (Bower et
ization conditions in the NLR gas and the inter-
al. 1995, Capetti et al. 1995a, Capetti et al.
actionofthegaswiththeradiojetwehavethere-
1995b). These results have provided compelling
fore obtained new long–slit spectroscopic obser-
evidence for strong dynamical interactions be-
vations with the Faint Object Camera f/48 spec-
tween the emission line gas and radio-emitting
trograph on board the HST. Ourresults allow us
ejecta on scales of ≤ 1kpc.
to resolve the spatial confusion and provide evi-
The physical foundation of this picture was
dence that shocks created by the jet play a key
firstdevelopedbyTaylor,DysonandAxon(1992)
roleinformingand,possibly,ionizingtheNLRof
who constructed fast bow shock models for the
NGC 1068. Throughoutthis paperwewill adopt
interaction between radio ejecta and the ISM
adistancetoNGC1068 of14.4 Mpc(Tully1988)
which included the effects of photo-ionization ′′
where 1 corresponds to 72 pc.
from the nucleus. Dopita and Sutherland (Do-
pita & Sutherland 1996a, Dopita & Sutherland 2. Observations and Data Reduction
1996b)haveemphasizedthatthehotshockedgas
coulditself makeanimportantlocalcontribution The Narrow Line Region of NGC 1068 was
to the ionizing continuum (becoming what they observed using the FOC f/48 long–slit spectro-
th
termed an auto-ionizing shock). graph on October 18 , 1996 at resolutions of
NGC 1068 is one of the closest Seyfert galax- 1.78˚A and 0′.′0287 per pixel along the dispersion
ies and it harbours a bright radio source, with and slit directions, respectively. The F305LP
a prominent radio-jet (Muxlow et al. 1996) ter- filter was used to isolate the first order spec-
minating in an extended radio-lobe (Wilson & trum which covers the 3650–5470 ˚A region and
Ulvestad 1983). Ground-based kinematic stud- therefore includes the [OII]λ3727, Hβλ4861 and
ies of the NLR have been carried out previously [OIII]λλ4959, 5007 ˚A emission lines. The slit,
′′ ′′
by a number of authors (Alloin et al. 1983, Bald- 0.063x13.5 in size, was placed at a position an-
◦
win et al. 1987, Meaburn and Pedlar 1986, Ce- gle of 83 andspectrawith exposuretimes of 627
cil, Bland & Tully 1990, Unger et al. 1992, Ar- seconds were taken in the 1024x512 non–zoomed
′′
ribas et al. 1996) in [OIII] λ5007. In particular mode at 6 locations separated by 1 . Unfortu-
two broad components straddling a narrow cen- nately, due to a guide star re-acquisition failure,
tral component in the NE radio lobe were iden- only 3 of the slit locations yielded usable spec-
tified. The narrow component appears to follow tra which we identify as POS1, POS2 and POS3
the expected rotation curve of the galaxy and is respectively.
3
N
E
Fig. 1.— The slit positions used in our study overlayed on a gray scale map of the [OIII] image of
Macchetto et al. 1994. The origin of the spatial axis on the borders of the image is centered on the
position of the hidden nucleus (Capetti et al. 1995c). The spatial coordinates used along the slit are
marked as crosses at POS1and POS2. Thezero point of this axis lies on cloud Gin POS3. Thefilaments
associated with the radio lobe intersect the slit at POS2 at location -1.5 arcseconds and POS1 at -1
arcseconds. The filled squares at POS3 indicate the limits of the contiguous regions where we extracted
the spectra shown in Fig. 3.
