The Astronomical Journal, 146:120 (22pp), 2013 November doi:10.1088/0004-6256/146/5/120
C
2013. The American Astronomical Society. All rights reserved. Printed in the U.S.A.
MOJAVE. X. PARSEC-SCALE JET ORIENTATION VARIATIONS AND SUPERLUMINAL
MOTION IN ACTIVE GALACTIC NUCLEI
M. L. Lister
1
, M. F. Aller
2
, H. D. Aller
2
,D.C.Homan
3
, K. I. Kellermann
4
, Y. Y. Kovalev
5,6
,
A. B. Pushkarev
6,7,8
, J. L. Richards
1
,E.Ros
6,9,10
, and T. Savolainen
6
1
Department of Physics, Purdue University, 525 Northwestern Avenue, West Lafayette, IN 47907, USA; mlister@purdue.edu
2
Department of Astronomy, University of Michigan, 817 Dennison Building, Ann Arbor, MI 48109, USA
3
Department of Physics, Denison University, Granville, OH 43023, USA
4
National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA 22903, USA
5
Astro Space Center of Lebedev Physical Institute, Profsoyuznaya 84/32, 117997 Moscow, Russia
6
Max-Planck-Institut f
¨
ur Radioastronomie, Auf dem H
¨
ugel 69, D-53121 Bonn, Germany
7
Pulkovo Observatory, Pulkovskoe Chaussee 65/1, 196140 St. Petersburg, Russia
8
Crimean Astrophysical Observatory, 98409 Nauchny, Crimea, Ukraine
9
Observatori Astron
`
omic, Universitat de Val
`
encia, Parc Cient
´
ıfic, C. Catedr
´
atico Jos
´
e Beltr
´
an 2, E-46980 Paterna, Val
`
encia, Spain
10
Departament d’Astronomia i Astrof
´
ısica, Universitat de Val
`
encia, C. Dr. Moliner 50, E-46100 Burjassot, Val
`
encia, Spain
Received 2013 June 6; accepted 2013 August 5; published 2013 October 8
ABSTRACT
We describe the parsec-scale kinematics of 200 active galactic nucleus (AGN) jets based on 15 GHz Very Long
Baseline Array (VLBA) data obtained between 1994 August 31 and 2011 May 1. We present new VLBA 15 GHz
images of these and 59 additional AGNs from the MOJAVE and 2 cm Survey programs. Nearly all of the 60 most
heavily observed jets show significant changes in their innermost position angle over a 12–16 yr interval, ranging
from 10
◦
to 150
◦
on the sky, corresponding to intrinsic variations of ∼0.
◦
5to∼2
◦
. The BL Lac jets show smaller
variations than quasars. Roughly half of the heavily observed jets show systematic position angle trends with time,
and 20 show indications of oscillatory behavior. The time spans of the data sets are too short compared to the fitted
periods (5–12 yr), however, to reliably establish periodicity. The rapid changes and large jumps in position angle
seen in many cases suggest that the superluminal AGN jet features occupy only a portion of the entire jet cross
section and may be energized portions of thin instability structures within the jet. We have derived vector proper
motions for 887 moving features in 200 jets having at least five VLBA epochs. For 557 well-sampled features,
there are sufficient data to additionally study possible accelerations. We find that the moving features are generally
non-ballistic, with 70% of the well-sampled features showing either significant accelerations or non-radial motions.
Inward motions are rare (2% of all features), are slow (<0.1 mas yr
−1
), are more prevalent in BL Lac jets, and are
typically found within 1 mas of the unresolved core feature. There is a general trend of increasing apparent speed
with distance down the jet for both radio galaxies and BL Lac objects. In most jets, the speeds of the features cluster
around a characteristic value, yet there is a considerable dispersion in the distribution. Orientation variations within
the jet cannot fully account for the dispersion, implying that the features have a range of Lorentz factor and/or
pattern speed. Very slow pattern speed features are rare, comprising only 4% of the sample, and are more prevalent
in radio galaxy and BL Lac jets. We confirm a previously reported upper envelope to the distribution of speed
versus beamed luminosity for moving jet features. Below 10
26
WHz
−1
there is a fall-off in maximum speed with
decreasing 15 GHz radio luminosity. The general shape of the envelope implies that the most intrinsically powerful
AGN jets have a wide range of Lorentz factors up to ∼40, while intrinsically weak jets are only mildly relativistic.
