35 research outputs found
Cosmological density perturbations in modified gravity theories
In the context of f(R) theories of gravity, we study the cosmological
evolution of scalar perturbations by using a completely general procedure. We
find that the exact fourth-order differential equation for the matter density
perturbations in the longitudinal gauge, reduces to a second-order equation for
sub-Hubble modes. This simplification is compared with the standard
(quasi-static) equation used in the literature. We show that for general f(R)
functions the quasi-static approximation is not justified. However for those
f(R) adequately describing the present phase of accelerated expansion and
satisfying local gravity tests, it does give a correct description for the
evolution of perturbations.Comment: 4 pages, 2 figures. Contribution to the proceedings of Spanish
Relativity Meeting 2008, Salamanca, Spain, 15-19 September 200
Does Arbuscular Mycorrhiza Determine Soil Microbial Functionality in Nutrient-Limited Mediterranean Arid Ecosystems?
Arbuscular mycorrhizal fungi (AMF) are determinant for the performance of plant communities and for the functionality of terrestrial ecosystems. In natural ecosystems, grazing can have a major impact on mycorrhizal fungi and consequently on plant growth. The objective of this study was to evaluate the statements referred above in Mediterranean arid areas in Tunisia. Root samples and rhizosphere soils of five dominant herbaceous plants were studied at six distinct arid sites differing on soil proprieties and grazing intensity. At each site, chemical and dynamic properties of the soil were characterized as well as the AMF colonization intensity and the soil functionality. Results showed that the mycorrhizal frequency and intensity and spore density, varied between plants in the same site and, for each plant, between sites and evidenced a positive effect of mycorrhized plants on soil microbial activity. Grazing and soil properties strongly affected AMF composition and the soil microbial and biochemical dynamics, which presented the lowest values at the sites with the highest grazing intensities. In conclusion, these results demonstrate that AMF improve soil biological properties, supporting the hypothesis that mycorrhiza and grazing compete for plant photosynthates, and highlight the importance of mycorrhizal symbiosis towards soil functionality under arid conditions.info:eu-repo/semantics/publishedVersio
Arbuscular mycorrhizal fungi in soil, roots and rhizosphere of Medicago truncatula: diversity and heterogeneity under semi-arid conditions
Mycorrhizal symbioses are considered indicators of ecosystem biodiversity. However, their diversity and relevance in arid and semi-arid ecosystems are poorly understood. This study addressed this subject, the main objective being to evaluate arbuscular mycorrhizal fungi (AMF) diversity and heterogeneity in a semi-arid region. Samples of bulk and rhizosphere soil and fine roots of Medicago truncatula were collected at four different sites with the same aridity index (6.1), in Bou-Hedma National Park, Tunisia, a semi-arid ecosystem. AMF taxa were assessed by 454- pyrosequencing and identified by BLAST matching of operational taxonomic units (OTUs) against the MaarjAM database, targeting AMF SSU rRNA gene diversity. Roots were the hotspots of AMF diversity (107 OTUs out of a total of 138). Of the 138 OTUs, 113 found correspondence in the MaarjAM database, with 32 AMF virtual taxa (VTX),19 Site-exclusive (SE) and 13 common to at least two sites (Non-site exclusive, NSE); the remaining 25 OTUs grouped in 16 putative new AMF taxa (pNTX), each one consisting of OTUs sharing pairwise distances not higher than 3%. We found a high diversity and heterogeneity of AMF across the four sites, which showed, in a regression analysis, significant relation to six out of the eight environmental parameters evaluated: grazing activity and soil texture, electrical conductivity, organic matter, total phosphorus and total nitrogen. AMF colonization of plants also presented significant differences among the four sites, as well as spore density, microbial biomass and several enzymatic activities (dehydrogenase, β-glucosidase and phosphatase) evaluated in rhizosphere soils. The four sites clustered in two groups in a hierarchical clustering evaluation based on their AMF diversity (total numbers of OTU, VTX and pNTX) and the parameters referred above. The crucial role of abiotic factors, other than aridity index, on AMF community composition, was evidenced by the high heterogeneity found between AMF communities across sites under identical aridity conditions
Merging history of three bimodal clusters
We present a combined X-ray and optical analysis of three bimodal galaxy
clusters selected as merging candidates at z ~ 0.1. These targets are part of
MUSIC (MUlti--Wavelength Sample of Interacting Clusters), which is a general
project designed to study the physics of merging clusters by means of
multi-wavelength observations. Observations include spectro-imaging with
XMM-Newton EPIC camera, multi-object spectroscopy (260 new redshifts), and
wide-field imaging at the ESO 3.6m and 2.2m telescopes. We build a global
picture of these clusters using X-ray luminosity and temperature maps together
with galaxy density and velocity distributions. Idealized numerical simulations
were used to constrain the merging scenario for each system. We show that A2933
is very likely an equal-mass advanced pre-merger ~ 200 Myr before the core
collapse, while A2440 and A2384 are post-merger systems ~ 450 Myr and ~1.5 Gyr
after core collapse, respectively). In the case of A2384, we detect a