4
The data reduction follows the procedure de- Turning to the emission line strengths, in Fig.
scribed in detail by Macchetto et al. 1997. Flux 3 we show plots of representative spectra from
calibration was performed using the observa- the five regions identified in Fig. 1. On the jet
tions of the spectrophotometric standard star axis (labeled as “Jet”) the gas is highly ionized;
LDS749b. [FeVII]λ3759˚Aand[ArIV]areclearlyvisibleand
To accurately determine the location of the the HeIIλ4686 is as high as 0.7 the strength of
slits we compared the surface brightness profile Hβ, while the [OII] emission appears to be de-
derived from the FOC, f/96, F501N HST image pressed. This increase in excitation is accompa-
ofMacchetto etal.1994withthatmeasuredfrom nied by the presence of an excess local contin-
the spectra at the three slit positions. The best uum, as shown in Fig. 4.
match is displayed in Fig. 1 and is accurate to Outside the jet the excitation conditions are
within half a slit width (≃ 0′.′03). much lower, with [OII] being relatively stronger,
The slit locations POS1 and POS2 cut across particularly to the E as shown in Fig. 4. The
theemission linefilament systemassociated with variation of the [OIII]/[OII] ratio along the slit,
the Northern radio lobe (Capetti, Axon, & Mac- plotted in Fig. 5, shows that there is a fac-
chetto 1997). While the slit at POS3 (see Fig. tor ≃5 difference between the ratio on the jet
1) traverses cloud G (Evans et al. 1991 ) and the axis and outside. The electron temperature
parallel feature to the West which form a funnel- measured from the [OIII]λλ4363/5007 ratio give
like structure where the jet meets the radio lobe Te = (1.15 ± 0.15)×104 K. A lower limit for
(see Gallimore et al. 1996). the electron density can be obtained from the
[ArIV]λλ4710/4740ratioandyieldsNe ≥ 104.5cm−3.
3. Results At similar distances from the nucleus, but off
thejet,groundbaseddensitymeasurementsfrom
The velocities derived from the [OIII] and [SII]λ6716,6731 ratio, obtained from our unpub-
[OII] lines by fitting gaussian line profiles are lished spectroscopy, indicate electron densities of
plotted in Fig. 2. Around the jet axis, at POS3 a few 100 cm−3. Thus the increase in excitation
(Fig. 2b),thelinesaresplitintotwovelocity sys- on the jet occurs despite an increase in density.
tems separated by ∼ 1500 km s−1. Outside this
region the gas motions are quiescent but with a 4. Discussion
−1
gradient of +500 km s from West to East. No-
tice that the velocity splitting is not symmetric Themorphologicalconnectionbetweentheop-
aboutthesystemicvelocity, butismuchlargeron tical and radio emission in the region of the jet
theblue-shiftedside. Theblueshiftedcomponent has been discussed in detail by Gallimore et al.
is also substantially brighter and has a larger (1996) and by Capetti, Macchetto and Lattanzi
velocity dispersion than the redshifted compo- (1997). They showed that there is a clear anti-
nent. At POS2 (Fig. 2a) the velocity field is correlation between radio and optical emission,
relatively flat, except at the filaments at slit co- with the brightest line emitting knots surround-
ordinates ∼ -1′′ – -2′′, where there is a large red- ing the radio-jet. In the jet-cloud interaction
shifted velocity perturbation of amplitude ∼ 300 picture, the radio jet clears a channel through
km s−1, at a similar radial velocity to that of the the cool ISM, leaving hot ionized gas in its wake
redshifted component of the split line region in and the line emission is highly enhanced along
POS3. POS1 (Fig. 2a) shows a similar pattern the edges of the radio-jet where the gas is com-
in velocity, but with a reduced amplitude in the pressed.
perturbed region. The overall velocity structure and ionization
5
Fig. 2.— Velocity fields measured at the threeslit positions. Lower panel: Aroundthejet axis, at POS3,
the gas is strongly kinematically perturbed and shows split lines with components separated by ∼ 1500
−1
km s . The dashed line marks the systemic velocity of NGC 1068 (Baan & Haschick 1983). Upper
panel: at POS2 and POS1 there are also large velocity perturbation at the location of the emission line
filaments.
