281 research outputs found
Ergodic Jacobi matrices and conformal maps
We study structural properties of the Lyapunov exponent and the
density of states for ergodic (or just invariant) Jacobi matrices in a
general framework. In this analysis, a central role is played by the function
as a conformal map between certain domains. This idea goes
back to Marchenko and Ostrovskii, who used this device in their analysis of the
periodic problem
Gas accretion as the origin of chemical abundance gradients in distant galaxies
It has recently been suggested that galaxies in the early Universe can grow
through the accretion of cold gas, and that this may have been the main driver
of star formation and stellar mass growth. Because the cold gas is essentially
primordial, it has a very low abundance of elements heavier than helium
(metallicity). As it is funneled to the centre of a galaxy, it will lead the
central gas having an overall lower metallicity than gas further from the
centre, because the gas further out has been enriched by supernovae and stellar
winds, and not diluted by the primordial gas. Here we report chemical
abundances across three rotationally-supported star-forming galaxies at z~3,
only 2 Gyr after the Big Bang. We find an 'inverse' gradient, with the central,
star forming regions having a lower metallicity than less active ones, opposite
to what is seen in local galaxies. We conclude that the central gas has been
diluted by the accretion of primordial gas, as predicted by 'cold flow' models.Comment: To Appear in Nature Oct 14, 2010; Supplementary Information included
her
Irradiation-induced Ag nanocluster nucleation in silicate glasses: analogy with photography
The synthesis of Ag nanoclusters in sodalime silicate glasses and silica was
studied by optical absorption (OA) and electron spin resonance (ESR)
experiments under both low (gamma-ray) and high (MeV ion) deposited energy
density irradiation conditions. Both types of irradiation create electrons and
holes whose density and thermal evolution - notably via their interaction with
defects - are shown to determine the clustering and growth rates of Ag
nanocrystals. We thus establish the influence of redox interactions of defects
and silver (poly)ions. The mechanisms are similar to the latent image formation
in photography: irradiation-induced photoelectrons are trapped within the glass
matrix, notably on dissolved noble metal ions and defects, which are thus
neutralized (reverse oxidation reactions are also shown to exist). Annealing
promotes metal atom diffusion, which in turn leads to cluster nuclei formation.
The cluster density depends not only on the irradiation fluence, but also - and
primarily - on the density of deposited energy and the redox properties of the
glass. Ion irradiation (i.e., large deposited energy density) is far more
effective in cluster formation, despite its lower neutralization efficiency
(from Ag+ to Ag0) as compared to gamma photon irradiation.Comment: 48 pages, 18 figures, revised version publ. in Phys. Rev. B, pdf fil
Modelling the Effects of Population Structure on Childhood Disease: The Case of Varicella
Realistic, individual-based models based on detailed census data are increasingly used to study disease transmission. Whether the rich structure of such models improves predictions is debated. This is studied here for the spread of varicella, a childhood disease, in a realistic population of children where infection occurs in the household, at school, or in the community at large. A methodology is first presented for simulating households with births and aging. Transmission probabilities were fitted for schools and community, which reproduced the overall cumulative incidence of varicella over the age range of 0–11 years old
Steep HIV prevalence declines among young people in selected Zambian communities: population-based observations (1995–2003)
BACKGROUND: Understanding the epidemiological HIV context is critical in building effective setting-specific preventive strategies. We examined HIV prevalence patterns in selected communities of men and women aged 15–59 years in Zambia. METHODS: Population-based HIV surveys in 1995 (n = 3158), 1999 (n = 3731) and 2003 (n = 4751) were conducted in selected communities using probability proportional to size stratified random-cluster sampling. Multivariate logistic regression and trend analyses were stratified by residence, sex and age group. Absence, <30% in men and <15% in women in all rounds, was the most important cause of non-response. Saliva was used for HIV testing, and refusal was <10%. RESULTS: Among rural groups aged 15–24 years, prevalence declined by 59.2% (15.7% to 6.4%, P < 0.001) in females and by 44.6% (5.6% to 3.1%, P < 0.001) in males. In age-group 15–49 years, declines were less than 25%. In the urban groups aged 15–24, prevalence declined by 47% (23.4% to 12.4%, P < 0.001) among females and 57.3% (7.5% to 3.2%, P = 0.001) among males but were 32% and 27% in men and women aged 15–49, respectively. Higher educated young people in 2003 had lower odds of infection than in 1995 in both urban [men: AOR 0.29(95%CI 0.14–0.60); women: AOR 0.38(95%CI 0.19–0.79)] and rural groups [men: AOR 0.16(95%CI 0.11–0.25), women: AOR 0.10(95%CI 0.01–7.34)]. Although higher mobility was associated with increased likelihood of infection in men overall, AOR, 1.71(95%CI 1.34–2.19), prevalence declined in mobile groups also (OR 0.52 95%CI 0.31–0.88). In parallel, urban young people with ≥11 school years were more likely to use condoms during the last casual sex (OR 2.96 95%CI 1.93–4.52) and report less number of casual sexual partners (AOR 0.33 95%CI 0.19–0.56) in the last twelve months than lower educated groups. CONCLUSION: Steep HIV prevalence declines in young people, suggesting continuing declining incidence, were masked by modest overall declines. The concentration of declines in higher educated groups suggests a plausible association with behavioural change
Star forming dwarf galaxies
Star forming dwarf galaxies (SFDGs) have a high gas content and low
metallicities, reminiscent of the basic entities in hierarchical galaxy
formation scenarios. In the young universe they probably also played a major
role in the cosmic reionization. Their abundant presence in the local volume
and their youthful character make them ideal objects for detailed studies of
the initial stellar mass function (IMF), fundamental star formation processes
and its feedback to the interstellar medium. Occasionally we witness SFDGs
involved in extreme starbursts, giving rise to strongly elevated production of
super star clusters and global superwinds, mechanisms yet to be explored in
more detail. SFDGs is the initial state of all dwarf galaxies and the relation
to the environment provides us with a key to how different types of dwarf
galaxies are emerging. In this review we will put the emphasis on the exotic
starburst phase, as it seems less important for present day galaxy evolution
but perhaps fundamental in the initial phase of galaxy formation.Comment: To appear in JENAM Symposium "Dwarf Galaxies: Keys to Galaxy
Formation and Evolution", P. Papaderos, G. Hensler, S. Recchi (eds.). Lisbon,
September 2010, Springer Verlag, in pres
The Formation and Evolution of the First Massive Black Holes
The first massive astrophysical black holes likely formed at high redshifts
(z>10) at the centers of low mass (~10^6 Msun) dark matter concentrations.
