311 research outputs found
Origin of the viewing-angle dependence of the optical continuum emission in quasars
The orientation-dependence of the optical continuum emission in radio-loud
quasars is investigated using a new, complete sample of low-frequency-selected
quasars, the Molonglo Quasar Sample (MQS). The optical continuum is found to be
highly anisotropic, brightening continuously from lobe- to core-dominated
quasars by 3-5 mag. It is argued that aspect-dependent extinction, rather than
relativistic boosting as has been previously proposed, provides the simplest
explanation consistent with the data. The reddening hypothesis is supported by
both the steeper optical slopes and the larger Balmer decrements found in
lobe-dominated quasars, as well as the stronger anisotropy seen at blue
wavelengths. The dust responsible is shown to be physically associated with the
quasar, lying mostly at radii between the broad and narrow-line regions in a
clumpy distribution. Such a geometry is reminiscent of a torus. However,
substantial numbers of dust clouds must lie within the torus opening angle,
contributing to an increasing average optical depth with increasing viewing
angle away from the jet axis.Comment: 14 pages, 21 figures, MNRAS accepte
Deep optical imaging of the field of PC1643+4631A&B, II: Estimating the colours and redshifts of faint galaxies
In an investigation of the cause of the cosmic microwave background decrement
in the field of the z = 3.8 quasar pair PC1643+4631, we have carried out a
study to photometrically estimate the redshifts of galaxies in deep
multi-colour optical images of the field taken with the WHT. To examine the
possibility that a massive cluster of galaxies lies in the field, we have
attempted to recover simulated galaxies with intrinsic colours matching those
of the model galaxies used in the photometric redshift estimation. We find that
when such model galaxies are added to our images, there is considerable scatter
of the recovered galaxy redshifts away from the model value; this scatter is
larger than that expected from photometric errors and is the result of
confusion, simply due to ground-based seeing, between objects in the field. We
have also compared the likely efficiency of the photometric redshift technique
against the colour criteria used to select z>3 galaxies via the strong colour
signature of the Lyman-limit break. We find that these techniques may
significantly underestimate the true surface density of z>3, due to confusion
between the high-redshift galaxies and other objects near the line of sight. We
argue that the actual surface density of z=3 galaxies may be as much as 6 times
greater than that estimated by previous ground-based studies, and note that
this conclusion is consistent with the surface density of high-redshift objects
found in the HDF. Finally, we conclude that all ground-based deep field surveys
are inevitably affected by confusion, and note that reducing the effective
seeing in ground-based images will be of paramount importance in observing the
distant universe.Comment: 18 pages, 60 figures, submitted to MNRAS, 2 large figure avaliable at
ftp://ftp.mrao.cam.ac.uk:/pub/PC1643/paper2.figure50.eps and
ftp://ftp.mrao.cam.ac.uk:/pub/PC1643/paper2.figure51.ep
Deep optical imaging of the field of PC1643+4631A&B, I: Spatial distributions and the counts of faint galaxies
We present deep optical images of the PC1643+4631 field obtained at the WHT.
This field contains two quasars at redshifts z=3.79 & 3.83 and a cosmic
microwave background (CMB) decrement detected with the Ryle Telescope. The
images are in U,G,V,R and I filters, and are complete to 25th magnitude in R
and G and to 25.5 in U. The isophotal galaxy counts are consistent with the
results of Metcalde et al. (1996), Hogg et al. (1997), and others. We find an
excess of robust high-redshift Ly-break galaxy candidates with 25.0<R<25.5
compared with the mean number found in the fields studied by Steidel et al. -we
expect 7 but find 16 - but we do not find that the galaxies are concentrated in
the direction of the CMB decrement. However, we are still not sure of the
distance to the system causing the CMB decrement. We have also used our images
to compare the commonly used object-finding algorithms of FOCAS and SExtractor:
we find FOCAS the more efficient at detecting faint objects and the better at
dealing with composite objects, whereas SExtractor's morphological
classification is more reliable, especially for faint objects near the
resolution limit. More generally, we have also compared the flux lost using
isophotal apertures on a real image with that on a noise-only image: recovery
of artificial galaxies from the noise-only image significantly overestimates
the flux lost from the galaxies, and we find that the corrections made using
this technique suffer a systematic error of some 0.4 magnitudes.Comment: 17 pages, 40 figures, submitted to MNRAS, 1 large figure avaliable at
ftp://ftp.mrao.cam.ac.uk:/pub/PC1643/paper1.figure18.p
The Molonglo Reference Catalog 1-Jy radio source survey IV. Optical spectroscopy of a complete quasar sample
Optical spectroscopic data are presented here for quasars from the Molonglo
Quasar Sample (MQS), which forms part of a complete survey of 1-Jy radio
sources from the Molonglo Reference Catalogue. The combination of low-frequency
selection and complete identifications means that the MQS is relatively free
from the orientation biases which affect most other quasar samples. To date,
the sample includes 105 quasars and 6 BL Lac objects, 106 of which have now
been confirmed spectroscopically. This paper presents a homogenous set of
low-resolution optical spectra for 79 MQS quasars, the majority of which have
been obtained at the Anglo-Australian Telescope. Full observational details are
given and redshifts, continuum and emission-line data tabulated for all
confirmed quasars.Comment: 40 pages, ApJS in pres
The Radio-Optical Correlation in Steep-Spectrum Quasars
Using complete samples of steep-spectrum quasars, we present evidence for a
correlation between radio and optical luminosity which is not caused by
selection effects, nor caused by an orientation dependence (such as
relativistic beaming), nor a byproduct of cosmic evolution. We argue that this
rules out models of jet formation in which there are no parameters in common
with the production of the optical continuum. This is arguably the most direct
evidence to date for a close link between accretion onto a black hole and the
fuelling of relativistic jets. The correlation also provides a natural
explanation for the presence of aligned optical/radio structures in only the
most radio luminous high-redshift galaxies.Comment: MNRAS in press. Uses BoxedEPS (included
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