A&A 448, 535-545 (2006)
DOI: 10.1051/0004-6361:20054338
Probing the ionized ISM of the CSS quasar 3C 277.1
I. Observations
W. D. Cotton1 - C. Fanti2,3 - R. Fanti2,3 - G. Bicknell4 - R. E. Spencer5
1 - National Radio Astronomy
Observatory
,
520 Edgemont Road, Charlottesville, VA 22903-2475, USA
2 - Istituto di Radioastronomia - INAF, via P. Gobetti 101, 40129
Bologna, Italy
3 - Dipartimento di Fisica, Universitá di Bologna, via
Irnerio 46, 40126 Bologna, Italy
4 - Australian National University, Mt. Stromlo Observatory,
Cotter Rd., Weston, ACT 2611, Australia
5 - NRAL-Jodrell Bank, University of Manchester, Macclesfield
Cheshire, SK11 9DL, UK
Received 11 October 2005 / Accepted 3 November 2005
Abstract
We present new multi-wavelength high resolution VLA polarimetric images of
the CSS quasar 3C 277.1 and analyze them with older Merlin and HST
[OIII] images.
These observations were made to test the hypothesis that the [OIII]
emission as well as the long wavelength radio opacity arise in shocks
that the quasar jet induces in the ISM.
The multi-wavelength radio polarimetric images allow the study of the
intervening magnetized plasma using Faraday effects.
The HST [OIII] image potentially reveals regions of shock that have
cooled sufficiently to radiate.
We find no compelling evidence that the jet is directly responsible
for the majority of the [OIII] emission or that there is significant
dense shocked material in the immediate vicinity of most of the jet.
There is evidence for jet-induced shocks on the northern edge of the
eastern lobe.
We find a spectral age of the radio source of
yr giving
advance speeds of the lobes of 0.15 c and 0.06 c for the western and
eastern lobes.
The strong Faraday effects appear to be confined within the inner 3 kpc
as does the [OIII] emission.
Key words: polarization -
galaxies: quasars: individual: 3C 277.1 -
galaxies: jets -
galaxies: nuclei -
radio continuum: galaxies
Compact Steep Spectrum (CSS) sources (Kapahi 1981;
Peacock & Wall 1982) are powerful extragalactic radio sources with
sub-galactic apparent linear dimensions and steep spectra
(
).
The angular dimensions of these sources are of the order of a few
arc-seconds and they are related as a class to the more compact
Giga-hertz Peaked Sources (GPS).
Fanti et al. (1995) and Readhead et al. (1996) suggest that GPS and CSS
sources represent part of the growth sequence leading to the large
scale double radio sources.
A detailed review is given by O'Dea (1998).
O'Dea (1998) has shown a tight inverse correlation between the
frequency of the spectral peak in CSS and GPS sources with linear
size.
This correlation could be the result of synchrotron self absorption
opacity (O'Dea 1998).
Alternately, Bicknell et al. (1997) and Bicknell et al. (2003) have
suggested that the opacity could be due to free-free absorption from
shock ionized gas in the ISM and this effect can also reproduce the
spectral peak-linear size relationship.
Observations of emission lines by Gelderman & Whittle (1994) have
shown that CSS sources have a denser than normal for AGNs ionized
component in the ISM and suggest this is the result of
jet-ISM interactions.
This is further supported by the HST observations of Axon et al. (2000)
of a number of CSS sources.
Most of the sources observed had extended [OIII] emission line gas that
appears related to the radio emission.
Further evidence for interaction of CSS jets with interstellar gas comes
from the polarization measurements of Saikia & Gupta (2003) where CSS
appear to be more asymmetric in polarization properties than larger radio
sources. They suggest that the asymmetry is caused by interaction of the
jets with in-falling gas which eventually fuels the central engine.
The effects of interaction asymmetries on the arm-length ratios of CSS
are also discussed in Jeyakumar et al. (2005).
This emission line gas would be expected to provide the free-free
opacity required in the Bicknell et al. (2003) model.
Cotton et al. (2003b) have shown that at low frequencies, the inner 3 kpc of CSS sources are strongly depolarized.
This depolarization is presumably by the same gas that produces the
line emission.
Optical images of line emission are only available with sufficient
resolution from very few of these sources so it is not clear how
strongly the ionized gas is associated with the radio jets.
Axon et al. (2000) only imaged 11 CSS sources so the optical properties
on the size scale of interest here are poorly known.
The Bicknell et al. (2003) shock ionization model predicts that the
thermal gas would be very filamentary with a low spatial filling
factor.
One way to test this hypothesis is through Faraday rotation and
depolarization measurements of the radio source as seen through this
medium.
The denser filaments would have a very high and variable Faraday depth,
and would largely depolarize any radio emission passing through them.
Emission passing through the holes in the Faraday screen could suffer
far less Faraday rotation and depolarization.
