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Chapter 2
Optical jets and their
properties
An important feature in the unied model of AGN, particularly radio-loud
AGN, is the jet. This is a relativistic out ow from the central regions of
the AGN, and is a major source of radio frequency emission (through the
synchrotron process) in radio-loud objects. The other major sources are the
large lobes often seen in radio galaxies, although these are stronger at low
radio frequencies than high.
Since the work in this thesis is concerned primarily with the optical and
near-infrared emission of radio-loud quasars, the amount of optical emission
that comes from the jet is of considerable interest. This chapter contains a
review of observations of optical counterparts to radio jets, and a summary
of the relevant properties, particularly the spectral indices of the jet and the
angle the jet makes with the line of sight.
2.1 Observed optical jets
Only a small number of optical counterparts to radio jets in active galaxies
have been observed, despite the large number of radio jets that are known.
The main reason for this is that, with a couple of notable exceptions, they
are faint and small (in breadth, if not length). The rapid increase in the
number of optical jets known in the past decade has been primarily due
to the advent of the Hubble Space Telescope (HST), with its unsurpassed
resolution, although some have been discovered via ground based telescopes,

16 Optical jets and their properties
Name Other names Redshift Host
of host type
3C 15 PKS 0034 014 0.073 E1
3C 66B 0220+424 0.0215 E
3C 78 0305+039 0.0289 S0/Sa
NGC 1218
3C 120 PKS 0430+052 0.033 S0
II Zw 14
PKS 0521 365 0.055 E
3C 200 0824+294 0.458 ?
3C 212 0855+143 1.049 ?
3C 245 1040+123 1.029 ?
3C 264 1142+198 0.0216 E
NGC 3862
3C 273 1226+023 0.16 E4
M87 1228+126 0.004 E1
3C 274
Virgo A
NGC 4486
3C 346 1641+173 0.161 E
3C 371 1807+698 0.051 E
3C 380 1828+487 0.692 E
PKS 2201+044 0.028 E
Table 2.1: Sources that exhibit optical jets that are coincident with radio synchrotron
jets. Other names lists common names by which the sources are known (positions are
B1950). The redshift and morphological type (where known) of the host galaxy is also
shown (generally taken from NED, the NASA/IPAC Extragalactic Database).
notably several using the Nordic Optical Telescope (NOT) on La Palma.
Those sources with optical jets are listed in Table 2.1, and descriptions of
the individual sources follow.
M87: The famous jet in M87 is the oldest known and most studied of all
optical jets, primarily because of its high surface brightness and proximity
to Earth. In 1918, the astronomer Heber Doust Curtis observed it during a
study of M87, describing it as a \curious, straight ray" (Curtis 1918). M87
was later discovered to be a strong radio source (Virgo A), and the non-
thermal nature of the jet was conrmed by Baade (1956), by detecting its
strong optical polarisation. It was already known to have a blue featureless
continuum and to be associated with radio emission { in the same manner

