14,339 research outputs found
Exact and Fast Numerical Algorithms for the Stochastic Wave Equation
On the basis of integral representations we propose fast numerical methods to solve the Cauchy problem for the stochastic wave equation without boundaries and with the Dirichlet boundary conditions. The algorithms are exact in a probabilistic sense
Molecular Gas Properties of the Giant Molecular Cloud Complexes in the Arms and Inter-arms of the Spiral Galaxy NGC 6946
Combining observations of multiple CO lines with radiative transfer modeling
is a very powerful tool to investigate the physical properties of the molecular
gas in galaxies. Using new observations as well as literature data, we provide
the most complete CO ladders ever generated for eight star-forming regions in
the spiral arms and inter-arms of the spiral galaxy NGC 6946, with observations
of the CO(1-0), CO(2-1), CO(3-2), CO(4-3), CO(6-5), 13CO(1-0) and 13CO(2-1)
transitions. For each region, we use the large velocity gradient assumption to
derive beam-averaged molecular gas physical properties, namely the gas kinetic
temperature (T_K), H2 number volume density n(H2) and CO number column density
N(CO). Two complementary approaches are used to compare the observations with
the model predictions: chi-square minimisation and likelihood. The physical
conditions derived vary greatly from one region to the next: T_K=10-250 K,
n(H2)=10^2.3-10^7.0 cm^-3 and N(CO)=10^15.0-10^19.3 cm^-2. The spectral line
energy distribution (SLED) of some of these extranuclear regions indicates a
star-formation activity that is more intense than that at the centre of our own
Milky Way. The molecular gas in regions with a large SLED turnover transition
(J_max>4) is hot but tenuous with a high CO column density, while that in
regions with a low SLED turnover transition (J_max<=4) is cold but dense with a
low CO column density. We finally discuss and find some correlations between
the physical properties of the molecular gas in each region and the presence of
young stellar population indicators (supernova remnants, HII regions, HI holes,
etc.)Comment: 23 pages, 11 figures, MNRAS, Accepte
The 2s atomic level in muonic 208-Pb
Relative intensities and energy measurements of 2s level in muonic Pb-20
Metallicities of Emission-Line Galaxies from HST ACS PEARS and HST WFC3 ERS Grism Spectroscopy at 0.6 < z < 2.4
Galaxies selected on the basis of their emission line strength show low
metallicities, regardless of their redshifts. We conclude this from a sample of
faint galaxies at redshifts between 0.6 < z < 2.4, selected by their prominent
emission lines in low-resolution grism spectra in the optical with the Advanced
Camera for Surveys (ACS) on the Hubble Space Telescope (HST) and in the
near-infrared using Wide-Field Camera 3 (WFC3). Using a sample of 11 emission
line galaxies (ELGs) at 0.6 < z < 2.4 with luminosities of -22 < M_B < -19,
which have [OII], H\beta, and [OIII] line flux measurements from the
combination of two grism spectral surveys, we use the R23 method to derive the
gas-phase oxygen abundances: 7.5 < 12+log(O/H) < 8.5. The galaxy stellar masses
are derived using Bayesian based Markov Chain Monte Carlo (\piMC^2) fitting of
their Spectral Energy Distribution (SED), and span the mass range 8.1 <
log(M_*/M_\sun) < 10.1. These galaxies show a mass-metallicity (M-L) and
Luminosity-Metallicity (L-Z) relation, which is offset by --0.6 dex in
metallicity at given absolute magnitude and stellar mass relative to the local
SDSS galaxies, as well as continuum selected DEEP2 samples at similar
redshifts. The emission-line selected galaxies most resemble the local "green
peas" galaxies and Lyman-alpha galaxies at z~0.3 and z~2.3 in the M-Z and L-Z
relations and their morphologies. The G-M_{20} morphology analysis shows that
10 out of 11 show disturbed morphology, even as the star-forming regions are
compact. These galaxies may be intrinsically metal poor, being at early stages
of formation, or the low metallicities may be due to gas infall and accretion
due to mergers.Comment: 24 pages with 7 figure
The relationship between the location of western mackerel spawning, larval drift and recruit distributions: A modelling study
Summary: Mackerel spawning distributions have changed over the last 20 years. From 1977-1995
there has been a progressive westerly shift in the distribution of spawning in May/June
(peak spawning), with a resultant increase in the proportion of spawning west of the shelf
break. Also during this time period there has been a marked increase in the proportion of
spawning in the north of the spawning area. This shift occurred most obviously between the
surveys carried out in 1986 and 1989. At the same time it is shown that there has been a
northerly shift in the distribution of first winter juveniles. These observations are derived
from data collected on the ICES coordinated winter bottom trawl surveys.