Thus, the prediction is that the portion of the radio source seen
through this screen will be largely depolarized, depending on the
blocking factor of the dense Faraday screen, but with only modest (few
100 rad/m2) rotation measure.
The depolarization should also not increase as rapidly with increasing
wavelength as is the case for a more homogeneous screen.
Recent observations of the CSS quasar 3C 138 (Cotton et al. 2003a),
suggest that the polarized emission from a moving component near the
core is largely seen through holes in a dense Faraday screen.
![\begin{figure}
\par\includegraphics[width=8.6cm,clip]{4338fg1a.eps} \includegrap...
....eps} \includegraphics[width=8.4cm,height=5cm,clip]{4338fg1d.eps}
\end{figure}](/articles/aa/full/2006/11/aa4338-05/Timg10.gif) |
Figure 1:
Upper left: Merlin image of 3C 277.1 at 6 cm and 0.06
resolution,
contours are total intensity,
vectors have lengths proportional to the bias corrected polarized
intensity and the orientation of the E-vectors.
Contours are powers of 2 times 200 Jy, negative contours are dashed.
The circle in the lower left corner gives the resolution of the
image.
The image is rotated on the sky by -42 .
Upper right: fractional polarization gray-scale with superposed
total intensity contours.
fractional polarization scale is shown in the wedge on the top.
Contours are powers of 4 times 200 Jy.
Lower: like upper but at 0.12
resolution. |
Open with DEXTER |
![\begin{figure}
\par {\hbox{
\includegraphics[height=9cm,angle=-90]{4338fg2a.eps} \includegraphics[height=9cm,angle=-90]{4338fg2b.eps} }}
\end{figure}](/articles/aa/full/2006/11/aa4338-05/Timg11.gif) |
Figure 2:
Left: VLA image of 3C 277.1 at 3.6 cm and 0.12
resolution, contours are total intensity,
vectors have lengths proportional to the bias corrected polarized
intensity and the orientation of the E-vectors.
Contours are powers of 2 times 100 Jy, negative contours are dashed.
The circle in the lower left corner gives the resolution of the
image.
The image is rotated by -42 .
Right: fractional polarization gray-scale with superposed
total intensity contours.
Fractional polarization scale is shown in the wedge on the top.
Contours are powers of 4 times 100 Jy. |
Open with DEXTER |
The only suitable candidate for such a test with HST observations
is 3C 277.1 (J1252+5634).
This is a 1.8
(8.3 kpc
)
radio source (Lüdke et al. 1998) in a quasar at z=0.32.
In the following we discuss new high resolution VLA+VLBA Pietown
observations of 3C 277.1 at 3.6, 2 and 1.3 cm wavelength and analyze
them together with the Merlin 6 cm data of Lüdke et al. (1998) and the
HST O[III] image of Axon et al. (2000).
In a subsequent paper (Paper II) we will present the results
of detailed modeling of this object.
3C 277.1 was observed using the NRAO VLA array on 7 September 2003 in
the "A'' configuration also using the VLBA antenna at Pietown, NM,
USA.
This gives baselines up to 70 km. in length and a reasonable uv
coverage due to the northerly declination of 3C 277.1;
the J2000 position of the core of 3C 277.1 is RA = 12 52 26.36, Dec = +56 34 19.6.
Observations were made over 12 h, alternating among wavelengths
of 3.6, 2, and 1.3 cm.
All data processing described here was done in the NRAO AIPS package.
The Merlin 6 cm data of Lüdke et al. (1998) were also reanalyzed.
The observations were made during 14 h on 18 April 1995 with a
single 15 MHz band-pass centered at 4.994 GHz in both RCP and LCP.
These data were calibrated as described in Lüdke et al. (1998).
After self calibration, images were made in Stokes I, Q, and U using 0.04
resolution.
For subsequent analysis this image was convolved to 0.06
and 0.12
resolutions.
Polarization amplitudes were bias corrected (AIPS task POLCO) before
forming the fractional polarization.
The Merlin 6 cm results are shown in Fig. 1.
The observations were made using two 50 MHz band-passes centered at 8.4351 and 8.4851 GHz in both RCP and LCP.
The data were obtained and reduced in the standard mode
using J1302+578 as the astrometric and instrumental polarization
calibrator and J1331+305 (3C 286) as the photometric and polarization
angle calibrator.
The assumed orientation of the electric vector of the linear
polarization of J1331+305 was 33
and its flux
density was 5.18 Jy.
After external calibration, several iterations of phase only
self-calibration were followed by a single amplitude and phase self
calibration.
After self-calibration, images were made in Stokes I, Q, and U
using 0.120
resolution.
Polarization amplitudes were bias corrected before
forming the fractional polarization.
The 3.6 cm VLA results are shown in Fig. 2.
The observations were made using two 50 MHz band-passes centered at
14.9649 and 14.9149 GHz in both RCP and LCP.
J1302+578 was used as the astrometric and instrumental polarization
calibrator and J1331+305 as the photometric and polarization
angle calibrator.
