1,697 research outputs found
Optical spectroscopy of two overlapping, flux-density-limited samples of radio sources in the North Ecliptic Cap, selected at 38 MHz and 151 MHz
We present the results of optical spectroscopy of two flux-density-limited
samples of radio sources selected at frequencies of 38 and 151 MHz in the same
region around the North Ecliptic Cap, the 8C-NEC and 7C-III samples
respectively. Both samples are selected at flux density levels ~20 times
fainter than samples based on the 3C catalogue. They are amongst the first
low-frequency selected samples with no spectral or angular size selection for
which almost complete redshift information has been obtained and they will
therefore provide a valuable resource for understanding the cosmic evolution of
radio sources and their hosts and environments. The 151-MHz 7C-III sample is
selected to have S_151 >=0.5 Jy and is the more spectroscopically complete; out
of 54 radio sources fairly reliable redshifts have been obtained for 44
objects. The 8C sample has a flux limit of S_38 >=1.3 Jy and contains 58
sources of which 46 have fairly reliable redshifts. We discuss possible biases
in the observed redshift distribution, and some interesting individual objects.
Using the 8C-NEC and 7C-III samples in conjunction, we form the first sample
selected on low-frequency flux in the rest-frame of the source, rather than the
usual selection on flux density in the observed frame. This allows us to remove
the bias associated with an increasing rest-frame selection frequency with
redshift. We investigate the difference this selection makes to correlations of
radio source properties with redshift and luminosity. We show in particular
that flux-density-based selection leads to an overestimate of the steepness of
the correlation of radio source size with redshift. (abridged)Comment: 22 pages, 10 figures, accepted by MNRA
NICMOS Imaging of the Host Galaxies of z ~ 2 - 3 Radio-Quiet Quasars
We have made a deep NICMOS imaging study of a sample of 5 z ~ 2 - 3
radio-quiet quasars with low absolute nuclear luminosities, and we have
detected apparent host galaxies in all of these. Most of the hosts have
luminosities approximately equal to present-day L*, with a range from 0.2 L* to
about 4 L*. These host galaxies have magnitudes and sizes consistent with those
of the Lyman break galaxies at similar redshifts and at similar rest
wavelengths, but are about two magnitudes fainter than high-z powerful radio
galaxies. The hosts of our high-z sample are comparable to or less luminous
than the hosts of the low-z RQQs with similar nuclear absolute magnitudes.
However, the high z galaxies are more compact than the hosts of the low z
quasars, and probably have only 10 - 20% of the stellar mass of their low-z
counterparts. Application of the M(bulge)/M(BH) relation found for present-day
spheroids to the stellar masses implied for the high z host galaxies would
indicate that they contain black holes with masses around 10^8 Msolar.
Comparison to their nuclear magnitudes implies accretion rates that are near or
at the Eddington limit. Although these high z hosts already contain
supermassive black holes, the galaxies will need to grow significantly to
evolve into present-day L* galaxies. These results are basically consistent
with theoretical predictions for the hierarchical buildup of the galaxy host
and its relation to the central supermassive black hole.Comment: 25 pages, 13 figures, accepted for publication in Ap
High-Resolution K' Imaging of the z = 1.786 Radio Galaxy 3C 294
We have obtained imaging in the K' band (~I-band rest frame) of the z=1.786
radio galaxy 3C 294 with the 36-element curvature-sensing adaptive optics
system Hokupa`a and the Canada-France-Hawaii Telescope. At a resolution of <
\~0."15, the galaxy is seen as a group of small but resolved knots distributed
over a roughly triangular region ~1."4 across. The interpretation of the
structure depends on the location of the nucleus, as indicated by the compact
radio core. Its position is uncertain by > ~0."5 (2-sigma) because of
uncertainties in the optical astrometry, but our best estimate places it at or
near the southern apex of the distribution. If this location is correct, the
most likely interpretation is that of a hidden quasar nucleus illuminating
dusty infalling dwarf-galaxy-like clumps having characteristic sizes of ~1.5
kpc.Comment: 8 pages. One figure in gif format. Postscript version including fig.
(240 kb) available at http://www.ifa.hawaii.edu/~canaguby/preprints.html To
appear in ApJ. Letter
A Description of Stroke Dynamics in 100 Meter Wheelchair Racing
Since wheelchair racing was introduced in the United States over thirty years ago, wheelchair sports have been experiencing a growing popularity.
