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1980MNRAS.191..607Porcas+
see also Radio sources selected at 966 MHz I.
Radio positions and optical IDs for sources selected at 966 MHz II
R.W. Porcas, C.M. Urry
National Radio Astronomy Observatory, Edgemont Road, Charlottesville,
Virginia 22901, USA
I.W.A. Browne, A.M. Cohen, E.J. Daintrce & D. Walsh
University of Manchester, Nuffield Radio Astronomy Laboratories,
Jodrell Bank, Macclesfield, Cheshire. SK11 9DL
SUMMARY
In a previous paper, accurate radio position measurements and optical
identifications were published for 538 sources selected from a survey made
at 966 MHz. The remaining 242 sources in the sample were either heavily
resolved or confused in these observations and no accurate radio positions
could be obtained. Here we present radio position measurements of those
remaining sources, made at 2.7 and 5.0 GHz using the 300-ft telescope
at NRAO. The positions obtained have rms errors of about 6" in
right ascension and 11" in declination. A few of the sources have
also been observed at 2.7 GHz with the NRAO three-element interferometer,
giving positions accurate to 1" rms.
These new positions have been used in a search for optical identifications on
the Palomar Sky Survey. Optical positions (with rms errors of
0.5") have been measured for candidate identifications. 135 identifications
are proposed, and the reliability of these is estimated at about 80 per cent.
1. INTRODUCTION
A study is in progress of radio sources from a 966-MHz survey made
at Jodrell Bank. The MKIa radio telescope (diameter 76m) was used to
find sources stronger than 0.7 Jy at 966 MHz, between declination +40^o
and +71^o . The 780 of these sources which lie more than 10^o from the
galactic plane form the basis of this study. In a previous paper (Cohen
et al. 1977; hereafter Paper I), we presented radio positions and optical
identifications for 538 of the sources, based on measurements made at
966 MHz with the Jodrell Bank MKIA-MKII interferometer. These sources
have relatively compact radio structures (less than about 1') and
positions accurate to about 2" rms were obtained. However,
the observations were insufficient for determining positions of large
sources or of those strongly confused by other sources.
In this paper we present positions for the remaining sources in the
sample. Most of these positions are derived from new measurements made
with the NRAO 300-ft transit telescope at Greenbank, West Virginia, at
frequencies of 5.0 and 2.7 GHz. For a few of the sources we have
obtained positions from measurements made at 2.7 GHz with the NRAO
three-element interferometer, and for some we have taken positions
published elsewhere in the literature. In Sections 2 and 3 we describe
these new observations, and also discuss the occurrence of multiple
source responses.
2. THE OBSERVATIONS
2.1 Pencil-beam measurements at 5.0 GHz
Most of these observations with the 300-ft telescope were carried
out during 1975 June and July, and additional observations were made in
1975 November and 1976 March. The feed system consisted of two linearly
polarized horns, symmetrically offset from the axis and separated by
7.2' on the sky, with beamwidths (FWHM) of 2.7' (see, e.g. Davis 1971).
A beam-switching mode was employed, the switched output
from the horns being fed to a single-channel, cooled, parametric amplifier
with a nominal 3 dB bandwidth of 150 MHz. Final balancing of the receiver
was done using synchronous IF gain modulation. The system temperature was
150 K. A rotatable mount enabled the separation of the feed horns to be
orientated at an angle from the east-west line (at which the E vector
position angle was 0^o). Thus orientations of 11^o or 22^o produce
separations of the two beams in declination of a half or one beamwidth,
respectively. Observations of a source at two transits are then sufficient
to derive the source position by modelling the data with a twodimensional
Gaussian function.
Drift observations were made of most of the 780 survey sources,
sampling and recording the data on magnetic tape every 2 s. The scan
length was chosen to allow sufficient data before and after transit
for the removal of a baseline (linear in most cases) but was longer
for those sources without precise positions. In many cases more than
two transits were required to obtain sufficient data on these sources.
The ratio of the gains of the two feeds was determined by observations
of a few sources with the feeds orientated at 0^o. The receiver gain
was calibrated by injecting noise from a noise tube in one side of the
system every fifteenth sample; in the subsequent data reduction these
samples were removed and used to calibrate the data in units of antenna
temperature.