6
Fig. 3.— Spectraextracted atthepositionsindicated inFig. 1. Thespectrumextracted ina0′.′14×0′.′063
aperture around cloud G, labeled ”Jet”, shows the high excitation gas on the jet with [FeVII], [ArIV]
and HeII emission lines.
7
[OII] POS 3 [OIII]
Fig. 4.— Grey scale image of the spectrum at POS3 showing the excess continuum associated with the
radio jet.
Fig. 5.— A plot of the [OIII]/[OII] ratio as a function of position along the slit at POS 3. The ratio
reaches a maximum on the jet axis where it is ∼ 5 times larger than that in the surrounding regions.
8
conditions of the gas associated with the jet of is approximately proportional to U; Penston et
NGC1068 can beunderstoodintermsofthis pic- al 1989). The enhancements in both density and
ture of an expanding and cooling cocoon around ionization parameter imply that the local ioniz-
the jet which creates very fast shocks with at ing radiation is a factor ∼ 500 higher at the lo-
least V ≃ 700 km s−1 (even when mass-loading cation of the jet in POS 3, than it is elsewhere.
of the expanding cocoon is taken into account). The exceptionally strong [Fe VII] and HeII lines
The viewing angle is such that to the NE the suggest that matter-bounded clouds might con-
lobe is projected on to the disk of NGC1068, so tribute significantly to the line emission near the
that the jet is seen emerging towards us out of jet. In this case, we will be overestimating the
the disk. The situation is very similar to that jump in U since low ionization lines such as [OII]
seen previously on the jet of 3C120 (Axon et arestronglysuppressedinsuchclouds. Neverthe-
al. 1989), even down to the clumpy structure of less, this is unlikely to explain the whole of the
the gas, which almost certainly arises because of inferred increase in the ionizing flux. Either a lo-
the onset of instabilities caused by the mixing of cal ionization source is present which dominates
hot and cold material. This morphology is again the AGN radiation field near the jet axis, or the
readily understood if it is due to the interaction AGN radiation field is itself highly anisotropic.
between the jet and the surrounding medium. A The most important local ionization source
viableexplanationfortheobservedasymmetryof is likely to be ionizing radiation (free-free and
the velocity splitting is that it is due to density high ionization line emission) from gas heated
stratification of the disk, that is the expanding and ionized in fast shocks driven by the radio
hot bubble is able to expand more rapidly along jet (Binette, Dopita & Tuohy, 1985; Sutherland,
the density gradient away from the disk plane, Bicknell & Dopita 1993; Dopita & Sutherland
than towards it. Further weight to this argu- 1996a,b). The strong [Fe VII] emission and the
ment is provided by the systematic blue-shift of excesscontinuumobservedonthejetaxismaybe
coronal lines with respect to the lower excitation a clue that the coronal gas expected in this pic-
lines (Marconi et al. 1996). This implies that the ture is indeed present. Alternatively, the on-axis
coronal lines comes from a different kinematic increase in the ionizing flux could be attributed
component which we would ascribe to the hot to intrinsic anisotropy of the nuclear continuum.
cooling gas in the cocoon. In this case, the size of the enhancement in flux,
The spectral diagnostics obtained from our and the fact that it is highly localized, require
long-slit spectra indicate that the kinematically a continuum beaming factor comparable with
disturbed line-emitting gas associated with the Doppler boosting in a moderately relativistic jet
radio jet has both a higher density and a higher (gamma a few). This idea can be ruled out
excitation level than the surrounding NLR. A because the region we have studied occurs after
similar result, based on narrow band imaging, a significant bend of the jet and therefore one
has been reported recently by Capetti, Axon, & would expect to see higher excitation along the
Macchetto 1997. The density enhancement is a orginal jet direction, which is not observed (cf.
factor > 100 if the ground-based measurements Capetti, Axon, & Macchetto 1997).