spectacular filament of galaxies and gas spreading over more than 1 h^{-1} Mpc,
which we infer to have been stripped during the previous collision. The
analysis of the MUSIC sample allows us to outline some general properties of
merging clusters: a strong luminosity segregation of galaxies in recent
post-mergers; the existence of preferential axes --corresponding to the merging
directions-- along which the BCGs and structures on various scales are aligned;
the concomitance, in most major merger cases, of secondary merging or accretion
events, with groups infalling onto the main cluster, and in some cases the
evidence of previous merging episodes in one of the main components. These
results are in good agreement with the hierarchical scenario of structure
formation, in which clusters are expected to form by successive merging events,
and matter is accreted along large--scale filaments
Singular shell embedded into a cosmological model
We generalize Israel's formalism to cover singular shells embedded in a
non-vacuum Universe. That is, we deduce the relativistic equation of motion for
a thin shell embedded in a Schwarzschild/Friedmann-Lemaitre-Robertson-Walker
spacetime. Also, we review the embedding of a Schwarzschild mass into a
cosmological model using "curvature" coordinates and give solutions with
(Sch/FLRW) and without the embedded mass (FLRW).Comment: 25 pages, 2 figure
Epidemiology and trends in non-fatal self-harm in three centres in England, 2000–2012: findings from the Multicentre Study of Self-Harm in England
Objectives: Self-harm is a major health problem in many countries, with potential adverse outcomes including suicide and other causes of premature death. It is important to monitor national trends in this behaviour. We examined trends in non-fatal self-harm and its management in England during the 13-year period, 2000–2012.
Design and setting: This observational study was undertaken in the three centres of the Multicentre Study of Self-harm in England. Information on all episodes of self-harm by individuals aged 15 years and over presenting to five general hospitals in three cities (Oxford, Manchester and Derby) was collected through face-to-face assessment or scrutiny of emergency department electronic databases. We used negative binomial regression models to assess trends in rates of self-harm and logistic regression models for binary outcomes (eg, assessed vs non-assessed patients).
Participants: During 2000–2012, there were 84 378 self-harm episodes (58.6% by females), involving 47 048 persons.
Results: Rates of self-harm declined in females (incidence rate ratio (IRR) 0.98; 95% CI 0.97 to 0.99, p<0.0001). In males, rates of self-harm declined until 2008 (IRR 0.96; 95% CI 0.95 to 0.98, p<0.0001) and then increased (IRR 1.05; 95% CI 1.02 to 1.09, p=0.002). Rates of self-harm were strongly correlated with suicide rates in England in males (r=0.82, p=0.0006) and females (r=0.74, p=0.004). Over 75% of self-harm episodes were due to self-poisoning, mainly with analgesics (45.7%), antidepressants (24.7%) and benzodiazepines (13.8%). A substantial increase in self-injury occurred in the latter part of the study period. This was especially marked for self-cutting/stabbing and hanging/asphyxiation. Psychosocial assessment by specialist mental health staff occurred in 53.2% of episodes.
Conclusions: Trends in rates of self-harm and suicide may be closely related; therefore, self-harm can be a useful mental health indicator. Despite national guidance, many patients still do not receive psychosocial assessment, especially those who self-injure
A2163: Merger events in the hottest Abell galaxy cluster I. Dynamical analysis from optical data
We present a detailed optical analysis of the galaxy cluster A2163 based on
new multicolor wide field imaging and medium-to-high resolution (R~2000)
spectroscopy. While X-ray observations have revealed that merging processes are
present in this cluster, the merging scenario is complex and not well-defined.
We undertook a complementary optical analysis, aiming to understand the
dynamics of the system, to add constraints to the merging scenario and to test
its effect on the properties of galaxies. We provide a spectroscopic catalog of
512 new objects, where 361 galaxies are identified as cluster members. We
derived estimates of the mean redshift and velocity dispersion: z= 0.2005 +/-
0.0003 and 1434 +/- 60 km/s, and performed a detailed dynamical analysis of
unprecedented accuracy. We detected multiple evidences of merging events: i)
strong subclustering in the galaxy density distribution, showing a main central
component A2163-A, a northern one A2163-B, visible both in optical and in
X-ray, and other substructures detected in optical, ii) bimodality in the
density distribution for A2163-A, strong segregation between gaz and galaxies,
and between galaxies of different luminosities, iii) low-density structures
oriented along specific axes: mainly the EW direction, and the NS one,
embedding the main substructures, iv) bimodality in the velocity distribution
and a velocity gradient (~1250 km/s) along the NE/SW axis of the cluster. A2163
is exceptionally massive, with Mvir = 3.8 +/- 0.4 10**15 Msol/h70. Our analysis
supports a scenario in which A2163-A has undergone a recent (t ~ 0.5 Gyr)
merger along a a NE/SW (or E--W) axis, and A2163-B is connected to the main
complex, probably infalling on A2163-A. (abridged)Comment: 23 pages, 22 figures, accepted for publication in Astronomy and
Astrophysic
3D superimposition of craniofacial imaging—The utility of multicentre collaborations
Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/149360/1/ocr12281.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/149360/2/ocr12281_am.pd