An ever increasing number of national, and international competitions are being held for the disabled athlete; and record times in racing events are being set on an almost routine basis. Much interest by coaches, athletes, and researchers exists in identifying optimal performance factors in wheelchair propulsion, Three major areas of interest relating to performance have been the topics of recent research, symposia, and conferenees. These include the following: (1) designing effective training programs; (2) improving chair design; and (3) optimizing technique.
Elite disabled athletes are being profiled by researchers from both physiological and biomechanical perspectives. All wheelchair users stand to benefit from wheelchair sports and research. Where many everyday chair users once were in a heavy, awkward «hospital-type» chair that fitted no one and certainly wasn't designed for sports use, now light weight, easily maneuverable chairs are in use. As equipment is improved and propulsion techniques become more efficient, all chair users can benefit from such knowledge.
The United States Olympic Committee sponsored their first Sports Medieine and Sports Science Conference for the Disabled Athlete in the United States in March of 1987. This conference provided the opportunity for coaches, athletes, researchers, and other persons interested in sports for the disabled athlete to come together to share knowledge and ideas, and to examine the unique needs of the disabled performer. While physical limitations may influence the disabled athlete's perfomance, today's athletes are vitally interested in learning how to maximize their individual physical abilities.
Although the major thrust of a great many of the research studies investigating wheelchair athletes has often been of a physiologic nature, a growing body of biomechanic research on wheelchair propulsion has been identified (Ridgwaw, Pope & Wilkerson, 1987; Siler, Martin & Mungiole, 1987; Higgs, 1986; Sanderson & Sommer, 1985; Cerquiglini, Figura, Marchetti & Ricci, 1981; King, 1981; and Perry, 1981). Many of these investigations have included small sample sizes, have been limited to male subjects, and have included relatively few classes of wheelchair athletes. Additionally, few have studied the elite wheelchair athlete during commpetition.
The purpose of this study was to develop a kinematic model of wheelchair propulsion during 100-meter racing as performed by three classes of elite male wheelchair athletes
Preoperative screening cultures in the identification of staphylococci causing wound and valvular infections in cardiac surgery
Cultures of nasal or presternal swabs form part of the routine preoperative screening of patients on the cardiac surgical ward. During a trial of antibiotic prophylaxis in 314 patients, preoperative isolates of Staphylococcus aureus and coagulase-negative staphylococci were compared with strains associated with postoperative sternal wound breakdown (24 patients) and prosthetic valve endocarditis (3 patients). Morphology, antibiotic sensitivity pattern, plasmid analysis and phage typing were used to differentiate strains. In only three cases of wound infection and one of prosthetic valve endocarditis were pathogenic staphylococci not distinguishable from preoperative isolates. The collection of superficial swabs for this purpose before cardiac surgery is therefore unlikely to be cost effective
Formation of diluted IIIâV nitride thin films by N ion implantation
iluted IIIâNââVâËâ alloys were successfully synthesized by nitrogen implantation into GaAs,InP, and AlyGa1âyAs. In all three cases the fundamental band-gap energy for the ion beam synthesized IIIâNââVâËâ alloys was found to decrease with increasing N implantation dose in a manner similar to that observed in epitaxially grownGaNâAs1âx and InNâPâËâalloys. In GaNâAsâËâ the highest value of x (fraction of âactiveâ substitutional N on As sublattice) achieved was 0.006. It was observed that NAs is thermally unstable at temperatures higher than 850â°C. The highest value of x achieved in InNâPâËâ was higher, 0.012, and the NP was found to be stable to at least 850â°C. In addition, the N activation efficiency in implantedInNâPâËâ was at least a factor of 2 higher than that in GaNâAsâËâ under similar processing conditions. AlyGa1âyNâAsâËâ had not been made previously by epitaxial techniques. N implantation was successful in producing AlyGa1âyNâAsâËâalloys. Notably, the band gap of these alloys remains direct, even above the value of y (y>0.44) where the band gap of the host material is indirect.This work was supported by the ââPhotovoltaic Materials
Focus Areaââ in the DOE Center of Excellence for the Synthesis
and Processing of Advanced Materials, Office of Science,
Office of Basic Energy Sciences, Division of Materials
Sciences under U.S. Department of Energy Contract No. DE-ACO3-76SF00098. The work at UCSD was partially supported
by Midwest Research Institute under subcontractor
No. AAD-9-18668-7 from NREL
The Spitzer mid-infrared AGN survey. II-the demographics and cosmic evolution of the AGN population
We present luminosity functions derived from a spectroscopic survey of AGN
