5 GHz EVN observations of GPS radio sources

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5 GHz EVN observations of GPS radio sources

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Title: 5 GHz EVN observations of GPS radio sources

1
5 GHz EVN observations of GPS radio sources

Liu Xiang C. Renolds R. Strom
Urumqi Observatory, NAOCAS

2
Properties of GPS sources

Giga-Hz Peaked Spectrum --- GPS
GPS radio sources make up a significant fraction
of the bright radio source sample (10), but
they are not well understood.
The GPS sources are powerful (P_1.4GHz
gt1025W/Hz), compact (lt1kpc),
have convex radio spectra that peak between
about 0.3-10GHz (observer's frame).
Only about 12 of GPS sources show extended radio
emission (gt 1kpc), and it is diffuse and very
faint. Most GPS sources appear to be truly
compact and isolated (O'Dea1998).

3
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4

Given the fact that GPSs are small radio sources,
within AGN, it is quite likely
that their low frequency radio emission will be
absorbed due to either
Synchrotron Self Absorption (SSA) and/or
Free-Free absorption (FFA),
giving rise to a peaked (GPS) radio spectrum. GPS
sources may be the
best objects for studying absorption and
scattering in AGNs,
for example, the HI absorption (Vermeulen et al.
2003), and free-free absorption (Morganti et al.
2004).

5
SSAFFA
Yang, Liu Shen 2005
6
CSO properties

Compact Symmetric Objects (CSOs) make up a class
of radio sources with distinctive radio
morphology.
They are powerful and compact sources with
overall size lt 1kpc, dominated by lobe/jet
emission on
both sides of the central engine, and are thought
to be relatively
free of beaming effects (Wilkinson et al. 1994).
Their small size is
most likely due to their youth (lt104 years) and
not due to a
dense confining medium (Owsianik Conway 1998).
A unification scenario assumes that
CSOs evolve into Medium-size Symmetric Objects
(MSOs, 1-15 kpc),
which, in turn, evolve into Large Symmetric
Objects
(LSOs, gt 15kpc), i.e. large FRII radio sources
(Fanti et al. 1995,
Readhead et al. 1996).

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8
Proper motion and ages
Liu Xiang et al. 2000
9

CSOs are of particular interest in the study of
the physics and
evolution of active galaxies as introduced in our
previous papers
(Liu Xiang et al. 2002, 2005, hereafter paper I,
II). A couple of CSOs
have been found to be very young radio sources
with ages of several
centuries (Fanti 2000, Gugliucci et al. 2005).
How were they
triggered? One suggestion is that CSO host
galaxies have been merging
or interacting, but it is still an open question.
It is possible that
a CSO study might find a difference between their
host galaxies and
other galaxies if we have a large CSO sample.
Secondly, how does a
young radio source evolve? Can we measure
intrinsic
hotspot-advance-speeds of young compact radio
sources? For compact
flat spectrum sources, which are often core-jet,
their jets may be
close to the line of sight and therefore Doppler
boosted. For CSOs,
the viewing angle is usually large, so the
intrinsic two-sided
jet/hotspot velocity can be readily determined. A
large CSO sample would
permit a statistical estimate of the speeds of
hotspots in CSOs.

10
FFA and scattering in CSO
Beaming, SSA and FFA It shows that two models
containing the same beaming parameters but having
quite different absorption mechanisms can fit two
lobes equally well. In one model, only FFA
(free-free absorption) is needed. In the other
model, both the SSA (synchrotron self-absorption)
in two lobes and an extra FFA toward the
southwest lobe are required. Our analysis cannot
unambiguously distinguish between two models.
Although the FFA process is invoked in both
models, the geometry of the absorbing gas could
be quite different. ---Xie, Jiang Shen 2005
Scattering in CSO Liu Yang 2003
11
Search CSOs from GPS sample