Key words: BL Lacertae objects: general – galaxies: active – galaxies: jets – quasars: general – radio continuum:
galaxies
Online-only material: color figures, figure sets, machine-readable and VO tables
1. INTRODUCTION
High-resolution multi-epoch radio observations of jetted out-
flows associated with active galactic nuclei (AGNs) have con-
tributed substantially to our understanding of the immediate
environments of supermassive black holes, by providing di-
rect measurements of jet flow kinematics and magnetic field
properties. Since its construction in 1994, the Very Long
Baseline Array (VLBA) has been used to regularly image the
brightest radio-loud AGN jets and study their evolution on
parsec-scales (Kellermann et al. 2004). The VLBA 2 cm Survey
(Kellermann et al. 1998) sampled the jet kinematics of 110
AGNs and was succeeded in 2002 by the MOJAVE program,
which added full polarization imaging and defined a com-
plete northern-sky radio flux-density-limited sample (Lister &
Homan 2005, hereafter Paper I). Kinematic results for 127
MOJAVE jets based on data spanning 1994–2007 were pre-
sented by Lister et al. (2009b, hereafter Paper VI) and Homan
et al. (2009, hereafter Paper VII). They showed that bright jet
features typically exhibit apparent superluminal speeds and ac-
celerated motions. These findings are consistent with the widely
accepted picture of high bulk Lorentz factor jets viewed at an-
gles very close to the line of sight, i.e., blazars. Although blazars
are quite rare in the general AGN parent population, their pre-
dominance in the flux-density-limited MOJAVE sample is a
direct result of Doppler orientation bias (Orr & Browne 1982),
since the observed flux densities of aligned, fast jets are highly
Doppler boosted by relativistic aberration effects.
The MOJAVE program has confirmed an important trend,
first reported by Vermeulen (1995) in the Caltech–Jodrell AGN
1
The Astronomical Journal, 146:120 (22pp), 2013 November Lister et al.
Tab le 1
General Properties of AGNs in the Combined Samples
B1950 Alias 2FGL Assoc. z Ref. Opt. Sample
(1) (2) (3) (4) (5) (6) (7)
0003+380 S4 0003+38 J0006.1+3821 0.229 Schramm et al. (1994)QL
0003−066 NRAO 005 ... 0.3467 Jones et al. (2005)BR
0007+106 III Zw 2 ... 0.0893 Sargent (1970)GR,L
0010+405 4C +40.01 ... 0.256 Thompson et al. (1992)QL
0015−054 PMN J0017−0512 J0017.6−0510 0.226 Shaw et al. (2012) Q G,L
0016+731 S5 0016+73 ... 1.781 Lawrence et al. (1986)QR
0048−097 PKS 0048−09 J0050.6−0929 0.635 Landoni et al. (2012) B G,R
0055+300 NGC 315 ... 0.0165 Huchra et al. (1999)GL
0059+581 TXS 0059+581 J0102.7+5827 0.644 Sowards-Emmerd et al. (2005)QR
0106+013 4C +01.02 J0108.6+0135 2.099 Hewett et al. (1995) Q G,R
0109+224 S2 0109+22 J0112.1+2245 0.265 Shaw et al. (2012) B G,R
0109+351 B2 0109+35 ... 0.450 Hook et al. (1996)QR
0110+318 4C +31.03 J0112.8+3208 0.603 Wills & Wills (1976)QG
0111+021 UGC 00773 ... 0.047 Wills & Wills (1976)BL
0116−219 OC −228 J0118.8−2142 1.165 Wright et al. (1983)QG
Notes. Columns are as follows: (1) B1950 name, (2) other name, (3) 2FGL catalog name, (4) redshift, (5) literature reference for redshift and optical
classification, (6) optical classification, where B = BL Lac, Q = quasar, G = radio galaxy, N = narrow-line Seyfert 1, and U = unidentified, (7) sample
membership, where G = 1FM γ -ray-selected sample, R = MOJAVE 1.5 Jy sample, and L = low-luminosity sample.
a
Known TeV emitter (http://tevcat.uchicago.edu).