ofthe[SII]λλ6716,6731 ratiocanbetakenastyp- Perhaps a more likely explanation is that the
ical of the regions extending away from the jet continuum anisotropy is largely produced by az-
axis. On the other hand, the variations in the imuthal variations in the optical depth to ioniz-
[OIII]5007/[OII]3727 ratio along the POS 3 slit ing photons. In this picture, the radio jet sweeps
imply a factor 5 enhancement in the ionization out a channel as it passes through the NLR with
parameter (U)across thejetaxis(sincethisratio the result that ionizing radiation from the cen-
9
tralsourcesuffersmuchlessattenuation alongits Inc., under NASA contract NAS 5–26555. A.C.
axis than in other directions. thanks the STScI visitor program for providing
Our results on POS1 and POS2 can also be financial support during the course of this work.
interpreted in terms of the interaction between A.R. is supported by a Royal Society Fellowship.
theradioplasmaandtheambientgas,butinthis We thank E. Oliva for kindly providing his com-
case it is the expanding lobe material which is pilation of atomic parameters and his code to
driving the motion. From our spectroscopic data derive line emissivities.
we can say little on the ionization conditions of
REFERENCES
these filaments because only the [O III] lines are
detected. Alloin, D., Pelat, D, Boksenberg, A, & Sargent,
W., 1983, ApJ, 275, 493.
5. Conclusions
Arribas, S., Mediavilla, E., Garcia-Lorenzo, B.,
We have presented new HST FOC f/48 long– 1996, ApJ 463, 509
slit spectra of the central 4 arcseconds of the
′′ Axon, D. J., Unger, S. W., Pedlar, A., Meurs,
NLR of NGC 1068. At a spatial scale of 0.0287
E. J. A, Whittle, D. M. & Ward, M. J., 1989,
per pixel these data provide an order of mag-
Nature, 341, 631-633.
nitude improvement in resolution over previous
ground based spectra and allow us to trace the Baan W.A. and Haschick A.D., 1983, AJ, 88,
interaction between the radio jet and the gas in 1088
the NLR. Our results show that, within ±0′.′5 of
Axon,D.J.,Capetti,A.,,Macchetto, F.,Sparks,
the radio-jet the emission lines are split into two
components whose velocity separation is ∼ 1500 W. B.& Boksenberg, A.1996, InScience with
−1 the Hubble Space Telescope- II, ed P. Ben-
km s and are clearly the result of the interac-
venuti, F.D. Macchetto & E. J. Schreier, 214-
tion between the radio jet and the ambient NLR
218.
material. The filaments associated with the ra-
dio lobe also show a redshifted kinematic distur-
Baldwin, J., Wilson, A.S., & Whittle, M., 1987,
−1
bance of the order of 300 km s which probably
ApJ, 319, 84.
is a consequence of the expansion of the radio
plasma. Binette, L., Dopita, M.A., Tuohy, I.R., 1985,
ApJ, 297, 476
Our results show that the highest excitation
gas is physically related to the location of the Bower, G., Wilson, A.S., Mulchaey, J., Miley,
jet and is accompanied by an excess continuum G.K., Heckman, T.M., and Krolik, J.H., 1994,
providing additional evidence for the interaction AJ, 107, 1686.
model.
Bower, G., Wilson, A.S., Morse, J.A., Gelder-
Themorphology,kinematicsand,possibly,the
man, R., Whittle, M., andMulchaey, J., 1995,
ionization structure of the NLR in the vicinity of
ApJ, 454, 106.
the jet of NGC 1068 are a direct consequence of
the interaction with the radio outflow. Capetti, A., Macchetto, F.D., Axon, D.J.,
Sparks, W.B., and Boksenberg, A. 1995a,
A.M. acknowledges partial support through ApJ, 448, 600.
GO grant G005.44800 from Space Telescope Sci-
Capetti, A., Axon, D.J., Kukula, M., Macchetto
ence Institute, which is operated by the Associ-
F.D., Pedlar, A., Sparks, W.B., and Boksen-
ation of Universities for Research in Astronomy,
berg, A. 1995b, ApJ, 454, 85.
10