Why search for CSOs in a sample of GPS sources? A
detailed explanation is given in paper II. In
brief,
assuming that most of the compact radio sources
are intrinsic
CSOs, by selection they will often show a
one-sided core-jet due
to Doppler boosting. Compact two-sided jets may
be detectable by
searching in a large flat-spectrum sample, as was
done by Taylor
Peck (2003). In a flux-limited complete sample
PRCJ1 (200
sources with S_5GHz gt 0.7 Jy and Dec gt35, Xu et
al.
1995), 10 out of 14 CSOs detected
are GPS sources. It is therefore efficient to
search for CSOs in
GPS samples (see paper I, II).
GPS sources share some common
properties with CSOs as they are both confined
within 1 kpc and
are probably young radio sources (Murgia 2003).

12
GPS samples

There are a few tens of sources in bright GPS
source samples, and a
couple of these sources have never (or poorly)
been imaged with
VLBI. From the lists of bright GPS source samples
(de Vries et
al. 1997, Stanghellini et al. 1998), we have
tried to image the GPS
sources with VLBI. The first and second
observation runs were made at
1.6, 2.3 and 8.4GHz with the EVN (European VLBI
Network) for 22
sources. Multi-frequency VLBI images were
obtained and 5 CSOs and 9
CSO candidates have been found (paper I, II).
In this follow up observation, we aim at imaging
the GPS sources at 5GHz to
confirm the CSO candidates, to find new CSOs from
the bright GPS samples, and
to measure the polarization from GPS sources. Our
search for CSOs from the GPS
samples is complementary to those by the COINS
group (Taylor \ Peck 2003)
which looks for CSOs based on samples consisting
mainly of flat spectrum radio
sources.

13
Polarization

GPS radio sources show very low polarization
(about 0.2 at
5GHz, O'Dea 1998). The low integrated
polarization could be due
to large Faraday depths around the radio source.
It is found that
some GPS galaxies are unpolarized while some GPS
quasars show
relatively higher fractional polarization
(Dallacasa 2004). This
may mean that the galaxies are depolarized more
than the quasars.
Higher frequency observations are required to
determine the
differences between GPS galaxies and quasars in
polarization. The
fractional polarization of Compact Steep Spectrum
(CSS) sources
tends to show depolarization (typically the
degree of polarization
is 1-3 at 5GHz, going up to 6-7 at 8.4GHz),
suggesting
that large Faraday depths are possible (O'Dea
1998). It is often
difficult to measure reliable Rotation Measures
(RMs) in GPS
sources because the polarization is so low that
it is often
difficult to detect polarization at multiple
wavelengths.
Observations at frequencies above the spectral
peak are needed to
determine if GPS sources generally have high RMs.

14
AGN and the EVN
15
VLBI observations

EVN observations at
1.6, 2.3/8.4 GHz, plus MERLIN observed in 1999,
2000, pulished in 2002, AA, Liu Xiang et al.
2.3/8.4 GHz, observed in 2002,
pulished in 2005, AA, Liu Xiang et al.
5 GHz, observed in 2004, wth new 6cm receiver in
Ur!,
accepted in 2005, AA, Liu Xiang et al.

16
5 GHz results
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18
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19
DA193

This radio loud quasar is extremely compact,
4mas, is only resolved with VLBI, and is often
used as a VLBI calibrator. The total intensity
map at 5GHz clearly shows a core and a jet.
The core is very compact with about 70 of the
total flux density,
the jet component is also compact with about 30
of the
total flux density. The jet position angle is
-48\circ in the north-west
direction, roughly consistent with -53\circ in
a VSOP map at 5GHz (Scott
et al. 2004). The distance between the jet
component and the core is 2.1 mas in
our 5GHz VLBI observation, which is greater than
the 0.7 mas in the previous
VSOP image. If the component detected here is the
same as that detected in the
VSOP image, then the increase over about 5.5
years corresponds to an apparent
velocity of 2.4/-0.6c (we use h0.75 here). It
has previously been suggested that the jet has a
superluminal motion of about 2.2/-0.7h-1c
(Lister Marscher 1998).