selected from Spitzer Space Telescope imaging surveys. Selection in the
mid-infrared is significantly less affected by dust obscuration. We can thus
compare the luminosity functions of the obscured and unobscured AGN in a more
reliable fashion than by using optical or X-ray data alone. We find that the
AGN luminosity function can be well described by a broken power-law model in
which the break luminosity decreases with redshift. At high redshifts
(), we find significantly more AGN at a given bolometric luminosity than
found by either optical quasar surveys or hard X-ray surveys. The fraction of
obscured AGN decreases rapidly with increasing AGN luminosity, but, at least at
high redshifts, appears to remain at \% even at bolometric
luminosities . The data support a picture in which the
obscured and unobscured populations evolve differently, with some evidence that
high luminosity obscured quasars peak in space density at a higher redshift
than their unobscured counterparts. The amount of accretion energy in the
Universe estimated from this work suggests that AGN contribute about 12\% to
the total radiation intensity of the Universe, and a high radiative accretion
efficiency is required to match current
estimates of the local mass density in black holes.Comment: 14 pages, accepted by Ap
Large amounts of optically-obscured star formation in the host galaxies of some type-2 quasars
We present Hubble Space Telescope images, and spectral energy distributions
from optical to infrared wavelengths for a sample of six 0.3<z<0.8 type-2
quasars selected in the mid-infrared using data from the Spitzer Space
Telescope. All the host galaxies show some signs of disturbance. Most seem to
possess dusty, star-forming disks. The disk inclination, estimated from the
axial ratio of the hosts, correlates with the depth of the silicate feature in
the mid-infrared spectra, implying that at least some of the reddening towards
the AGN arises in the host galaxy. The star formation rates in these objects,
as inferred from the strengths of the PAH features and far-infrared continuum,
range from 3-90 Msun/yr, but are mostly much larger than those inferred from
the [OII]3727 emission line luminosity, due to obscuration. Taken together with
studies of type-2 quasar hosts from samples selected in the optical and X-ray,
this is consistent with previous suggestions that two types of extinction
processes operate within the type-2 quasar population, namely a component due
to the dusty torus in the immediate environment of the AGN, and a more extended
component due to a dusty, star forming disk.Comment: 5 pages, 3 figures. Accepted by ApJ Letter
Static Black Hole Solutions without Rotational Symmetry
We construct static black hole solutions that have no rotational symmetry.
These arise in theories, including the standard electroweak model, that include
charged vector mesons with mass . In such theories, a magnetically
charged Reissner-Nordstrom black hole with horizon radius less than a critical
value of the order of is classically unstable against the development
of a nonzero vector meson field just outside the horizon, indicating the
existence of static black hole solutions with vector meson hair. For the case
of unit magnetic charge, spherically symmetric solutions of this type have
previously been studied. For other values of the magnetic charge, general
arguments show that any new solution with hair cannot be spherically symmetric.
In this paper we develop and apply a perturbative scheme (which may have
applicability in other contexts) for constructing such solutions in the case
where the Reissner-Nordstrom solution is just barely unstable. For a few low
values of the magnetic charge the black holes retain a rotational symmetry
about a single axis, but this axial symmetry disappears for higher charges.
While the vector meson fields vanish exponentially fast at distances greater
than , the magnetic field and the metric have higher multipole
components that decrease only as powers of the distance from the black hole.Comment: 42 pages, phyzzx. 4 figures (PostScript, 1.7 MB when uncompressed)
available by email from the Authors on reques
MODIS Science Algorithms and Data Systems Lessons Learned
For almost 10 years, standard global products from NASA's Earth Observing System s (EOS) two Moderate Resolution Imaging Spectroradiometer (MODIS) sensors are being used world-wide for earth science research and applications. This paper discusses the lessons learned in developing the science algorithms and the data systems needed to produce these high quality data products for the earth sciences community. Strong science team leadership and communication, an evolvable and scalable data system, and central coordination of QA and validation activities enabled the data system to grow by two orders of magnitude from the initial at-launch system to the current system able to reprocess data from both the Terra and Aqua missions in less than a year. Many of the lessons learned from MODIS are already being applied to follow-on missions
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