(This table is available in its entirety in machine-readable and Virtual Observatory (VO) forms in the online journal. A portion is shown here for guidance
regarding its form and content.)
survey, in which jets with the fastest superluminal speeds all tend
to have high Doppler boosted radio luminosities. To first order,
such a trend might be expected from orientation and Doppler
boosting effects, but an analysis by Cohen et al. (2007) and
Monte Carlo simulations presented in Paper VI indicated that
there is a correlation between intrinsic jet speed and intrinsic (de-
beamed) luminosity present in the population. In the absence of
such a correlation, we would expect to see highly superluminal
jets at much lower boosted radio luminosities.
In order to further investigate these issues, we expanded the
MOJAVE program in 2006 to include regular VLBA imaging of
additional low-luminosity AGN jets. In 2009, we expanded the
sample again to encompass new γ -ray loud blazar jets detected
by Fermi (Lister et al. 2011).
We present new VLBA 15 GHz images of the original
135 source MOJAVE flux-density-limited sample obtained be-
tween 2007 September 6 and 2011 May 1. We also present
VLBA images of 124 additional AGNs from three new AGN
jet samples, based on 15 GHz VLBA data obtained between
1994 August 31 and 2011 May 1 from the NRAO archive and
the MOJAVE program (Lister et al. 2009a, hereafter Paper V).
These include a complete radio-selected sample above 1.5 Jy,
a complete γ -ray-selected sample, and a representative
low-luminosity AGN jet sample. We use these data to present
an updated kinematics analysis of the 135 jets in the original
MOJAVE flux-density-limited sample and first ever kinematics
analyses of 65 jets for which we have obtained at least five
VLBA epochs.
The overall layout of the paper is as follows. In Sections 2
and 3 we discuss the samples and observational data, respec-
tively. In Section 4 we describe our method of modeling the
individual jet features and their kinematic properties. We dis-
cuss overall trends in the data in Section 5 and summarize our
findings in Section 6. We adopt a cosmology with Ω
m
= 0.27,
Ω
Λ
= 0.73, and H
0
= 71 km s
−1
Mpc
−1
. We refer to the
radio sources throughout using either B1950 nomenclature or
commonly used aliases, which we list in Table 1.
2. AGN SAMPLE DEFINITIONS
2.1. Radio-selected MOJAVE 1.5 Jy Sample
Unlike blazar surveys in the optical or soft X-ray regimes,
the radio emission from the brightest radio-loud blazars is not
substantially obscured by or blended with emission from the host
galaxy. A VLBA-selected sample thus provides a very “clean”
blazar sample, namely, one selected on the basis of (beamed
synchrotron) jet emission.
In Paper V we described the original radio-selected MOJAVE
sample of 135 AGNs, which was based on the 15 GHz VLBA
flux density exceeding 1.5 Jy (2 Jy for declination <0
◦
)at
any epoch during the period 1994.0–2004.0. The sky region
was limited to declination −20
◦
and the Galactic plane
region |b| < 10
◦
. In order to encompass a broader range of
Fermi-detected AGNs, particularly those that recently entered
an active state, we updated our radio-selection criteria in 2011
to form the complete MOJAVE 1.5 Jy sample. The latter now
consists of all known non-gravitationally lensed AGNs with
J2000 declination > − 30
◦
(no Galactic plane restriction) and
VLBA flux density S
15 GHz
> 1.5 Jy at any epoch between
1994.0 and 2010.0. The new list results in a larger overlap
with the Fermi AGN catalog (Ackermann et al. 2011) and
simplifies the determination of luminosity functions (e.g., Cara
& Lister 2008), which are useful for studies of the extragalactic
background light and blazar parent populations.
The overall properties of the sample are summarized in
Table 1, where the “R” notation in Column (7) indicates
1.5 Jy radio sample membership. There are 183 AGNs in total
(see Table 2), with the sample being heavily dominated by
flat-spectrum radio quasars (78%) and BL Lac objects (16%).
The optical classifications are 98% complete, with redshifts
available for 96% of the sample.
2.2. γ -ray-selected 1FM Sample
The continuous all-sky coverage of the LAT instrument
on board the Fermi satellite has significantly improved our
2
The Astronomical Journal, 146:120 (22pp), 2013 November Lister et al.
