A SHRINKING COMPACT SYMMETRIC OBJECT: J11584+2450?
S. E. Tremblay,
1
G. B. Taylor,
1,2
J. F. Helmboldt,
3
C. D. Fassnacht,
4
and T. J. Pearson
5
Recei
v
ed 2007 December 5; accepted 2008 May 22
ABSTRACT
We present multifrequency, multiepoch, Very Long Baseline Array (VLBA) observations of J11584+2450. These
observations clearly show this source, previously classified as a core jet, to be a compact symmetric object (CSO).
Comparisons between these new data and data taken over the last 11 years shows the edge-brightened hot spots retreat-
ing toward the core (and slightly to the west) at approximately 0.3
c
. Whether this motion is strictly apparent or actu-
ally physical in nature is discussed, as well as possible explanations, and what implications a physical contraction of
J11584+2450 would have for current CSO models.
Subject headin
g
g
s:
galaxies: active — galaxies: evolution — galaxies: individual (J11584+2450) — galaxies: jets —
galaxies: nuclei — radio continuum: galaxies
1. INTRODUCTION
Compact symmetric objects (CSOs) are now a well-established
class of radio sources loosely defined as sources with emission
on both sides of the core (which itself is not always detected) on
a size scale of 1 kpc or less (Wilkinson et al. 1994). The generally
accepted explanation for the small size of these objects is that they
are young radio sources which could grow into larger FR II ob-
jects (Readhead et al. 1996; O’Dea 1998). Alternately, it has been
proposed that the small size of these structures is due to their
growth being frustrated by a dense environment (van Breugel
et al. 1984; O’Dea 1998).
Due to their rapid growth, age estimates for the emission from
these objects can be obtained kinematically (Owsianik & Conway
1998; Taylor et al. 2000), yielding ages ranging from tens to thou-
sands of years. Less accurate spectroscopic models (Readhead
et al. 1996; Murgia et al. 1999; Gugliucci et al. 2005) place CSOs
at a few thousands of years old. This generally supports the theory
that these are young active galactic nuclei (AGNs) in the early
stages of evolution. However, si
nce the age distribution of cur-
rently known CSOs is heavily weighted on the younger side
(Gugliucci et al. 2005), this indicates that the evolution might
not be straightforward. This distribution should not be taken as
definitive, however, since Gugliucci et al. concede it may be in-
fluenced by selection effects. One theory suggests that CSOs could
be generally short-lived objects with only a small fraction of them
surviving to become larger scale objects, while the remaining
galaxies become permanently radio-quiet (Owsianik & Conway
1998). In a competing theory there exists a cyclic process where
unsuccessful CSOs have multiple op
portunities to grow into larger
objects (O’Dea & Baum 1997). Alternatively, the current distri-
bution could be an artifact of the small statistical sample from
which it is derived combined with selection effects.
The picture presented in the above models might be overly
simplistic. For example, all of these models predict continuous
radial expansionof the lobes, but CSOs such as 1031+567 (Taylor
et al. 2000) have been observed with nonradial motion. Here we
present observations of J11584+2450 (PKS 1155+251, SDSS
J115825.79+245018.0), a galaxy with a redshift of 0
:
20160
0
:
00040 (Zensus et al. 2002) that appears to be contracting to-
ward its core on both sides.
Throughout this discussion, we assume
H
0
¼
73 km s
1
Mpc
1
,
m
¼
0
:
27,
¼
0
:
73, so 1 mas
¼
3
:
213 pc.
2. OBSERVATIONS AND DATA REDUCTION
Multifrequency observations of J11584+2450 were performed
on 2006 September 19 with the Very Long Baseline Array (VLBA).