2.1 Observed optical jets 17
as the Crab Nebula.
M87 itself is a giant elliptical (in fact, one of the most massive known {
it has a mass of up to  10 14 M (Mathews 1978)) at the heart of the Virgo
cluster. It was rst discovered by Charles Messier, in his famous catalogue
of Nebulae and Star Clusters (Messier 1781), and has been the subject of
much study throughout this century.
Narrow-band imaging and spectroscopy by the HST have revealed the
presence of a disk of ionised gas, surrounding a black hole of mass 2:40:7
10 9
M (Ford et al. 1994; Harms et al. 1994). This mass is actually obtained
from the velocities of the disk and is the mass internal to a radius of 0 00 :24
(or 18{20 pc), which, with a mass to light ratio of M=L  170, implies the
presence of a black hole.
The jet itself shows remarkably similar structure at both optical and
radio frequencies, which is seen in both continuum images and polarisation
images. The brighter knots in particular show evidence for a steepening of
the spectrum in the optical (Meisenheimer et al. 1996), indicating that the
spectrum turns over at optical frequencies.
3C 273: The jet in 3C 273 is the second oldest known optical jet. It
was rst noted by Schmidt (1963) (describing it as \a faint wisp or jet") in
the paper that rst reported 3C 273's large redshift.
The structure of the 3C 273 jet is atypical for optical jets. The radio
emission is much more extended, both laterally and longitudinally, than the
optical emission, with the optical jet running along the ridge line of the
radio emission (Morrison and Sadun 1992; Thomson et al. 1993). The radio
features generally coincide with the optical knots, although the bright radio
head of the jet is not seen in the optical (Bahcall et al. 1995). It is possible
that the optical jet emission is not pure synchrotron (Thomson et al. 1993),
but may include either some scattered quasar light or starlight.
The host galaxy has been shown (Bahcall et al. 1997) to be a bright E4
elliptical. Indeed, at M V = 22:1, it is brighter than the brightest galaxy
of a rich cluster. It seems to be in a group of several galaxies.
3C 15: This source is an elliptical galaxy, classied as an intermediate
FR I / FR II. The optical counterpart to the jet was discovered by Martel
et al. (1998), using HST. The optical jet matches the inner part of the radio
jet, although the two radio knots are undetected in the optical.
Interestingly enough, the nucleus of 3C 15 appears to lack a sharp point-

18 Optical jets and their properties
like AGN, possibly indicating that the source is in an inactive state. What is
seen, however, are three ridges of emission extending out from the nucleus,
along with a dark dust band, which seem to point to a merger event.
3C 66B: This optical jet was detected by Butcher et al. (1980) using
a vidicon video camera on the 4m Mayall telescope at Kitt Peak. At HST
resolution (Macchetto et al. 1991a), the jet shows lamentary structure,
with evidence for two strands. This has not been seen in any other optical
jet except M87, which shows a more tightly bounded structure, possibly
indicating a dierent interaction with the medium. Fraix-Burnet (1997)
found evidence for an optical counter-jet (3C 66B is the only optical jet
source that shows a radio counter-jet), which, if conrmed, would cast some
doubt on the beaming model for jets.
3C 78: The optical jet in 3C 78 was discovered by Sparks et al. (1995).
The jet shows a \remarkable" coincidence between the radio and optical
morphologies and direction (that is, the position angles are the same). The
host galaxy is notable for being a spiral (an S0 at least), as well as showing
Seyfert 1 characteristics.
3C 120: 3C 120 is classed as a Seyfert 1 galaxy, and so is rare in not
being an elliptical host. It has a variable AGN, is a powerful X-ray source
(Maraschi et al. 1991), and has a radio jet with superluminal motion (Walker
et al. 1988). A faint optical counterpart to the jet was detected using the
NOT (Hjorth et al. 1995). One of the \condensations" was found to be
polarised (p = 12%  4%), thus linking it with the radio jet. The overall
structure of the optical jet coincided with the radio, although no counter-
parts to the radio knots could be discerned. It is possible that the optical
emission is not pure synchrotron (similar to 3C 273) { there could be some
scattered starlight present as well.
PKS 0521 365: This is an elliptical radio galaxy that exhibits a bright
radio nucleus, along with extended radio emission. In HST observations
(Macchetto et al. 1991b), a bright knot was seen at the same place as in
VLA data. After this, the jet has roughly constant surface brightness.
However, in HST snapshot survey observations (Scarpa et al. 1999), this
bright knot is not seen. In these observations, the jet has an almost constant
width along its length, which implies that the plasma is moving in a well-
dened cylindrical funnel. There is also a general one-to-one correspondence
between the radio and optical morphologies.

