96 research outputs found
Redshifts of CLASS Radio Sources
Spectroscopic observations of a sample of 42 flat-spectrum radio sources from
the Cosmic Lens All-Sky Survey (CLASS) have yielded a mean redshift of with an RMS spread of 0.95, at a completeness level of 64%. The sample
consists of sources with a 5-GHz flux density of 25-50 mJy, making it the
faintest flat-spectrum radio sample for which the redshift distribution has
been studied. The spectra, obtained with the Willam Herschel Telescope (WHT),
consist mainly of broad-line quasars at and narrow-line galaxies at
. Though the mean redshift of flat-spectrum radio sources exhibits
little variation over more than two orders of magnitude in radio flux density,
there is evidence for a decreasing fraction of quasars at weaker flux levels.
In this paper we present the results of our spectroscopic observations, and
discuss the implications for constraining cosmological parameters with
statistical analyses of the CLASS survey.Comment: 10 pages, AJ accepte
A multi-epoch VLBI survey of the kinematics of CFJ sources
Context. This is the second in a series of papers presenting VLBI observations of the 293 Caltech-Jodrell Bank Flat-spectrum (hereafter CJF) sources and their analysis.
Aims. We obtain a consistent motion dataset large enough to allow the systematic properties of the population to be studied.
Methods. We present detailed kinematic analysis of the complete flux-density limited CJF survey. We computed 2D kinematic models based on the optimal model-fitting parameters of multi-epoch VLBA observations. This allows us to calculate not only radial, but also orthogonal motions, and thus to study curvature and acceleration. Statistical tests of the motions measured and their reliability were performed. A correlation analysis between the derived apparent motions, luminosities, spectral indices, and core dominance and the resulting consequences is described.
Results. With at least one velocity in each of the 237 sources, this sample is much larger than any available before, so it allows a meaningful statistical investigation of apparent motions and any possible correlations with other parameters in AGN jets. The main results to emerge are as follows: - In general motions are not consistent with a single uniform velocity applicable to all components along a jet. - We find a slight trend towards a positive outward acceleration and also adduce some evidence for greater acceleration in the innermost regions. - We find a lack of fast components at physical distances less than a few pc from the reference feature. - Only ~4% of the components from galaxies and <2% of those from quasars undergo large bends i.e. within 15° of ± 90°. - The distribution of radial velocities shows a broad distribution of velocities (apparent velocities up to 30 c). Fifteen percent of the best-sampled jet components exhibit low velocities that may need to be explained in a different manner to the fast motions. - Some negative superluminal motions are seen, and in 15 cases (6%) these are definitely significant. - We find a strong correlation between the 5 GHz luminosity and the apparent velocity. - The CJF galaxies, on average, show slower apparent jet-component velocities than the quasars. - The mean velocity in the VLBA 2 cm survey (Kellermann et al. 2004, ApJ, 609, 539) is substantially higher than in the CJF survey, the ratio could be roughly a factor of 1.5-2. This supports the observed trend toward increasing apparent velocity with increasing observing frequency.
Conclusions. This AGN survey provides the basis for any statistical analysis of jet and jet-component properties
Redshifts and Neutral Hydrogen Observations of Compact Symmetric Objects in the COINS Sample
Compact Symmetric Objects (CSOs) are young radio galaxies whose jet axes lie
close to the plane of the sky, and whose appearance is therefore not dominated
by relativistic beaming effects. The small linear sizes of CSOs make them
valuable for studies of both the evolution of radio galaxies and testing
unified schemes for active galactic nuclei (AGN). A parsec-scale region of gas
surrounding the central engine is predicted by both accretion and obscuration
scenarios. Working surfaces, or ``hot spots,'' and the radio jets of CSOs are
close enough to the central engines that this circumnuclear gas can be seen in
absorption.