Tab le 2
Optical Classification Summary of AGN Samples
1.5 Jy Radio 1FM γ -ray Low-luminosity
Quasars 142 72 7
BL Lac objects 29 42 20
Radio galaxies 8 1 16
Narrow-line Seyfert 1s 0 1 0
Unidentified 4 0 0
Total 183 116 43
Note. Some AGNs belong to two or more of the samples listed above (see
Figure 1).
knowledge of the blazar population at γ -ray energies above
100 MeV by identifying nearly 1000 AGNs associated with
γ -ray sources (Ackermann et al. 2011). For the joint LAT
team–MOJAVE study of Lister et al. (2011), we constructed
a γ -ray sample based on the initial 11 month First Fermi AGN
catalog (Abdo et al. 2010). The specific criteria were: (1) average
integrated >0.1 GeV energy flux 3 × 10
−11
erg cm
−2
s
−2
between 2008 August 4 and 2009 July 5, (2) J2000 declination
>30
◦
, (3) galactic latitude |b| > 10
◦
, and (4) source not
associated with a gravitational lens. The sample is complete
with respect to γ -ray flux, with the exception of two γ -ray
sources (1FGL J1653.6−0158 and 1FGL J2339.7−0531) that
were dropped since they had no unambiguous radio counterpart.
The overall properties of the sample are described by Lister
et al. (2011) and summarized in Table 1, where the “G” notation
in Column (7) indicates γ -ray sample membership. There are
116 AGNs in total (Table 2), 56 of which are in common with
the MOJAVE 1.5 Jy sample. Like our radio-selected sample, it
is heavily dominated by blazars, but contains a larger fraction of
BL Lac objects (36%). The remainder of the sample are quasars,
with the exception of the nearby radio galaxy NGC 1275 (3C
84) and the narrow-line Seyfert 1 galaxy PMN J0948+0022.
2.3. Low-luminosity Compact AGN Sample
In 2006 we expanded the MOJAVE program to include reg-
ular VLBA observations of 16 AGNs with VLBA 15 GHz
luminosities below <10
26
WHz
−1
. These were chosen from
the VLBA Calibrator Survey (Beasley et al. 2002; Fomalont
et al. 2003; Petrov et al. 2005, 2006; Kovalev et al. 2007;
Petrov et al. 2008), based on the following criteria: (1) 8 GHz
VLBA flux density greater than 0.35 Jy, (2) z 0.3, and (3)
J2000 declination > − 30
◦
. By adding the AGN already in the
MOJAVE program that met these criteria, we obtained a final
sample of 43 low-luminosity compact AGNs, as indicated by
the “L” notation in Column (7) of Table 1. Although the latter
would not typically be considered as low-luminosity among the
general radio-loud AGN population, we will refer to them as the
“low-luminosity” sample, in comparison with the typically
high-luminosity blazars in our radio- and γ -ray-selected sam-
ples. Due to the lack of redshift information for the full VLBA
Calibrator Survey, our low-luminosity sample is not complete.
However, it is a useful representative set for examining the
kinematics of weaker jets and in this respect complements other
small-sample low-luminosity AGN very long baseline interfer-
ometry (VLBI) monitoring programs (e.g., Giovannini et al.
2001; Piner et al. 2010).
These three samples, comprising 259 AGNs in total, provide
a broad cross section of AGN types among bright, compact radio
sources. The overlap among the samples is shown in Figure 1.
Figure 1. Area-proportional Venn diagram with labels indicating the total
number of AGNs in each subset of the MOJAVE 1.5 Jy, 1FM γ -ray-selected,
and low-luminosity AGN samples.
(A color version of this figure is available in the online journal.)
Since only 200 of these sources had at least five VLBA epochs
as of 2011 May, the kinematic data are incomplete. In particular,
the AGNs with missing data tend to be among the weaker radio-
and γ -ray-selected AGNs that were added later in the MOJAVE
survey. A statistical inter-comparison of these samples will be
presented in future papers in this series once a full, unbiased
data set has been collected.
3. VLBA OBSERVATIONS AND DATA REDUCTION
In Paper V, we presented 15 GHz VLBA images of the
135 AGNs in the original MOJAVE flux-density-limited sample
based on data from the MOJAVE programs spanning 1994
August 31 to 2007 September 9, the VLBA 2m Survey, and
the NRAO archive.
