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The quasar fraction in a sample of giant
radio sources
By Ga r r e t Co t t e r
e­mail: garret@mrao.cam.ac.uk
Mullard Radio Astronomy Observatory, Cavendish Laboratory, Madingley Road, Cambridge
CB3 0HE, UNITED KINGDOM
A classic test of the standard orientation­dependent unified schemes has been the assessment of
the quasar fraction in complete radio samples. Using a sample of giant radiosources (projected
size D ú 1 Mpc) out to redshifts of z ¸ 1, I demonstrate that the fraction of broad line objects
is smaller than that obtained by comparison of quasars and galaxies in the 3CR sample. I stress
that to make more accurate predictions about proposed torus opening angles, one must carefully
bin objects by size and apply a knowledge of the distribution of D in the population.
1. Introduction
Giant radio sources (D ? ¸ 1 Mpc) have traditionally been of great interest for several
reasons. Their large angular sizes give excellent opportunities for the study of source
physics, and as they lie at the extreme size limit of the radio source population, they
can be used to investigate and constrain models of source lifetime evolution and cosmic
population evolution. Taking candidates from a sample of possible distant giant radio
sources (Dingley (1992)), we have obtained spectrophotometric measurements on the
WilliamHerschel Telescope and have identified 16 giant radio sources at redshifts between
z = 0:3 and z = 1:6 (Cotter, Rawlings & Saunders (1995)).
This sample of giants provides an interesting new position from which to discuss uni­
fication schemes for radio loud quasars and powerful radio galaxies (Barthel (1989), Ant­
onucci (1993)). These schemes propose that both types of object are intrinsically similar
but present different appearances to observers viewing them from different directions.
An obscuring torus hides the nuclear broad line and non--thermal continuum emission
from a large fraction of the sky; the opening angle of this torus determines the relative
numbers of galaxies and quasars seen by one observer. By comparing the numbers and
projected sizes of galaxies and quasars in the 3CR sample, Barthel (1989) concluded
that the opening angle of the torus is approximately 90 ffi . There has since been much
debate on this issue, centered on the objectivity of the samples used and the difficulty
in determining whether or not some objects have broad emission lines. Aside from these
points, the calculation of opening angles by this method can suffer from an important
bias: for objects of any particular projected size, one selects quasars which are intrins­
ically larger than galaxies. If one does not have a sample which adequately represents
the entire range of D in the population, the results obtained by simply comparing the
numbers and relative projected sizes of quasars and galaxies may be inaccurate.
2. The sample
The 7C giants sample is drawn from a selection of 35 radio sources from the 7C survey
(Dingley (1992), McGilchrist et al. (1990)). The selection criteria were that the flux
density at 151 MHz should lie between 0.4 and 1.0 Jy and the angular size should lie
between 1.5 and 3.0 arcmin. Of these objects, 30 were identified to a limiting magnitude
1

2 Garret Cotter: The quasar fraction in a sample of giant radio sources
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Figure 1. Spectra of a selection of objects from the 7C giants sample, obtained with the
WHT. Wavelength is shown in units of š Angstrom and flux in units of 10 \Gamma17 erg cm \Gamma2 s \Gamma1 š A \Gamma1 .
of R = 22 using the University of Hawaii 2.2 m telescope (Dingley (1992)) and the
University of Texas 2.7 m telescope (Cotter, Rawlings & Saunders (1995)). Using the
4.2 m William Herschel Telescope we obtained spectrophotometric measurements of the
identified objects and a blind spectrum (Rawlings, Eales & Warren (1990)) of one of
the unidentified objects. The giants sample now contains 16 objects with spectroscopic
redshifts and is complete in the magnitude range 18 Ÿ R Ÿ 22, corresponding to a
redshift range of 0:3 ! ¸ z ! ¸ 1:0. Objects brighter than this are to close to be giants and
there are several objects fainter than R = 22 which remain unidentified.
Some typical spectra of broad and narrow emission line objects in the giants sample
are show in Figure 1. The distribution of the 7C giants on the radio luminosity--linear
size (PD) diagram is shown in Figure 2. Radio galaxies from the revised 3C ``LRL''
sample of Laing, Riley & Longair (1983) are shown for comparison.
3. Unified schemes
In the redshift range through which the giants sample is complete, there are two
objects w broad emission lines and and 13 radio galaxies showing only narrow emission
lines. These relative numbers are immediately consistent with the unified schemes--since

