478 research outputs found
A Complete Spectroscopic Survey of the Milky Way satellite Segue 1: Dark matter content, stellar membership and binary properties from a Bayesian analysis
We introduce a comprehensive analysis of multi-epoch stellar line-of-sight
velocities to determine the intrinsic velocity dispersion of the ultrafaint
satellites of the Milky Way. Our method includes a simultaneous Bayesian
analysis of both membership probabilities and the contribution of binary
orbital motion to the observed velocity dispersion within a 14-parameter
likelihood. We apply our method to the Segue 1 dwarf galaxy and conclude that
Segue 1 is a dark-matter-dominated galaxy at high probability with an intrinsic
velocity dispersion of 3.7^{+1.4}_{-1.1} km/sec. The dark matter halo required
to produce this dispersion must have an average density of 2.5^{+4.1}_{-1.9}
solar mass/pc^3 within a sphere that encloses half the galaxy's stellar
luminosity. This is the highest measured density of dark matter in the Local
Group. Our results show that a significant fraction of the stars in Segue 1 may
be binaries with the most probable mean period close to 10 years, but also
consistent with the 180 year mean period seen in the solar vicinity at about 1
sigma. Despite this binary population, the possibility that Segue 1 is a bound
star cluster with the observed velocity dispersion arising from the orbital
motion of binary stars is disfavored by the multi-epoch stellar velocity data
at greater than 99% C.L. Finally, our treatment yields a projected
(two-dimensional) half-light radius for the stellar profile of Segue 1 of
28^{+5}_{-4} pc, in excellent agreement with photometric measurements.Comment: 15 pages, 19 figure
Detection of Gram-Negative Histamine-Producing Bacteria in Fish: A Comparative Study
ABSTRACT Poisoning due to ingestion of foods with elevated levels of biogenic amines (histamine, putrescine, cadaverine, and tyramine) is well documented. Histamine fish poisoning largely is due to growth of naturally occurring bacteria associated with scombroid fish species. A rapid and reliable method is needed to screen for the presence of histamine-forming bacteria in fish. This study included a comparison of three methods for the detection of histamine-producing bacteria. A total of 152 histamineproducing and non-histamine-producing bacteria from multiple sources were screened using a modified Niven's agar method, a potentiometric method, and a PCR-based assay targeting a 709-bp fragment of the histidine decarboxylase gene. Histamine production by bacterial isolates was confirmed by high-performance liquid chromatography (HPLC). Bacterial strains were categorized as producing high amounts of histamine, low amounts of histamine, or no histamine. Of the 152 strains tested, 128 (84%) were positive with the Niven's agar method, 73 (48%) were positive with the potentiometric technique, and 74 (49%) were positive with the PCR assay. Overall, a 38% false-positive rate was observed with the modified Niven's agar method, although this method detected both low-histamine and high-histamine strains. There was a high degree of concordance (.99%) between results of the potentiometric and PCR methods, but neither of these methods detected low-histamine bacteria. These observations support the need for a simple and straightforward yet sensitive method for detecting histamine-producing bacteria in seafood and environmental samples
Wide-Field Imaging and Polarimetry for the Biggest and Brightest in the 20GHz Southern Sky
We present wide-field imaging and polarimetry at 20GHz of seven of the most
extended, bright (Stot >= 0.50 Jy), high-frequency selected radio sources in
the southern sky with declinations < -30 deg. Accompanying the data are brief
reviews of the literature for each source, The results presented here aid in
the statistical completeness of the Australia Telescope 20GHz Survey's bright
source sample. The data are of crucial interest for future cosmic microwave
background missions as a collection of information about candidate calibrator
sources. We are able to obtain data for seven of the nine sources identified by
our selection criteria. We report that Pictor A is thus far the best
extragalactic calibrator candidate for the Low Frequency Instrument of the
Planck European Space Agency mission due to its high level of integrated
polarized flux density (0.50+/-0.06 Jy) on a scale of 10 arcmin. Six of the
seven sources have a clearly detected compact radio core, with either a null or
less than two percent detection of polarized emission from the nucleus. Most
sources with detected jets have magnetic field alignments running in a
longitudinal configuration, however PKS1333-33 exhibits transverse fields and
an orthogonal change in field geometry from nucleus to jets.Comment: 17 pages, 9 figures, 2 table
High--frequency predictions for number counts and spectral properties of extragalactic radio sources. New evidences of a break at mm wavelengths in spectra of bright blazar sources
We present models to predict high frequency counts of extragalactic radio
sources using physically grounded recipes to describe the complex spectral
behaviour of blazars, that dominate the mm-wave counts at bright flux
densities. We show that simple power-law spectra are ruled out by
high-frequency (nu>100 GHz) data. These data also strongly constrain models
featuring the spectral breaks predicted by classical physical models for the
synchrotron emission produced in jets of blazars (Blandford & Konigl 1979;
Konigl 1981). A model dealing with blazars as a single population is, at best,
only marginally consistent with data coming from current surveys at high radio
frequencies. Our most successful model assumes different distributions of break
frequencies, nu_M, for BL Lacs and Flat-Spectrum Radio Quasars (FSRQs). The
former objects have substantially higher values of nu_M, implying that the
synchrotron emission comes from more compact regions; therefore, a substantial
increase of the BL Lac fraction at high radio frequencies and at bright flux
densities is predicted. Remarkably, our best model is able to give a very good
fit to all the observed data on number counts and on distributions of spectral
indices of extragalactic radio sources at frequencies above 5 and up to 220