The assumed orientation of the electric vector of the linear
polarization of J1331+305 was 33
and its flux
density was 3.42 Jy.
Antenna pointing errors were measured and corrected every 1-1.5 h using measurements of J1302+578 at 3.6 cm.
Observations of J1331+305 were calibrated using a standard model of
the source.
Measurements of 3C 277.1 were immediately preceded and followed by
observations of the astrometric phase calibrator with 12 min
duration observations of 3C 277.1.
A correction was determined for the coherence loss in the phase
reference calibrator due to atmospheric phase fluctuations by
comparing the ratio of the total flux density in a derived image of
J1331+305 with and without self calibration.
After external calibration of 3C 277.1, several iterations of phase only
self-calibration were made.
After self-calibration, images were made in Stokes I, Q, and U using 0.06
resolution. Polarization amplitudes were bias corrected before
forming the fractional polarization.
The 2 cm VLA results are shown in Fig. 3.
![\begin{figure}
\par {\hbox{
\includegraphics[height=9cm,angle=-90]{4338fg3a.eps...
...g3c.eps} \includegraphics[height=9cm,angle=-90]{4338fg3d.eps} }}
\end{figure}](/articles/aa/full/2006/11/aa4338-05/Timg14.gif) |
Figure 3:
Upper: VLA image of 3C 277.1 at 2 cm and 0.06
resolution,
contours are total intensity,
vectors have lengths proportional to the bias corrected polarized
intensity and the orientation of the E-vectors.
Contours are powers of 2 times 100 Jy, negative contours are dashed.
The circle in the lower left corner gives the resolution of the
image.
The image is rotated by -42 .
Upper right: fractional polarization gray-scale with superposed
total intensity contours.
Fractional polarization scale is shown in the wedge on the top.
Contours are powers of 4 times 100 Jy.
Lower: like upper but at 0.12
resolution. |
Open with DEXTER |
![\begin{figure}
\par {\hbox{
\includegraphics[height=9cm,angle=-90]{4338fg4a.eps...
....eps} \includegraphics[height=9cm,angle=-90]{4338fg4d.eps} }}
\par\end{figure}](/articles/aa/full/2006/11/aa4338-05/Timg15.gif) |
Figure 4:
Upper Left: VLA image of 3C 277.1 at 1.3 cm and 0.06
resolution,
contours are total intensity,
vectors have lengths proportional to the bias corrected polarized
intensity and the orientation of the E-vectors.
Contours are powers of 2 times 100 Jy, negative contours are dashed.
The circle in the lower left corner gives the resolution of the
image.
The image is rotated by -42 .
Upper right: fractional polarization gray-scale with superposed
total intensity contours.
Fractional polarization scale is shown in the wedge on the top.
Contours are powers of 4 times 100 Jy.
Lower: like upper but at 0.12
resolution. |
Open with DEXTER |
The observations were made using two 50 MHz band-passes centered at
22.4851 and 22.4351 GHz in both RCP and LCP.
The observations and reduction were done in the same manner as the 2 cm observations.
The assumed orientation of the electric vector of the linear
polarization of J1331+305 was 33
and its flux
density was 2.50 Jy.
After self-calibration, images were made in Stokes I, Q, and U using 0.06
resolution. Polarization amplitudes were bias corrected before
forming the fractional polarization.
The 1.3 cm VLA results are shown in Fig. 4.
Two sets of images were made, at 0.12
for all wavelengths and at
0.06
for 6, 2 and 1.3 cm.
In the latter cases, the 0.06
image was convolved to obtain a
resolution of 0.12
.
The core is strong and well defined in all images and was used for the
fine alignment.
All images were then re-sampled onto the same grid for subsequent
analysis.
The rms off-source noise levels in the images given in Figs. 1-4 are summarized in Table 1.
The total intensity (I) and fractional polarization (FPol) at 0.12
resolution is given in Table 2 for selected positions:
the peak of the east lobe, the core and the peak of the west lobe.
Faraday rotation images were derived using a nonstandard AIPS program,
ROTM, which determined rotation measures by fitting a
law
using a least squares procedure and weights determined from the SNR of
each measurement.
Ambiguities in the rotation measure at each pixel were resolved using
the closely spaced measurements in
to predict the phase
of measurements with larger
.
This fitting procedure was used to derive the rotation measure and the
intrinsic polarization angle in each pixel with sufficient signal.
The data presented here has a good
distribution; the
shorter wavelengths are sufficiently closely spaced that there are no
phase ambiguities and these data easily predict ambiguities at 6 cm.
The derived Faraday rotation image is shown in Fig. 5.
Any Faraday rotation in the vicinity of the source needs to be
corrected by (1+z)2, or 1.74 for 3C 277.1 to correct for the
difference in emitted and observed wavelengths.
The derived intrinsic polarization vectors are shown in Fig. 6.
