699 research outputs found
Lie Algebras and Suppression of Decoherence in Open Quantum Systems
Since there are many examples in which no decoherence-free subsystems exist
(among them all cases where the error generators act irreducibly on the system
Hilbert space), it is of interest to search for novel mechanisms which suppress
decoherence in these more general cases. Drawing on recent work
(quant-ph/0502153) we present three results which indicate decoherence
suppression without the need for noiseless subsystems. There is a certain
trade-off; our results do not necessarily apply to an arbitrary initial density
matrix, or for completely generic noise parameters. On the other hand, our
computational methods are novel and the result--suppression of decoherence in
the error-algebra approach without noiseless subsystems--is an interesting new
direction.Comment: 7 page
On the number of representations providing noiseless subsystems
This paper studies the combinatoric structure of the set of all
representations, up to equivalence, of a finite-dimensional semisimple Lie
algebra. This has intrinsic interest as a previously unsolved problem in
representation theory, and also has applications to the understanding of
quantum decoherence. We prove that for Hilbert spaces of sufficiently high
dimension, decoherence-free subspaces exist for almost all representations of
the error algebra. For decoherence-free subsystems, we plot the function
which is the fraction of all -dimensional quantum systems which
preserve bits of information through DF subsystems, and note that this
function fits an inverse beta distribution. The mathematical tools which arise
include techniques from classical number theory.Comment: 17 pp, 4 figs, accepted for Physical Review
The Mid-Infrared Period-Luminosity Relations for the Small Magellanic Cloud Cepheids Derived from Spitzer Archival Data
In this paper we derive the Spitzer IRAC band period-luminosity (P-L)
relations for the Small Magellanic Cloud (SMC) Cepheids, by matching the
Spitzer archival SAGE-SMC data with the OGLE-III SMC Cepheids. We find that the
3.6micron and 4.5micron band P-L relations can be better described using two
P-L relations with a break period at log(P)=0.4: this is consistent with
similar results at optical wavelengths for SMC P-L relations. The 5.8micron and
8.0micron band P-L relations do not extend to sufficiently short periods to
enable a similar detection of a slope change at log(P)=0.4. The slopes of the
SMC P-L relations, for log(P)>0.4, are consistent with their LMC counterparts
that were derived from a similar dataset. They are also in agreement with those
obtained from a small sample of Galactic Cepheids with parallax measurements.Comment: 14 pages, 5 figures and 2 tables. ApJ accepte
Holographic Normal Ordering and Multi-particle States in the AdS/CFT Correspondence
The general correlator of composite operators of N=4 supersymmetric gauge
field theory is divergent. We introduce a means for renormalizing these
correlators by adding a boundary theory on the AdS space correcting for the
divergences. Such renormalizations are not equivalent to the standard normal
ordering of current algebras in two dimensions. The correlators contain contact
terms that contribute to the OPE; we relate them diagrammatically to
correlation functions of compound composite operators dual to multi-particle
states.Comment: 18 pages, one equation corr., further comments and refs. adde
High-resolution radio continuum survey of M33 II. Thermal and nonthermal emission
We determine the variation in the nonthermal radio spectral index in the
nearby spiral galaxy M33 at a linear resolution of 360 pc. We separate the
thermal and nonthermal components of the radio continuum emission without the
assumption of a constant nonthermal spectral index. Using the Spitzer FIR data
at 70 and 160 m and a standard dust model, we deredden the H
emission. The extinction corrected H emission serves as a template for
the thermal free-free radio emission. Subtracting from the observed 3.6 cm and
20 cm emission (Effelsberg and the VLA) this free-free emission, we obtain the
nonthermal maps. A constant electron temperature used to obtain the thermal
radio intensity seems appropriate for M~33 which, unlike the Milky Way, has a
shallow metallicity gradient. For the first time, we derive the distribution of
the nonthermal spectral index across a galaxy, M33. We detect strong nonthermal
emission from the spiral arms and star-forming regions. Wavelet analysis shows
that at 3.6 cm the nonthermal emission is dominated by contributions from
star-forming regions, while it is smoothly distributed at 20 cm. For the whole
galaxy, we obtain thermal fractions of 51% and 18% at 3.6 cm and 20 cm,
respectively. The thermal emission is slightly stronger in the southern than in
the northern half of the galaxy. We find a clear radial gradient of mean
