3,011 research outputs found
The rotation curves of dwarf galaxies: a problem for Cold Dark Matter?
We address the issue of accuracy in recovering density profiles from
observations of rotation curves of galaxies. We ``observe'' and analyze our
models in much the same way as observers do the real galaxies. We find that the
tilted ring model analysis produces an underestimate of the central rotational
velocity. In some cases the galaxy halo density profile seems to have a flat
core, while in reality it does not. We identify three effects, which explain
the systematic biases: (1) inclination (2), small bulge, and (3) bar. The
presence of even a small non-rotating bulge component reduces the rotation
velocity. In the case of a disk with a bar, the underestimate of the circular
velocity is larger due to a combination of non-circular motions and random
velocities. Signatures of bars can be difficult to detect in the surface
brightness profiles of the model galaxies. The variation of inclination angle
and isophote position angle with radius are more reliable indicators of bar
presence than the surface brightness profiles. The systematic biases in the
central ~ 1 kpc of galaxies are not large. Each effect separately gives
typically a few kms error, but the effects add up. In some cases the error in
circular velocity was a factor of two, but typically we get about 20 percent.
The result is the false inference that the density profile of the halo flattens
in the central parts. Our observations of real galaxies show that for a large
fraction of galaxies the velocity of gas rotation (as measured by emission
lines) is very close to the rotation of stellar component (as measured by
absorption lines). This implies that the systematic effects discussed in this
paper are also applicable both for the stars and emission-line gas.Comment: ApJ, in press, 30 pages, Latex, 21 .eps figure
Searching for tidal tails around Centauri using RR Lyrae Stars
We present a survey for RR Lyrae stars in an area of 50 deg around the
globular cluster Centauri, aimed to detect debris material from the
alleged progenitor galaxy of the cluster. We detected 48 RR Lyrae stars of
which only 11 have been previously reported. Ten among the eleven previously
known stars were found inside the tidal radius of the cluster. The rest were
located outside the tidal radius up to distances of degrees from the
center of the cluster. Several of those stars are located at distances similar
to that of Centauri. We investigated the probability that those stars
may have been stripped off the cluster by studying their properties (mean
periods), calculating the expected halo/thick disk population of RR Lyrae stars
in this part of the sky, analyzing the radial velocity of a sub-sample of the
RR Lyrae stars, and finally, studying the probable orbits of this sub-sample
around the Galaxy. None of these investigations support the scenario that there
is significant tidal debris around Centauri, confirming previous
studies in the region. It is puzzling that tidal debris have been found
elsewhere but not near the cluster itself.Comment: 11 pages, 11 figures, Accepte
Thermal spin transport and spin-orbit interaction in ferromagnetic/non-magnetic metals
In this article we extend the currently established diffusion theory of
spin-dependent electrical conduction by including spin-dependent
thermoelectricity and thermal transport. Using this theory, we propose new
experiments aimed at demonstrating novel effects such as the spin-Peltier
effect, the reciprocal of the recently demonstrated thermally driven spin
injection, as well as the magnetic heat valve. We use finite-element methods to
model specific devices in literature to demonstrate our theory. Spin-orbit
effects such as anomalous-Hall, -Nernst, anisotropic magnetoresistance and
spin-Hall are also included in this model
Amplitude Zeros in Production
We demonstrate that the Standard Model amplitude for at the Born-level exhibits an approximate zero located at
at
high energies, where the () are the left-handed couplings
of the -boson to fermions and is the center of mass scattering
angle of the -boson. The approximate zero is the combined result of an exact
zero in the dominant helicity amplitudes and strong gauge
cancelations in the remaining amplitudes. For non-standard couplings
these cancelations no longer occur and the approximate amplitude zero is
eliminated.Comment: 11 pages, 4 figures submitted separately as uuencoded tar-ed
postscript files, FSU-HEP-940307, UCD-94-
Electron Wave Function in Armchair Graphene Nanoribbons
By using analytical solution of a tight-binding model for armchair
nanoribbons, it is confirmed that the solution represents the standing wave
formed by intervalley scattering and that pseudospin is invariant under the
scattering. The phase space of armchair nanoribbon which includes a single
Dirac singularity is specified. By examining the effects of boundary
perturbations on the wave function, we suggest that the existance of a strong
boundary potential is inconsistent with the observation in a recent scanning
tunneling microscopy. Some of the possible electron-density superstructure
patterns near a step armchair edge located on top of graphite are presented. It
is demonstrated that a selection rule for the G band in Raman spectroscopy can
be most easily reproduced with the analytical solution.Comment: 7 pages, 4 figure
Impact of time-ordered measurements of the two states in a niobium superconducting qubit structure
Measurements of thermal activation are made in a superconducting, niobium
Persistent-Current (PC) qubit structure, which has two stable classical states
of equal and opposite circulating current. The magnetization signal is read out
by ramping the bias current of a DC SQUID. This ramping causes time-ordered
measurements of the two states, where measurement of one state occurs before
the other. This time-ordering results in an effective measurement time, which
can be used to probe the thermal activation rate between the two states.