Since Faraday effects increase with the square of the wavelength,
images at longer wavelengths can be depolarized relative to those at
shorter wavelengths.
The ratios of the fractional polarization at 6, 3.6 and 2 cm are
shown relative to 1.3 cm in Fig. 7.
The derived spectral index between 6 and 1.3 cm is shown in Fig. 8.
The core of 3C 277.1 has a flat spectral index of -0.1, and the lobes
have steep, power law, spectra with indices of -0.8 to -1.0
.
The observed wavelengths of all images reported on here are too short
be effected by free-free absorption for any reasonable value of electron
density.
Table 1:
rms noise in images.
Table 2:
Selected derived values.
![\begin{figure}
\par {\hbox{
\includegraphics[height=9cm,angle=-90]{4338fig5.eps} }}
\end{figure}](/articles/aa/full/2006/11/aa4338-05/Timg20.gif) |
Figure 5:
The gray scale shows the rotation measure image at
0.12
resolution with superposed contours of the VLA 3.6 cm
image.
Rotation measures are in the observer's frame.
Plots of the polarization angle as a function of wavelength squared
are shown with lines representing the fitted rotation measure for a
number of locations.
The estimated errors are either given by the vertical size of the
cross or are smaller.
The bar at the top gives the values of the gray scale in rad m-2.
Contours are powers of 4 times 200 Jy.
The circle in the lower left corner gives the resolution.
The image is rotated by -42 . |
Open with DEXTER |
![\begin{figure}
\par {\hbox{
\includegraphics[height=9cm,angle=-90]{4338fig6.eps} }}
\end{figure}](/articles/aa/full/2006/11/aa4338-05/Timg21.gif) |
Figure 6:
Contours of the 2 cm 0.12
resolution image with
vectors whose length are proportional to the 2 cm
fractional polarization and the derived intrinsic polarization angle.
Contours are powers of 2 times 200 Jy.
The image is rotated by -42 . |
Open with DEXTER |
![\begin{figure}
\par {\hbox{
\includegraphics[height=9cm,angle=-90]{4338fg7a.eps...
...{\hbox{
\includegraphics[height=9cm,angle=-90]{4338fg7c.eps} }}
\end{figure}](/articles/aa/full/2006/11/aa4338-05/Timg22.gif) |
Figure 7:
Top: the gray scale shows the ratio of the
fractional polarization at 6 cm to that at 1.3 cm with superposed
contours of the 2 cm 0.12
resolution image.
Contours are powers of 4 times 100 Jy.
The image is rotated by -42 .
Middle: like top but the ratio of 3.6 to 1.3 cm fractional
polarizations.
Bottom: like top but the ratio of 6 to 1.3 cm fractional
polarizations. |
Open with DEXTER |
![\begin{figure}
\par {\hbox{
\includegraphics[height=9cm,angle=-90]{4338fig8.eps} }}
\end{figure}](/articles/aa/full/2006/11/aa4338-05/Timg23.gif) |
Figure 8:
The gray scale shows the spectral index at 0.12
resolution between 6 and 1.3 cm with superposed contours of the 2 cm
0.12'' resolution image.
Contours are powers of 4 times 100 Jy.
The image is rotated by -42 . |
Open with DEXTER |
The work by Burn (1966) and Tribble (1991) have attempted to
explain the depolarization with wavelength seen in many sources in
terms of a variable Faraday screen within or in front of the source.
This variable screen consists of cells with different Faraday
properties.
The Burn model considers the case where all the cells are the same
size and are either much smaller or larger that the resolution of the
observations.
In the latter case, the result is largely Faraday rotation and only in
the former one is depolarization important.
In the case of unresolved cells and with the assumption that the
rotation measure is randomly distributed with a dispersion
,
the fractional polarization (m) as a function of
is:
The Tribble (1991) model extends the Burn analysis by allowing a
distribution of cell sizes as well as the Faraday rotation of the
cells.
In this case:
where s0 is a characteristic scale of the largest cells and b is
the size of the beam.
However, when
is very large this reduces to:
In this regime is is not possible to separate s0/b and
.
The integrated properties of a complete sample of CSS sources are
compared to these models by Fanti et al. (2004).
In the results given above for 3C 277.1 it is possible to do an
analysis on a pixel-by-pixel basis comparing the model to the
observations.
Unfortunately, for most of our data, the Tribble parameters
s0/b and
are degenerate so we applied the
model of Burn (1966) only.
For each pixel at which fractional polarizations were available at a
minimum of two wavelengths, the two parameter (m and
)
Burn model was fitted.
This fitting was done by a multi-stage direct parameter search
procedure.
The results of this fitting are shown in Fig. 9; data
and model fits for selected positions are shown in plots at the bottom.
The behavior shown in Fig. 9 is relatively
consistent over the western lobe but varies dramatically between lobes and
in the core and across the eastern lobe.
In the western lobe, the derived intrinsic fractional polarization is
typically 10-20% and the Faraday dispersion low,
150 rad/
.
