36,333 research outputs found
Probing the Low Surface Brightness Dwarf Galaxy Population of the Virgo Cluster
We have used public data from the Next Generation Virgo Survey (NGVS) to
investigate the dwarf galaxy population of the Virgo cluster beyond what has
previously been discovered. We initially mask and smooth the data, and then use
the object detection algorithm Sextractor to make our initial dwarf galaxy
selection. All candidates are then visually inspected to remove artefacts and
duplicates. We derive Sextractor parameters to best select low surface
brightness galaxies using g band central surface brightness values of 22.5 to
26.0 mag sq arc sec and exponential scale lengths of 3.0 - 10.0 arc sec to
identify 443 cluster dwarf galaxies - 303 of which are new detections. These
new detections have a surface density that decreases with radius from the
cluster centre. We also apply our selection algorithm to 'background',
non-cluster, fields and find zero detections. In combination, this leads us to
believe that we have isolated a cluster dwarf galaxy population. The range of
objects we are able to detect is limited because smaller scale sized galaxies
are confused with the background, while larger galaxies are split into numerous
smaller objects by the detection algorithm. Using data from previous surveys
combined with our data, we find a faint end slope to the luminosity function of
-1.35+/-0.03, which does not significantly differ to what has previously been
found for the Virgo cluster, but is a little steeper than the slope for field
galaxies. There is no evidence for a faint end slope steep enough to correspond
with galaxy formation models, unless those models invoke either strong feedback
processes or use warm dark matter.Comment: Accepted for publication in MNRA
Condensation risk: comparison of steady-state and transient methods
Accurate assessment of both surface and interstitial condensation risk is important not only to reduce the damaging effect of moisture within the structure of buildings, but also to provide a healthy environment free from mould growth. The current British Standard (BS EN ISO 13788: 2002) contains an assessment procedure based on the assumption of a steady-state heat flow through the building envelope, neglecting the transient nature of the problem. This paper compares and evaluates numerical results of the condensation risk calculation under both steady-state and transient conditions using the existing numerical codes. Significant differences are apparent between the predictions of the simple (steady-state) and complex (transient) methods for all construction details modelled
The effect of stellar-mass black holes on the structural evolution of massive star clusters
We present the results of realistic N-body modelling of massive star clusters
in the Magellanic Clouds, aimed at investigating a dynamical origin for the
radius-age trend observed in these systems. We find that stellar-mass black
holes, formed in the supernova explosions of the most massive cluster stars,
can constitute a dynamically important population. If a significant number of
black holes are retained (here we assume complete retention), these objects
rapidly form a dense core where interactions are common, resulting in the
scattering of black holes into the cluster halo, and the ejection of black
holes from the cluster. These two processes heat the stellar component,
resulting in prolonged core expansion of a magnitude matching the observations.
Significant core evolution is also observed in Magellanic Cloud clusters at
early times. We find that this does not result from the action of black holes,
but can be reproduced by the effects of mass-loss due to rapid stellar
evolution in a primordially mass segregated cluster.Comment: Accepted for publication in MNRAS Letters; 2 figures, 1 tabl
SCUBA imaging of NGC 7331 dust ring
We present observations of the spiral galaxy NGC 7331 using the
Sub-millimetre Common User Bolometer Array (SCUBA) on the James Clark Maxwell
Telescope. We have detected a dust ring of 45 arcsec radius (3.3 kpc) at
wavelengths of 450 and 850-micron. The dust ring is in good correspondence with
other observations of the ring in the mid-infrared (MIR), CO and
radio-continuum, suggesting that the observed dust is associated with the
molecular gas and star formation. A B-K colour map shows an analogous ring
structure with an asymmetry about the major axis, consistent with the
extinction being produced by a dust ring. The derived temperature of the dust
lies between 16 and 31 K and the gas-to-dust ratio between 150 and 570,
depending on the assumed dust emission efficiency index (beta=1.5 or 2.).Comment: 5 pages, 6 figures, to be published in MNRA
The central molecular gas structure in LINERs with low luminosity AGN: evidence for gradual disappearance of the torus
We present observations of the molecular gas in the nuclear environment of
three prototypical low luminosity AGN (LLAGN), based on VLT/SINFONI AO-assisted
integral-field spectroscopy of H2 1-0 S(1) emission at angular resolutions of
~0.17". On scales of 50-150 pc the spatial distribution and kinematics of the
molecular gas are consistent with a rotating thin disk, where the ratio of
rotation (V) to dispersion (sigma) exceeds unity. However, in the central 50
pc, the observations reveal a geometrically and optically thick structure of
molecular gas (V/sigma10^{23} cm^{-2}) that is likely to be
associated with the outer extent of any smaller scale obscuring structure. In
contrast to Seyfert galaxies, the molecular gas in LLAGN has a V/sigma<1 over
an area that is ~9 times smaller and column densities that are in average ~3
times smaller. We interpret these results as evidence for a gradual
disappearance of the nuclear obscuring structure. While a disk wind may not be
able to maintain a thick rotating structure at these luminosities, inflow of
material into the nuclear region could provide sufficient energy to sustain it.