2.1 Observed optical jets 19
3C 200: In their HST snapshot survey of 3CR sources, de Ko et al.
(1996) detected a narrow feature extending southeast from the core of this
quasar. The direction and morphology of the radio jet is \practically iden-
tical", and so they suggest this is a very strong candidate for an optical jet.
However, a longer exposure image than that taken in the snapshot survey
would be required to obtain better analysis of this jet.
3C 212: This is one of the highest redshift sources known to exhibit
optical counterparts to radio jet structures. With an absolute magnitude
of M V < 24, it is more accurately described as a quasar, rather than a
radio galaxy. It was imaged with HST and Keck by Ridgway and Stockton
(1997). It shows a peculiar morphological structure, with a number of blue
optical components lying along the radio jet axis. It also shows an extended
structure that Ridgway and Stockton speculated could be the optical coun-
terpart to the radio lobe, but was shown (Stockton and Ridgway 1998) to
more likely be a neighbouring galaxy.
3C 245: This, too, is a bright, high redshift quasar (M V < 26), also
imaged by Ridgway and Stockton (1997). These images show a linear feature
that coincides exactly with the radio jet, and since there is good correspon-
dence between the optical and radio structure, it is likely the optical emission
is due to synchrotron radiation.
3C 264: This optical jet was discovered by Crane et al. (1993). The
radio and optical jets show a very good spatial coincidence. There is also a
`ring' of enhanced optical emission at a radius of 300-400 pc from the nucleus,
which Baum et al. (1997) interpret as most likely due to the presence of a
face-on dusty disk. The jet widens dramatically after passing through this
disk and becomes turbulent, changes direction, and fades rapidly. This is
most likely due to decreased pressure in the ambient medium outside the
disk/ring.
Lara et al. (1999) have shown that the optical emission is most likely due
to synchrotron emission (and not Inverse Compton or scattering), but that
likely explanations of re-acceleration have some problems. In particular, well
dened knots are required along the jet, and these are not seen. Also, it is
unclear how the radio-optical spectral index is kept approximately constant
along the jet.
3C 346: Optical emission associated with a radio knot was discovered
(Dey and van Breugel 1994) in this FR II radio galaxy, using ground based

20 Optical jets and their properties
imaging in the optical/ultraviolet. HST snapshot imaging by de Ko et al.
(1996) resolves this knot into a curved jet and a triple hot spot that coincides
perfectly with the radio emission.
3C 371: 3C 371 is classed as a BL Lac object and exhibits a one-sided
radio jet. The optical jet was discovered using the NOT by Nilsson et al.
(1997). The radio and optical morphologies do not exactly coincide, with
the radio jet being several times longer than the optical (Scarpa et al. 1999),
in a manner somewhat reminiscent of the jet of 3C 273, and unlike other
jets such as those of M87 and PKS 0521 365.
3C 380: This object is a radio-loud quasar, classied as a compact steep-
spectrum source by Fanti et al. (1990). The radio jet shows two prominent
knots that are seen in the optical (O'Dea et al. 1999). There is a good
one-to-one correspondence between the optical and radio emission in both
the knots. Due to the large redshift of the source, these knots are the most
luminous known.
PKS 2201+044: The optical jet in this source was discovered (Scarpa
et al. 1999) during the HST snapshot survey of BL Lac objects, and so the
exposure time was less than ideal. However, the jet was able to be resolved,
and the association with the radio jet was established. A nal point to note
is that, although 2201+044 is classied as a BL Lac, it shows broad emission
lines in its spectrum, and so the beaming cannot be very strong (perhaps
due to a large angle to the line of sight).
There are a number of other sources that are possible candidates for
optical jets. De Ko et al. (1996) found a further 8 candidates for optical
synchrotron jet emission, including 3C 20 (the optical counterpart of a hot
spot was detected by Hiltner et al. (1994)), 3C 133 and 3C 410. As these
observations were made with HST in snapshot mode (which involves shorter
exposures than are ideal), longer exposure imaging may be expected to
resolve these jets. In addition to these sources, a number of sources have
been shown to exhibit optical emission associated with a radio hot spot (Dey
and van Breugel 1994; Meisenheimer et al. 1989).
2.2 Observed properties of optical jets
In this section we list and discuss some of the observed properties of the
optical jets, in particular their length and viewing angle, and the spectral

