Of all the major radio surveys described in Sect. 2, only FIRST has sufficient positional accuracy and resolution for the optical identification of R > 20 objects. We present FIRST maps of 139 WN and 8 TN sources in Appendices B.2 and B.4.
Outside the area covered by FIRST, we have observed all the remaining WN sources, 30% of the TN sample,
and 71% of the MP sources at 0
3 to 5
resolution using the Very Large Array (VLA;
Napier et al. 1983) and Australia Telescope Compact Array (ATCA; Frater et al.
1992) telescopes. A log of the radio observations is given in Table 4. We observed
targets for our VLA runs on the basis of declination (A-array for
0
and BnA-array for
0
)
and sky coverage of the WN and TN samples, which were still incomplete at the time of
the 1996 observations. We observed all WN, and most TN sources with the VLA, and all MP sources with the
ATCA. We observed TN sources between
with either VLA or ATCA,
depending on the progress of the NVSS at the time of the observations.
We observed all sources in the standard 4.86 GHz C-band with a 50 MHz bandwidth, resulting in a
resolution of 0
3 in the A-array and
1
in the BnA-array. We spent 5 minutes on
each source, implying a theoretical rms level of 75
Jy, or a ratio of total integrated signal over
map noise of 110 for the weakest sources, assuming no spectral curvature beyond 1.4 GHz. We performed
calibration and data editing in
, the Astronomical Image Processing System from NRAO. We used 3C 286
as the primary flux calibrator in all runs. Comparison of the flux density of 3C 48 with the predicted
values indicated the absolute flux density scale was accurate up to 2%. We observed nearby (within
15
)
secondary flux calibrators every 15 to 20 minutes to calibrate the phases. After flagging of
bad data, we spilt the uv-data up into separate data sets for imaging and self-calibration in DIFMAP,
the Caltech difference mapping program (Sheppard et al. 1997). We used field sizes of
164
(A-array) or 256
(BnA-array) with pixel scales of 0
08/pixel (A-array) or
0
25/pixel (BnA-array). Even the smallest field of view is still four times larger than the
resolution of the NVSS, so all components of an unresolved NVSS source will be covered.
We cleaned each source brighter than the 5
level, followed by a phase-only self-calibration. We
repeated the latter for all sources in the field of a source. Next, we made a new model from the
(self-calibrated) uv-data, and subsequently cleaned to the level reached before. The last stage in the
mapping routine was a deep clean with a 1% gain factor over the entire field. Most of the resulting maps
have noise levels in the range 75 to 100
Jy, as expected.
We used the ATCA in the 6C configuration, which has a largest baseline of 6 km. We observed at a central
frequency of 1.384 GHz, which was selected to avoid local interference. We used 21 of the 30 frequency
channels that had high enough signal, which resulted in an effective central frequency of 1.420 GHz, with
a 84 MHz bandwidth. In order to obtain a good uv-coverage, we observed each source eight to ten times
for three minutes, spread in hour angle. The primary flux calibrator was the source 1934-648; we used
secondary flux calibrators within 20
of the sources to calibrate the phases. We performed editing
and calibration in
, following standard procedures. We made maps using the automated
mapping/self-calibration procedure MAPIT in
. The resulting 1.420 GHz maps
(Fig. B.6.) have
noise levels of
mJy.
Of all 343 WN sources, 139 have FIRST maps (Appendix B.2). All remaining 204 sources were observed, and 141 were detected. The remaining 30% were too faint at 4.86 GHz to be detected in 5 min snapshots, because their high frequency spectral index steepens more than expected, or they were over-resolved. Because they are significantly brighter, all the observed 89 TN and 41 MP sources were detected. We present contour maps of all the detections in Appendices B.1, B.3, B.5, and B.6 and list the source parameters in Tables A.1 to A.3.
We have subdivided our sources into 5 morphological classes, using a classification similar to that used by Röttgering et al. (1994). Note that this classification is inevitably a strong function of the resolution, which varies by a factor of 20 between the VLA A-array and the ATCA observations.
