4,919 research outputs found
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
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
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
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
High Prevalence of Non-Typhoid Salmonella Bacteraemia Among Febrile HIV Adult Patients Admitted at a Tertiary Hospital, North-Western Tanzania.
Bacterial blood stream infections constitute a significant public-health problem and it is an important cause of morbidity and mortality in HIV infected patients. Little is known in developing countries regarding salmonella bacteraemia among HIV patients. The purpose of this study was to determine the bacterial pathogens causing blood stream infection among febrile adults attending in a tertiary hospital North-Western, Tanzania. A prospective cross-sectional study involving 346 consecutive, febrile adult patients admitted at Bugando Medical Centre was conducted. Demographic and other data were collected using standardized questionnaires. Blood culture was done followed by susceptibility testing using disc diffusion method. HIV testing was also performed as per Tanzania national algorithm and total white blood cell counts and CD4+ counts determined. Of 346 febrile adult patients 33 (9.5%) had blood stream infections. The common isolates were Salmonella spp 13(39.4%), Escherichia coli 8 (24.2%), Streptococcus pneumonia 5(15.2%), Staphylococcus aureus 4(12.1%), Citrobacter spp 1(3%), Streptococcus pyogenes 1(3%) and Klebsiella pneumonia 1(3%). A total of 156 (45.1%) patients were HIV infected; of whom 12/156 (7.6%) were infected by non-typhoid Salmonella spp compared to 1/190 (0.5%) of non-HIV infected patients (RRR 11.2, p=0.029) infected with Salmonella typhi. HIV infected patients with bacteraemia had significantly lower CD4+ count than those without bacteraemia (median 28 vs. 88 cells/ml, p=0.01). Patients with salmonella bacteraemia had significantly lower median of WBC than those with non-salmonella as well as those without bacteraemia (median, 3.6 vs. 17.5 vs. 9.8x109, p=0.0001). All Salmonella spp were sensitive to ceftriaxone and imipenem, while being 84%, 69.2%, 38% and 8% resistant to chloramphenicol, ampicillin, sulphamethaxazole/trimethoprim and ciprofloxacin respectively. Predictors of mortality were HIV infection (OR 2.3, p=0.006), Glasgow coma score of less than 15 (OR 3.4, p=0.0001) and night sweats (OR 2.4, p=0.014). Non-typhoid Salmonella spp that are highly resistant to common antibiotics are predominant cause of bacterial blood stream infection among HIV patients attending Bugando Medical Centre. Continuous surveillance and intervention strategies should be put in place to monitor and manage cases of bloodstream infections in HIV-positive patients in Mwanza, Tanzania
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
HI Observations of the Supermassive Binary Black Hole System in 0402+379
We have recently discovered a supermassive binary black hole system with a
projected separation between the two black holes of 7.3 parsecs in the radio
galaxy 0402+379. This is the most compact supermassive binary black hole pair
yet imaged by more than two orders of magnitude. We present Global VLBI
observations at 1.3464 GHz of this radio galaxy, taken to improve the quality
of the HI data. Two absorption lines are found toward the southern jet of the
source, one redshifted by 370 +/- 10 km/s and the other blueshifted by 700 +/-
10 km/s with respect to the systemic velocity of the source, which, along with
the results obtained for the opacity distribution over the source, suggests the
presence of two mass clumps rotating around the central region of the source.
We propose a model consisting of a geometrically thick disk, of which we only
see a couple of clumps, that reproduces the velocities measured from the HI
absorption profiles. These clumps rotate in circular Keplerian orbits around an
axis that crosses one of the supermassive black holes of the binary system in
0402+379. We find an upper limit for the inclination angle of the twin jets of
the source to the line of sight of 66 degrees, which, according to the proposed
model, implies a lower limit on the central mass of ~7 x 10^8 Msun and a lower
limit for the scale height of the thick disk of ~12 pc .Comment: 20 pages, 7 figures. Accepted on the Astrophysical Journa
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
Luminous Infrared Galaxies With the Submillimeter Array. III. The Dense Kiloparsec Molecular Concentrations of Arp 299
We have used high resolution (~2.3") observations of the local (D = 46 Mpc)
luminous infrared galaxy Arp 299 to map out the physical properties of the
molecular gas which provides the fuel for its extreme star formation activity.
The 12CO J=3-2, 12CO J=2-1 and 13CO J=2-1 lines were observed with the
Submillimeter Array and the short spacings of the 12CO J=2-1 and J=3-2
observations have been recovered using James Clerk Maxwell Telescope single
dish observations. We use the radiative transfer code RADEX to estimate the
physical properties (density, column density and temperature) of the different
regions in this system. The RADEX solutions of the two galaxy nuclei, IC 694
and NGC 3690, are consistent with a wide range of gas components, from warm
moderately dense gas with T_{kin} > 30 K and n(H_{2}) ~ 0.3 - 3 x 10^{3}
cm^{-3} to cold dense gas with T_{kin} ~ 10-30 K and n(H_{2}) > 3 x 10^{3}
cm^{-3}. The overlap region is shown to have a better constrained solution with
T_{\rm{kin}}$ ~ 10-50 K and n(H_{2}) ~ 1-30 x 10^{3} cm^{-3}. We estimate the
gas masses and star formation rates of each region in order to derive molecular
gas depletion times. The depletion times of all regions (20-60 Myr) are found
to be about 2 orders of magnitude lower than those of normal spiral galaxies.
This rapid depletion time can probably be explained by a high fraction of dense
gas on kiloparsec scales in Arp 299. We estimate the CO-to-H_{2} factor,
\alpha_{co} to be 0.4 \pm 0.3 (3 x 10^{-4}/ x_{CO}) M_{sol} (K km s^{-1}
pc^{2})^{-1} for the overlap region. This value agrees well with values
determined previously for more advanced merger systems.Comment: 24 pages, 4 figures, ApJ accepte
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