A summary of these and other observations referred to in this
paper is presented in Table 1. These observations consisted of
four 8 MHz wide intermediate frequencies (IFs) in the C, X, and
U bands with full polarization centered at 4605.5, 4675.5, 4990.5,
5091.5, 8106.0, 8176.0, 8491.0, 8590.0, 14902.5, 14910.5, 15356.5,
and 15364.5 MHz at an aggregate bit rate of 256 Mbps to maxi-
mize (
u
,
v
) coverage and sensitivity. When the data in each band
were combined, the three central frequencies were 4844.7, 8344.7,
and 15137.5 MHz. The integrations were performed in blocks
(
2minutesfor5and8GHz,
7.5 minutes for 15 GHz), and
these blocks were spread out over a 9.5 hr period to maximize
(
u
,
v
) coverage of the source.
Most of the calibration and initial imaging of the new data were
carried out by automated AIPS (Greisen 2003) and DIFMAP
(Shepherd 1997) scripts similar to those used in reducing the
VIPS 5 GHz survey data (Helmboldt et al. 2007; Taylor et al.
2005). To summarize, flagging of bad data and calibration were
performed using the VLBA data calibration pipeline (Sjouwerman
et al. 2005), while imaging was performed using DIFMAP scripts
described in Taylor et al. (2005). Final imaging was performed
manually using the DIFMAP program, with beam sizes of 1
:
906
;
3
:
16 in position angle
6.89
,1
:
195
;
1
:
788 in position angle
3.58
,and0
:
6876
;
0
:
9794 in position angle
2.717
for 5,
8, and 15 GHz, respectively.
3. RESULTS
3.1.
Ima
g
es
Kellermann et al. (2004) observed J11584+2450 (B1155+251)
as part of the VLBA 2 cm Survey. Since these observations were
only at 15 GHz, they typically identified the brightest component
1
Department of Physics and Astronomy, University of New Mexico, Albu-
querque, NM 87131; tremblay@unm.edu, gbtaylor@unm.edu.
2
National Radio Astronomy Observatory, Socorro, NM 87801.
3
Naval Research Laboratory, Code 7213, Washington, DC 20375; joe
.helmboldt@nrl.navy.mil.
4
Department of Physics, University of California, Davis, CA 95616; fassnacht@
solid.physics.ucdavis.edu.
5
Astronomy Department, California Institute of Technology, Pasadena, CA
91125; tjp@astro.caltech.edu.
153
The Astrophysical Journal
, 684:153
Y
159, 2008 September 1
#
2008. The American Astronomical Society. All rights reserved. Printed in U.S.A.
in an image to be the core, and consequently classified this source
as a core jet with the core being the southern, bright component.
Figure 1 shows the 5, 8, and 15 GHz VLBA images made from
the 2006 September observations of J11584+2450. The 15 GHz
map shows the clearest structure, and so is used to label compo-
nents of the source. The 15 GHz image shows a compact unre-
solved component (C) with resolved emission both to the north
(N1) and the south (S1). The southern emission then seems to
have another component that expands out toward the west (W2).
There also exists some weak emission on the western edge of the
image (W1; with a peak flux density of 0.55 mJy beam
1
). S1 is
the brightest component in the image (52.3 mJy beam
1
peak),
and is what was previously identified as the core. In the 8 GHz
image the edges of C, N1, S1, and W2 become indistinguishable,
but these components can still be identified by local peaks within
the image. Interestingly, an eastern spur develops from the south-
ern edge of N1, extending in the opposite direction from the ma-
jority of the diffuse emission. Overall the emission appears more
extended, and W1 has a more significant detection. The 5 GHz
map further smears the interior components together until only
N1 and S1 are clearly visible as local maxima of the map. The spur
mentioned above becomes brighter, and the western emission
stretches out farther toward a very well detected W1 (3.83 mJy
beam
1
peak flux density). There is emission from the eastern spur
toward the south in this image (hereafter the 5 GHz southeastern
clump), which has an integrated flux density of 3.78 mJy.
The geometry between N1, C, and S1 was measured using the
2006 15 GHz data, since those components are most distinguish-
able. The axial ratio, N1/S1
¼
1
:
59, and the angle subtended be-
tween the arms, N1-C-S1, is 166.9
.