>From observations of 250 of the survey sources with accurately
known positions, the declination dependences of the telescope pointing
offsets and beamshape were determined. The calibration parameters
found in this way were then used to correct the positions of the
remaining sources. The position errors quoted in Table 1 are those
derived from the uncertainties in the calibration procedure or the
Gaussian fitting errors if larger. Where the response differed
significantly from that expected from a point source, a rough estimate
of the source size was determined.
2.2 Pencil-beam measurements at 2.7 GHz
These observations were carried out with the 300-ft telescope
during 1975 December, 1976 August and September. For these observations
the telescope was equipped with a three-feed system, the central feed being
on-axis and straddled by feeds offset by 9.8' on the sky, with
beamwidths of 5' (see, e.g. Owen 1975). The outer feeds responded
to a single circular-polarization mode and the central feed to both
left- and right-hand circular polarizations. All four outputs were
switched against a reference load and fed to a four-channel receiver with
parametric amplifiers of bandwidths 40 MHz and system temperatures 120 K.
IF gain modulation was employed to balance the receiver.
The drift scans and data analysis were similar to those used for the
5.0-GHz observations. A feed box orientation of 14^o from an east-west
line was used, and an orientation of 0^o was used to determine the
relative gains of the feeds. In many cases a single transit observation
was sufficient to model the source response.
A pointing problem was encountered during daytime observations, due to
strong sunlight producing differential heating of the telescope support
towers and panels. This resulted in systematic pointing excursions of
up to 40" in declination and 8" in right ascension. (This
effect was not noticeable during the 5.0-GHz observing sessions which were
mainly during overcast weather). The effect was removed by calibrating
on sources of known position.
Flux densities derived from these 2.7-GHz observations and those at
5.0 GHz described above will be published separately.
2.3 Three-element interferometer measurements at 2.7 GHz
These observations were made for just a few sources which were
suspected quasars on the basis of a preliminary search for identifications,
and hence had a relatively high probality of being compact. The NRAO
three-element interferometer has been described by Hogg et al. (1969).
The observations were taken during 1976 October with a baseline configuration
of 300-1200-1500m, and during 1976 December with a configuration of
900-1800-2700m. Each source was observed for about 8 min at each of 3 - 5
different hour angles in either one or both of the observing sessions. The
instrumental phase was determined every hour by observing compact radio
sources of accurately known position, and the calibrated visibility data
were Fourier transformed and 'CLEANED' to produce rough source maps. Because
of the very sparse nature of the u.v. coverage, these maps were only
sufficient to indicate which sources were unresolved. For these unresolved
sources only, positions were obtained, using the phase of the visibility
data.
3. RESULTS
3.1 The Radio Positions
Radio positions for the sources are listed in Table 1, which is to be found
on Microfiche MN191/1. For most sources we have given the most accurate
position from our measurements. In five cases, the position from the 966-MHz
survey is the best available. A reference number (1 - 4) indicates the origin
of each position. For a few sources, previously published positions are
quoted and references are given in the footnotes.
3.2 Multiple Sources
The observations made with the NRAO 300-ft telescope have approximately
six times the angular resolution of those of the 966-MHz survey, and they
show that 38 of the 'sources' listed in the survey consist of more than
one peak of radio emission. In 37 cases there were just two peaks and in
one there were three. It is probable that the majority of these multiple
responses arise from physically unrelated radio sources. We suggest this
because the original 966 MHz survey was confusion limited and a number of
weaker sources in it will be heavily confused or blends of two (or more)
sources of comparable flux density. Such multiple responses near the
position of a survey source are designated by A, B ... etc.in Table 1. In
a few cases, weak sources were detected in the 300-ft scans, generally at
large distances from the survey source position and clearly did not
contribute to the original 966-MHz 'source'. Such sources are mentioned
in the notes on individual sources.
All the east-west blends should have been found on the drift scans,
but most of the north-south ones will have been missed. Using the
distributions of separations and position angles of those multiple sources
actually found, we estimate the number of multiple sources we are likely
to have missed as about 50, giving an estimated total of 90 multiple
sources. This is approximately the number expected for such a
confusion-limited survey.