In this context, LLAGN may represent the final phase of accretion in current
theories of torus evolution. While the inflow rate is considerable during the
Seyfert phase, it is slowly decreasing, and the collisional disk is gradually
transitioning to become geometrically thin. Furthermore, the nuclear region of
these LLAGN is dominated by intermediate-age/old stellar populations (with
little or no on-going star formation), consistent with a late stage of
evolution.Comment: 15 pages, including 4 figures and 1 table, Accepted for publication
in ApJ Letter
An initial evaluation of a biohygrothermal model for the purpose of assessing the risk mould growth in UK dwellings
Moulds are organisms that may be found in both the indoor and outdoor environment. Moulds play an important rolebreaking down and digesting organic material, but, if they are significantly present in the indoor environment they mayaffect the health of the occupants. A relative humidity of 80% at wall surfaces is frequently stated as the decisivecriterion for mould growth and methods used to assess the risk of mould growth are often based on steady stateconditions. However, considering the dynamic conditions typically found in the indoor environment, a betterunderstanding of the conditions required for mould to grow would seem desirable. This paper presents initialexploratory work to evaluate and assess ‘WUFI-bio’ - ‘biohygrothermal’ software that predicts the likelihood of mould growth under transient conditions. Model predictions are compared with large monitored data set from 1,388 UKdwellings before and after insulation and new heating systems are installed (‘Warm Front’), the suitability of thissoftware as a tool to predict mould growth will ultimately be assessed. This paper presents some initial, exploratorywork
Formal Definitions of Unbounded Evolution and Innovation Reveal Universal Mechanisms for Open-Ended Evolution in Dynamical Systems
Open-ended evolution (OEE) is relevant to a variety of biological, artificial
and technological systems, but has been challenging to reproduce in silico.
Most theoretical efforts focus on key aspects of open-ended evolution as it
appears in biology. We recast the problem as a more general one in dynamical
systems theory, providing simple criteria for open-ended evolution based on two
hallmark features: unbounded evolution and innovation. We define unbounded
evolution as patterns that are non-repeating within the expected Poincare
recurrence time of an equivalent isolated system, and innovation as
trajectories not observed in isolated systems. As a case study, we implement
novel variants of cellular automata (CA) in which the update rules are allowed
to vary with time in three alternative ways. Each is capable of generating
conditions for open-ended evolution, but vary in their ability to do so. We
find that state-dependent dynamics, widely regarded as a hallmark of life,
statistically out-performs other candidate mechanisms, and is the only
mechanism to produce open-ended evolution in a scalable manner, essential to
the notion of ongoing evolution. This analysis suggests a new framework for
unifying mechanisms for generating OEE with features distinctive to life and
its artifacts, with broad applicability to biological and artificial systems.Comment: Main document: 17 pages, Supplement: 21 pages Presented at OEE2: The
Second Workshop on Open-Ended Evolution, 15th International Conference on the
Synthesis and Simulation of Living Systems (ALIFE XV), Canc\'un, Mexico, 4-8
July 2016 (http://www.tim-taylor.com/oee2/
Condensation risk – impact of improvements to Part L and robust details on Part C Final report: BD2414
This report summarises the main findings of the project ‘Impacts of Improvements to Part L and Robust Construction Details (RCD) on Part C’. The work consisted of a fieldwork element, undertaken by Leeds Metropolitan University and a modelling element carried out by University College London. Details of the work programme are contained in Appendix 1. The fieldwork consisted of the analysis of design material and site surveys from 16 housing developments constructed to Part L 2002 and adopting the Robust Construction Detail route to compliance. The modelling element of the project sought to identify the extent to which the ‘as built’ details give rise to a significantly increased condensation risk as compared to the relevant ‘standard’ robust construction details, as defined in the guidance. In addition to assessing ‘as built’ performance, the modelling phase of the project has investigated the suitability of the relevant calculation methods used to assess the risk of surface and interstitial condensation and mould growth. This report draws together the important conclusions from the project which has previously been presented in several very detailed interim reports and also for the first time presents the results of a workshop where these results were discussed to obtain industry feedback. The overall conclusions, future work and dissemination plans are also presented
An evaluation of the hygrothermal performance of 'standard' and 'as built' construction details using steadystate and transient modelling
Accurate assessment of both surface and interstitial condensation risk at the design stage of buildings is of great importance - not just to minimise the damaging effects moisture can cause to building envelopes, but also to contribute to the provision of adequate indoor air quality. Guidance certainly does exist with regards to limiting thermal bridging in order to prevent condensation occurring on new constructions. However, a recent study has provided clear evidence that the reality, both in translating the available guidance into a specific design and in construction on site is often rather different from the 'ideal'. This paper reports on that study and compares and evaluates the hygrothermal performance of construction details for different phases during the building life cycle. The results of both the surface and interstitial condensation risk simulations under both steady-state and transient conditions are presented and discussed. Significant differences in the hygrothermal performance of 'standard' and 'as built' construction details are observed
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