These black holes grow by mergers and gas accretion, evolve into the population
of bright quasars observed at lower redshifts, and eventually leave the
supermassive black hole remnants that are ubiquitous at the centers of galaxies
in the nearby universe. The astrophysical processes responsible for the
formation of the earliest seed black holes are poorly understood. The purpose
of this review is threefold: (1) to describe theoretical expectations for the
formation and growth of the earliest black holes within the general paradigm of
hierarchical cold dark matter cosmologies, (2) to summarize several relevant
recent observations that have implications for the formation of the earliest
black holes, and (3) to look into the future and assess the power of
forthcoming observations to probe the physics of the first active galactic
nuclei.Comment: 39 pages, review for "Supermassive Black Holes in the Distant
Universe", Ed. A. J. Barger, Kluwer Academic Publisher
Protein and lipid kinase inhibitors as targeted anticancer agents of the Ras/Raf/MEK and PI3K/PKB pathways
The identification and characterization of the components of individual signal transduction cascades, and advances in our understanding on how these biological signals are integrated in cancer initiation and progression, have provided new strategies for therapeutic intervention in solid tumors and hematological malignancies. To this end, pharmaceutical efforts have been directed to target different components of the Ras/Raf/MEK and PI3K/PKB pathways. This review article covers recent salient achievements in the identification and development of Raf, MEK, and PI3K inhibitors
Functional, Non-Clonal IgMa-Restricted B Cell Receptor Interactions with the HIV-1 Envelope gp41 Membrane Proximal External Region
The membrane proximal external region (MPER) of HIV-1 gp41 has several features that make it an attractive antibody-based vaccine target, but eliciting an effective gp41 MPER-specific protective antibody response remains elusive. One fundamental issue is whether the failure to make gp41 MPER-specific broadly neutralizing antibodies like 2F5 and 4E10 is due to structural constraints with the gp41 MPER, or alternatively, if gp41 MPER epitope-specific B cells are lost to immunological tolerance. An equally important question is how B cells interact with, and respond to, the gp41 MPER epitope, including whether they engage this epitope in a non-canonical manner i.e., by non-paratopic recognition via B cell receptors (BCR). To begin understanding how B cells engage the gp41 MPER, we characterized B cell-gp41 MPER interactions in BALB/c and C57BL/6 mice. Surprisingly, we found that a significant (∼7%) fraction of splenic B cells from BALB/c, but not C57BL/6 mice, bound the gp41 MPER via their BCRs. This strain-specific binding was concentrated in IgMhi subsets, including marginal zone and peritoneal B1 B cells, and correlated with enriched fractions (∼15%) of gp41 MPER-specific IgM secreted by in vitro-activated splenic B cells. Analysis of Igha (BALB/c) and Ighb (C57BL/6) congenic mice demonstrated that gp41 MPER binding was controlled by determinants of the Igha locus. Mapping of MPER gp41 interactions with IgMa identified MPER residues distinct from those to which mAb 2F5 binds and demonstrated the requirement of Fc CH regions. Importantly, gp41 MPER ligation produced detectable BCR-proximal signaling events, suggesting that interactions between gp41 MPER and IgMa determinants may elicit partial B cell activation. These data suggest that low avidity, non-paratopic interactions between the gp41 MPER and membrane Ig on naïve B cells may interfere with or divert bnAb responses
Human papillomavirus and vaccination: knowledge, attitudes, and behavioural intention in adolescents and young women in Italy
This study assesses knowledge, attitudes, and behavioural intention towards human papillomavirus (HPV) infection and vaccination in a random sample of 1348 adolescents and young women aged 14–24 years in Italy. A self-administered anonymous questionnaire covered demographics; knowledge about HPV infection, cervical cancer, and HPV vaccine; the perceived risk for contracting HPV infection and/or for developing cervical cancer, the perceived benefits of a vaccination to prevent cervical cancer, and willingness to receive an HPV vaccine. Only 23.3% have heard that HPV is an infection of the genital mucosa and about cervical cancer. Those older, with at least one parent who is a health care professional, with personal, familiar, or friendly history of cervical cancer, and having underwent a health checkup in the last year with information about HPV vaccination were significantly more knowledgeable. Risk perception scores (range: 1–10) of contracting HPV infection and of developing cervical cancer were 5.8 and 6.5. Older age, not having a parent who is a health care professional, having had a personal, familiar, or friendly history of cervical cancer, and need of additional information were predictors of the perceived susceptibility of developing cervical cancer. The vast majority professed intent to receive an HPV vaccine and the significant predictors were having at least one parent who is a health care professional, a high perceived risk of contracting HPV infection and of developing cervical cancer, and a high belief towards the utility of a vaccination for preventing cervical cancer. Knowledge about HPV infection and cervical cancer should be improved with more attention to the benefit of HPV vaccination
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