11
In Figure 2 we show naturally weighted
15 GHz VLBA images derived from newly acquired VLBA
data on these AGNs up to 2011 May 1, as well as VLBA data
from 1994 August 31 to 2011 May 1 on 124 AGNs in our new
samples.
The multi-epoch observations for each AGN, along with
the corresponding image parameters, are listed in Table 3.
Column 3 gives the VLBA project code for each observation,
along with an indicator as to whether it is from the MOJAVE
program, the VLBA 2 cm Survey, or the NRAO archive. For the
latter, we considered only archival data with at least four scans
spanning a range of hour angle, and which included eight or
more VLBA antennas. The VLBA 2 cm Survey observations
(1994–2002) analyzed by Kellermann et al. (2004) consist
of approximately 1 hr integrations on each AGN, broken up
into approximately 6–8 minute scans separated in hour angle
to improve the interferometric coverage. A similar observing
method and integration times were used in the full polarization
MOJAVE observations from 2002 May to 2007 September
(VLBA codes BL111, BL123, BL137, and BL149; see Table 3)
and are described in Paper I. During 2006 (VLBA code BL137),
the 15 GHz integration times were shortened by a factor of
∼3 to accommodate interleaved scans at three other observing
frequencies (8.1, 8.4, and 12.1 GHz). The latter data were
presented by Hovatta et al. (2012) and Pushkarev et al. (2012).
The MOJAVE and 2 cm Survey observations were recorded at
11
http://archive.nrao.edu/
3
The Astronomical Journal, 146:120 (22pp), 2013 November Lister et al.
Figure 2. Naturally weighted 15 GHz total intensity VLBA contour images of individual epoch observations of the MOJAVE AGN sample. The contours are in
successive powers of
√
2 times the base contour level, as listed in Table 3 and at the top of each panel. Because of self-calibration, in some cases the origin may be
coincident with the brightest feature in the image, rather than the putative core feature listed in Table 4.
(The complete figure set (1753 images) is available in the online journal.)
4
The Astronomical Journal, 146:120 (22pp), 2013 November Lister et al.
Tab le 3
Summary of 15 GHz Image Parameters
VLBA Freq. B
maj
B
min
B
pa
I
tot
rms I
base
Fig.
Source Epoch Code (GHz) (mas) (mas) (
◦
) (Jy) (mJy bm
−1
) (mJy bm
−1
)Num.
(1) (2) (3) (4) (5) (6) (7) (8) (9) (10)
0003+380 2006 Mar 9 BL137B
a
15.4 1.01 0.73 18 0.649 0.4 1.3 2.1
2006 Dec 1 BL137L
a
15.4 0.85 0.58 −17 0.511 0.4 1.2 2.2
2007 Mar 28 BL137P
a
15.4 0.86 0.61 −15 0.602 0.3 1.0 2.3
2007 Aug 24 BL149AM
a
15.4 0.92 0.58 −28 0.554 0.3 0.8 2.4
2008 May 1 BL149AO
a
15.4 0.82 0.57 −9 0.806 0.2 0.7 2.5
2008 Jul 17 BL149AK
a
15.4 0.84 0.55 −12 0.725 0.2 0.6 2.6
2009 Mar 25 BL149BJ
a
15.4 0.84 0.62 −12 0.435 0.2 0.5 2.7
2010 Jul 12 BL149CL
a
15.4 0.89 0.54 −12 0.438 0.2 0.5 2.8
0003−066 2008 Jul 30 BL149AL
a
15.4 1.38 0.53 −8 1.951 0.2 0.7 2.9
2009 May 2 BL149BK
a
15.4 1.21 0.48 −7 2.513 0.2 0.6 2.10
2009 Oct 27 BL149CC
a
15.4 1.71 0.56 −13 2.141 0.2 1.0 2.11
2010 Aug 6 BL149CM
a
15.4 1.33 0.59 1 2.143 0.2 0.6 2.12
2010 Nov 29 BL149CY
a
15.4 1.48 0.53 −8 2.055 0.2 0.5 2.13
0007+106 2008 Aug 25 BL149BB
a
15.4 1.17 0.49 −11 0.511 0.3 0.8 2.14
Notes. Columns are as follows: (1) B1950 name, (2) date of VLBA observation, (3) VLBA experiment code, (4) observing frequency (GHz), (5) FWHM
major axis of restoring beam (milliarcseconds), (6) FWHM minor axis of restoring beam (milliarcseconds), (7) position angle of major axis of restoring beam
(degrees), (8) total I flux density (Jy), (9) rms noise level of image (mJy per beam), (10) lowest I contour (mJy per beam), and (11) figure number.
a
Full polarization MOJAVE epoch.
b
2 cm VLBA Survey epoch.
