58,722 research outputs found
Hoag's Object: Evidence for Cold Accretion onto an Elliptical Galaxy
(Abridged) We present new photometric and spectroscopic observations of the
famous Hoag's Object, a peculiar ring galaxy with a central roundish core. The
nature of Hoag's Object is still under controversial discussion. Previous
studies demonstrated that a major accretion event that took place at least 2-3
Gyr ago can account for the observational evidence. However, the role of
internal nonlinear mechanisms in forming the outer ring was not yet completely
ruled out. These new data, together with HI and optical information from the
literature, are used to demonstrate that Hoag's Object is a relatively isolated
system surrounded by a luminous quasi-spiral pattern and a massive, low-density
HI disc. The main stellar body is an old, mildly triaxial elliptical galaxy
with very high angular momentum. We review previous formation scenarios of
Hoag's Object in light of the new data and conclude that the peculiar
morphology could not represent a late phase in barred early-type galaxies
evolution. In addition, no observational evidence supports late merging events
in the evolution of the galaxy, although further tests are required before
safely dismissing this idea. We propose a new scenario where the elliptical
core formed in the early Universe with the HI disc forming shortly after the
core by prolonged "cold" accretion of primordial gas from the intergalactic
medium. The low gas density does not allow intense star formation to occur
everywhere in the disc, but only along a tightly wound spiral pattern of
enhanced density induced by the triaxial gravitational potential.Comment: 18 pages, 12 figures. MNRAS in press, minor changes to match
published versio
Neural Mechanisms for Information Compression by Multiple Alignment, Unification and Search
This article describes how an abstract framework for perception and cognition may be realised in terms of neural mechanisms and neural processing.
This framework — called information compression by multiple alignment, unification and search (ICMAUS) — has been developed in previous research as a generalized model of any system for processing information, either natural or
artificial. It has a range of applications including the analysis and production of natural language, unsupervised inductive learning, recognition of objects and patterns, probabilistic reasoning, and others. The proposals in this article may be seen as an extension and development of
Hebb’s (1949) concept of a ‘cell assembly’.
The article describes how the concept of ‘pattern’ in the ICMAUS framework may be mapped onto a version of the cell
assembly concept and the way in which neural mechanisms may achieve the effect of ‘multiple alignment’ in the ICMAUS framework.
By contrast with the Hebbian concept of a cell assembly, it is proposed here that any one neuron can belong in one assembly and only one assembly. A key feature of present proposals, which is not part of the Hebbian concept, is that any cell assembly may contain ‘references’ or ‘codes’ that serve to identify one or more other cell assemblies. This mechanism allows information to be stored in a compressed form, it provides a robust mechanism by which assemblies may be connected to form hierarchies and other kinds of structure, it means that assemblies can express
abstract concepts, and it provides solutions to some of the other problems associated with cell assemblies.
Drawing on insights derived from the ICMAUS framework, the article also describes how learning may be achieved with neural mechanisms. This concept of learning is significantly different from the Hebbian concept and appears to provide a better account of what we know about human learning
Dust yields in clumpy SN shells: SN 1987A revisited
We present a study of the effects of clumping on the emergent spectral energy
distribution (SED) from dusty supernova (SN) shells illuminated by a diffuse
radiation source distributed throughout the medium. (...) The fully 3D
radiation transport problem is solved using a Monte Carlo code, MOCASSIN, and
we present a set of models aimed at investigating the sensitivity of the SEDs
to various clumping parameters. We find that, contrary to the predictions of
analytical prescriptions, the combination of an optical and IR observational
data set is sufficient to constrain dust masses even in the case where
optically thick clumps are present. Using both smoothly varying and clumped
grain density distributions, we obtain new estimates for the mass of dust
condensed by the Type II SN 1987A by fitting the optical and infrared
spectrophotometric data of Wooden et al. (1993) at two epochs (day 615 and day
775). (...) From our numerical models we derive dust masses for SN 1987A that
are comparable to previous analytic clumped graphite grain mass estimates, and
at least two orders of magnitude below the 0.1-0.3 Msol that have been
predicted to condense as dust grains in primordial core collapse supernova
ejecta. This low condensation efficiency for SN 1987A is in contrast to the
case of SN 2003gd, for which a dust condensation efficiency as large as 0.12
has recently been estimated. (Abridged)Comment: accepted for publication in MNRAS. The paper contains 15 figures and
1 tabl
Hierarchical modelling of species sensitivity distribution: development and application to the case of diatoms exposed to several herbicides
The Species Sensitivity Distribution (SSD) is a key tool to assess the
ecotoxicological threat of contaminant to biodiversity. It predicts safe
concentrations for a contaminant in a community. Widely used, this approach
suffers from several drawbacks: i)summarizing the sensitivity of each species
by a single value entails a loss of valuable information about the other
parameters characterizing the concentration-effect curves; ii)it does not
propagate the uncertainty on the critical effect concentration into the SSD;
iii)the hazardous concentration estimated with SSD only indicates the threat to
biodiversity, without any insight about a global response of the community
related to the measured endpoint. We revisited the current SSD approach to
account for all the sources of variability and uncertainty into the prediction
and to assess a global response for the community. For this purpose, we built a
global hierarchical model including the concentration-response model together
with the distribution law for the SSD. Working within a Bayesian framework, we
were able to compute an SSD taking into account all the uncertainty from the
original raw data. From model simulations, it is also possible to extract a
quantitative indicator of a global response of the community to the
contaminant. We applied this methodology to study the toxicity of 6 herbicides
to benthic diatoms from Lake Geneva, measured from biomass reduction
On the efficiency of stochastic volume sources for the determination of light meson masses
We investigate the efficiency of single timeslice stochastic sources for the
calculation of light meson masses on the lattice as one varies the quark mass.
Simulations are carried out with Nf = 2 flavours of non-perturbatively O(a)
improved Wilson fermions for pion masses in the range of 450 - 760 MeV. Results
for pseudoscalar and vector meson two-point correlation functions computed
using stochastic as well as point sources are presented and compared. At fixed
computational cost the stochastic approach reduces the variance considerably in
the pseudoscalar channel for all simulated quark masses. The vector channel is
more affected by the intrinsic stochastic noise. In order to obtain stable
estimates of the statistical errors and a more pronounced plateau for the
effective vector meson mass, a relatively large number of stochastic sources
must be used.Comment: 18 pages, 6 figure
An investigation into the effect of floor colour on the behaviour of the horse
Adverse reactions of the domestic horse to environmental stimuli can be problematic in training and management. Hesitation and alarm reactions to visual features of the ground can occur in both ridden work and when handling horses. To assess the effect of one visual feature (colour) on the behaviour of the domestic horse, the reactions of sixteen riding horses to eight different coloured mats were recorded. The effect of stimulus position on these reactions was assessed by presenting them in two different positions, either on the ground (where the horses had to walk over them) or against a wall (where the horses walked past them). Each colour/position combination was presented twice in order to assess the effect of previous experience. An alleyway was constructed to allow the horses to be tested unconstrained and freely walking throughout. The time taken to traverse the alleyway and the observed reaction to the colour was recorded
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