extinction in the galactic plane. The nonthermal spectral index map indicates
that the relativistic electrons suffer energy-loss when diffusing from their
origin in star-forming regions towards interarm regions and the outer parts of
the galaxy. We also conclude that the radio emission is mostly nonthermal at R
5 kpc in M33.Comment: 15 pages, 14 figures, accepted for publication in the Astronomy and
Astrophysics journa
A multi-scale study of infrared and radio emission from Scd galaxy M33
We investigate the energy sources of the infrared (IR) emission and their
relation to the radio continuum emission at various spatial scales within the
Scd galaxy M33. We use the wavelet transform to analyze IR data at the Spitzer
wavelengths of 24, 70, and 160m, as well as recent radio continuum data at
3.6cm and 20cm. An H map serves as a tracer of the star forming regions
and as an indicator of the thermal radio emission. We find that the dominant
scale of the 70m emission is larger than that of the 24m emission,
while the 160m emission shows a smooth wavelet spectrum. The radio and
H maps are well correlated with all 3 MIPS maps, although their
correlations with the 160m map are weaker. After subtracting the bright
HII regions, the 24 and 70m maps show weaker correlations with the 20cm
map than with the 3.6cm map at most scales. We also find a strong correlation
between the 3.6cm and H emission at all scales. Comparing the results
with and without the bright HII regions, we conclude that the IR emission is
influenced by young, massive stars increasingly with decreasing wavelength from
160 to 24m. The radio-IR correlations indicate that the warm dust-thermal
radio correlation is stronger than the cold dust-nonthermal radio correlation
at scales smaller than 4kpc. A perfect 3.6cm-H correlation implies that
extinction has no significant effect on H emitting structures.Comment: 15 pages, 10 figures, accepted for publication in the Astronomy and
Astrophysics Journa
High resolution radio continuum survey of M33: I. The radio maps
We study the exponential scale length of total radio emission, the spectral
index distribution, and the linear radio polarization in the Scd galaxy M33.
Observations were carried out using the 3.6 cm dual channel and the 6.2 cm four
channel receivers of the 100--m Effelsberg telescope along with the L-band VLA
D--array at 20 cm. High spatial resolution and sensitivity in both total and
linearly polarized radio continuum emission from M33 were achieved. We found
considerable extended emission, not only from the main arms I S and I N, but
also from the weaker arms. The large--scale magnetic field exhibits
well--ordered spiral structure with almost the same orientation as that of the
optical spiral arms, however, it does not show a clear structural correlation
or anti--correlation with the optical arms. There is a north-south asymmetry in
polarization that is frequency-dependent. We found that the ring mean spectral
index versus radius increases faster beyond = 4 kpc. At each wavelength,
the exponential scale length is larger inside than outside = 4 kpc. From
the larger scales lengths at 4 kpc, we conclude that star forming
regions are mainly spread over the region 4 kpc without a dominant
nuclear concentration. Furthermore, at 4 kpc, a spatial correlation
between cosmic rays and star forming regions may exist. From the behaviour of
the mean spectral indices obtained from different pairs of the radio continuum
data at 3.6, 6.2, and 20 cm, we confirm that a decrease in the thermal fraction
causes an increase in the spectral index. The frequency-dependent asymmetry in
the polarization hints to an asymmetry in Faraday depolarization.Comment: 16 pages, 13 figures, accepted for publication in the Astronomy and
Astrophysics Journa
Towards a Precision Cosmology from Starburst Galaxies at z>2
This work investigates the use of a well-known empirical correlation between
the velocity dispersion, metallicity, and luminosity in H beta of nearby HII
galaxies to measure the distances to HII-like starburst galaxies at high
redshifts. This correlation is applied to a sample of 15 starburst galaxies
with redshifts between z=2.17 and z=3.39 to constrain Omega_m, using data
available from the literature. A best-fit value of Omega_m = 0.21 +0.30 -0.12
in a Lambda-dominated universe and of Omega_m = 0.11 +0.37 -0.19 in an open
universe is obtained. A detailed analysis of systematic errors, their causes,
and their effects on the values derived for the distance moduli and Omega_m is
carried out. A discussion of how future work will improve constraints on
Omega_m by reducing the errors is also presented.Comment: 7 pages, 3 figures, accepted for publication in MNRA
High levels of childhood obesity observed among 3- to 7-year-old New Zealand Pacific children is a public health concern.