Garret Cotter: The quasar fraction in a sample of giant radio sources 3
Figure 2. The distribution in the linear size--radio luminosity plane of the sources in the 7C
giants sample (diamonds) and the 3C LRL sample (crosses)
the largest objects seen will mostly lie in the plane of the sky, we expect them to be
predominantly radio galaxies.
It is notable that the 7C objects fill a region of PD space which is relatively empty
in the LRL sample. This ``hole'' identified by Saunders (1982) is thought to be caused
by sources dropping out of the 3C sample as they fall in P during their own lifetimes
(Cotter, Rawlings & Saunders (1995)). The implication is the even LRL does not fully
sample the entire range of D, so one must try to reconstruct a more complete picture of
the PD distribution by adding in other surveys.
With this new sample one may make a comparison with Barthel's result by combining
a selected number of the giants with an equivalent selection of 3C sources. This gives
a value of the quasar fraction appropriate to all radio sources in one particular radio
luminosity bin at one particular cosmic epoch. I choose the 7C giants in the redshift
range 0:5 Ÿ z Ÿ 1:0, the range used by Barthel, in which the giants sample is complete.
These redshifts limits correspond to a range of radio luminosity 10 25:5 Ÿ P151MHz Ÿ
10 26:5 W \Gamma1 Hz \Gamma1 sr \Gamma1 . In this range the 7C giants sample contains 2 quasars and 8 galaxies
of 800 Ÿ (D=kpc) Ÿ 1500. Objects of this radio luminosity are detected in the LRL
sample from zero redshift out to z = 0:4. In the LRL sample, 16 galaxies and 9 quasars
fall into the same radio luminosity bin but have linear sizes outside the range of the
7C objects. However we cannot simply take the D ¸ 1 Mpc objects from 7C and the
D ! ¸ 1 Mpc objects from 3C and add them together to define a new sample. The 3C
objects sample a co­moving volume of 3:8 \Theta 10 10 Mpc 3 and the 7C objects sample a

4 Garret Cotter: The quasar fraction in a sample of giant radio sources
volume of 1:1 \Theta 10 10 Mpc 3 , so the contributions from each sample must be weighted
accordingly. Further, a correction must be made for cosmic evolution. In effect, the size
of the sample of 3C objects must be back­evolved to correct for the change in co--moving
density which we know has taken place. I use the current best estimate of the radio
luminosity function (Dunlop & Peacock (1990)) to make this cosmic evolution correction.
The Dunlop and Peacock radio luminosity function estimates that the number density
of objects in this radio power range decreased by a factor of approximately four between
the high redshift bin 0:5 Ÿ z Ÿ 1:0 and the low redshift bin z Ÿ 0:4. The number of
3C objects must be divided by 3.4 because it samples a larger volume, and multiplied
by 4 because it samples a region of redshift where the source density is known to be
less that the high redshift bin of the 7C giants. Making these corrections, the combined
sample contains effectively 12.6 quasars and 27.2 galaxies. These numbers give a torus
opening angle of 90 ffi , in agreement with Barthel's estimate for sources of higher P , but
using objects of similar radio power to those sometimes used to ``disprove'' unification
schemes.
The problem with this analysis is that the radio luminosity function contains no in­
formation about the cosmic evolution of D in the population. When turning the clock
back for the 3C sources, I have to assume that their D distribution does not change.
So although the above estimate of the opening angle includes sources at large D that
were missed in 3C, there is no indication whether or not the relative sizes of the quasars
and galaxies involved are consistent with this fraction. The object of the exercise is not
to determine conclusively the torus opening angle; rather to demonstrate the need for
careful selection of samples at different flux levels which span as large a range of D as
possible.
4. Conclusions
This sample provides clear supporting evidence that powerful radio galaxies and radio
loud quasars are indeed intrinsically similar objects. Simple techniques of counting the
total numbers of quasars and galaxies in entire samples can be influenced by various
biases. To determine the opening angle of the torus, and to investigate how this may
change with radio source power, one must calculate the quasar/galaxy ratio at different
points in the PD diagram. One may then attempt to convolve a proposed opening
angle with the true D distribution in the population, for comparison with the observed
quasar/galaxy ratio as a function of D at any particular radio luminosity.
REFERENCES
Barthel P.D., 1989, ApJ, 336, 606.
Cotter G., Rawlings S. & Saunders R.D.E., 1995, MNRAS, submitted.
Dingley S.J., 1992, PhD thesis, (University of Cambridge).
Antonucci R., 1993, ARA&A, 31, 473.
McGilchrist M.M., Baldwin J.E., Riley J.M., Titterington D.J., Waldraw E.M. & Warner P.J.,
1990, MNRAS, 246, 110.
Laing R.A., Riley J.M. & Longair M.S., 1983, MNRAS, 204, 151. (LRL)
Saunders R., 1982, PhD thesis, (University of Cambridge).
Rawlings S., Eales S. & Warren S., 1990, MNRAS, 243, 14P.
Dunlop J.S. & Peacock J.A., 1990, MNRAS, 247, 19.