This lobe is only weakly depolarized but the Burn model gives a poor
fit to the fractional polarization data, especially at 3.6 cm.
The fractional polarization at the core is low, of order 1% or less
at all wavelengths (see Fig. 7) hence, a low
Faraday dispersion. This is consistent with the relatively low
rotation measure seen in Fig. 5.
The eastern lobe shows more strongly the expected effects of a Faraday
screen.
The intrinsic fractional polarization varies from 
in the
southern regions of the lobe and drops to below the detection
threshold of a couple percent on the northern edge.
The Faraday dispersion in this lobe is large everywhere, from
600 rad/
near the center to
1500 rad/
near the edges.
This is to be compared to the observed rotation measures of order
-1000 rad/
detected in this lobe in Fig. 5.
Although this lobe is strongly depolarized at longer wavelengths, the
Burn model can adequately represent the observations in contrast to
the western lobe.
The innermost knot in the western jet can be seen in Fig. 9 has an intrinsic degree of polarization, m0,
of 10% which is comparable to typical values in the western lobe.
![\begin{figure}
\par {\hbox{
\includegraphics[height=9cm,angle=-90]{4338fg9a.eps...
...{\hbox{
\includegraphics[height=9cm,angle=-90]{4338fg9b.eps} }}
\end{figure}](/articles/aa/full/2006/11/aa4338-05/Timg29.gif) |
Figure 9:
Results of pixel-by-pixel fitting of Burn (1966)
model at 0.12
resolution.
Top: the gray scale shows the derived fractional polarization at
zero wavelength with superposed contours of the 3.6 cm total intensity
image.
Contours are powers of 4 times 200 Jy; the gray scale values are
indicated by the wedge at the top.
The image is rotated by -42 .
Bottom: the gray scale gives the derived Faraday dispersion ( )
in rad/ .
Shown below is the fractional polarization as a function of
wavelength squared for selected locations represented as pluses (+).
The solid line is the fitted model, errors equal or are less than the
vertical extent of the pluses. |
Open with DEXTER |
A detailed comparison of the radio and the HST O[III]
(Axon et al. 2000) images is given in Fig. 10.
The relative astrometry of the two images is insufficient for this
comparison.
However, the nucleus in this source is relatively prominent so it was
used to align the radio and [OIII] images.
The optical emission is more extended along the axis of the radio jet
than across, suggesting a relationship.
The North-West radio lobe extends beyond the line emission whereas the
South East lobe is closer to the core and appears to be embedded in the
line emitting gas.
However, since it takes some time for the shocks to become radiative,
of order 104 years (Bicknell et al. 2003), it is not clear from this
figure if the North-West jet has emerged from the dense region of the
ISM or the shocks associated with it have not become radiative.
We note that the projected distance of this lobe is larger than the
"magic'' 3 kpc radius seen by Cotton et al. (2003b) at which
depolarization effects dropped in a large sample of GPS/CSS sources.
Bicknell et al. (1997) and Bicknell et al. (2003) have suggested that the
[OIII] emission outside of the nuclear region arises from shocks
produced from the interaction of the jet and clouds of gas in the
ISM.
In 3C 277.1 the [OIII] emission is not confined to the region of the
jet and prominent emission regions are visible to a projected distance of
1.5 kpc.
These regions may be photionized by radiation escaping from other
shocks or by radiation from the nucleus.
They could also be excited by shocks induced by a high-pressured lobe
that is not apparent at these relatively high frequencies.
![\begin{figure}
\par {\hbox{
\includegraphics[width=9cm]{4338fg10.eps} }}
\end{figure}](/articles/aa/full/2006/11/aa4338-05/Timg31.gif) |
Figure 10:
Negative gray scale of the O[III] emission image with
superposed contours of the VLA 3.6 cm image.
The bar at the top gives the values of the gray scale.
Contours are powers of 4 times 200 Jy.
The circle in the lower left corner gives the resolution of the 3.6 cm
image.
The images are rotated by -42 . |
Open with DEXTER |
A study by Labiano et al. (2005) found an [OIII]/H
line ratio of
9 in the nucleus and 5 in the extended regions.
This suggests the gas in the core is photo-ionized and shocked in the
extended emission.
3C 277.1 was included in the study of O'Dea et al. (2002) which found that
the emission from the North-West lobe was 250-300 km s-1 higher than
systemic.
This velocity suggests that this is the receding side of the source.
This is in disagreement with the prominence of the jet in this
direction and the ratio of the arm lengths of the two sides of the
source which both suggest this is the approaching side.
Side-to-side differences in the ISM are likely responsible for these
discrepancies.
The images presented in this paper are all at wavelengths well above
the long wavelength turnover in the integrated sources spectrum so do
not directly bear on the source(s) of the long wavelength opacity.
However, these short wavelength images together with integrated flux
densities from the literature can be used in a fitting procedure to
determine the parameters of simple models of the long wavelength
opacity.