20
The polarization image shows a weak feature, with
integrated fractional polarization of 1 and chi
149\circ. This fraction at 5GHz is less than
that of about 1.5 at 43GHz (Lister Marscher
1998), the polarization angle is nearly
perpendicular to that at 43GHz. The result
may support the idea that the fractional
polarization in GPS sources decreases as the
frequency decreases (O'Dea 1998). The WSRT data
of DA193 in the VLBI observation has been used to
calibrate the polarization angle in the VLBI
images. The polarization fraction measured in the
WSRT data is 1.3 for DA193.
21
PKS 0554-026

The 5GHz VLBI image shows a compact core, with a
jet or diffuse
emission within 20pc . The total flux seen
in the Westerbork local interferometry data has
been recovered in
this image. The total flux density has changed
from 290 mJy
(observed 1980, Wright Otrupcek, 1990 Parkes
Catalogue) to 171
mJy in this observation, indicating this source
is variable and
probably a core-jet source. The structure at
5GHz is similar to
that found at 2.3/8.4GHz (paper II) in general.
There is no clear
detection of polarized flux.

22
PKS 091411

The 5GHz VLBI image shows a symmetric double
source,
with a tail associated with the eastern component
C'.
From the displacement between the two components
detected in this
5GHz VLBI image, it is clear that they
correspond to components
A' and C' in paper II, meaning that components
B' and D' from
paper II are undetected here. We have tentatively
registered the
single component at 8.4GHz (paper II) as
component A' in the
5GHz image by consideration of its spectrum.
Such an
identification means that the spectral index of
both components A
and C is steeper between 8.4 and 5GHz than
between 5 and 2.3GHz
which is typical of the components in
CSOs above the turnover frequency. Component C'
has a rather
steep spectral index of 1.95 between 2.3 and
5GHz and so could
reasonably be expected to be undetectable at
8.4GHz.
From the steep spectra of the components and its
symmetrical
structure, we classify the source as a CSO. The
total flux density
has changed from 140 mJy (observed 1979, Parkes
Catalogue 1990) to

23
B3 1133432

This is an empty field in the optical. The 5GHz
VLBI image shows a double source. The double
structure exhibits two opposite edge-brightened
hotspot/lobes with
spectral indices of 1.08 and 1.35 between 2.3 and
5GHz for the
northern and southern lobe respectively, also
see. We confirm that the source is a CSO.
Orienti et al (2004) imaged the source with the
VLBA at 5GHz, and
it is similar to in general.
95 of the Effelsberg flux density has been
recovered in our 5GHz image.
No polarization is detected for this source.

24
1333589

It is an empty field in the optical. The GPS
source peaks at
4.9GHz. The 5GHz VLBI image shows a double
structure. The northern component is more
extended
in PA 26\circ than was seen at 2.3/8.4GHz
(paper I) or in
a 5GHz image by Xu et al. (1995, snapshot
observation with MKII
recording in 1.8 MHz bandwidth). The southern
component was
measured to have 325 mJy by Xu et al. (1995), but
only 175 mJy is
seen in our image, so it has decreased by 46 in
13 years while
the northern component has increased 46 from 324
mJy to 472 mJy.
There seems to be a new jet-like extension to the
northern
component, which may be responsible for the
increasing flux
density of the northern component in the past 13
years. The
distance between the two components is 12.8 mas,
the same as in Xu
et al. (1995). The northern component shows an
inverted spectrum
if it is considered as a single component, and
like the southern component it has an
edge-brightened hotspot/lobe and steep inverted
spectrum. We
classify the source as a CSO. The Effelsberg flux
density has been
recovered in the VLBI image. No polarization is
detected in the
5GHz VLBI observation.

25
1404286 (OQ 208)

It shows a similar structure to previous
observations at 5GHz (e.g. Wang et al. 2003).
This is a CSO as
confirmed in paper I. No polarization is detected
in the 5GHz
VLBI observation. It is previously reported that
its linear
polarization is less than 0.2 at 5GHz
(Stanghellini et al.
1998). The source has been found to be a
Compton-thick AGN from
the X-ray (Guainazzi et al. 2004) and radio (Yang
Liu 2005),
indicating a dense confining medium in the
source. The medium may
have depolarized the radio source.