This cross-sectional, community-based survey was designed to assess attained growth and body composition of 3- to 7-y-old Pacific children (n = 21 boys and 20 girls) living in Dunedin, New Zealand, and to examine nondietary factors associated with the percentage of body fat. Fat mass, lean tissue mass and the percentage of body fat were measured using dual energy X-ray absorptiometry. One trained anthropometrist also measured height, weight, skinfolds (triceps, subscapular) and circumferences (mid-upper arm, chest, waist, calf). Compared with the National Center for Health Statistics and National Health and Examination Surveys I and II reference data, these Pacific children were tall and heavy for their age with high arm-muscle-area-for-height. Median (quartiles) Z-scores for height and BMI-for-age and arm-muscle-area-for-height were 1.33 (0.60, 2.15), 1.20 (0.74, 4.43) and 1.09 (0.63, 1.85), respectively. Their median (quartile) percentage of body fat was 21.8% (15.0, 35.5) of which 38.5% was located in the trunk. The estimated percentage of children classified as obese ranged from 34 to 49% depending on the criterion used. Over 60% of the children had levels of trunk fat above 1 SD of reported age- and sex-specific Z-scores for New Zealand children. The nondietary factors examined (hours of television viewing and hours playing organized sports, as reported by parents) were not associated with variations in the percentage of body fat, after adjusting for age, sex and birth weight. These extremely high levels of obesity and truncal fat among very young New Zealand children will have major public health implications as these children age
A photoionization model of the spatial distribution of the optical and mid-IR properties in NGC595
We present a set of photoionization models that reproduce simultaneously the
observed optical and mid-infrared spatial distribution of the HII region NGC595
in the disk of M33 using the code CLOUDY. Both optical (PMAS-Integral Field
Spectroscopy) and mid-infrared (8 mi and 24 mi bands from Spitzer) data provide
enough spatial resolution to model in a novel approach the inner structure of
the HII region. We define a set of elliptical annular regions around the
central ionizing cluster with an uniformity in their observed properties and
consider each annulus as an independent thin shell structure. For the first
time our models fit the relative surface brightness profiles in both the
optical (Halpha, [OII], [OIII]) and the mid-infrared emissions (8 mi and 24
mi), under the assumption of a uniform metallicity (12+log(O/H) = 8.45; Esteban
et al. 2009) and an age for the stellar cluster of 4.5 Myr (Malumuth et al.
1996). Our models also reproduce the observed uniformity of the R23 parameter
and the increase of the [OII]/[OIII] ratio due to the decrease of the
ionization parameter. The variation of the Halpha profile is explained in terms
of the differences of the occupied volume (the product of filling factor and
total volume of the shell) in a matter-bounded geometry, which also allows to
reproduce the observed pattern of the extinction. The 8 mi/24 mi ratio is low
(ranging between 0.04 and 0.4) because it is dominated by the surviving of
small dust grains in the HII region, while the PAHs emit more weakly because
they cannot be formed in these thin HII gas shells. The ratio is also well
fitted in our models by assuming a dust-to-gas ratio in each annulus compatible
with the integrated estimate for the whole HII region after the 70 mi, and 160
mi Spitzer observations.Comment: Accepted for publication in MNRAS, 9 pages, 17 figure
- âŠ