522 research outputs found
Interdisciplinarity and transdisciplinarity in landscape studies - the Wageningen DELTA approach
Explanation of the Dutch approach of landscape research, planning and managemen
Periodicity makes galactic shocks unstable - I. Linear analysis
We study the dynamical stability of stationary galactic spiral shocks. The
steady-state equilibrium flow contains a shock of the type derived by Roberts
in the tightly wound approximation. We find that boundary conditions are
critical in determining whether the solutions are stable or not. Shocks are
unstable if periodic boundary conditions are imposed. For intermediate
strengths of the spiral potential, the instability disappears if boundary
conditions are imposed such that the upstream flow is left unperturbed as in
the classic analysis of D'yakov and Kontorovich. This reconciles apparently
contradictory findings of previous authors regarding the stability of spiral
shocks. This also shows that the instability is distinct from the
Kelvin-Helmholtz instability, confirming the findings of Kim et al. We suggest
that instability is a general characteristics of periodic shocks, regardless of
the presence of shear, and provide a physical picture as to why this is the
case. For strong spiral potentials, high post-shock shear makes the system
unstable also to parasitic Kelvin-Helmholtz instability regardless of the
boundary conditions. Our analysis is performed in the context of a simplified
problem that, while preserving all the important characteristics of the
original problem, strips it from unnecessary complications, and assumes that
the gas is isothermal, non self-gravitating, non-magnetised.Comment: Accepted for publication in MNRA
Exact and Asymptotic Inference in Clinical Trials with Small Event Rates under Inverse Sampling
In this paper we discuss statistical inference for a two-by-two table under inverse sampling, where the total number of
cases is fixed by design. We demonstrate that the exact unconditional distributions of some relevant statistics differ from
the distributions under conventional sampling, where the sample size is fixed by design. This permits us to define a simple
unconditional alternative to Fisher’s exact test. We provide an asymptotic argument including simulations to demonstrate
that there is little power-loss associated with the alternative test when the expected response rates are rare. We then
apply the method to design a clinical trial in cataract surgery, where a rare side effect occurs in one in one-thousand
patients. Objective of the trial is to demonstrate that adjuvant treatment with an antibiotic will reduce this risk to one in
two-thousand. We use an inverse sampling design and demonstrate how to set this up in a sequential manner. Particularly
simple stopping rules can be defined when using the unconditional alternative to Fisher’s exact test
A theoretical explanation for the Central Molecular Zone asymmetry
It has been known for more than thirty years that the distribution of
molecular gas in the innermost 300 parsecs of the Milky Way, the Central
Molecular Zone, is strongly asymmetric. Indeed, approximately three quarters of
molecular emission comes from positive longitudes, and only one quarter from
negative longitudes. However, despite much theoretical effort, the origin of
this asymmetry has remained a mystery. Here we show that the asymmetry can be
neatly explained by unsteady flow of gas in a barred potential. We use
high-resolution 3D hydrodynamical simulations coupled to a state-of-the-art
chemical network. Despite the initial conditions and the bar potential being
point-symmetric with respect to the Galactic Centre, asymmetries develop
spontaneously due to the combination of a hydrodynamical instability known as
the "wiggle instability" and the thermal instability. The observed asymmetry
must be transient: observations made tens of megayears in the past or in the
future would often show an asymmetry in the opposite sense. Fluctuations of
amplitude comparable to the observed asymmetry occur for a large fraction of
the time in our simulations, and suggest that the present is not an exceptional
moment in the life of our Galaxy.Comment: Accepted for publication in MNRAS. Videos of the simulations are
available at http://www.ita.uni-heidelberg.de/~mattia/download.htm
SHARDS: Constraints on the dust attenuation law of star-forming galaxies at z~2
We make use of SHARDS, an ultra-deep (<26.5AB) galaxy survey that provides
optical photo-spectra at resolution R~50, via medium band filters (FWHM~150A).