Integrated flux densities of the two lobes and the core were
determined from integrals in the image (AIPS IMEAN). These values as
well as integrated flux densities were then used in a direct parameter
search least squares procedure to determine best fit models of the
components.
The component model included two opacity types, synchrotron and
free-free:
where
is the flux density at frequency
,
is the
reference frequency (for synchrotron opacity, this is the frequency of
the maximum in the spectrum), S0 is the flux density at frequency
,
is the spectral index and f is a free-free
absorption factor.
In all fittings, the East lobe was the one with the lowest frequency
cutoff, either free-free or synchrotron.
This result is counter-intuitive as this component is the more
compact which should result in a higher synchrotron cutoff and is
behind more thermal plasma which should result in a higher free-free
cutoff.
Since the opacity source in the western component is very weakly
constrained and there appears to be little plasma in its direction,
the opacity source fitted in this component was always synchrotron.
The best fit model parameters are given in Table 3
and the fitted spectra shown in Figs. 11 and 12.
Table 3:
Best fit spectral models.
![\begin{figure}
\par {\hbox{
\includegraphics[width=6cm,angle=-90]{4338fg11.eps} }}
\end{figure}](/articles/aa/full/2006/11/aa4338-05/Timg37.gif) |
Figure 11:
Fitted spectral decomposition with only synchrotron opacity for all
components.
Fitted data are shown with vertical error bars.
The solid line is the total model flux density and the dashed lines
are individual component spectra, "C'' = core, "W'' = west lobe, "E'' = east lobe.
The west lobe has its low frequency cutoff around 1 GHz, the core has
a flat spectrum and the east component has a low frequency cutoff
around 0.06 GHz. |
Open with DEXTER |
![\begin{figure}
\par {\hbox{
\includegraphics[width=6cm,angle=-90]{4338fg12.eps} }}
\end{figure}](/articles/aa/full/2006/11/aa4338-05/Timg38.gif) |
Figure 12:
Fitted spectral decomposition with only free-free opacity in the east
lobe and only synchrotron opacity for the other components.
Fitted data are shown with vertical error bars.
The solid line is the total model flux density and the dashed lines
are individual component spectra, "C''=core, "W'' = west lobe, "E'' = east lobe.
The west lobe has its low frequency cutoff around 1 GHz, the core has
a flat spectrum and the east component has a low frequency cutoff
around 0.06 GHz. |
Open with DEXTER |
![\begin{figure}
\par {\hbox{
\includegraphics[width=6cm,angle=-90]{4338fg13.eps} }}\end{figure}](/articles/aa/full/2006/11/aa4338-05/Timg39.gif) |
Figure 13:
Scatter plot of the radio continuum at 3.6 cm versus the [OIII]
emission on a pixel-by-pixel basis of the images shown in Fig. 10 but excluding the nuclear region.
Brightnesses are given in arbitrary units.
The near anti-correlation is shown by the "L'' shape of the
distribution. |
Open with DEXTER |
The regions of strong radio and [OIII] emission shown in Fig. 10 are nearly anti-correlated.
A further examination of this is shown in Fig. 13 which
shows the distribution of radio continuum and line emission on a
pixel-by-pixel basis outside of the nuclear region.
Most of the [OIII] emission arises quite far from the jet and is
unlikely to be directly shock excited by the jet.
Furthermore, there is no evidence in Figs. 10 and 13 that the jet is causing local enhancements of the
[OIII] emission.
Due to the finite time needed for the shocks to cool to the point that
there is significant line emission, and the limited lifetime in this
phase, [OIII] emission would not be expected everywhere along the jet,
but, except for a region near the western edge of the line emitting
region, the [OIII] emission appears to avoid the jet.
Due to potential projection effects, even an association here is
unclear; this [OIII] emission appears to be part of a large scale
"spiral'' apparently unassociated with the jet.
There is another region of [OIII] which is possibly associated with
the jet.
This is near the northern side of the eastern lobe where the northern
extension of the lobe appears to be diverted further to the east, see
Fig. 10.
If jet-induced shocks are at a very high temperature, the cooling
time until there is significant line emission can be quite long and
the absence of [OIII] emission does not rule out such shocks.
However, the plasma in these shocks will still affect radio emission
propagating through them in the forms of free-free absorption and
Faraday rotation if there is also a magnetic field.
The Bicknell et al. (2003) model predicts that the shocked material would be
filamentary.
Large variations in the Faraday depth through different lines of sight
would largely depolarize radio signals propagating through the
filaments and the signals passing between the filaments would be
largely unaffected.
Thus, the expectation is that radio emission passing through such a
shocked region would be weakly polarized, depending on the covering
factor of the filaments, and that the polarized components of the
signal would have relatively low Faraday rotation.
This is an accurate description of what is seen in the nuclear region
of the source; a low polarization at all frequencies and relatively
low Faraday rotation of what remains.