We used the VLA in the D configuration to investigate what
appeared to be an extension of the western emission to kiloparsec
scale from the NRAO VLA Sky Survey (NVSS; Condon et al.
1998), but found no indication
of any western emission from
J11584+2450 (Fig. 2) and the previous extension to be a result
of the higher rms (0.45 mJy beam
1
) of the NVSS compared to
our image with rms
¼
0
:
085 mJy beam
1
.
In addition, we acquired a visual image (Fig. 3) from the Sloan
Digital Sky Survey Data Release 5 (SDSS DR5; Adelman-
McCarthy et al. 2007). This shows the source to have a galaxy
4
00
to the southeast, which is uncataloged outside of the SDSS.
Two different algorithms have been used to determine a multi-
color photometric redshift for this object from the SDSS image.
The first algorithm utilized the template fitting method and yields
z
¼
0
:
0008
0
:
0226 (Csabai et al. 2003), while the second al-
gorithm used a neural network method and yields
z
¼
0
:
065
0
:
123 (Oyaizu et al. 2008), placing an upper limit of
z
¼
0
:
19,
which is comparable to J11584+2450’s redshift.
3.2.
Spectral Index Distribution
The 2006 VLBA images at 8 and 15 GHz were matched in
resolutioninordertoobtainaspec
tral index distribution across
the source that was overlaid onto a 5 GHz image to show overall
source structure (Fig. 4). This distribution clearly shows a com-
pact flat-spectrum component (
0
:
276, where
F
/
)
situated between two steeper spectrum lobes (
1
:
03). More
steep-spectrum emission is found to the west of these lobes (
1
:
36 to
1.54), where it then fades below the detection thresh-
old at high frequencies. Looking at the 5 GHz southeastern clump
and using its peak, the spectral index would have to be steeper
than
4.35 for the emission to fall below the rms of the 8 GHz
image. Alternatively, the absence of this feature could be due to
having fewer short spacings at 8 GHz, or it could merely be an
imaging artifact at 5 GHz.
4. DISCUSSION
4.1.
Reclassification of J11584+2450
The compact flat-spectrum component seen in Figure 4 is
compatible with emission from the nucleus, or core, of a galaxy
(Begelman et al. 1984). The steeper spectrum components ex-
tending north and south from the core are accordant with the
spectral signature of jets or hot spots. This overall structure is
clearly consistent with J11584+2450 being a CSO.
4.2.
Component Motions
To characterize intrinsic motions within the source, a multi-
component elliptical Gaussian model was made of the 1999 data
and then applied to the 1995, 2001, and 2006 15 GHz data, vary-
ing the flux and position parameters of each Gaussian component
(Table 2). The extended dual-lobed structure in the 1995 data (but
notably missing from the 1999, 2001, and 2006 images) of what
is now considered the core was modeled using a single compo-
nent, since it is likely this extension is only an artifact.
Each of the four IFs of the 2006 data was then individually
modeled and compared to each other to determine the systematic
error (
sys
) associated with modeling each component. The total
position error (
tot
) was then calculated for individual components
using
2
tot
¼
2
stat
þ
2
sys
,where
stat
is the expected statistical
errorassociatedwithmodelingGaussiancomponentsintwo-
dimensional polar coordinates (Table 3; adapted from one-
dimensional case per Fomalont [1999]). Since all model positions
TABLE
1
VLA and VLBA Observat
ions of J1158+2450
Frequency
(GHz)
Date
Time
(minutes)
BW
(MHz)
Pol.
IFs
Peak
(mJy beam
1
)
rms
(mJy beam
1
)
1.3649
a
......................
2007 Mar 08
24.7
100
4
2
1073.7
0.1
4.8447
b
......................
2006 Sep 19
24.9
32
4
4
193.38
0.11
8.3447
b
......................
2006 Sep 19
26.9
32
4
4
118.38
0.17
8.3541
b
......................
2000 May 06
20.6
64
1
8
133.1
0.2
15.138
b
......................