This dataset is combined with ancillary optical and NIR fluxes to constrain the
dust attenuation law in the rest-frame NUV region of star-forming galaxies
within the redshift window 1.5<z<3. We focus on the NUV bump strength (B) and
the total-to-selective extinction ratio (Rv), targeting a sample of 1,753
galaxies. By comparing the data with a set of population synthesis models
coupled to a parametric dust attenuation law, we constrain Rv and B, as well as
the colour excess, E(B-V). We find a correlation between Rv and B, that can be
interpreted either as a result of the grain size distribution, or a variation
of the dust geometry among galaxies. According to the former, small dust grains
are associated with a stronger NUV bump. The latter would lead to a range of
clumpiness in the distribution of dust within the interstellar medium of
star-forming galaxies. The observed wide range of NUV bump strengths can lead
to a systematic in the interpretation of the UV slope () typically used
to characterize the dust content. In this study we quantify these variations,
concluding that the effects are ~0.4.Comment: 13 pages, 11+2 figures, 3 tables. MNRAS, in pres
A deeper look at the dust attenuation law of star-forming galaxies at high redshift
A diverse range of dust attenuation laws is found in star-forming galaxies. In particular, Tress et al. (2018) studied the SHARDS survey to constrain the NUV bump strength (B) and the total-to-selective ratio (R-V) of 1753 star-forming galaxies in the GOODS-N field at 1.5 < z < 3. We revisit here this sample to assess the implications and possible causes of the correlation found between R-V and B. The UVJ bicolour plot and main sequence of star formation are scrutinized to look for clues into the observed trend. The standard boundary between quiescent and star-forming galaxies is preserved when taking into account the wide
range of attenuation parameters. However, an additional degeneracy - regarding the effective attenuation law - is added to the standard loci of star-forming galaxies in the UVJ diagram. A simple phenomenological model with an age-dependent extinction (at fixed dust composition) is compatible with the observed trend between R-V and B, whereby the
opacity decreases with the age of the populations, resulting in a weaker NUV bump when the overall attenuation is shallower (greyer). In addition, we compare the constraints obtained by the SHARDS sample with dust models from the literature, supporting a scenario where geometry could potentially drive the correlation between R-V and B
Constraints on the dust extinction law of the Galaxy with Swift/UVOT, Gaia, and 2MASS
We explore variations of the dust extinction law of the Milky Way by selecting stars from the Swift/UVOT Serendipitous Source Catalogue, cross-matched with Gaia DR2 and 2MASS to produce a sample of 10 452 stars out to ∼4 kpc with photometry covering a wide spectral window. The near ultraviolet passbands optimally encompass the 2175 Å bump, so that we can simultaneously fit the net extinction, quoted in the V band (A_{V}), the steepness of the wavelength dependence (δ), and the bump strength (E_{b}). The methodology compares the observed magnitudes with theoretical stellar atmospheres from the models of Coelho. Significant correlations are found between these parameters, related to variations in dust composition that are complementary to similar scaling relations found in the more complex dust attenuation law of galaxies – that also depend on the distribution of dust among the stellar populations within the galaxy. We recover the strong anticorrelation between AV and Galactic latitude, as well as a weaker bump strength at higher extinction. δ is also found to correlate with latitude, with steeper laws towards the Galactic plane. Our results suggest that variations in the attenuation law of galaxies cannot be fully explained by dust geometry
Dynamically driven inflow onto the galactic center and its effect upon molecular clouds
Funding: ERC via the ERC Synergy Grant “ECOGAL” (grant 855130) (M.C.S., R.G.T., S.C.O.G., and R.S.K). R.J.S. gratefully acknowledges an STFC Ernest Rutherford fellowship (grant ST/N00485X/1).The Galactic bar plays a critical role in the evolution of the Milky Way's Central Molecular Zone (CMZ), driving gas toward the Galactic Center via gas flows known as dust lanes. To explore the interaction between the CMZ and the dust lanes, we run hydrodynamic simulations in arepo, modeling the potential of the Milky Way's bar in the absence of gas self-gravity and star formation physics, and we study the flows of mass using Monte Carlo tracer particles. We estimate the efficiency of the inflow via the dust lanes, finding that only about a third (30% ± 12%) of the dust lanes' mass initially accretes onto the CMZ, while the rest overshoots and accretes later. Given observational estimates of the amount of gas within the Milky Way's dust lanes, this suggests that the true total inflow rate onto the CMZ is 0.8 ± 0.6 M⊙ yr−1. Clouds in this simulated CMZ have sudden peaks in their average density near the apocenter, where they undergo violent collisions with inflowing material. While these clouds tend to counter-rotate due to shear, co-rotating clouds occasionally occur due to the injection of momentum from collisions with inflowing material (∼52% are strongly counter-rotating, and ∼7% are strongly co-rotating of the 44 cloud sample). We investigate the formation and evolution of these clouds, finding that they are fed by many discrete inflow events, providing a consistent source of gas to CMZ clouds even as they collapse and form stars.Peer reviewe
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