This study used data on mackerel egg distributions during May, derived from the ICES
triennial mackerel egg surveys as input to the NORWECOM transport model, using real
weather fields. The outputs from the model indicated that most eggs and larvae could
expect to be transported south after spawning and that there had been no significant change
in this transport pattern during the period studied. The combination of more northerly
spawning and the prevailing transport pattern may explain, in part, the increase in the
recruit population in the northern nursery areas. It is concluded that passive transport can
explain the juvenile distribution in some areas, but that active migration must also play a
role in the area of the Celtic Sea.
The interactions between the modelled transport patterns and the real egg distribution data
are discussed with reference to these topics and to the potential survival of larvae in the
first weeks after hatching
Streptococcus pneumoniae NanC. Structural insights into the specificity and mechanism of a sialidase that produces a sialidase inhibitor
This work was supported by the Biotechnology and Biological Sciences Research Council (UK) and the Medical Research Council (UK).Streptococcus pneumoniae is an important human pathogen that causes a range of disease states. Sialidases are important bacterial virulence factors. There are three pneumococcal sialidases: NanA, NanB, and NanC. NanC is an unusual sialidase in that its primary reaction product is 2-deoxy-2,3-didehydro-N-acetylneuraminic acid (Neu5Ac2en, also known as DANA), a nonspecific hydrolytic sialidase inhibitor. The production of Neu5Ac2en from α2-3-linked sialosides by the catalytic domain is confirmed within a crystal structure. A covalent complex with 3-fluoro-β-N-acetylneuraminic acid is also presented, suggesting a common mechanism with other sialidases up to the final step of product formation. A conformation change in an active site hydrophobic loop on ligand binding constricts the entrance to the active site. In addition, the distance between the catalytic acid/base (Asp-315) and the ligand anomeric carbon is unusually short. These features facilitate a novel sialidase reaction in which the final step of product formation is direct abstraction of the C3 proton by the active site aspartic acid, forming Neu5Ac2en. NanC also possesses a carbohydrate-binding module, which is shown to bind α2-3- and α2-6-linked sialosides, as well as N-acetylneuraminic acid, which is captured in the crystal structure following hydration of Neu5Ac2en by NanC. Overall, the pneumococcal sialidases show remarkable mechanistic diversity while maintaining a common structural scaffold.Publisher PDFPeer reviewe
3D Spectrophotometry of Planetary Nebulae in the Bulge of M31
We introduce crowded field integral field (3D) spectrophotometry as a useful
technique for the study of resolved stellar populations in nearby galaxies. As
a methodological test, we present a pilot study with selected extragalactic
planetary nebulae (XPN) in the bulge of M31, demonstrating how 3D spectroscopy
is able to improve the limited accuracy of background subtraction which one
would normally obtain with classical slit spectroscopy. It is shown that due to
the absence of slit effects, 3D is a most suitable technique for
spectrophometry. We present spectra and line intensities for 5 XPN in M31,
obtained with the MPFS instrument at the Russian 6m BTA, INTEGRAL at the WHT,
and with PMAS at the Calar Alto 3.5m Telescope. Using 3D spectra of bright
standard stars, we demonstrate that the PSF is sampled with high accuracy,
providing a centroiding precision at the milli-arcsec level. Crowded field 3D
spectrophotometry and the use of PSF fitting techniques is suggested as the
method of choice for a number of similar observational problems, including
luminous stars in nearby galaxies, supernovae, QSO host galaxies,
gravitationally lensed QSOs, and others.Comment: (1) Astrophysikalisches Institut Potsdam, (2) University of Durham.
