5,673 research outputs found
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
Identifying Compact Symmetric Objects in the Southern Sky
We present results of multifrequency polarimetric VLBA observations of 20
compact radio sources. The observations represent the northern and southern
extensions of a large survey undertaken to identify Compact Symmetric Objects
(CSOs) Observed in the Northern Sky (COINS). 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). In this paper we
report on observations made of 20 new CSO candidates discovered in the northern
and southern extremities of the VLBA Calibrator Survey. We identify 4 new CSOs,
and discard 12 core-jet sources. The remaining 4 sources remain candidates
pending further investigation. We present continuum images at 5 GHz and 15 GHz
and, where relevant, images of the polarized flux density and spectral index
distributions for the 8 new CSOs and CSO candidates.Comment: accepted to Ap
The COINS Sample - VLBA Identifications of Compact Symmetric Objects
We present results of multifrequency polarimetric VLBA observations of 34
compact radio sources. The observations are part of a large survey undertaken
to identify CSOs Observed in the Northern Sky (COINS). Compact Symmetric
Objects (CSOs) are of particular interest in the study of the physics and
evolution of active galaxies. Based on VLBI continuum surveys of ~2000 compact
radio sources, we have defined a sample of 52 CSOs and CSO candidates. In this
paper, we identify 18 previously known CSOs, and introduce 33 new CSO
candidates. We present continuum images at several frequencies and, where
possible, images of the polarized flux density and spectral index distributions
for the 33 new candidates and one previously known but unconfirmed source. We
find evidence to support the inclusion of 10 of these condidates into the class
of CSOs. Thirteen candidates, including the previously unconfirmed source, have
been ruled out. Eleven sources require further investigation. The addition of
the 10 new confirmed CSOs increases the size of this class of objects by 50%.Comment: 24 pages, incl 8 figures. Accepted for publication in ApJ. Figure
quality degraded in the interests of space, full gzipped PS version also
available at http://www.ee.nmt.edu/~apeck/papers
Monitoring the Bi-Directional Relativistic Jets of the Radio Galaxy 1946+708
We report on a multi-frequency, multi-epoch campaign of Very Long Baseline
Interferometry observations of the radio galaxy 1946+708 using the VLBA and a
Global VLBI array. From these high-resolution observations we deduce the
kinematic age of the radio source to be 4000 years, comparable with the
ages of other Compact Symmetric Objects (CSOs). Ejections of pairs of jet
components appears to take place on time scales of 10 years and these
components in the jet travel outward at intrinsic velocities between 0.6 and
0.9 c. From the constraint that jet components cannot have intrinsic velocities
faster than light, we derive H_0 > 57 km s^-1 Mpc^-1 from the fastest pair of
components launched from the core. We provide strong evidence for the ejection
of a new pair of components in ~1997. From the trajectories of the jet
components we deduce that the jet is most likely to be helically confined,
rather than purely ballistic in nature.Comment: 20 pages, 8 figures, accepted to Ap
Cell wall protection by the Candida albicans class I chitin synthases
Open Access funded by Medical Research Council Acknowledgments We thank Kevin Mackenzie in the Microscopy and Histology Core Facility (Institute of Medical Sciences, University of Aberdeen), and Donna MacCallum for helpful statistical advice. This work was supported by grants from the Wellcome Trust (0868827 and 080088) including a Wellcome Trust Strategic Award (097377) and an Investigator Award to NG (101873), an MRC New Investigator Award to ML (MR/J008230/1) and a PhD scholarship awarded to KP from the Ministry of Sciences and Technology and Chiang Mai University, Thailand. Author contributions are as follows: KP constructed strains, performed the majority of the experiments, analyzed the data and contributed to the preparation of the manuscript. JA produced Fig. S1 using the data from the phosphoproteomic analysis conducted by SP and AB. NG conceived and designed experiments, analyzed data and commented on drafts of the manuscript. ML constructed strains, conceived, designed and performed experiments, analyzed data and wrote the manuscript.Peer reviewedPublisher PD
Spittlebugs: Bioecology, Host Plant Resistance and Advances in IPM