GHz. Predictions for the forthcoming sub-mm blazar counts from Planck, at the
highest HFI frequencies, and from Herschel surveys are also presented.Comment: 21 pages, 16 figures; Eq.B.2 and labels in few Figures correcte
Pseudoscalar Glueball Mass: QCD vs. Lattice Gauge Theory Prediction
We study whether the pseudoscalar glueball mass in full QCD can differ from
the prediction of quenched lattice calculations. Using properties of the
correlator of the vacuum topological susceptibility we derive an expression for
the upper bound on the QCD glueball mass. We show that the QCD pseudoscalar
glueball is lighter than the pure Yang-Mills theory glueball studied in
quenched lattice calculations. The mass difference between those two states is
of order of . The value calculated for the QCD glueball mass
can not be reconciled with any physical state observed so far in the
corresponding channel. The glueball decay constant and its production rate in
radiative decays are calculated. The production rate is large enough
to be studied experimentally.Comment: 18 pages, LaTex fil
Follow-up observations at 16 and 33 GHz of extragalactic sources from WMAP 3-year data: I - Spectral properties
We present follow-up observations of 97 point sources from the Wilkinson
Microwave Anisotropy Probe (WMAP) 3-year data, contained within the New
Extragalactic WMAP Point Source (NEWPS) catalogue between declinations of -4
and +60 degrees; the sources form a flux-density-limited sample complete to 1.1
Jy (approximately 5 sigma) at 33 GHz. Our observations were made at 16 GHz
using the Arcminute Microkelvin Imager (AMI) and at 33 GHz with the Very Small
Array (VSA). 94 of the sources have reliable, simultaneous -- typically a few
minutes apart -- observations with both telescopes. The spectra between 13.9
and 33.75 GHz are very different from those of bright sources at low frequency:
44 per cent have rising spectra (alpha < 0.0), where flux density is
proportional to frequency^-alpha, and 93 per cent have spectra with alpha <
0.5; the median spectral index is 0.04. For the brighter sources, the agreement
between VSA and WMAP 33-GHz flux densities averaged over sources is very good.
However, for the fainter sources, the VSA tends to measure lower values for the
flux densities than WMAP. We suggest that the main cause of this effect is
Eddington bias arising from variability.Comment: 12 pages, 13 figures, submitted to MNRA
Galaxy Number Counts and Implications for Strong Lensing
We compare galaxy number counts in HST/ACS fields containing
moderate-redshift (0.2<z<1.0) strong gravitational lenses with those in two
control samples: (1) the first square degree of the COSMOS survey, comprising
259 ACS fields and (2) 20 "pure parallel" fields randomly located on the sky.
Through a Bayesian analysis we determine the expectation values (mu_0) and
confidence levels of the underlying number counts for a range of apertures and
magnitude bins. Our analysis has produced the following results: (i) We infer
that our control samples are not consistent, with the number counts in the
COSMOS sample being significantly higher than in the pure parallel sample for
21 <= F814W <= 23. This result matches those found in previous analyses of
COSMOS data using different techniques. (ii) We find that small-size apertures,
centered on strong lenses, are overdense compared with randomly placed
apertures in the control samples, even compared to the COSMOS sample.
Correcting for the local clustering of elliptical galaxies, based on the
average two-point correlation function reduces this overdensity to the 1-2
sigma level. Thus, the overdensity of galaxies seen along a typical line of
sight to a lens can be explained mostly by the natural clustering of galaxies,
rather than being due to lenses lying along otherwise biased lines of sight.
However, a larger sample of lenses is needed to see if the remaining bias
persists when the lens-field uncertainties are smaller. (iii) There is
considerable scatter in the lines of sight to _individual_ lens systems, but
quantities that are linearly dependent on the external convergence (e.g., H_0)
should become unbiased if the extra galaxies that cause the bias can be
accounted for in the lens models either through direct modeling or via an
informed prior on the external convergence. The number counts can used to set
such an informed prior.Comment: Modified to match the version accepted for publication in MNRAS. 15
Pages, 10 figure
Catalog of Radio Galaxies with z>0.3. I:Construction of the Sample
The procedure of the construction of a sample of distant () radio
galaxies using NED, SDSS, and CATS databases for further application in
statistical tests is described. The sample is assumed to be cleaned from
objects with quasar properties. Primary statistical analysis of the list is
performed and the regression dependence of the spectral index on redshift is
found.Comment: 9 pages, 6 figures, 2 table
Status of Muon Collider Research and Development and Future Plans
The status of the research on muon colliders is discussed and plans are
outlined for future theoretical and experimental studies. Besides continued
work on the parameters of a 3-4 and 0.5 TeV center-of-mass (CoM) energy
collider, many studies are now concentrating on a machine near 0.1 TeV (CoM)
that could be a factory for the s-channel production of Higgs particles. We
discuss the research on the various components in such muon colliders, starting
from the proton accelerator needed to generate pions from a heavy-Z target and
proceeding through the phase rotation and decay ()
channel, muon cooling, acceleration, storage in a collider ring and the
collider detector. We also present theoretical and experimental R & D plans for
the next several years that should lead to a better understanding of the design
and feasibility issues for all of the components. This report is an update of
the progress on the R & D since the Feasibility Study of Muon Colliders
presented at the Snowmass'96 Workshop [R. B. Palmer, A. Sessler and A.
Tollestrup, Proceedings of the 1996 DPF/DPB Summer Study on High-Energy Physics
(Stanford Linear Accelerator Center, Menlo Park, CA, 1997)].Comment: 95 pages, 75 figures. Submitted to Physical Review Special Topics,
Accelerators and Beam
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