We have determined the source parameters by fitting two-dimensional Gaussian profile to all the components of a source. The results are listed in Tables A.1 to A.3 which contain:
WN J0043+4719: The source 18
north of the NVSS position is not detected in the
NVSS. This is therefore not a real USS source because the NVSS flux density was underestimated.
WN J0048+4137: Our VLA map probably doesn't go deep enough to detect all the flux of this source.
WN J0727+3020: The higher resolution FIRST map shows that both components of this object are indeed identified on the POSS, even though the NVSS position is too far off to satisfy our identification criterion.
WN J0717+4611: Optical and near-IR spectroscopy revealed this object as a red quasar at z=1.462 (De Breuck et al. 1998b).
WN J0725+4123: The extended POSS identification suggest this source is located in a galaxy cluster.
WN J0829+3834: The NVSS position of this unresolved source is 7
(
)
from the FIRST position, which itself is only at 2
from the WENSS position.
WN J0850+4830: The difference with the NVSS position indicates that our VLA observations are not deep enough to detect a probable north-eastern component.
WN J0901+6547: This 38
large source is over-resolved in our VLA observations, and
probably even misses flux in the NVSS, and is therefore not a real USS source.
WN J1012+3334: The bend morphology and bright optical sources to the east indicate this object is probably located in a galaxy cluster.
WN J1101+3520: The faint FIRST component 20
north of the brighter Southern
component is not listed in the FIRST catalog, but is within 1
of a faint optical object. This
might be the core of a 70
triple source.
WN J1152+3732: The distorted radio morphology and bright, extended POSS identification suggest this source is located in a galaxy cluster.
WN J1232+4621: This optically identified and diffuse radio source suggest this source is located in a galaxy cluster.
WN J1314+3515: The diffuse radio source appears marginally detected on the POSS.
WN J1329+3046A,B, WN J1330+3037, WN J1332+3009 & WN J1333+3037: The noise in the FIRST image is almost ten times higher than average due to the proximity of the S1400=15 Jy source 3C 286.
WN J1330+5344: The difference with the NVSS position indicates that our VLA observations are not deep enough to detect a probable south-eastern component.
WN J1335+3222: Although the source appears much like the hotspot of a larger source with
the core 90
to the east, no other hotspot is detected in the FIRST within 5
.
WN J1359+7446: The extended POSS identification suggests this source is located in a galaxy cluster.
WN J1440+3707: The equally bright galaxy 30
south of the POSS identification
suggests that this source is located in a galaxy cluster.
WN J1509+5905: The difference with the NVSS position indicates that our VLA observations are not deep enough to detect a probable western component.
WN J1628+3932: This is the well studied galaxy NGC 6166 in the galaxy cluster Abell 2199 (e.g. Zabludoff et al. 1993).
WN J1509+5905: The difference with the NVSS position indicates that our VLA observations are not deep enough to detect a probable west-south-western component.
WN J1821+3601: The source 35
south-west of the NVSS position is not detected in the
NVSS. This is therefore not a real USS source because the NVSS flux density was underestimated.
WN J1832+5354: The source 19
north-east of the NVSS position is not detected in the
NVSS. This is therefore not a real USS source because the NVSS flux density was underestimated.
WN J1852+5711: The extended POSS identification suggests this source is located in a galaxy cluster.
WN J2313+3842: The extended POSS identification suggests this source is located in a galaxy cluster.
TN J0233+2349: This is probably the north-western hotspot of a 35
source, with the
south-eastern component barely detected in our VLA map.
TN J0309-2425: We have classified this source as a 13
double, but the western
component might also be the core of a 45
source, with the other hotspot around
.
TN J0349-1207: The core-dominated structure is reminiscent of the red quasar WN J0717+4611.
TN J0352-0355: This is probably the south-western hotspot of a 30
source.
TN J0837-1053: Given the 10
difference between the positions of the NVSS and
diffuse VLA source, this is probably the northern component of a larger source.
TN J0408-2418: This is the z=2.44 source MRC 0406-244 (McCarthy et al. 1996). The bright object on the POSS is a foreground star to the north-east of the R=22.7galaxy.