2006 Sep 19
90.8
32
4
4
52.27
0.11
15.335
b,c
....................
2001 Mar 04
57.0
56
1
1
74.85
0.31
15.335
b,c
....................
1999 May 21
37.6
56
1
1
82.73
0.33
15.350
b,c
....................
1995 Apr 07
44.9
32
1
1
110.0
0.3
a
VLA observation.
b
VLBA observation.
c
These data were taken as part of the VLBA 2 cm Survey (Kellermann et al. 2004).
TREMBLAY ET AL.
154
Vol. 684
are referenced to C,
sys
for the core is accounted for by the other
component uncertainties. Since the calibrated data sets from the
VLBA 2 cm Survey are each averaged to one frequency, the
sys
attained from the 2006 data was applied to them as well. Simi-
larly, a multicomponent Gaussian model was made of the 2000
data and then applied to the 2006 8 GHz data, varying the flux
and position parameters of each Gaussian component (Table 4).
The
sys
values were obtained for each component using the four
IFs as above in the 2006 data, and using the four IFs closest to
those same frequencies in the 2000 data (Table 5).
These models were used to calculate component velocities
relative to the core for each band, which are plotted in Figure 5
with the tail of each vector located at the earliest data position in
the band. The components representing the hot spots at the work-
ing surface of the jets which were modeled (N1 and S1) appear to
have contracted toward the core as well as traveled westward,
and this motion is consistent between the two bands. In addition,
Fig.
1.—VLBA observations from 2006 September of J11584+2450 at fre-
quencies of (
from top to bottom
) 4.84, 8.34, and 15.13 GHz. Contour levels begin
at 0.375 mJy beam
1
and increase by factors of 2
1
=
2
.
Fig.
2.—VLA observations from 2007 March centered on J11584+2450 at
1.3649 GHz in D configuration. The contour levels begin at 0.336 mJy beam
1
and increase by levels of 2. The source shows no significant structure at this fre-
quency and resolution, but does show possible extension to the southeast.
Fig.
3.—This
R
-band SDSS image of J11584+2450 also shows a second
source 4
00
to the southeast (SDSS J115826.16+245014.9). If these sources have
the same redshift, then there is 13 kpc separation between them. In this image,
the magnitude of J11854+2450 is 17
:
68
0
:
01, while the magnitude of SDSS
J115826.16+245014.9 is 21
:
99
0
:
06.
SHRINKING CSO: J11584+2450?
155
No. 1, 2008
the W2 component moves toward the northwest in both bands.
Performing a least-squares fit to solve for the velocities in each
band separately, and then using these independent values to re-
duce the error, yields a radial contraction velocity (normalized to
the speed of light) of 0
:
42
0
:
03
c
for S1 and 0
:
20
0
:
07
c
for
N1. Overlaying the contour maps of different epochs (see Fig. 6
for one example) is also supportive of contraction, since the 1995
contours are interior to the 2006 contours, suggesting that the
component motion is not an artifact of the modeling.
This motion was not detected by the VLBA 2 cm Survey over
the 6 yr interval between 1995 and 2001, since the velocities
involved (
0.03 mas yr
1
) are well within most of their stated
velocity errors for this source (Kellermann et al. 2004). The
total flux density of the source has been decreasing steadily
since at least 1995; the models show that this drop can be at-
tributed to S1 decreasing steadily (40% decrease at 15 GHz over
this 11 yr period), while the other components exhibit small
fluctuations.
4.3.
Apparent Motion Interpretation
Since actual contraction of the source toward the core is some-
thing that has not been previously observed, we first examine the
reasons behind an effect that would merely cause apparent mo-
tion in the system. One possible explanation for seeing the con-
traction of this source is that if hot spots are advancing out away
from the core and expanding and younger hot spots are brighten-
ing due to interactions at the end of the jet, then the models might
not be fit to the same components. The largest problems with this
hypothesis are its lack of explanation of both the western emis-
sion and the western component to the hot spot velocities, which
means these properties require a separate, unrelated explanation
if the contraction is to be explained by hot spot dimming and
advance.