26
1433-040

The source is identified as a galaxy with
m_r22.3 (Stanghellini et al.\
1993). It appeared in the O'Dea (1996) and early
GPS sample lists (Spoelstra
1985). This is the first VLBI image for the
source (Fig.\ref1433-040_ip) it
shows a core-jet appearance. It can be fitted
with two Gaussian components a
central compact one and a weaker but broader one
to the north-east
(PA13.9\circ). It is likely that the more
compact component is the
core.
The spectrum of the source is inverted at about
1GHz with a
spectral index \alpha \sim -0.18. The total
flux density varies
from 200 mJy (observed 1980, Parkes Catalogue
1990) to 246 mJy
(this observation). 88\ of the total flux
density has been
detected in the 5GHz VLBI image. The core-jet
classification is
supported by the positive detection of
polarization in this
source. Integrated polarization of 3.6\ is
detected in the 5GHz
VLBI observation, with an EVPA of 143\circ
(Fig.\ref1433-040_p). The polarization seems
largely to come
from the jet component because the polarized
emission is located

27
1509054

This object has been classified as a Seyfert-1
galaxy with a
redshift of 0.084 (Chavushyan et al. 2001). The
5GHz VLBI image
shows an asymmetric double, which is
similar to the 8.4GHz image (paper I). The
eastern component is
more compact than the western one, and the latter
can be resolved
into three subcomponents. The VCS2 image
at 8.4GHz is similar but missing component C'
(Fomalont et al.
2003). The spectral indices of the eastern and
western component
are -1.8 and 0.67 respectively between 5 and
8.4GHz. From the
compactness and steep rising spectrum, the
eastern component is
likely to be the core of the source. The total
flux density of the
source has increased from 526 mJy (Dallacasa et
al. 2000) to 688
mJy in this observation, suggesting it is
variable and may be a
core-jet source. From its triple-like structure
we still retain
the source as a CSO candidate for future
classification. Weak
polarization is possibly detected at the central
component
in the 5GHz VLBI image -- we use it as an upper
limit, although the EVPA
differs from that of the WSRT result. The source
may be associated

28
1518046

The 5GHz VLBI image shows a classical double,
with two
hotspot/lobes. The identification of these
components as hotspots/lobes is
supported by the spectra, as given in paper I.
The total flux density is stable
from 1.03 Jy (Parkes Catalogue 1990) to 1.06 Jy
in this observation. We
conclude that this is a CSO or MSO (for its size
is nearly 1kpc). We tried to
measure the lobe expansion speed by comparing our
map to an early 5GHz map
(observed in April 1983, Mutel \ Hodges 1985).
We measure an expansion of
2.2/-1.1 mas in 21.5 years, or a hotspot/lobe
proper motion of 0.9/-0.5c
(we use h0.75 here). Although it is only a
2sigma detection of motion, it
may suggest that this MSO has higher hotspot/lobe
proper motion that is typical
of CSOs. This is important because we have
previously had little knowledge
about MSO hotspot velocity. Possible polarization
is detected at the hotspots
in the 5GHz image, we use it as an upper limit
although the EVPA differs from that of the WSRT
result.

29
1751278

The 5GHz VLBI image exhibits asymmetric double
structure. The
northern component is about 10 times brighter
than the southern
one. We derive a spectral index of 0.59
for the strong one, and 0.84 for the weaker one,
between 1.6 and
5GHz. Both have steep spectra, suggesting that
the source is a
double-lobe source rather than a core-jet source
as we discussed
in paper I. We think that component D' in paper
I may not be a
genuine component. The total flux density is
stable within 7
from 280 mJy (Griffith et al. 1990) to 260 mJy in
this
observation. The total flux density at 5GHz is
recovered in the
VLBI image. Possible polarization is detected at
the strong
component in the 5GHz image, which we use as an
upper limit.