2.2 Observed properties of optical jets 21
indices (both optical and ratio-optical). All of these parameters are im-
portant in constraining the models that will be used for the synchrotron
emission from the Parkes quasars, as well as beaming models for quasars
and BL Lacs (Chapter 7).
2.2.1 Power law indices
A relatively easy parameter of optical jets to measure is the spectral index,
which is dened (for this discussion) as , where F  /   . This is usually
determined on the basis of two points: a radio and an optical observation,
giving the radio{optical slope  ro , or two dierent optical wavelengths for
the optical slope  o . The spectral index gives one an idea of the average
slope of the continuum over the frequency range under consideration, which
allows the emission mechanisms to be constrained.
Since all the optical jets have radio counterparts,  ro gives information
on how the optical emission is connected to the radio emission, and this can
help determine whether the optical emission is synchrotron as well (since
the radio emission is widely accepted to be due to the synchrotron process).
The optical slope can also give information on the emission processes at the
higher optical frequencies, without worrying about the radio emission.
Table 2.2 lists the measured spectral indices for all optical jets as taken
from the literature. At rst glance, several things are immediately apparent.
The average values for the spectral indices are h o i = 1:410:33 and h ro i =
0:79  0:13. Even though there is some scatter in the values, it is clear that
the  o values are steeper than the  ro values.
This is consistent with a synchrotron component that cuts o or turns
over at some maximum frequency. This in fact has been postulated as one
reason for the low number of sources seen with optical jets (as compared
to the number of sources with radio jets), as most of the sources without
optical jets have this cut o at a frequency lower than optical frequencies.
We note that, from Table 2.2, the values of  o , while there are a couple
above 2:0, are mostly clustered around values of 1:3 { 1:4. Is this signicant?
If the optical slopes were much steeper, the jet emission would be harder to
detect, since they will in general be fainter, and so jets with steeper optical
indices will tend to be selected against. On the other hand, a atter optical
slope would not be selected against, and so the absence of at optical slopes
(i.e. slopes that are closer to or the same as the radio{optical slope) appears

22 Optical jets and their properties
Name  o  ro ref
3C 15 1.2 | 8
| 0.95 7
| 0.86 10
3C 66B 1.6 0.80 1
2.4 | 8
3C 78 1.2 | 8
|  0:69 3
| 0.81 10
3C 120 1.3 0.65{0.69 2
| 0.69 10
3C 200 | 0.74 10
PKS 0521 365 2.0 0.76 1
1.4 | 8
| 0.73 7
3C 212 1.4 | 8
| 1.06 10
3C 245 1.2 | 8
| 1.03 10
3C 264 1.4 0.58 1
1.34 0.63 6
1.4 | 8
| 0.85 10
3C 273 1.33 0.90 1
1.3 | 8
| 1.07 10
M87 1.2 0.66 1
0.97 0.65 4
1.2 | 8
| 0.71 10
3C 346 1.8 | 8
| 0.87 10
3C 371 | 0.81 5
| 0.75 9
| 0.74 10
3C 380 1.2 | 8
PKS 2201+044 | 0.85 9
Table 2.2: Table showing spectral indices  (where S /   ). The indices given are the
optical (o) and the radio{optical (ro ). References: (1) Crane et al. (1993); (2) Hjorth
et al. (1995); (3) Sparks et al. (1995); (4) Meisenheimer et al. (1996); (5) Nilsson et al.
(1997); (6) Lara et al. (1999); (7) Martel et al. (1998); (8) O'Dea et al. (1999); (9) Scarpa
et al. (1999); (10) Scarpa and Urry (2001)

