3,674 research outputs found
A Common Origin for Quasar Extended Emission-Line Regions and Their Broad-Line Regions
We present a correlation between the presence of luminous extended
emission-line regions (EELRs) and the metallicity of the broad-line regions
(BLRs) of low-redshift quasars. The result is based on ground-based [O III]
5007 narrow-band imaging and Hubble Space Telescope UV spectra of 12 quasars at
0.20 < z < 0.45. Quasars showing luminous EELRs have low-metallicity BLRs (Z <
0.6 Z_Solar), while the remaining quasars show typical metal-rich gas (Z >
Z_Solar). Previous studies have shown that EELRs themselves also have low
metallicities (Z < 0.5 Z_Solar). The correlation between the occurrence of
EELRs and the metallicity of the BLRs, strengthened by the sub-Solar
metallicity in both regions, indicates a common external origin for the gas,
almost certainly from the merger of a gas-rich galaxy. Our results provide the
first direct observational evidence that the gas from a merger can indeed be
driven down to the immediate vicinity (< 1 pc) of the central black hole.Comment: Accepted for publication in ApJ Letters. 4 pages, 1 figure, 1 tabl
The evolution of the stellar populations in low surface brightness galaxies
We investigate the star formation history and chemical evolution of low
surface brightness (LSB) disk galaxies by modelling their observed
spectro-photometric and chemical properties using a galactic chemical and
photometric evolution model incorporating a detailed metallicity depen dent set
of stellar input data. For a large fraction of the LSB galaxies in our sample,
observed properties are best explained by models incorporating an exponentially
decreasing global star formation rate (SFR) ending at a present-day gas
fraction (M_{gas}/(M_{gas}+M_{stars}) = 0.5 for a galaxy age of 14 Gyr. For
some galaxies small amplitude star formation bursts are required to explain the
contribution of the young (5-50 Myr old) stellar population to the galaxy
integrated luminosity. This suggests that star formation has proceeded in a
stochastic manner.
The presence of an old stellar population in many late-type LSB galaxies
suggests that LSB galaxies roughly follow the same evolutionary history as HSB
galaxies, except at a much lower rate. In particular, our results imply that
LSB galaxies do not form late, nor have a delayed onset of star formation, but
simply evolve slowly.Comment: To be published in A&
Design and structural characterisation of olfactomedin-1 variants as tools for functional studies
Background Olfactomedin-1 (Olfm1; also known as Noelin or Pancortin) is a highly-expressed secreted brain and retina protein and its four isoforms have different roles in nervous system development and function. Structural studies showed that the long Olfm1 isoform BMZ forms a disulfide-linked tetramer with a V-shaped architecture. The tips of the Olfm1 "V" each consist of two C-terminal beta-propeller domains that enclose a calcium binding site. Functional characterisation of Olfm1 may be aided by new biochemical tools derived from these core structural elements. Results Here we present the production, purification and structural analysis of three novel monomeric, dimeric and tetrameric forms of mammalian Olfm1 for functional studies. We characterise these constructs structurally by high-resolution X-ray crystallography and small-angle X-ray scattering. The crystal structure of the Olfm1 beta-propeller domain (to 1.25 angstrom) represents the highest-resolution structure of an olfactomedin family member to date, revealing features such as a hydrophilic tunnel containing water molecules running into the core of the domain where the calcium binding site resides. The shorter Olfactomedin-1 isoform BMY is a disulfide-linked tetramer with a shape similar to the corresponding region in the longer BMZ isoform. Conclusions These recombinantly-expressed protein tools should assist future studies, for example of biophysical, electrophysiological or morphological nature, to help elucidate the functions of Olfm1 in the mature mammalian brain. The control over the oligomeric state of Olfm1 provides a firm basis to better understand the role of Olfm1 in the (trans-synaptic) tethering or avidity-mediated clustering of synaptic receptors such as post-synaptic AMPA receptors and pre-synaptic amyloid precursor protein. In addition, the variation in domain composition of these protein tools provides a means to dissect the Olfm1 regions important for receptor binding
In en om de lever
Niet UB, maar tijdelijk ter bevordering van de PDF bestanden in het Leids Repositorium
Some rare and new caddis flies recorded for the Netherlands (Trichoptera)
Contains fulltext :
35412.pdf (publisher's version ) (Open Access)Since Higler published his list of extinct and endangered caddis flies species in 1995 a number of new and rare species were recorded for the Netherlands. These new findings can be partly attributed to the growing interest among researchers to study caddis flies, which has resulted in a more thorough investigation of certain water types. Other records of rare species were discovered by collecting and identifying pupae and adults, such as Hydroptila vectis and Oxyethira falcata. Some rare species, which have not been recorded since 1950, are common nowadays, possibly caused by the improved water quality of the river systems in the Netherlands. Other new species to the Dutch fauna are the result of new taxonomic insights
Innovatie in de hepatologie door klinisch en translationeel onderzoek
Afscheidscollege door prof. dr. Bart van Hoek uitgesproken bij zijn emeritaat als hoogleraar Maag-, Darm-, Leverziekten, in het bijzonder de Hepatologie aan de Universiteit Leiden op vrijdag 26 mei 2023LUMC / Geneeskund
Simple Axioms for Local Properties
Correspondence theory allows us to create sound and complete axiomatizations
for modal logic on frames with certain properties. For example, if we restrict
ourselves to transitive frames we should add the axiom which, among other things, can be interpreted
as positive introspection. One limitation of this technique is that the frame
property and the axiom are assumed to hold globally, i.e., the relation is
transitive throughout the frame, and the agent's knowledge satisfies positive
introspection in every world.
In a modal logic with local properties, we can reason about properties that
are not global. So, for example, transitivity might hold only in certain parts
of the model and, as a result, the agent's knowledge might satisfy positive
introspection in some worlds but not in others. Van Ditmarsch et al. (2012)
introduced sound and complete axiomatizations for modal logics with certain
local properties. Unfortunately, those axiomatizations are rather complex.
Here, we introduce far simpler axiomatizations for a wide range of local
properties.Comment: In Proceedings TARK 2023, arXiv:2307.0400
- …