In the nuclear region we interpret our results as indicating a dense
Faraday screen with a large, but less than unity, covering factor.
However, in the lobes the situation is different.
The western lobe has relatively few apparent Faraday effects which is
consistent with its apparent emergence from the line emitting region.
Figure 9 illustrates this with 10-20% intrinsic
polarization and low Faraday dispersion.
The first knot in the western jet has polarization properties similar
to the western lobe; relatively high intrinsic polarization and low
Faraday dispersion and Faraday rotation.
This portion of the jet appears to have very little shocked plasma
around it.
The eastern lobe is strongly depolarized with wavelength and has a
relatively large overall Faraday rotation and Faraday dispersion which
is well described by the Burn model.
The intrinsic polarization, m0 shown in Fig. 9
varies from 
in the south of this lobe to less than a few
percent in the north.
This strong depolarization, even at relatively short wavelength is
the signature we expect from viewing the source through a dense
Faraday screen with large covering factor.
Furthermore, the depolarization becomes stronger toward a clump of
[OIII] emission discussed above.
A closeup view of this region is given in Fig. 14;
the left side shows the decreasing intrinsic polarization toward
regions of higher [OIII] emission.
Higher resolution (0.06
)
1.3 cm results are given on the right
showing that the southern portion of the lobe is far more polarized
than the north.
The Faraday rotation over most of this lobe is relatively constant
near
-1000 rad/m2 except for the northern edge where it jumps to
+2400 rad/m2.
This suggests that the overall Faraday rotation and dispersion seen in
this lobe are from a large scale distribution of magnetized plasma
but there are additional and stronger Faraday effects in the north
which may be due to a dense Faraday screen created by shocks from the
northward expansion of the eastern lobe.
![\begin{figure}
\par {\hbox{
\includegraphics[height=9cm]{4338f14a.eps} \includegraphics[height=9cm]{4338f14b.eps} }}
\end{figure}](/articles/aa/full/2006/11/aa4338-05/Timg41.gif) |
Figure 14:
Left: closeup of east lobe of 3C 277.1 with the intrinsic
polarization (m0 from Burn model fit to 0.12
resolution data) in
negative gray-scale and the HST [OIII] emission as contours.
The decreasing polarization fraction toward the region of [OIII]
emission is readily apparent.
The scale of the intrinsic polarization is given by the wedge at the top.
The [OIII] contour are factors of
and are arbitrary units
The image is rotated by -42 .
Right: same area as left but showing 1.3 cm 0.06
resolution bias corrected polarized intensity as grayscale and total
intensity as contours.
The gray-scale is given in mJy by the wedge at the top;
contours are are factors of
times 0.5 mJy. |
Open with DEXTER |
The spectral decomposition shown in Figs. 11 and 12 is difficult to understand.
If the basic opacity at long wavelengths is either synchrotron or
free-free, the eastern component should have the higher frequency
turnover as it is both the more compact and behind more thermal
material.
The spectral fitting has been assigning the apparent kink in the
spectrum around 1 GHz as the turnover of the western lobe.
An alternate interpretation is that this feature is due to the aging
of the emitting relativistic electrons.
After subtracting the core from the integrated spectrum, it
is well fitted by a continuous injection model with a break frequency
at 4.1 GHz and a possible turn-over at about 50 MHz.
Assuming an equipartition magnetic field of
1 mG, this gives an age
yr and average projected velocities of the heads
of the lobes of 0.15 c and 0.06 c for the western and eastern lobes.
The evidence from the radio source largely points in the direction of the
western jet being the approaching one, these are jet sidedness and arm
length ratio.
The difference in morphology does suggest an intrinsic
difference in the ISM encountered by the two jets.
The eastern jet appears to be broadened near the head whereas the
western one is not.
The much stronger Faraday effects on the eastern side of the source
suggest that this is the receding one of the jet (Laing-Garrington
effect).
The optical evidence of O'Dea et al. (2002) is that [OIII] emission in
the area of the Western lobe was 250-300 km s-1 higher than systemic.
If this gas is physically associated with the jet, this would suggest
that this jet were the receding one.
From the arguments given above, it is unclear if this gas is
associated with the jet.
If the emitting cloud is associated with the jet but were on the far
side of it, the cloud could be accelerated away from us.
It is not possible on the basis of material presented here to
determine the origin of the long wavelength opacity in 3C 277.1.
If the opacity is free-free in shocked material surrounding the jet,
then this material has not cooled sufficiently to begin emitting
in the [OIII] line.
There is thermal plasma in front of the eastern lobe as evidenced by
the Faraday effects seen in the radio emission.
However, free-free opacity and Faraday rotation depend on different
powers of the density, as well as the magnetic field dependence of
Faraday effects, so it is not possible to convert from Faraday effects
to free-free opacity.
There is no evidence suggesting that free-free absorption is
significant in the western lobe.