18 pages, 11 figures, accepted for publication in Ap
The effect of heterospecific and conspecific competition on inter-individual differences in tungara frog tadpole (Engystomops pustulosus) behavior:Effect of competition on variation in tadpole behavior
Repeated social interactions with conspecifics and/or heterospecifics during early development may drive the differentiation of behavior among individuals. Competition is a major form of social interaction and its impacts can depend on whether interactions occur between conspecifics or heterospecifics and the directionality of a response could be specific to the ecological context that they are measured in. To test this, we reared tungara frog tadpoles (Engystomops pustulosus) either in isolation, with a conspecific tadpole or with an aggressive heterospecific tadpole, the whistling frog tadpole (Leptodactylus fuscus). In each treatment, we measured the body size and distance focal E. pustulosus tadpoles swam in familiar, novel and predator risk contexts six times during development. We used univariate and multivariate hierarchical mixed effect models to investigate the effect of treatment on mean behavior, variance among and within individuals, behavioral repeatability and covariance among individuals in their behavior between contexts. There was a strong effect of competition on behavior, with different population and individual level responses across social treatments. Within a familiar context, the variance in the distance swam within individuals decreased under conspecific competition but heterospecific competition caused more variance in the average distance swam among individuals. Behavioral responses were also context specific as conspecific competition caused an increase in the distance swam within individuals in novel and predator risk contexts. The results highlight that the impact of competition on among and within individual variance in behavior is dependent on both competitor species identity and context
Relationships between Larval and Juvenile Abundance of Winter-Spawned Fishes in North Carolina, USA
We analyzed the relationships between the larval and juvenile abundances of selected estuarine-dependent fishes that spawn during the winter in continental shelf waters of the U.S. Atlantic coast. Six species were included in the analysis based on their ecological and economic importance and relative abundance in available surveys: spot Leiostomus xanthurus, pinfish Lagodon rhomboides, southern flounder Paralichthys lethostigma, summer flounder Paralichthys dentatus, Atlantic croaker Micropogonias undulatus, and Atlantic menhaden Brevoortia tyrannus. Cross-correlation analysis was used to examine the relationships between the larval and juvenile abundances within species. Tests of synchrony across species were used to find similarities in recruitment dynamics for species with similar winter shelf-spawning life-history strategies. Positive correlations were found between the larval and juvenile abundances for three of the six selected species (spot, pinfish, and southern flounder). These three species have similar geographic ranges that primarily lie south of Cape Hatteras. There were no significant correlations between the larval and juvenile abundances for the other three species (summer flounder, Atlantic croaker, and Atlantic menhaden); we suggest several factors that could account for the lack of a relationship. Synchrony was found among the three southern species within both the larval and juvenile abundance time series. These results provide support for using larval ingress measures as indices of abundance for these and other species with similar geographic ranges and winter shelf-spawning life-history strategies
The Stromlo Missing Satellites Survey
The Stromlo Missing Satellites (SMS) program is a critical endeavor to
investigate whether cold dark matter cosmology is flawed in its ability to
describe the matter distribution on galaxy scales or proves itself once again
as a powerful theory to make observational predictions. The project will
deliver unprecedented results on Milky Way satellite numbers, their
distribution and physical properties. It is the deepest, most extended survey
for optically elusive dwarf satellite galaxies to date, covering the entire
20,000 sq deg of the Southern hemisphere. 150TB of CCD images will be analysed
in six photometric bands, 0.5-1.0 mag fainter than SDSS produced by the ANU
SkyMapper telescope over the next five years. (For more details see:
http://msowww.anu.edu.au/~jerjen/SMS_Survey.html)Comment: 4 pages, 1 figure, in "Galaxies in the Local Volume" (Sydney, 8-13
July 2007), eds B. Koribalski and H. Jerjen, Springer Astrophysics and Space
Science Proceedings, p. 18
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