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The quasar fraction in a sample of giant radio sources
Garret
Cotter
e-mail:
garret@mrao.cam.ac.uk
Mullard Radio Astronomy
Observatory,
Cavendish Laboratory, Madingley Road, Cambridge CB3 0HE, UNITED KINGDOM
Abstract:
A classic test of the standard orientation-dependent unified schemes has been
the assessment of the quasar fraction in complete radio samples. Using a sample
of giant radiosources (projected size 
) out to redshifts
of 
, 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 
in the population.
Giant radio sources (
) 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 William Herschel Telescope and
have identified 16 giant radio sources at redshifts between 
and 
(Cotter, Rawlings & Saunders (1995)).
This sample of giants provides an interesting new position from which to
discuss unification schemes for radio loud quasars and powerful radio galaxies
(Barthel (1989), Antonucci (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 
. 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 intrinsically larger than galaxies. If one
does not have a sample which adequately represents the entire range of 
in
the population, the results obtained by simply comparing the numbers and
relative projected sizes of quasars and galaxies may be inaccurate.
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 of 
using the University of Hawaii 2.2m telescope
(Dingley (1992)) and the University of Texas 2.7m telescope (Cotter, Rawlings & Saunders (1995)). Using
the 4.2m 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 
, corresponding to a redshift range of 
. Objects
brighter than this are to close to be giants and there are several objects
fainter than 
which remain unidentified.
Figure: Spectra of a selection of objects from the 7C giants sample, obtained
with the WHT. Wavelength is shown in units of Ångstrom and flux in units of
.
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 (
) diagram is shown in Figure 2.
Radio galaxies from the revised 3C ``LRL'' sample of Laing, Riley & Longair (1983) are shown for
comparison.
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)
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 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 
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
during their own lifetimes (Cotter, Rawlings & Saunders (1995)). The implication is the even LRL
does not fully sample the entire range of 
, so one must try to reconstruct a
more complete picture of the 
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 
, the range
used by Barthel, in which the giants sample is complete. These redshifts limits
correspond to a range of radio luminosity 
. In this range the 7C giants sample
contains 2 quasars and 8 galaxies of 
.
Objects of this radio luminosity are detected in the LRL sample from zero
redshift out to 
. 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 
objects from 7C
and the 
objects from 3C and add them together to define a
new sample. The 3C objects sample a co-moving volume of 
and the 7C objects sample a volume of 
, 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 back-evolved 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 
and the low redshift bin 
. 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 
, in
agreement with Barthel's estimate for sources of higher 
, 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 information about the cosmic evolution of 
in the population. When
turning the clock back for the 3C sources, I have to assume that their 
distribution does not change. So although the above estimate of the opening
angle includes sources at large 
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 
as possible.
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 
diagram. One may then
attempt to convolve a proposed opening angle with the true 
distribution in
the population, for comparison with the observed quasar/galaxy ratio as a
function of 
at any particular radio luminosity.
- 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.
YERAC 94 Account
Wed Feb 22 16:49:34 GMT 1995