4.4.
Physical Motion Interpretations
Leaving open the possibility that the data represent physical
motions in the system, we include
discussion along those lines.
One interpretation is that we are viewing a projection effect caused
by rotation of the source. While solid-body rotation is an unphys-
ical scenario, it gives us an idea about what fluid rotation would
look like for this system, so we consider it as a first approximation
of rotational motion. The angular velocity of the rotation is de-
pendent on the initial orientation. Assuming the axis of rota-
tion lies in the plane of the sky, and the inclination angle is less
than 45
, since larger values would yield Doppler boosting,
the jets would have a rotational period between just 260 and
1880 yr.
Since the system is a fluid and not a rigid body, it would actu-
ally have differential rotation. The core would therefore be spinning
Fig.
4.—Multifrequency observations of a newly identified CSO (J11584+
2450); 5 GHz contours overlaid on an 8
Y
15 GHz spectral index image. Note the
flat-spectrum core, as well as the symmetric dual-lobed structure in the source.
Also, the emission abruptly bends to the west. This sudden path change, and the
steep-spectrum compact knot at the western edge, are not clearly understood.
TABLE
2
15 GH
z
Gaussian Model Components
Component
Epoch
S
(Jy)
r
(mas)
r
(mas)
(deg)
(deg)
a
(mas)
b
/
a
(deg)
C...........................
1995
0.0167
0.000
0.007
0.00
0.00
0.283
1.00
18.57
1999
0.0331
0.000
0.003
0.00
0.00
0.283
1.00
18.57
2001
0.0170
0.000
0.007
0.00
0.00
0.283
1.00
18.57
2006
0.0120
0.000
0.003
0.00
0.00
0.283
1.00
18.57
S1 .........................
1995
0.1267
3.647
0.014
161.28
0.15
0.272
0.86
50.59
1999
0.0945
3.514
0.014
159.79
0.15
0.272
0.86
50.59
2001
0.0798
3.400
0.014
159.03
0.15
0.272
0.86
50.59
2006
0.0765
3.360
0.014
160.12
0.15
0.272
0.86
50.59
N1.........................
1995
0.0327
5.610
0.052
9.55
1.12
3.02
0.345
29.62
1999
0.0288
5.801
0.049
8.37
0.95
3.02
0.345
29.62
2001
0.0257
5.497
0.045
9.52
1.78
3.02
0.345
29.62
2006
0.0399
5.325
0.038
6.82
0.50
3.02
0.345
29.62
W2........................
1995
0.0637
3.226
0.034
127.77
0.46
2.72
0.32
81.22
1999
0.0541
3.775
0.033
115.06
0.46
2.72
0.32
81.22
2001
0.0369
3.746
0.033
115.65
2.65
2.72
0.32
81.22
2006
0.0630
3.435
0.030
116.83
0.34
2.72
0.32
81.22
Notes.—
Parameters of each Gaussian component of the model brightness distribution are as follows: Component, Gaussian component;
Epoch, year of observation (see Table 1);
S
,fluxdensity;
r
;
r
;;
, polar coordinates (and the associated errors) of the center of the component
relative to the center of component C;
a
, semimajor axis;
b
/
a
, axial ratio; and
, component orientation. All angles are measured from north
through east.
TREMBLAY ET AL.
156
TABLE
3
15 GH
z
Gaussian Model Components from Selected IF
s
Component
Epoch
r
1
(mas)
r
2
(mas)
r
3
(mas)
r
4
(mas)
r
(sys)
(mas)
1
(deg)
2
(deg)
3
i
(deg)
4
(deg)
(sys)
(deg)
S1 ............................
2006
3.368
3.353
3.341
3.377
0.014
159.93
160.10
160.36
160.13
0.15
N1............................
2006
5.276
5.343
5.376
5.318
0.037
7.20
6.19
6.75
7.19
0.41
W2...........................
