1,518 research outputs found
The new risk management: the good, the bad, and the ugly
At one time, risk management was limited to insurance and the avoidance of lawsuits and accidents. The new risk management also includes using tools developed for pricing financial options for the management of financial risks within the firm. Trading in financial markets based on these tools can insulate companies from the risk of changes in interest rates, input prices, or currency fluctuations. In this article Philip H. Dybvig and William J. Marshall introduce the new risk management and the policy choices firms should be considering.Management ; Risk
RingFinder: automated detection of galaxy-scale gravitational lenses in ground-based multi-filter imaging data
We present RingFinder, a tool for finding galaxy-scale strong gravitational
lenses in multiband imaging data. By construction, the method is sensitive to
configurations involving a massive foreground early-type galaxy and a faint,
background, blue source. RingFinder detects the presence of blue residuals
embedded in an otherwise smooth red light distribution by difference imaging in
two bands. The method is automated for efficient application to current and
future surveys, having originally been designed for the 150-deg2 Canada France
Hawaii Telescope Legacy Survey (CFHTLS). We describe each of the steps of
RingFinder. We then carry out extensive simulations to assess completeness and
purity. For sources with magnification mu>4, RingFinder reaches 42% (resp. 25%)
completeness and 29% (resp. 86%) purity before (resp. after) visual inspection.
The completeness of RingFinder is substantially improved in the particular
range of Einstein radii 0.8 < REin < 2. and lensed images brighter than g =
22.5, where it can be as high as 70%. RingFinder does not introduce any
significant bias in the source or deflector population. We conclude by
presenting the final catalog of RingFinder CFHTLS galaxy-scale strong lens
candidates. Additional information obtained with Hubble Space Telescope and
Keck Adaptive Optics high resolution imaging, and with Keck and Very Large
Telescope spectroscopy, is used to assess the validity of our classification,
and measure the redshift of the foreground and the background objects. From an
initial sample of 640,000 early type galaxies, RingFinder returns 2500
candidates, which we further reduce by visual inspection to 330 candidates. We
confirm 33 new gravitational lenses from the main sample of candidates, plus an
additional 16 systems taken from earlier versions of RingFinder. First
applications are presented in the SL2S galaxy-scale Lens Sample paper series.Comment: 32 pages (aastex 2col format), 6 figs, ApJ Accepte
The SL2S Galaxy-scale Lens Sample. III. Lens Models, Surface Photometry and Stellar Masses for the final sample
We present Hubble Space Telescope (HST) imaging data and CFHT Near IR
ground-based images for the final sample of 56 candidate galaxy-scale lenses
uncovered in the CFHT Legacy Survey as part of the Strong Lensing in the Legacy
Survey (SL2S) project. The new images are used to perform lens modeling,
measure surface photometry, and estimate stellar masses of the deflector
early-type galaxies. Lens modeling is performed on the HST images (or CFHT when
HST is not available) by fitting the spatially extended light distribution of
the lensed features assuming a singular isothermal ellipsoid mass profile and
by reconstructing the intrinsic source light distribution on a pixelized grid.
Based on the analysis of systematic uncertainties and comparison with inference
based on different methods we estimate that our Einstein Radii are accurate to
\sim3%. HST imaging provides a much higher success rate in confirming
gravitational lenses and measuring their Einstein radii than CFHT imaging does.
Lens modeling with ground-based images however, when successful, yields
Einstein radius measurements that are competitive with spaced-based images.
Information from the lens models is used together with spectroscopic
information from the companion paper IV to classify the systems, resulting in a
final sample of 39 confirmed (grade-A) lenses and 17 promising candidates. The
redshifts of the main deflector span a range 0.3<zd< 0.8, providing an
excellent sample for the study of the cosmic evolution of the mass distribution
of early-type galaxies over the second half of the history of the Universe.Comment: Submitted to The Astrophysical Journa
Extraordinarily high leaf selenium to sulfur ratios define ‘se-accumulator’ plants
Background and Aims: Selenium (Se) and sulfur (S) exhibit similar chemical properties. In flowering plants (angiosperms) selenate and sulfate are acquired and assimilated by common transport and metabolic pathways. It is hypothesized that most angiosperm species show little or no discrimination in the accumulation of Se and S in leaves when their roots are supplied a mixture of selenate and sulfate, but some, termed Se-accumulator plants, selectively accumulate Se in preference to S under these conditions.