Several species and genera of spittlebugs (Homoptera: Cercopidae) are economic pests of grasses in tropical America. These insects compete with grazing animals by reducing forage availability and quality. They may cause serious losses on millions of hectares of improved pastures based on cultivars of several species of Brachiaria (signal grasses). Except for the cultivar Marandu, most of the available commercial cultivars of Brachiaria are susceptible to spittlebugs. In spite of their economic importance, much research need to be done yet. Such insect-plant system encompasses a diverse group of spittlebug species, a diverse group of forage grass species, which are under different management systems, in a broad range of ecological zones. Control efforts have been directed to host plant resistance, alternative that has been recognized as being of easy adoption and of low cost to farmers. It is probably the best control measure for controlling insect pests in low value crops, like pastures, widely established over vast areas. Screening for spittlebug resistance has been conducted both at CIAT and Embrapa-Beef Cattle Center, and promising accessions have been found. It is important however, that additional biological and behavioral studies of these insects, together with evaluations of other control techniques, like biological control and cultural practices, are also performed. Promising control measures and future research needs are discussed
Obscuration of the Parsec Scale Jets in the Compact Symmetric Object 1946+708
We present results of VLA and VLBA observations of the 1.420 GHz neutral
hydrogen absorption associated with the Compact Symmetric Object 1946+708
(z=0.101). We find significant structure in the gas on parsec scales. The peak
column density in the HI (N_HI~2.2x10^23 cm^-2 (T_s/8000K)) occurs toward the
center of activity of the source, as does the highest velocity dispersion
(FWHM~350 \kms). In addition, we find that the continuum spectra of the various
radio components associated with these jets strongly indicate free-free
absorption. This effect is particularly pronounced toward the core and inner
components of the receding jet, suggesting the presence of a screen local to
the source, perhaps part of an obscuring torus.Comment: revised version, some text added, 1 figure changed; accepted to
Astrophysical Journal, 22 page LaTeX document includes 8 postscript figure
Disks, Tori, and Cocoons: Emission and Absorption Diagnostics of AGN Environments
One of the most important problems in the study of active galaxies is
understanding the detailed geometry, physics, and evolution of the central
engines and their environments. The leading models involve an accretion disk
and torus structure around a central dense object, thought to be a supermassive
black hole. Gas found in the environment of AGN is associated with different
structures: molecular accretion disks, larger scale atomic tori, ionized and
neutral "cocoons" in which the nuclear regions can be embedded. All of them can
be studied at radio wavelengths by various means. Here, we summarize the work
that has been done to date in the radio band to characterize these structures.
Much has been learned about the central few parsecs of AGN in the last few
decades with contemporary instruments but the picture remains incomplete. In
order to be able to define a more accurate model of this region, significant
advances in sensitivity, spectral and angular resolution, and bandpass
stability are required. The necessary advances will only be provided by the
Square Kilometer Array and we discuss the possibilities that these dramatic
improvements will open for the study of the gas in the central region of AGN.Comment: To appear in "Science with the Square Kilometer Array," eds. C.
Carilli and S. Rawlings, New Astronomy Reviews (Elsevier: Amsterdam); 17
pages, 7 figures (four of them in separate gif/tif files) The full paper with
high resolution images can be downloaded from
http://www.astron.nl/~morganti/Papers/AGNenvironment.ps.g
Refractive Index of Humid Air in the Infrared: Model Fits
The theory of summation of electromagnetic line transitions is used to
tabulate the Taylor expansion of the refractive index of humid air over the
basic independent parameters (temperature, pressure, humidity, wavelength) in
five separate infrared regions from the H to the Q band at a fixed percentage
of Carbon Dioxide. These are least-squares fits to raw, highly resolved spectra
for a set of temperatures from 10 to 25 C, a set of pressures from 500 to 1023
hPa, and a set of relative humidities from 5 to 60%. These choices reflect the
prospective application to characterize ambient air at mountain altitudes of
astronomical telescopes.Comment: Corrected exponents of c0ref, c1ref and c1p in Table
Barriers to the Success of 100% Maritime Cargo Container Scanning
The attacks of September 11, 2001 revealed national security vulnerabilities that had previously not received high level priority in the United States, such as insecure transportation and infrastructure networks. In response, airport securityâincluding passenger and baggage scanningâhas been improved. Yet seaport security policies have been slow to change. Five years after 9/11, only 5% of the six million cargo containers that arrive at U.S. seaports are scanned for threats
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