TN J0443-1212: Using the higher resolution VLA image, we can identify this radio source with a faint object on the POSS.
TN J2106-2405: This is the z=2.491 source MRC 2104-242 (McCarthy et al. 1996). The identification is an R=22.7 object, not the star to the north-north-west of the NVSS position.
![]() |
Figure 9:
Radio "color-color'' plots for the WN sample. The abscissa is the
![]() |
We have used the CATS database at the Special Astronomical Observatory (Verkhodanov et al. 1997) to search for all published radio measurements of the sources in our samples. In Appendix C, we show the radio spectra for all sources with flux density information for more than two frequencies (the S4860 points from our VLA observations are also included). These figures show that most radio spectra have curved spectra, with flatter spectral indices below our selection frequencies, as has been seen in previous USS studies (see e.g. Röttgering et al. 1994; Blundell et al. 1998).
This low frequency flattening and high frequency steepening is obvious in the radio "color-color
diagrams'' of the WN sample (Fig. 9). The median spectral index at low frequencies ( MHz) is -1.16, while the median
.
At higher frequencies (
MHz), the steepening continues to a median
.
Note that the real
value of the latter is probably even steeper, as 30% of the WN sources were not detected in our 4.86 GHz
VLA observations, and may therefore have even more steepened high-frequency spectral indices.
In Table 5, we give the distribution of the radio structures of the 410 USS sources for which we have good radio-maps. At first sight, all three our samples have basically the same percentage of resolved sources, but the similar value for the MP sample is misleading, as it was observed at much lower resolution.
USS Samples | ||||
Morphology | WN | TN | MP | Combined |
Single | 157 (56 ![]() |
43 (48 ![]() |
23 (56 ![]() |
223 (54 ![]() |
Double | 81 (29 ![]() |
28 (31 ![]() |
16 (39 ![]() |
125 (31 ![]() |
Triple | 22 (8 ![]() |
9 (10 ![]() |
0 (0 ![]() |
31 (8 ![]() |
Multiple | 2 (1 ![]() |
4 (5 ![]() |
0 (0 ![]() |
6 (1 ![]() |
Diffuse | 18 (6 ![]() |
5 (6 ![]() |
2 (5 ![]() |
25 (6 ![]() |
# Observed | 280 | 89 | 41 | 410 |
![]() |
Figure 10:
Median angular size for the flux density limited, spectrally unbiased WSRT samples of Oort
1988 (open triangles), and for our combined USS samples (filled squares). The sources have been
binned in equal number bins, and errors represent the 35% and 65% levels of the distribution. Note that
our USS selection does not affect the value of the median, and that our USS samples also exclude sources
that fall below the break at
![]() |
Our results are different from the USS sample of
Röttgering et al. (1994),
which contains only 18%
unresolved sources at comparable resolution (1
5). To check if this effect is due to the fainter
sources in our sample, we compared our sample with the deep high resolution VLA observations of spectrally
unbiased sources (Oort 1988; Coleman & Condon 1985). The resolution of our
observations is significantly better than the median angular size for
S1400 > 1 mJy sources, allowing
us to accurately determine the median angular sizes in our samples. We find that our USS sources have a
constant median
angular size of
between 10 mJy and 1 Jy (Fig. 10). This
is indistinguishable from the results from samples without spectral index selection. It indicates that our
USS selection of sources with
and
does not bias the angular size
distribution in the resulting sample. The "downturn'' in angular sizes that occurs at
mJy is
probably due to a different radio source population, which consist of lower redshift sources in spiral
galaxies (see e.g., Coleman & Condon 1985; Oort et al. 1987; Benn et al.
1993). By selecting only sources with
S1400 > 10 mJy, we have avoided "contamination'' of
our sample by these foreground sources.
We have searched for further correlations between spectral index or spectral curvatures and angular size or flux density, but found no significant results, except for a trend for more extended sources to have lower than expected 4.86 GHz flux densities, but this effect can be explained by missing flux at large scales in our VLA observations.