2006
3.473
3.430
3.390
3.440
0.030
116.45
116.63
117.06
117.30
0.34
Notes.—
Parameters of each Gaussian component of the IF model position distribution are as follows: Component, Gaussian component; Epoch, year of observa-
tion (see Table 1);
r
1
Y
r
4
, radial positions of model components in IFs 1 through 4, respectively;
r
(
sys
)
, the standard deviation in radial position;
1
Y
4
, the polar angular
position of model components in IFs 1 through 4, respectively; and
(
sys
)
, the standard deviation in polar angular position. All angles are measured from north through
east.
TABLE
4
8GH
z
Gaussian Model Components
Component
Epoch
S
(Jy)
r
(mas)
r
(mas)
(deg)
(deg)
a
(mas)
b
/
a
(deg)
C...........................
2000
0.0273
0.000
0.001
0.00
0.00
0.214
1.00
63.74
2006
0.0115
0.000
0.003
0.00
0.00
0.214
1.00
63.74
S1 .........................
2000
0.1581
3.240
0.017
157.29
0.15
0.4875
0.71
23.38
2006
0.1507
3.082
0.022
155.45
0.19
0.4875
0.71
23.38
N1.........................
2000
0.1005
5.597
0.055
6.75
0.31
2.195
0.76
38.51
2006
0.1098
5.508
0.049
5.27
0.10
2.195
0.76
38.51
W2........................
2000
0.1505
3.653
0.028
109.43
0.32
1.57
0.80
40.08
2006
0.1489
3.703
0.011
105.95
0.47
1.57
0.80
40.08
Notes.—
Parameters of each Gaussian component of the model brightness distribution are as follows: Component, Gaussian component;
Epoch, year of observation (see Table 1);
S
, flux density;
r
;
r
;;
, polar coordinates (and the associated errors) of the center of the component
relative to the center of component C;
a
, semimajor axis;
b
/
a
, axial ratio; and
, component orientation. All angles are measured from north
through east.
TABLE
5
8GH
z
Gaussian Model Components from Selected IF
s
Component
Epoch
r
1
(mas)
r
2
(mas)
r
3
(mas)
r
4
(mas)
r
(sys)
(mas)
1
(deg)
2
(deg)
3
i
(deg)
4
(deg)
(sys)
(deg)
S1 ............................
2000
3.225
3.270
3.250
3.237
0.017
157.64
157.48
157.26
157.29
0.15
2006
3.106
3.079
3.051
3.101
0.022
155.60
155.27
155.24
155.67
0.19
N1............................
2000
5.573
5.675
5.535
5.551
0.055
7.31
6.65
7.05
6.54
0.31
2006
5.467
5.501
5.586
5.467
0.049
5.17
5.34
5.19
5.39
0.09
W2...........................
2000
3.607
3.673
3.675
3.642
0.028
109.24
109.98
110.07
109.45
0.32
2006
3.692
3.721
3.696
3.707
0.011
106.46
105.96
105.23
106.29
0.47
Notes.—
Parameters of each Gaussian component of the IF model position distribution are as follows: Component, Gaussian component; Epoch, year of observa-
tion (see Table 1);
r
1
Y
r
4
, radial positions of model components in IFs 1 through 4, respectively;
r
(
sys
)
, the standard deviation in radial position;
1
Y
4
, the polar angular
position of model components in IFs 1 through 4, respectively; and
(
sys
)
, the standard deviation in polar angular position. All angles are measured from north through
east.
even faster than the observed jet components, and we would ex-
pect the core to appear highly variable, since the base of the jet
would frequently be pointed toward us. A second physical inter-
pretation of the motion is precession. The westward component
to the velocities of both jets strongly argues against precession.
If the jets were precessing and thus appearing shorter, they would
move in opposite directions (i.e., one moves east, while the other
moves west) as well as inward. In addition, these hypotheses fail
to explain the older western emission.