2.2 Observed properties of optical jets 23
to be a real one. That is, there are not signicant numbers of jets with
synchrotron spectra extending to UV and X-ray energies.
2.2.2 Jet length and viewing angles
The length of observed optical jets can be a diфcult parameter to measure.
Quite obviously, it depends on the capabilities of the telescope being used, as
the end of the jet needs to be able to be resolved and detected signicantly
above the background. Even when the end of the jet is able to be dened,
the apparent length of the jet is still only the projected length { that is, the
jet is seen projected onto the plane of the sky. For this reason, jet length
and the viewing angle, or the angle that the jet makes to the line of sight,
are tied together.
The length of the jet can provide information on both the power of the
central source (depending on models of how jets are formed and therefore
how they connect to the central black hole), as well as the nature of the inter-
stellar and intergalactic medium. A list has been compiled of jet lengths (in
both arcseconds and kiloparsecs) in Table 2.3. The lengths span nearly two
orders of magnitude, with the powerful quasar 3C 273 having the longest.
However, these lengths are of course projected lengths, so viewing angle
eects have not been accounted for.
Since the jets are seen in projection against the plane of the sky, this
viewing angle is quite a diфcult parameter to measure. As well as being im-
portant for determining the correct lengths of jets, this angle is also impor-
tant for determining the Doppler factor, and hence the amount of relativistic
beaming that is occurring (see Chapter 3 and Chapter 7). The viewing an-
gle, therefore, is very important in understanding the overall energetics of
the jet. Often, determinations of the viewing angle will be coupled with
determinations of the Lorentz factor . This is the \bulk Lorentz factor"
(rather than the Lorentz factor of the individual radiating particles), which
comes from the speed at which the \blobs" or features in the jet are moving.
One determination of the angle (and in this case, ) was made by Scarpa
and Urry (2001). They discussed the energy budget of the known optical
jets, and found that the above parameters lay in a \most probable" region
centered around  7:5 and   20 ф . These values were reached by requir-
ing the jets to transport enough kinetic energy to power an average radio
lobe, since the kinetic power of the jet depends on . The \most probable"

24 Optical jets and their properties
Name Length Length
(arcsec) (kpc)
3C 15 4.2 5.3
3C 66B 8.0 3.2
3C 78 1.5 0.8
3C 120 15.0 24.8
PKS 0521 365 6.5 6.4
3C 200 0.8 3.6
3C 212 2.2  12.5
3C 245 1.6  9.1
3C 264 2.2 0.9
3C 273 23.0 56.2
M87 25.0 2.1
3C 346 3.6 9.0
3C 371 4.5 4.1
3C 380 1.4 8.5
PKS 2201+044 2.1 1.1
Table 2.3: Projected lengths of optical jets, in both arcseconds and kiloparsecs. The
kiloparsec lengths are calculated using H0 = 75 km s 1 Mpc 1 . Note that an  indicates
that the length shown was calculated by the author from published images.
region was found by considering the amplication or de-amplication of the
jet emission due to relativistic beaming (a de-amplied jet is less likely to
be detected), which depends on both and .
An alternative method is that used by Sparks et al. (2000). They imaged
the nearest ve 3CR galaxies that host optical jets, and found evidence for
almost circular dust disks in four of them. They interpreted these disks
as being face-on, so that the optical jets emerge close to perpendicular to
these disks. From this, they nd a critical line of sight angle of 30 ф { 40 ф ,
above which an optical jet is not seen. Taking the beam angular width to
be  1=, this implies a minimum value of  1:4 2.
A somewhat dierent way of obtaining viewing angles for jets has been
the approach of Ghisellini et al. (1993). They used observations of super-
luminal motion of radio jets to obtain an upper limit on the viewing angle.
To do this, the Doppler factor ф needs to be found, and then the angle can
be calculated from the apparent transverse speed  a . The Doppler factor
is calculated by comparing the predicted self-Compton ux at X-rays with
that observed { this gives an expression for ф in terms of the radio and