The CSOs Observed in the Northern Sky (COINS) sample is comprised of 52 CSO
candidates identified in three VLBI surveys. Of these, 27 have now been
confirmed as CSOs. Optical redshifts are available in the literature for 28 of
the CSO candidates, and HI absorption has been detected toward four. We present
new optical spectroscopic redshifts for three of the candidates and summarize
the current status of optical identifications. We further report on the
discovery of HI in absorption towards the CSO J1816+3457 and summarize the
results of neutral hydrogen absorption studies of the sources in this sample.Comment: 12 pages, Accepted for publication in Ap
Polarimetry of Compact Symmetric Objects
We present multi-frequency VLBA observations of two polarized Compact
Symmetric Objects (CSOs), J0000+4054 and J1826+1831, and a polarized CSO
candidate, J1915+6548. Using the wavelength-squared dependence of Faraday
rotation, we obtained rotation measures (RMs) of -180 \pm 10 rad m^-2 and 1540
\pm 7 rad m^-2 for the latter two sources. These are lower than what is
expected of CSOs (several 1000 rad m^-2) and, depending on the path length of
the Faraday screens, require magnetic fields from 0.03 to 6 \mu G. These CSOs
may be more heavily affected by Doppler boosting than their unpolarized
counterparts, suggesting that a jet-axis orientation more inclined towards the
line of sight is necessary to detect any polarization. This allows for low RMs
if the polarized components are oriented away from the depolarizing
circumnuclear torus. These observations also add a fourth epoch to the proper
motion studies of J0000+4054 and J1826+1831, constraining their kinematic age
estimates to >610 yrs and 2600 \pm 490 yrs, respectively. The morphology,
spectrum, and component motions of J1915+6548 are discussed in light of its new
classification as a CSO candidate, and its angle to the line of sight (~50\deg)
is determined from relativistic beaming arguments.Comment: 29 pages, including 9 figures; Accepted by Astrophysical Journal, 16
Feb 0
Architecture of North Atlantic Contourite Drifts Modified by Transient Circulation of the Icelandic Mantle Plume
Overflow of Northern Component Water, the precursor of North Atlantic Deep Water, appears to have varied during Neogene times. It has been suggested that this variation is moderated by transient behavior of the Icelandic mantle plume, which has influenced North Atlantic bathymetry through time. Thus pathways and intensities of bottom currents that control deposition of contourite drifts could be affected by mantle processes. Here, we present regional seismic reflection profiles that cross sedimentary accumulations (Björn, Gardar, Eirik and Hatton Drifts). Prominent reflections were mapped and calibrated using a combination of boreholes and legacy seismic profiles. Interpreted seismic profiles were used to reconstruct solid sedimentation rates. Björn Drift began to accumulate in late Miocene times. Its average sedimentation rate decreased at ∼2.5 Ma and increased again at ∼0.75 Ma. In contrast, Eirik Drift started to accumulate in early Miocene times. Its average sedimentation rate increased at ∼5.5 Ma and decreased at ∼2.2 Ma. In both cases, there is a good correlation between sedimentation rates, inferred Northern Component Water overflow, and the variation of Icelandic plume temperature independently obtained from the geometry of diachronous V-shaped ridges. Between 5.5 and 2.5 Ma, the plume cooled, which probably caused subsidence of the Greenland-Iceland-Scotland Ridge, allowing drift accumulation to increase. When the plume became hotter at 2.5 Ma, drift accumulation rate fell. We infer that deep-water current strength is modulated by fluctuating dynamic support of the Greenland-Scotland Ridge. Our results highlight the potential link between mantle convective processes and ocean circulation
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
A Multi-Epoch VLBI Survey of the Kinematics of CJF Sources; Part I: Model-Fit Parameters and Maps
Context: This is the first of a series of papers presenting VLBI observations
of the 293 Caltech-Jodrell Bank Flat-Spectrum (hereafter CJF) sources and their
analysis. Aims: One of the major goals of the CJF is to make a statistical
study of the apparent velocities of the sources. Methods: We have conducted
global VLBI and VLBA observations at 5 GHz since 1990, accumulating thirteen
separate observing campaigns. Results: We present here an overview of the
observations, give details of the data reduction and present the source
parameters resulting from a model-fitting procedure. For every source at every
observing epoch, an image is shown, built up by restoring the model-fitted
components, convolved with the clean beam, into the residual image, which was
made by Fourier transforming the visibility data after first subtracting the
model-fitted components in the uv-plane. Overplotted we show symbols to
represent the model components. Conclusions: We have produced VLBI images of
all but 5 of the 293 sources in the complete CJF sample at several epochs and
investigated the kinematics of 266 AGN.Comment: Figure 1 and Table 2 are only available in electronic form at the CDS
and soon at http://www.mpifr-bonn.mpg.de/staff/sbritzen/cjf.htm
Caltech-Jodrell Bank VLBI Surveys
The Caltech-Jodrell Bank VLBI surveys of bright extragalactic radio sources north of declination 35° were carried out between 1990 and 1995 using the Mark-II system, achieving images with a resolution of about 1 mas at 5 GHz. The CJl survey (together with the older “PR” survey) includes 200 objects with 5 GHz flux density greater than 0.7 Jy; the CJ2 survey includes 193 flat-spectrum sources with 5 GHz flux density greater than 0.35 Jy; and we have defined a complete flux-density limited sample, CJF, of 293 flat-spectrum sources stronger than 0.35 Jy. We summarize the definition of the samples and the VLBI, VLA, MERLIN, and optical observations, and present some highlights of the astrophysical results. These include: (1) superluminal motion and cosmology; (2) morphology and evolution of the “compact symmetric objects” (CSOs); (3) two-sided motion in some CSOs; (4) the angular-size-redshift diagram; (5) misalignment of parsec-scale and kiloparsec-scale jets
- …