There appears to be no detailed association of [OIII] emission with
the radio jet.
Much of the optical emission is sufficiently far from the jet that it
is unlikely to arise directly from shocks induced by the passage of
the jet through the ISM.
The finite cooling time for the shock to become radiative may make the
association of shocks and jets using only the optical line emission
more difficult.
Significant Faraday effects are seen in the radio emission from the
jet but there is also little evidence that the Faraday screen comes
from the immediate vicinity of the jet and could be due to passage
of the radiation through the NLR.
The western lobe appears to have emerged from the region of dense
plasma but the eastern lobe is behind a considerable amount of
magnetized plasma.
In addition, the northern edge of the east lobe is strongly
depolarized, even at relatively short wavelengths and with large
variations in the Faraday rotation.
We interpret these effects as possibly being due to an especially
dense plasma screen arising from jet-induced shocks.
Further supporting this interpretation is the presence of higher [OIII]
emission in this area.
Thus, overall, the jet seems to have little interaction with the ISM
except possibly in a limited region of the eastern lobe.
However, we cannot rule out the possibility that some of the
[OIII] emission not clearly associated with the radio emission is
shock excited by an extended, very steep spectrum radio lobe not
detected in the current data.
Interpretation of the integrated radio source spectrum in terms of a
continuous injection model gives a spectral age of
yr and projected velocities of the heads of the lobes of 0.15 c and
0.06 c for the western and eastern lobes.
The source of the opacity causing the long wavelength turnover in the
radio spectrum remains uncertain.
Near the nucleus of the source, the low fractional polarization and
low Faraday rotation suggest that the emission has passed through a
dense Faraday screen with a high covering factor.
Emission within a projected distance of the nucleus of 3 kpc show
strong Faraday effects whereas outside of this distance show much
weaker Faraday effects, in agreement with Cotton et al. (2003b) and
Fanti et al. (2004).
Acknowledgements
The authors would like to thank A. Capetti for providing the HST
O[III] image.
- Axon, D. J.,
Capetti, A., Fanti, R., et al. 2000, AJ, 120, 2284 [NASA ADS] [CrossRef] (In the text)
- Bicknell,
G. V., Dopita, M. A., & O'Dea, C. P. 1997, AJ,
485, 112 [NASA ADS] [CrossRef] (In the text)
- Bicknell,
G. V., Saxton, C. J., & Sutherland, R. S. 2003,
PASA, 20, 102 [NASA ADS] (In the text)
- Burn, B. J.
1966, MNRAS, 133, 67 [NASA ADS] (In the text)
- Cotton,
W. D., Dallacasa, D., Fanti, C., et al. 2003a, A&A,
406, 43 [EDP Sciences] [NASA ADS] [CrossRef] (In the text)
- Cotton,
W. D., Dallacasa, D., Fanti, C., et al. 2003b, PASA, 20,
12 [NASA ADS] (In the text)
- Fanti, C., Fanti,
R., Dallacasa, D., et al. 1995, A&A, 302, 317 [NASA ADS] (In the text)
- Fanti, C.,
Branchesi, M., Cotton, W. D., et al. 2004, A&A, 427,
465 [EDP Sciences] [NASA ADS] [CrossRef] (In the text)
-
Gelderman, R., & Whittle, M. 1994, ApJS, 91, 491 [NASA ADS] [CrossRef] (In the text)
-
Jeyakumar, S., Wiita, P. J., Saikia, D. J., & Hooda,
J. S. 2005, A&A, 432, 823 [EDP Sciences] [NASA ADS] [CrossRef] (In the text)
- Kapahi,
V. K. 1981, A&AS, 43, 381 [NASA ADS] (In the text)
- Labiano, A.,
O'Dea, C. P., Gelderman, R., et al. 2005, A&A, 436,
493 [EDP Sciences] [NASA ADS] [CrossRef] (In the text)
- Lüdke, E.,
Garrington, S. T., Spencer, R. E., et al. 1998,
MNRAS, 299, 467 [NASA ADS] [CrossRef] (In the text)
- O'Dea, C. P.
1998, PASP, 110, 493 [NASA ADS] [CrossRef] (In the text)
- O'Dea, C. P.,
de Vries, W. H., Koekemoer, A. M., et al. 2002,
AJ, 123, 2333 [NASA ADS] [CrossRef] (In the text)
- Peacock,
J. A., & Wall, J. V. 1982, MNRAS, 198, 843 [NASA ADS] (In the text)
- Readhead,
A. C. S., Taylor, G. B., Pearson, T. J., &
Wilkinson, P. N. 1996, ApJ, 460, 634 [NASA ADS] [CrossRef] (In the text)
- Saikia,
D. J., & Gupta, N. 2003, A&A, 405, 499 [EDP Sciences] [NASA ADS] [CrossRef] (In the text)
- Tribble,
P. C. 1991, MNRAS, 250, 726 [NASA ADS] (In the text)
Copyright ESO 2006