Another physical elucidation of the motion is that there exists
some reason for the pressure of the environment to increase, then
this could leave the jets underpressured and lead to contraction of
J11584+2450. Such a pressure change could result from a rela-
tive motion between a clumpy environment and the CSO. The
1.59 N1/S1 axial ratio is also indicative of the jets encountering a
dense environment. If the departure of the ratio from 1 was due
to Doppler boosting, then the brighter hot spot would be farther
from the core (S1 in the case of J11584+2450). This would yield
a ratio smaller than 1; therefore, the ratio is likely due to the jet
running into difficulty as it tunnels through the environment,
causing it to be both shorter and brighter. Both the angle N1-C-S1
and the western deviation of the emission are consistent with
the source moving eastward and being influenced by ram pres-
sure similar to what has been observed in wide-angle tailed
radio sources (e.g., 3C 465; Hardcastle et al. 2005; Sakelliou &
Merrifield 2000), but on a smaller spatial scale. Relative mo-
tion could exist between J11584+2450 and its host galaxy, caus-
ing the interstellar medium to produce ram pressure against the
radio jet. If the companion galaxy to the southeast (Fig. 3) is
at a similar redshift and these two galaxies are members of a
cluster, then J11584+2450 might be moving toward the center
of the gravitational potential. However, this is highly specula-
tive, and more observations are needed to determine the cluster
environment.
5. CONCLUSIONS
After analyzing multifrequency (5, 8, and 15 GHz) VLBA data
from radio source J11584+2450 we reclassify it as a CSO. Fitting
the data with multicomponent Gaussian models and overlaying
images of different epochs on each other not only shows that this
source is not growing at the usual rate of
0.1
c
Y
0.3
c
, but also
that each jet is apparently shrinking in size at
0.3
c
and each is
additionally moving westward at
0.2
c
. Confirmation of other
CSOs having either recessive behavior or nonradial motion like
1031+567 (Taylor et al. 2000) would mean the current models
need to be modified to allow for possible nonlineargrowth periods
during the evolution of AGNs. The prospect of nonlinear growth
for CSOs would further bring into question the validity of kine-
matic ages. Gugliucci et al. (2005) found sevenout of the13CSOs
they dated to be under 500 yr old and commented that the expec-
tation for a steady state population of CSOs would have a uniform
distribution of ages. While this is a small statistical sample, it is
also what one would expect to see if CSOs spend a greater frac-
tion of time as small sources. However, in kinematic observations
of
10 CSOs we have seen contraction in just one source.
Future VLBA observations of this source are planned to follow
the motion of the components and to see whether they continue
to recede toward the core, and over what timescale this occurs.
Lower frequency (1.4 GHz) observations should be carried out
to confirm the existence of the 5 GHz southeastern clump. If
the emission from both jets is actually flowing toward the west,
then lower frequency observations might also show W1 merg-
ing with the diffuse western emission, and could reveal larger
scale structures.
We thank an anonymous referee for constructive suggestions.
The National Radio Astronomy Observatory is a facility of the
National Science Foundation operated under cooperative agree-
ment by Associated Universities, Inc.
Facilities:
VLA, VLBA
Fig.
5.—Relative velocity of model components. Velocity of each Gaussian
model component is plotted (1 mas
¼
0
:
2
c
) with the tail of each vector originat-
ing at the model component’s position at its earliest observation (1995 for 15 GHz
and 2000 for 8 GHz). The model fits are in agreement with the contour overlay
plots in showing this source to be shrinking.
Fig.
6.—Two-epoch overlay of J11584+2450; 15 GHz data from 1995 (
red
contours
) are plotted with the 15 GHz data from 2006 (
blue contours
). This fig-
ure shows contraction of the source toward the core (
cross
) over time. The con-
tour levels begin at 1.10 mJy beam
1
and increase by factors of 2
1
=
2
.
TREMBLAY ET AL.
158
Vol. 684
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SHRINKING CSO: J11584+2450?
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No. 1, 2008