Sometimes when only one source is found it has a flux density much lower
than expected from other flux density data. In these cases we suspect
the existence of another undetected source and mention this possibility
in the footnotes.
There also exist very extended sources (e.g. DA240, 3C236) which,
whilst being single sources, would nevertheless produce a two-peaked
response in the pencil-beam observations. This possibility is considered
below in the discussion of the optical identification procedure.
4. OPTICAL IDENTIFICATIONS
Using the best available radio position for each source, a search has
been made on the Palomar Sky Survey prints for optical identifications.
Survey sources which give rise to widely separated responses were generally
treated as two independent sources, and the position of each has been
searched. However, the line joining them was also checked for bright
galaxies and, for the double sources 0157+405, 0247+467 and 1127+553, both
components are now thought to be associated with a bright galaxy lying
between them. The remaining 34 pairs appear to consist of unrelated sources,
many of them being separately identifiable, and one further source (0730+504)
has three distinct peaks of emission. This brings the total number of
independent sources in the present list up to 280.
Candidate identifications are classified according to the scheme in
Table 2. Their optical positions have been measured to an accuracy of
about 0.5" rms, and red and blue apparent magnitudes mr and mb, have
been estimated (using the methods described in Paper I).
Table 2. Identification Classification
BSO,NSO,RSO Blue, neutral or red stellar object respectively.
QSO Quasi-stellar object confirmed by optical spectroscopy.
BO, NO, RO Blue, neutral or red object whose image is too faint to
class as stellar or extended.
G Galaxy (extended image on at least one print).
DG Double galaxy, or two very close galaxies.
GCL Galaxy in a cluster.
CL Cluster.
E Empty field.
NI Not identified (see text).
...: A tentative identification only.
4.1 Identification Criteria
In deciding whether to accept or reject any particular object as an
identification, the possible radio extent of the present sources has been
borne in mind. Although detailed structural information is not available
for the majority of these sources, it is known that many of them have angular
sizes in excess of 60". The radio position errors are typically
6 x 11" rms in RA and Dec respectively, and thus radio-optical
position differences of three or four times these errors are possible for
the larger sources. To accept as an identification any object lying
within the possible radio extent of the source, or even within three times
the position errors, would give too high a number of misidentifications
with chance background objects (mostly galactic stars and faint galaxies).
To obtain a reasonable level of reliability for the identifications, we
have adopted as rough guidelines the following criteria. These are based
on the background densities of various types of optical objects given in
Paper I.
(i) Galaxies brighter than mr ~ 16 are accepted up to 1' from the
radio position. The number of these occurring by chance in 280 random
fields of radius 1' is about five. In fact 45 such galaxies have
been found near the radio positions, implying a reliability of 90 per cent
for these identifications.
(ii) Galaxies with 16.0 < mr < 18.0 are accepted up to four times the
typical radio position errors. Again about five such galaxies are
expected by chance in 280 search fields, but 27 were actually found,
implying a reliability of over 80 per cent for these identifications.
(iii) Stellar objects: at high galactic latitudes (|b| > 20^o) QSOs are
accepted out to twice the typical position errors. In the 205 high-latitude
fields, about five chance BSOs are expected. Thirty were actually
found, again implying a reliability of over 80 per cent. However, at low
galactic latitudes the background sky density of blue stars rises sharply
and, for sources with 10^o < |b| < 20^o, only two identifications with BSOs
have been made, both of which are less than one standard deviation from the
radio position.
The density of red and neutral stellar objects is everywhere too
high for reliable indentifications to be made with such objects. One
tentative identification has been made with an RSO which is possibly a
compact galaxy.
(iv) Faint galaxies and other faint objects cannot generally be accepted
as identifications as their background density is too high. A total of
16 such objects which lie within only one standard deviation of the radio
positions have been proposed as tentative identifications, but their
reliability is low.
(v) Few sources can confidently be identified as empty fields since the
number of chance objects in the optical fields is so high. Sources for
which no reliable identification can be found using the above criteria
are classified 'NI' (not identified). Objects which are possible
candidates in such fields are mentioned in the notes on individual
sources.
4.2 Identification content
Identifications are proposed for 135 of the 280 sources, and are
listed in Table l which also gives their optical positions and magnitudes.
These identifications are summarized in Table 3.