Fitting the magnetization signal as a function of temperature and ramp time
allows one to estimate a quality factor of 10^6 for our devices, a value
favorable for the observation of long quantum coherence times at lower
temperatures.Comment: 14 pages, 4 figure
Determining MAOD Using a Single Exhaustive Severe Intensity Test
International Journal of Exercise Science 13(4): 702-713, 2020. Maximal accumulated oxygen deficit (MAOD) provides a measure of anaerobic capacity. However, its measurement is a time-consuming process. The purpose of this study was to evaluate a measure of anaerobic capacity that avoids contentious assumptions and demands of the MAOD method. Twelve women and eight men volunteered for the study and completed cycle ergometer tests that resulted in exhaustion after ~4 min and ~8 min. In each test, anaerobic capacity was determined as (i) the MAOD and (ii) the sum of the phosphocreatine and glycolytic contributions (PCr+glycolysis). MAOD was determined by subtraction of the accumulated oxygen uptake from the total oxygen cost. Phosphocreatine and glycolytic contributions were calculated from post-exercise VO2 and blood lactate responses. MAOD in the 4-min and 8-min tests (79.1 ± 7.6 mL∙kg–1 and 79.6 ± 7.4 mL∙kg–1) and PCr+glycolysis in these tests (80.0 ± 7.3 mL∙kg–1 and 79.0 ± 6.9 mL∙kg–1) were correlated (r ≥ 0.91) and not significantly different. These results support the use of post-exercise measures to quantify the phosphocreatine and glycolytic contributions and to provide an alternative to MAOD for measurement of anaerobic capacity
Interaction induced collapse of a section of the Fermi sea in in the zig-zag Hubbard ladder
Using the next-nearest neighbor (zig-zag) Hubbard chain as an one
dimemensional model, we investigate the influence of interactions on the
position of the Fermi wavevectors with the density-matrix renormalization-group
technique (DMRG). For suitable choices of the hopping parameters we observe
that electron-electron correlations induce very different renormalizations for
the two different Fermi wavevectors, which ultimately lead to a complete
destruction of one section of the Fermi sea in a quantum critical point
The Increase in Oxygen Demand During Severe Intensity Exercise Must be Included in Calculation of Oxygen Deficit
International Journal of Exercise Science 13(4): 645-655, 2020. A contentious element in the traditional method of calculating accumulated oxygen deficit (AOD) is the assumption that the oxygen demand remains constant throughout a bout of exercise. The purpose of this study was to investigate the appropriateness of this assumption. Twelve women and eight men volunteered for the study and completed cycle ergometer tests that resulted in exhaustion after ~4 min and ~8 min. In each test, AOD was calculated by subtracting accumulated oxygen uptake (in mL∙kg–1) from estimated total oxygen cost (in mL∙kg–1), which was estimating two ways: (i) assuming that oxygen demand (in mL∙kg–1∙min–1) increases over the course of the exercise bout and (ii) assuming it remains constant. Values for AOD in the 4-min and 8-min tests were expected to be the same (maximal). Mean values for AOD in the 4-min and 8-min tests were similar (79.1 ± 7.6 mL∙kg–1 and 79.6 ± 8.3 mL∙kg–1) when calculated assuming an increase in oxygen demand, but different (71.0 ± 7.9 mL∙kg–1 and 42.5 ± 7.6 mL∙kg–1) when the demand was kept constant. These results support the hypothesis that oxygen demand increases during exhaustive severe intensity cycling exercise. This increase must be included in calculation of AOD
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