425 research outputs found
Brans-Dicke Gravity from Entropic Viewpoint
We interpret the Brans-Dicke gravity from entropic viewpoint. We first apply
the Verlinde's entropic formalism in the Einstein frame, then perform the
conformal transformation which connects the Einstein frame to the Jordan frame.
The transformed result yields the equation of motion of the Brans-Dicke theory
in the Jordan frame.Comment: Title changed, minor changes to match the published versio
Intrinsic Axis Ratio Distribution of Early-type Galaxies From Sloan Digital Sky Survey
Using Sloan Digital Sky Survey Data Release 5, we have investigated the
intrinsic axis ratio distribution (ARD) for early-type galaxies. We have
constructed a volume-limited sample of 3,922 visually-inspected early-type
galaxies at carefully considering sampling biases
caused by the galaxy isophotal size and luminosity. We attempt to de-project
the observed ARD into three-dimensional types (oblate, prolate, and triaxial),
which are classified in terms of triaxiality. We confirm that no linear
combination of -distributed axis ratios of the three types can
reproduce the observed ARD. However, using Gaussian intrinsic distributions, we
have found reasonable fits to the data with preferred mean axis ratios for
oblate, prolate, and triaxial (triaxials in two axis ratios), where the fractions of
oblate, prolate and triaxial types are
\textrm{O:P:T}=0.29^{\pm0.09}:0.26^{\pm0.11}:0.45^{\pm0.13}-23.3 < M_r \leq -21.2-21.2 < M_r <-19.3$) sample does. Oblate is
relatively more abundant among the less luminous galaxies. Interestingly, the
preferences of axis ratios for triaxial types in the two luminosity classes are
remarkably similar. We have not found any significant influence of the local
galaxy number density on ARD. We show that the results can be seriously
affected by the details in the data selection and type classification scheme.
Caveats and implications on galaxy formation are discussed.Comment: 9 pages, 11 figures, Accepted for publication in Ap
Understanding the escape of LyC and LyĪ± photons from turbulent clouds
Understanding the escape of Lyman continuum (LyC) and Lyman alpha (Lya)
photons from molecular clouds is one of the keys to constraining the
reionization history of the Universe and the evolution of galaxies at high
redshift. Using a set of radiation-hydrodynamic simulations with adaptive mesh
refinement, we investigate how photons propagate and escape from turbulent
clouds with different masses, star formation efficiencies (SFEs), and
metallicities, as well as with different models of stellar spectra and
supernova feedback. We find that the escape fractions in both LyC and Lya are
generally increasing with time if the cloud is efficiently dispersed by
radiation and supernova feedback. When the total SFE is low (1% of the cloud
mass), 0.1-5% of LyC photons leave the metal-poor cloud, whereas the fractions
increase to 20-70% in clouds with a 10% SFE. LyC photons escape more
efficiently if gas metallicity is lower, if the upper mass limit in the stellar
initial mass function is higher, if binary interactions are allowed in the
evolution of stars, or if additional strong radiation pressure, such as Lya
pressure, is present. As a result, the number of escaping LyC photons can
easily vary by a factor of on cloud scales. The escape fractions of Lya
photons are systemically higher (60-80%) than those of LyC photons despite
large optical depths at line centre (). Scattering of Lya
photons is already significant on cloud scales, leading to double-peaked
profiles with peak separations of during
the initial stage of the cloud evolution, while it becomes narrower than
in the LyC bright phase. Comparisons
with observations of low-redshift galaxies suggest that Lya photons require
further interactions with neutral hydrogen to reproduce their velocity offset
for a given LyC escape fraction
The SPHINX cosmological simulations of the first billion years: The impact of binary stars on reionization
We present the SPHINX suite of cosmological adaptive mesh refinement
simulations, the first radiation-hydrodynamical simulations to simultaneously
capture large-scale reionization and the escape of ionizing radiation from
thousands of resolved galaxies. Our and co-moving Mpc volumes resolve
haloes down to the atomic cooling limit and model the inter-stellar medium with
better than pc resolution. The project has numerous goals in
improving our understanding of reionization and making predictions for future
observations. In this first paper we study how the inclusion of binary stars in
computing stellar luminosities impacts reionization, compared to a model that
includes only single stars. Owing to the suppression of galaxy growth via
strong feedback, our galaxies are in good agreement with observational
estimates of the galaxy luminosity function. We find that binaries have a
significant impact on the timing of reionization: with binaries, our boxes are
percent ionized by volume at , while without them our
