8,750 research outputs found
Fish for the city: meta-analysis of archaeological cod remains and the growth of London’s northern trade
The growth of medieval cities in Northern Europe placed new demands on food supply, and led to the import of fish from increasingly distant fishing grounds. Quantitative analysis of cod remains from London provides revealing insight into the changing patterns of supply that can be related to known historical events and circumstances. In particular it identifies a marked increase in imported cod from the thirteenth century AD. That trend continued into the fifteenth and sixteenth centuries, after a short downturn, perhaps attributable to the impact of the Black Death, in the mid fourteenth century. The detailed pattern of fluctuating abundance illustrates the potential of archaeological information that is now available from the high-quality urban excavations conducted in London and similar centres during recent decades
Nonlinearity and chaos in economic models: implications for policy decisions
This survey paper discusses the policy implications that can be expected from the recent research on nonlinearity and chaos in economic models. Expected policy implications are interpreted as a driving force behind the recent proliferation of research in this area. In general, it appears that no new justification for policy intervention is developed in models of endogenous fluctuations, although this conclusion depends in part on the definition of equilibrium. When justified, however, policy tends to be very effective in these models.Macroeconomics ; Economic stabilization
An annotated bibliography of tax compliance and tax compliance costs
An annotated bibliography of tax compliance and tax compliance costs.tax; tax compliance; compliance costs; bibliography; tax evasion; tax avoidance; auditing; tax simplification
Access and use of weather and climate information by women and men farmers: Rwanda Climate Services for Agriculture qualitative evaluation preliminary findings
The Rwanda Climate Services for Agriculture project has sought to build capacity of the country’s national institutions to provide climate information tailored to the needs of the agriculture sector, deliver climate services to farmers across Rwanda’s 30 districts, and help them to effectively use the information to manage climate risk. Project interventions include: training Farmer Promoters,
who are part of Rwanda’s national agricultural extension service, to guide farmers in the Participatory Integrated Climate Services for Agriculture (PICSA) process (Dorward et al., 2015); and organizing farmers into Radio Listeners’ Clubs that meet weekly to participate in climate services radio programs and discuss management responses
Unbound Star-forming Molecular Clouds
We explore whether observed molecular clouds could include a substantial
population of unbound clouds. Using simulations which include only turbulence
and gravity, we are able to match observed relations and naturally reproduce
the observed scatter in the cloud size-linewidth coefficient, at fixed surface
density. We identify the source of this scatter as a spread in the intrinsic
virial parameter. Thus these observational trends do not require that clouds
exist in a state of dynamical equilibrium. We demonstrate that cloud virial
parameters can be accurately determined observationally with an appropriate
size estimator. All our simulated clouds eventually form collapsing cores,
regardless of whether the cloud is bound overall. This supports the idea that
molecular clouds do not have to be bound to form stars or to have observed
properties like those of nearby low-mass clouds.Comment: 9 pages, 6 figures, Accepted for publication by MNRA
X-rays across the galaxy population - III. The incidence of AGN as a function of star formation rate
We map the co-eval growth of galaxies and their central supermassive black
holes in detail by measuring the incidence of Active Galactic Nuclei (AGN) in
galaxies as a function of star formation rate (SFR) and redshift (to z~4). We
combine large galaxy samples with deep Chandra X-ray imaging to measure the
probability distribution of specific black hole accretion rates (LX relative to
stellar mass) and derive robust AGN fractions and average specific accretion
rates. First, we consider galaxies along the main sequence of star formation.
We find a linear correlation between the average SFR and both the AGN fraction
and average specific accretion rate across a wide range in stellar mass () and to at least z~2.5, indicating that AGN in
main-sequence galaxies are driven by the stochastic accretion of cold gas. We
also consider quiescent galaxies and find significantly higher AGN fractions
than predicted, given their low SFRs, indicating that AGN in quiescent galaxies
are fuelled by additional mechanisms (e.g. stellar winds). Next, we bin
galaxies according to their SFRs relative to the main sequence. We find that
the AGN fraction is significantly elevated for galaxies that are still
star-forming but with SFRs below the main sequence, indicating further
triggering mechanisms enhance AGN activity within these sub-main-sequence
galaxies. We also find that the incidence of high-accretion-rate AGN is
enhanced in starburst galaxies and evolves more mildly with redshift than
within the rest of the galaxy population, suggesting mergers play a role in
driving AGN activity in such high-SFR galaxies.Comment: 19 pages, 15 figures. This is a pre-copyedited, author-produced
version of an article accepted for publication in MNRAS following peer revie
Evolving Molecular Cloud Structure and the Column Density Probability Distribution Function
The structure of molecular clouds can be characterized with the probability
distribution function (PDF) of the mass surface density. In particular, the
properties of the distribution can reveal the nature of the turbulence and star
formation present inside the molecular cloud. In this paper, we explore how
these structural characteristics evolve with time and also how they relate to
various cloud properties as measured from a sample of synthetic column density
maps of molecular clouds. We find that, as a cloud evolves, the peak of its
column density PDF will shift to surface densities below the observational
threshold for detection, resulting in an underlying lognormal distribution
which has been effectively lost at late times. Our results explain why certain
observations of actively star-forming, dynamically older clouds, such as the
Orion molecular cloud, do not appear to have any evidence of a lognormal
distribution in their column density PDFs. We also study the evolution of the
slope and deviation point of the power-law tails for our sample of simulated
clouds and show that both properties trend towards constant values, thus
linking the column density structure of the molecular cloud to the surface
density threshold for star formation.Comment: 10 pages, 9 figures, Accepted for publication by MNRA
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