Methods: This paper surveys Se and S accumulation in leaves of 39 angiosperm species, chosen to represent the range of plant Se accumulation phenotypes, grown hydroponically under identical conditions.
Results: The data show that, when supplied a mixture of selenate and sulfate: (1) plant species differ in both their leaf Se ([Se]leaf) and leaf S ([S]leaf) concentrations; (2) most angiosperms show little discrimination for the accumulation of Se and S in their leaves and, in non-accumulator plants, [Se]leaf and [S]leaf are highly correlated; (3) [Se]leaf in Se-accumulator plants is significantly greater than in other angiosperms, but [S]leaf, although high, is within the range expected for angiosperms in general; and (4) the Se/S quotient in leaves of Se-accumulator plants is significantly higher than in leaves of other angiosperms.
Conclusion: The traits of extraordinarily high [Se]leaf and leaf Se/S quotients define the distinct elemental composition of Se-accumulator plants
The SWELLS Survey. VI. hierarchical inference of the initial mass functions of bulges and discs
The long-standing assumption that the stellar initial mass function (IMF) is
universal has recently been challenged by a number of observations. Several
studies have shown that a "heavy" IMF (e.g., with a Salpeter-like abundance of
low mass stars and thus normalisation) is preferred for massive early-type
galaxies, while this IMF is inconsistent with the properties of less massive,
later-type galaxies. These discoveries motivate the hypothesis that the IMF may
vary (possibly very slightly) across galaxies and across components of
individual galaxies (e.g. bulges vs discs). In this paper we use a sample of 19
late-type strong gravitational lenses from the SWELLS survey to investigate the
IMFs of the bulges and discs in late-type galaxies. We perform a joint analysis
of the galaxies' total masses (constrained by strong gravitational lensing) and
stellar masses (constrained by optical and near-infrared colours in the context
of a stellar population synthesis [SPS] model, up to an IMF normalisation
parameter). Using minimal assumptions apart from the physical constraint that
the total stellar mass within any aperture must be less than the total mass
within the aperture, we find that the bulges of the galaxies cannot have IMFs
heavier (i.e. implying high mass per unit luminosity) than Salpeter, while the
disc IMFs are not well constrained by this data set. We also discuss the
necessity for hierarchical modelling when combining incomplete information
about multiple astronomical objects. This modelling approach allows us to place
upper limits on the size of any departures from universality. More data,
including spatially resolved kinematics (as in paper V) and stellar population
diagnostics over a range of bulge and disc masses, are needed to robustly
quantify how the IMF varies within galaxies.Comment: Accepted for publication in MNRAS. 15 pages, 8 figures. Code
available at https://github.com/eggplantbren/SWELLS_Hierarchica
On the detectability of strong lensing in near-infrared surveys
We present new lensing frequency estimates for existing and forthcoming deep
near-infrared surveys, including those from JWST and VISTA. The estimates are
based on the JAdes extraGalactic Ultradeep Artificial Realisations (JAGUAR)
galaxy catalogue accounting for the full photometry and morphologies for each
galaxy. Due to the limited area of the JAGUAR simulations, they are less suited
to wide-area surveys, however we also present extrapolations to the surveys
carried out by Euclid and the Nancy Grace Roman Space Telescope. The
methodology does not make assumptions on the nature of the lens itself and
probes a wide range of lens masses. The lenses and sources are selected from
the same catalogue and extend the analysis from the visible bands into the
near-infrared. After generating realistic simulated lensed sources and
selecting those that are detectable with SNR>20, we verify the lensing
frequency expectations against published lens samples selected in the visible,
finding them to be broadly consistent. We find that JWST could yield ~ 65
lensed systems in COSMOS-Web, of which ~ 25 per cent have source redshifts >4.
Deeper, narrower programs (e.g. JADES-Medium) will probe more typical source
galaxies (in flux and mass) but will find fewer systems (~ 25). Of the surveys
we investigate, we find 55-80 per cent have detectable multiple imaging.
Forthcoming NIR surveys will likely reveal new and diverse strong lens systems
including lensed sources that are at higher redshift (JWST) and dustier, more
massive and older (Euclid NISP) than those typically detected in the
corresponding visible surveys.Comment: 14 pages, 9 figures, accepted for publication by MNRA
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