We have searched for optical identifications of our USS sources on the digitized POSS-I. We used the
likelihood ratio identification criterion as described by e.g.
de Ruiter et al. (1977).
In short, this
criterion compares the probability that a radio and optical source with a certain positional difference
are really associated with the probability that this positional difference is due to confusion with a
field object (mostly a foreground star), thereby incorporating positional uncertainties in both radio and
optical positions. The ratio of these probabilities is expressed as the
likelihood ratio LR. In the
calculation, we have assumed a density of POSS objects
10-4''-2,
independent of galactic latitude b. We have adopted a likelihood ratio cutoff
= 1.0, slightly lower
than the values used by
de Ruiter et al. (1977) and
Röttgering et al. (1994).
We list sources with LR> 1.0
for our USS samples in Tables A.4 to A.6. We have included four WN sources (WN J0704+6318,
WN J1259+3121, WN J1628+3932 and WN J2313+3842), two TN sources (TN J0510-1838 and TN J1521+0741) and
four MP sources (MP J0003-3556, MP J1921-5431, MP J1943-4030 and MP J2357-3445) as
identifications because both their optical and radio morphologies are diffuse and overlapping, making it
impossible to measure a common radio and optical component, while they are very likely to be associated.
Figure 11 shows the identification fraction of USS sources on the POSS (
).
Because the distributions for the WN and TN are very similar, we have combined both samples to calculate
the identification fraction. Unlike the results for 4C USS
(Tielens et al. 1979;
Blumenthal & Miley 1979),
we do not detect a
decrease of the identification fraction with steepening spectral index
. We interpret the
constant
15% identification fraction from our sample as a combined population of foreground
objects, primarily consisting of clusters (see next section).
Our extremely steep spectral index criterion
would then select only radio galaxies too distant to be detected on the POSS (
).
Using the NASA Extragalactic Database (NED), the SIMBAD database and the W3Browse at the High Energy
Astrophysics Science Archive Research Center, we have searched for known optical and X-ray identifications
of sources in our samples (see Appendices A.7 to A.9). Of the bright optical (
)
identifications, only one source is a known as a K0-star, three (TN J0055+2624, TN J0102-2152, and
TN J1521+0742) are "Relic radio galaxies''
(Komissarov & Gubanov 1994; Giovannini
et al. 1999), while all others are known galaxy clusters.
All optical cluster identifications, except MP J1943-4030, are also detected in the ROSAT All-Sky survey
Bright Source Catalogue (RASS-BSC; Voges et al. 1999). Conversely, of the 23 X-ray sources,
seven are known galaxy clusters, and three known galaxies. The remaining 13 sources are good galaxy
cluster candidates because they either show a clear over-density of galaxies on the POSS (eight sources),
or they have low X-ray count rates (< 0.02 counts s-1), suggesting that these might be more distant
galaxy clusters too faint to be detected on the POSS. We conclude that probably >3% of our USS sources
are associated with galaxy clusters, and that the combined USS + X-ray selection is an efficient (up to
85%) selection technique to find galaxy clusters.
Three of our USS sources (WN J2313+4253, TN J0630-2834 and TN J1136+1551) are previously known pulsars
(Kaplan et al. 1998). It is worth noting that two out of nine sources in our USS samples
with
are known pulsars. Because
Lorimer et al. (1995)
found the median spectral index of pulsars to
be
,
we examined the distribution of
spectral indices as a function of Galactic latitude. In Fig. 12, we plot the percentage of
pulsar candidates as a function of Galactic latitude. The four times higher
density near the Galactic plane strongly suggests that the majority of these
sources are indeed pulsars, which are confined to our Galaxy. A sample of such
sources at
would be an efficient pulsar search method.
We also note that no known quasars are present in our sample. Preliminary results from our optical
spectroscopy campaign
(De Breuck et al. 1998b, 2000) indicate that 10% of our sample are
quasars.
We interpret this lack of previously known quasars as a selection bias in quasar samples against USS
sources.
At
,
all five USS sources with known redshift are HzRGs, indicating a selection of sources
without detections on the POSS strongly increases our chances of finding HzRGs.
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