volumes fail to reionize by . These results are robust to changes in
volume size, resolution, and feedback efficiency. The escape of ionizing
radiation from individual galaxies varies strongly and frequently. On average,
binaries lead to escape fractions of percent, about times
higher than with single stars only. The higher escape fraction is a result of a
shallower decline in ionizing luminosity with age, and is the primary reason
for earlier reionization, although the higher integrated luminosity with
binaries also plays a sub-dominant role
Implications of the remarkable homogeneity of galaxy groups and clusters
We measure the diversity of galaxy groups and clusters with mass M>1E13/h
Msun, in terms of the star formation history of their galaxy populations, for
the purpose of constraining the mass scale at which environmentally-important
processes play a role in galaxy evolution. We consider three different group
catalogues, selected in different ways, with photometry and spectroscopy from
the Sloan Digital Sky Survey. For each system we measure the fraction of
passively-evolving galaxies within R200 and brighter than either Mr=-18 (and
with z<0.05) or Mr=-20 (and z<0.1). We use the (u-g) and (r-i) galaxy colours
to distinguish between star-forming and passively-evolving galaxies. By
considering the binomial distribution expected from the observed number of
members in each cluster, we are able to either recover the intrinsic scatter in
this fraction, or put robust 95% confidence upper-limits on its value. The
intrinsic standard deviation in the fraction of passive galaxies is consistent
with a small value of <0.1 in most mass bins for all three samples. There is no
strong trend with mass; even groups with M=1E13/h Msun are consistent with such
a small, intrinsic distribution. We compare these results with theoretical
models of the accretion history to show that, if environment plays a role in
transforming galaxies, such effects must occur first at mass scales far below
that of rich clusters, at most M=1E13 Msun.Comment: 5 pages, MNRAS Letters, in pres
Predicting biochemical acclimation of leaf photosynthesis in soybean under in-field canopy warming using hyperspectral reflectance
Traditional gas exchange measurements are cumbersome, which makes it difficult to capture variation in biochemical parameters, namely the maximum rate of carboxylation measured at a reference temperature (Vcmax25) and the maximum electron transport at a reference temperature (Jmax25), in response to growth temperature over time from days to weeks. Hyperspectral reflectance provides reliable measures of Vcmax25 and Jmax25; however, the capability of this method to capture biochemical acclimations of the two parameters to high growth temperature over time has not been demonstrated. In this study, Vcmax25 and Jmax25 were measured over multiple growth stages during two growing seasons for field-grown soybeans using both gas exchange techniques and leaf spectral reflectance under ambient and four elevated canopy temperature treatments (ambient+1.5, +3, +4.5, and +6Ā°C). Spectral vegetation indices and machine learning methods were used to build predictive models for Vcmax25 and Jmax25, based on the leaf reflectance. Results showed that these models yielded an R2 of 0.57ā0.65 and 0.48ā0.58 for Vcmax25 and Jmax25, respectively. Hyperspectral reflectance captured biochemical acclimation of leaf photosynthesis to high temperature in the field, improving spatial and temporal resolution in the ability to assess the impact of future warming on crop productivity
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
Licence to be active: parental concerns and 10ā11-year-old children's ability to be independently physically active
Background Physical activity independent of adult supervision is an important component of youth physical activity. This study examined parental attitudes to independent activity, factors that limit licence to be independently active and parental strategies to facilitate independent activity. Methods In-depth phone interviews were conducted with 24 parents (4 males) of 10ā11-year-old children recruited from six primary schools in Bristol. Results Parents perceived that a lack of appropriate spaces in which to be active, safety, traffic, the proximity of friends and older children affected childrenās ability to be independently physically active. The final year of primary school was perceived as a period when children should be afforded increased licence. Parents managed physical activity licence by placing time limits on activity, restricting activity to close to home, only allowing activity in groups or under adult supervision. Conclusions Strategies are needed to build childrenās licence to be independently active; this could be achieved by developing parental self-efficacy to allow children to be active and developing structures such as safe routes to parks and safer play areas. Future programmes could make use of traffic-calming programmes as catalysts for safe independent physical activity
- ā¦