138 research outputs found
On the interpretation of the observed angular-size-flux-density relation for extragalactic radio sources
The interpretation of the observed relation between median angular sizes (θm) of extragalactic radio sources and flux density at 408 MHz has been examined. The predicted θm-S relations based on well-observed strong sources in parent samples selected at 178 and 1400 MHz, and existing models of the evolving radio luminosity function can be made to fit the observed relation only by invoking cosmological evolution in linear sizes even for the q0 = 0 universe. Predictions based on a parent sample at 2.7 GHz are shown to overestimate the contribution of steep-spectrum, compact (SSC) sources in low-frequency samples unless the downward curvature in the spectra of such sources is taken into account. When approximate corrections are made for this effect, predictions based on the 2.7 GHz parent sample cannot obviate the need for linear size evolution as claimed in the literature
VLA observations of hot spots in high luminosity radio sources
VLA observations at 6 cm have been made of 16 distant luminous 3C sources that appeared to be unresolved or slightly resolved in Cambridge maps but which were known from VLBI observations to contain significant fine structure on the scale of about 1 kpc in their lobes. The general morphology of these sources is very similar to that of their nearby powerful counterparts; most of the lobes contain low brightness tails which are often directed from the hot spots towards the associated optical object. The hot spots are generally unresolved by the VLA observations; in 3C 254, 3C 268.4 and 3C 352, one of the lobes contains two hot spots
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
Extragalactic sources with asymmetric radio structure I. Observations of 17 sources
We present total-intensity and linear-polarization observations with the Very Large Array (VLA) at λ6 and 2 cm of 17 sources, almost all of which were suspected to have extended emission only on one side of the nucleus. Five of them are still one-sided, three appear unresolved, while seven have radio lobes on both sides of the nucleus. The outer components in the double-lobed sources, however, have significantly different surface brightness or are very asymmetrically located with respect to the nucleus
Aspect dependent optical continuum emission in radio quasars
We have defined a new, complete, low-frequency selected sample of southern radio quasars, the Molonglo Quasar Sample, with the aim of studying the aspect dependence of the radio and optical emission. As a test for enhancement of the optical continuum, we find that the narrow [O II] and [O III] emission line equivalent widths decrease systematically with radio core dominance. This effect is consistent with the optical continnum being relativistically boosted at angles close to the line of sight. However, such an interpretation seems to be in conflicts aith the aspect independent behaviour of the broad lines, most notably Kβ
VLA Observations of a New Population of Blazars
We present the first deep VLA radio images of flat-spectrum radio quasars
(FSRQ) with multiwavelength emission properties similar to those of BL Lacs
with synchrotron X-rays. Our observations of twenty-five of these sources show
that their radio morphologies are similar to those of other radio quasars.
However, their range of extended powers is more similar to that of BL Lacertae
objects (BL Lacs) and extends down to the low values typical of FR I radio
galaxies. Five out of our nine lobe-dominated sources have extended radio
powers in the range typical of both FR I and FR II radio galaxies, but their
extended radio structure is clearly FR II-like. Therefore, we have not yet
found a large population of radio quasars hosted by FR Is. Two thirds of our
sources have a core-dominated radio morpholgy and thus X-rays likely dominated
by the jet. We find that their ratios of radio core to total X-ray luminosity
are low and in the regime indicative of synchrotron X-rays. This result shows
that also blazars with strong emission lines can produce jets of high-energy
synchrotron emission and undermines at least in part the ``blazar sequence''
scenario which advocates that particle Compton cooling by an external radiation
field governs the frequency of the synchrotron emission peak.Comment: 26 pages, 33 figures. Accepted for publication in Ap
High-redshift radio galaxies from the Molonglo catalogue
We report the result of radio and optical observations of a sample of southern hemisphere radio galaxies, selected from the Molonglo Reference Catalogue. The source were selected to have 408 MHz flux densities greater than 0.9 Jy and low-frequency spectral indices steeper than -0.9 VLA 6 cm maps and optical identifications of 13 sources are presented. All of these sources are indentified with faint galaxies, many of which have extended optical structures with morphologies characteristics od high redshift (Z > 0.8) radio galaxies. Long - slit spectroscopic observation have yielded redshifts for 11 of the galaxies, 5 of which are at Z > 2. The most distant source in the present sample, 0316 - 257, has a redshift of 3.13, while the median redshift for the sample is 1.6. Our result for this subsample suggest that our total sample of 150 sources may contain ~ 40 galaxies with Z > 2. We have imaged four of our galaxies having Z > 2 in the light of Lyα. Three of these have highly extended Lyα emission distributed along their radio axes
Multiwaveband studies of the Molonglo 1-Jy sample of radio galaxies
This article does not have an abstract
Parsec-scale Magnetic-Field Structures in HEAO-1 BL Lacs
We present very long baseline interferometry polarization images of an X-ray
selected sample of BL Lacertae objects belonging to the first High Energy
Astronomy Observatory (HEAO-1) and the ROSAT-Green Bank (RGB) surveys. These
are primarily high-energy-peaked BL Lacs (HBLs) and exhibit core-jet radio
morphologies on pc-scales. They show moderately polarized jet components,
similar to those of low-energy-peaked BL Lacs (LBLs). The fractional
polarization in the unresolved cores of the HBLs is, on average, lower than in
the LBLs, while the fractional polarizations in the pc-scale jets of HBLs and
LBLs are comparable. However a difference is observed in the orientation of the
inferred jet magnetic fields -- while LBL jets are well-known to preferentially
exhibit transverse magnetic fields, the HBL jets tend to display longitudinal
magnetic fields. Although a `spine-sheath' jet velocity structure, along with
larger viewing angles for HBLs could produce the observed magnetic field
configuration, differences in other properties of LBLs and HBLs, such as their
total radio power, cannot be fully reconciled with the different-angle scenario
alone. Instead it appears that LBLs and HBLs differ intrinsically, perhaps in
the spin rates of their central black holes.Comment: 41 pages, 21 figures, accepted for publication in MNRA
Minimal Angular Size of Distant Sources in Open, CDM, and Scalar Field Cosmologies
We propose a simple method for determining the redshift at which the
angular size of an extragalactic source with fixed proper diameter takes its
minimal value. A closed analytical expression, which is quite convenient for
numerical evaluation is derived. The method is exemplified with the following
FRW type expanding universes: the open matter dominated models
(), a critical density model with cosmological constant
(), and the class of scalar field cosmologies proposed
by Ratra and Peebles. The influence of systematic evolutionary effects is
briefly discussed.Comment: 8 pages, 1 postscript figures, uses revtex macro
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