(This table is available in its entirety in machine-readable and Virtual Observatory (VO) forms in the online journal. A portion is shown here for guidance
regarding its form and content.)
a data rate of 128 Mbps, which was increased to 256 Mbps in
the epochs from 2007 July 3 to 2008 September 12 inclusive,
and 512 Mbps thereafter. Beginning with the 2007 January 6
epoch, we increased the number of AGNs observed in each
24 hr MOJAVE session from 18 to 25 AGNs to accommodate
our expanded monitoring sample described in Section 2.On
2009 February 25 we increased this further to 30 AGNs per
session.
4. DATA ANALYSIS
4.1. Gaussian Model Fitting
As in Paper VI, we modeled the (u, v) visibility data at all
AGN epochs using a series of Gaussian components in the
Difmap software package (Shepherd 1997). In the majority
of cases, we used circular Gaussians for jet features and
occasionally (when necessary) elliptical Gaussians for the core
feature. The latter was typically the brightest feature at the
extreme end of a one-sided jet in most sources (see the Appendix
and Paper VI for a discussion of core identifications and
two-sided jets in the sample). The parameters of the Gaussian
fits are listed in Table 4. In some instances, it was not possible
to robustly cross-identify the same components in a jet from
one epoch to the next. Those components with robust cross-
identifications over at least five epochs for the purpose of
kinematics analysis are indicated in Column 10 of Table 4.
For the non-robust components, we note that the assignment of
the same identification number across epochs in Table 4 does
not necessarily indicate a reliable cross-identification.
We estimate errors on the component sizes to be roughly
twice the positional error, according to Fomalont (1999). The
errors on the peak flux density values are approximately 5%
(see Appendix A of Homan et al. 2002). Based on our previous
analysis from Paper VI, we estimate the typical uncertainties in
the Gaussian centroid positions to be ∼1/5 of the FWHM beam
dimensions. For isolated bright and compact components the
positional errors are smaller by approximately a factor of 2. A
more quantitative estimate for individual components can be ob-
tained using the scatter of our kinematic fit residuals (Columns
14 and 15 of Table 5). These residuals represent only estimates of
the uncertainty of the fits and are likely underestimates in some
cases due to possible errors in component cross-identification
and/or a low number of epochs. Small variations in the apparent
core position, due to changes in opacity and/or newly emerging
features, can also contribute to the positional errors. Deviations
from linear or simple accelerated motion can also increase the
magnitude of the fit residuals (see Section 5.5).
In Paper VI there were eight jets that had no robust jet
components. After our analysis using the new data, six re-
mained in this category: 0235+164, 0727−115, 1124−186,
1324+224, 1739+522, and 1741−038. In the case of 0109+224
and 0742+103, we did not consider any components to be robust
in Paper VI, due to gaps in temporal coverage. We have subse-
quently obtained several closely spaced VLBA epochs and now
consider several slow-moving components in these two jets to
be robust.
In Paper VI we listed 0048−097 and 1958−179 as having
one robust component each. However, after re-examining the
original model fits along with the new data, we have determined
that these two jets are too compact at 15 GHz to classify any of
their components as robust. Of the new AGNs not in Paper VI,
there are six with no robust components: 0716+332, 0946+006,
1921−293, 1959+650, 2023+335, and 2247−283.
4.2. Jet Kinematics Analysis
We performed two sets of kinematics analyses on the ro-
bust Gaussian jet components in our sample. The first assumed
a simple non-accelerating, two-dimensional vector fit to the
component position over time, referenced to the core com-
ponent (which we presumed to be stationary). For the com-
ponents that had measurements at 10 or more epochs, we
also performed a constant acceleration fit (as described in
5