Table 3. Identification content.
Bright galaxies (mr < 18.0) 72
Faint galaxies 10
QSO 7
BSO 23
RSO 1
Faint objects (RO+NO) 6
Clusters 16
---
Total positive identifications 135
E 10
NI 135
---
Total number of sources 280
The percentage of galaxies brighter than mr ~ 18 amongst these sources
(26%) is much higher then the 7% found for the more compact sources in
Paper I. This is consistent with the correlation of angular size with
apparent magnitude for galaxies first noted by Minkowski (1961).
Since BSOs and bright galaxies can be identified with reasonable
confidence, few such identifications should have been missed. However,
almost half of the present sources remain unidentified and thus these are
probably mostly empty fields and faint galaxies.
4.3 Finding charts
Finding charts for new identifications are given in Plate 1. The
reproductions are from the 'E' (red) prints of the Palomar Sky Survey
(copyright 1957, National Geographic Society). The fields measure
8.5'; the upper right-hand corner is north preceding.
4.4 Two amendments to the identifications for sources in Paper I
1050+542. The observations with the 300-ft telescope show a lobe error in
the MKIA-MK II interferometer position. The 5-GHz radio position (1950) is:
RA: 10h 50m 57s.1 (+-6"). dec +54^o 17' 52'' (+- 11")
The new identification is a BSO at -6.1",-3.9" from this position,
mr = 18.2, mb = 18.9.
1636+473. This source was previously listed as an empty field. Unpublished
measurements at 2.7 and 8.1 GHz made using the NRAO 3-element interferometer
indicate that this source has a double structure with component separation
of 19". One component has an inverted spectrum and within 1"
there is a QSO, mr = 17.5, mb = 18.0, at
RA: 16h 36m 19s.18. dec: +47^o 23' 28".2 (+- 0.5").
Finding charts for these two new identifications are given in Plate 1.
5. CONCLUDING REMARKS
The new radio positions presented here have enabled us to propose
identifications for about half of the extended or confused sources in
this sample. Some structural information is necessary to identify the
remaining 135 sources.
A paper is in preparation discussing the identifications presented
here together with those already published in Paper I.
ACKNOWLEDGMENTS
We are grateful to Bev and Derek Wills and to Pat Moore for
communicating unpublished data.
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Catalogue of Galaxies and Clusters of Galaxies, California Institute of
Technology, Pasadena.
Key to Table 1
Column
- (1) Source name expressed as a truncated RA-dec. For multiple sources
this refers to the position of the single survey response.
- (2) Name from other catalogues or surveys, if available: 3C, Edge et al.
(1959), Bennett (1962); 4C, Gower, Scott & Wills (1967); 4CP,
(Pencil Beam) Caswell & Crowther (1969); OA-OZ, Brundage et al. (1971);
NRAO, Pauliny-Toth, Wade & Heeschen (1966); HB, Hanbury Brown & Hazard
(1953); V, MacLeod et al. (1965); DA, Galt & Kennedy (1968);WKB,
Williams, Kenderdine & Baldwin (1966).
- (3) 966-MHz flux density in Jy, measured during the initial survey. Rms
error = [(0.09S)^2 + (0.2)^2]^(1/2).
- (4) Radio Right Ascension (1950) in h, min, s.
- (5) Rms error in RA, in".
- (6) Radio Declination (1950) in degrees, arcmin, ".
- (7) Rms error in dec, in".
- (8) Reference to origin of radio position: 1, 5.0-GHz data; 2, 2.7-GHz data;
3, 2.7-GHz interferometer observations; 4, 966-MHz survey scans; -,
previously published position (see individual footnotes).
- (9) Identification (see Table 2).
- (10), Optical magnitudes of the identification, estimated from the red and
blue prints respectively.
- (11) Rms error ~0.5 mag. 'NV' indicates that the object is not visible on that
print. Bracketed values are rough estimates only.
- (12), Optical position of the identification expressed as offsets in"
in RA and dec from the quoted
- (13) radio position (in the sense 'optical position minus radio position').
Unless otherwise stated in the footnotes, the optical positions were
measured at Jodrell Bank and have rms errors of ~0.5". Galaxies
with mr < 14.0 have very extended optical images and were not generally
measured.
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