2.3 X-ray jets 25
X-ray uxes, and the angular size of the radio core. The derived angles,
for those sources with optical jets (only four of them have calculated val-
ues), are typically . 10 ф (although one source, 3C 345, has  = 17 ф ). We do
note, however, that these Doppler factors are derived using non-simultaneous
data, which may lead to erroneous results (particularly for the more variable
sources).
Superluminal motion has also been observed in the optical for one optical
jet { that of M87. Biretta et al. (1999) observed M87 with HST at ve
dierent epochs from 1994 to 1998, and found superluminal motion with
apparent speeds from 4c to 6c. For the fastest features, they nd that the
orientation angles must satisfy  . 19 ф and the Lorentz factors must be
& 6 (with the sense that larger angles require larger Lorentz factors and
produce smaller Doppler factors { an angle of  = 19 ф requires = 40 and
gives ф = 0:5, compared with  = 10 ф requiring = 6 and ф = 5:7 (see
Table 3 of Biretta et al. (1999))).
2.3 X-ray jets
Finally, we will move from the optical region to brie y discuss jets at X-
ray energies. The two best-known optical jets, those of M87 and 3C 273,
have both been detected in X-rays, as has a knot in the jet of 3C 120.
The detection of the M87 jet was rst reported by Schreier et al. (1982),
based on observations with the Einstein satellite, and were further developed
by Biretta et al. (1991). Due to the comparatively poor resolution of this
satellite, the detection of the jet was dominated by the emission from the
core and from the strongest of the emission knots. A similar detection was
made with ROSAT (Neumann et al. 1997). The emission process may be
synchrotron, although it is likely to be due to a dierent population of
electrons to that emitting the radio{optical spectrum. Alternatively, it may
be due to some other X-ray process (such as thermal bremsstrahlung or
inverse Compton scattering) (Biretta et al. 1991; Meisenheimer et al. 1996).
The ROSAT satellite also detected X-ray emission from a radio knot in
3C 120 (Harris et al. 1998). Since this radio knot has not been detected
at optical frequencies (the optical jet is much closer in to the core), it is
unlikely that the X-ray emission is due to synchrotron. However, since
no simple thermal bremsstrahlung or inverse Compton model ts the data,

26 Optical jets and their properties
Harris et al. speculate on the existence of a separate, high-energy population
of electrons that radiate a at-spectrum X-ray component.
The launch last year of the Chandra X-ray Observatory provided a
startling new example of an X-ray jet. The rst target for the observatory
was the radio quasar PKS 0637 752, a point source (so it was believed)
that was to be used to calibrate the telescope optics. However, it was imme-
diately apparent that there was an X-ray jet (Schwartz et al. 2000), which
was coincident with the known radio jet (Tingay et al. 1998). At the source
redshift of z = 0:651, the jet has a length of some 100kpc.
The origin of the X-rays is not certain (Chartas et al. 2000). A single-
component power law synchrotron model does not work, as the optical emis-
sion is several orders of magnitude below the interpolation between the radio
and X-ray data. Other models, such as inverse Compton, are diфcult to t
eectively (Schwartz et al. 2000).
The jet of 3C 273 has also been imaged with Chandra (Marshall et al.
2000). Morphologically, the X-ray emission is strongest at the end of the
jet closest to the core, and then decreases as the jet gets further away. This
is the opposite sense to the radio emission, which is strongest at the head
of the jet. The spectrum of the rst knot in the jet (i.e. the one closest to
the core) is consistent with a single power law extending from radio to X-
ray energies, which may be due to synchrotron emission (although it is also
possible that at least some of the X-ray emission originates from inverse-
Compton scattering of the microwave background by relativistic motion in
the jet). The likelihood of synchrotron emission being responsible for the
X-ray ux decreases the further along the jet you go.