30
1824271

The 5GHz VLBI image shows a symmetric
double structure. The spectral indices between
5GHz and 8.4GHz
(paper II) are steeper than 1 for both
components, and are 1.0 and
0.8 between 2.3GHz and 5GHz for components A
and B respectively, indicating the source is a
double-lobe source. We confirm that the source is
a CSO. The total
flux density has slightly changed from 98 mJy
(Griffith et al.
1990) to 111 mJy in this observation. All of the
total flux
density has been detected in the VLBI image. No
polarization is
detected in the 5GHz VLBI and WSRT observations.

31
TXS 2121-014

The 5GHz VLBI image exhibits double lobes and a
weak core-like
component between the lobes. It is
difficult to estimate the spectral index of the
core' since it is
not separated from the western lobe at 2.3GHz
(paper II). The
spectral indices between 2.3GHz and 5GHz are
1.1 and 0.8 for the
eastern and western lobe respectively, we confirm
that the source is a CSO. The
total flux density seems stable from 320 mJy
(Parkes Catalogue
1990) to 327 mJy in this observation. No
polarization is detected
in the 5GHz VLBI observation, and the WSRT value
is similarly low.

32
PKS 2322-040

With a typical GPS spectrum peaked at 1.4 GHz,
the 5GHz VLBI
image exhibits double lobe/hotspots and a weak
jet-like emission
to the northern lobe. It is similar to
that at 2.3GHz (paper II) where a sign of the
jet is also seen
but is not well resolved because of the lower
resolution at
2.3GHz. The total flux density seems stable to
within 10\, from
500 mJy (Parkes Catalogue 1990) to 548 mJy in
this observation.
The spectral indices of components A' and B'
are 0.55 and 1.48
respectively between 2.3 and 5GHz. From the
steep spectra and symmetric
edge-brightened lobe/hotspots, we classify the
source as a CSO.
No polarization is detected in the 5GHz VLBI and
WSRT observations.

33
2323790

This is a galaxy. The NVSS image exhibits
double circular components which are separated by
/-2arcmin.
It is not clear if they are separate sources. The
WENSS map
indicates the source has multiple components. The
5GHz VLBI image
shows a bright component and two jet-like
components, but the
source looks like a double (Fig.\ref2323790_i)
. It is the
first VLBI image for the source and we cannot
give a proper
classification. This is a S5 source, the total
flux density seems
stable at 5GHz from 448 mJy (S5 data, observed
in 1978, K\"uhr et
al. 1981) to 438 mJy in this observation. We
retain this source as
a CSO candidate for further classification. 94
of the total flux
density has been detected in the VLBI image. No
polarization is
detected in either the 5GHz VLBI or the WSRT
observation.

34
Discussion

CSOs are defined as compact radio sources with
relatively steep
spectrum double lobes on the opposite sides of a
core.
Ideally a core component with a spectrum flatter
than the lobes must be identified before sources
can be confirmed as CSOs.
For some CSOs
with a jet axis very close to the plane of sky
the core may be so
weak as to be undetectable, yet a CSO
identification can still be
secured if there are symmetric edge-brightened
lobes (Taylor
Peck 2003).
For example, of the CSOs 0914114, 1133432,
1333589,1518046, 1824271, 2121-014 and
2322-040 in this paper, only the source 2121-014
may show core emission.

35
Discussion

In our sample, the optical counterparts of the 14
sources comprise
2 quasars and 10 galaxies and 2 empty fields.
1 quasar (DA193) and 1 galaxy (1433-040) show
linear polarizations gt1, and 1 quasar (1518046)
and 2 galaxies (1509054 and 1751278) show
possible polarizations of lt0.5, and 9 galaxies
show no
polarization, perhaps supporting the idea that
quasars may have
relatively higher fractional polarization than
galaxies, although
the statistics of this sample are rather small.
The relatively
high polarizations in the core-jet sources may be
the result of
less depolarization in these sources. Further
study of a large
sample is required.