34 research outputs found
SuperNova Acceleration Probe (SNAP): Investigating Photometric Redshift Optimization
The aim of this paper is to investigate ways to optimize the accuracy of
photometric redshifts for a SNAP like mission. We focus on how the accuracy of
the photometric redshifts depends on the magnitude limit and signal-to-noise
ratio, wave-length coverage, number of filters and their shapes and observed
galaxy type. We use simulated galaxy catalogs constructed to reproduce observed
galaxy luminosity functions from GOODS, and derive photometric redshifts using
a template fitting method. By using a catalog that resembles real data, we can
estimate the expected number density of galaxies for which photometric
redshifts can be derived. We find that the accuracy of photometric redshifts is
strongly dependent on the signal-to-noise (S/N) (i.e., S/N>10 is needed for
accurate photometric redshifts). The accuracy of the photometric redshifts is
also dependent on galaxy type, with smaller scatter for earlier type galaxies.
Comparing results using different filter sets, we find that including the
U-band is important for decreasing the fraction of outliers, i.e.,
``catastrophic failures''. Using broad overlapping filters with resolution
~4gives better photometric redshifts compared to narrower filters (resolution
>~5) with the same integration time. We find that filters with square response
curves result in a slightly higher scatter, mainly due to a higher fraction of
outliers at faint magnitudes. We also compare a 9-filter set to a 17-filter
set, where we assume that the available exposure time per filter in the latter
set is half that of the first set. We find that the 9-filter set gives more
accurate redshifts for a larger number of objects and reaches higher redshift,
while the 17-filter set is gives better results at bright magnitudes.Comment: 30 pages, 10 figures. Submitted to A
The Oxford-Dartmouth Thirty Degree Survey II: Clustering of Bright Lyman Break Galaxies - Strong Luminosity Dependent Bias at z=4
We present measurements of the clustering properties of bright ()
z4 Lyman Break Galaxies (LBGs) selected from the Oxford-Dartmouth Thirty
Degree Survey (ODT). We describe techniques used to select and evaluate our
candidates and calculate the angular correlation function which we find best
fitted by a power law, with
(with in arcseconds), using a constrained slope of . Using
a redshift distribution consistent with photometric models, we deproject this
correlation function and find a comoving
h Mpc in a flat cosmology for
. This corresponds to a linear bias value of
(assuming ). These data show a
significantly larger and than previous studies at . We
interpret this as evidence that the brightest LBGs have a larger bias than
fainter ones, indicating a strong luminosity dependence for the measured bias
of an LBG sample. Comparing this against recent results in the literature at
fainter (sub-) limiting magnitudes, and with simple models describing
the relationship between LBGs and dark matter haloes, we discuss the
implications on the implied environments and nature of LBGs. It seems that the
brightest LBGs (in contrast with the majority sub- population), have
clustering properties, and host dark matter halo masses, that are consistent
with them being progenitors of the most massive galaxies today.Comment: Accepted for Publication in MNRAS. 15 Pages, 13 Figure
The Oxford-Dartmouth Thirty Degree Survey I: Observations and Calibration of a Wide-Field Multi-Band Survey
The Oxford Dartmouth Thirty Degree Survey (ODTS) is a deep, wide, multi-band
imaging survey designed to cover a total of 30 square degrees in BVRi'Z, with a
subset of U and K band data, in four separate fields of 5-10 deg^2 centred at
00:18:24 +34:52, 09:09:45 +40:50, 13:40:00 +02:30 and 16:39:30 +45:24.
Observations have been made using the Wide Field Camera on the 2.5-m Isaac
Newton Telescope in La Palma to average limiting depths (5 sigma Vega, aperture
magnitudes) of U=24.8, B=25.6, V=25.0, R=24.6, and i'=23.5, with observations
taken in ideal conditions reaching the target depths of U=25.3, B=26.2, V=25.7,
R=25.4, and i'=24.6. The INT Z band data was found to be severely effected by
fringing and, consequently, is now being obtained at the MDM observatory in
Arizona. A complementary K-band survey has also been carried out at MDM,
reaching an average depth of K_{5\sigma}~18.5. At present, approximately 23
deg^2 of the ODTS have been observed, with 3.5 deg^2 of the K band survey
completed. This paper details the survey goals, field selection, observation
strategy and data reduction procedure, focusing on the photometric calibration
and catalogue construction. Preliminary photometric redshifts have been
obtained for a subsample of the objects with R <= 23. These results are
presented alongside a brief description of the photometric redshift
determination technique used. The median redshift of the survey is estimated to
be z~0.7 from a combination of the ODTS photometric redshifts and comparison
with the redshift distributions of other surveys. Finally, galaxy number counts
for the ODTS are presented which are found to be in excellent agreement with
previous studies.Comment: 18 pages, 21 figures, Accepted for publication in MNRA
Prime Focus Spectrograph (PFS) for the Subaru Telescope: Overview, recent progress, and future perspectives
PFS (Prime Focus Spectrograph), a next generation facility instrument on the
8.2-meter Subaru Telescope, is a very wide-field, massively multiplexed,
optical and near-infrared spectrograph. Exploiting the Subaru prime focus, 2394
reconfigurable fibers will be distributed over the 1.3 deg field of view. The
spectrograph has been designed with 3 arms of blue, red, and near-infrared
cameras to simultaneously observe spectra from 380nm to 1260nm in one exposure
at a resolution of ~1.6-2.7A. An international collaboration is developing this
instrument under the initiative of Kavli IPMU. The project is now going into
the construction phase aiming at undertaking system integration in 2017-2018
and subsequently carrying out engineering operations in 2018-2019. This article
gives an overview of the instrument, current project status and future paths
forward.Comment: 17 pages, 10 figures. Proceeding of SPIE Astronomical Telescopes and
Instrumentation 201
The Hyper Suprime-Cam SSP survey: Overview and survey design
Hyper Suprime-Cam (HSC) is a wide-field imaging camera on the prime focus of the 8.2-m Subaru telescope on the summit of Mauna Kea in Hawaii. A team of scientists from Japan, Taiwan, and Princeton University is using HSC to carry out a 300-night multi-band imaging survey of the high-latitude sky. The survey includes three layers: the Wide layer will cover 1400âdeg2 in five broad bands (grizy), with a 5âÏ point-source depth of r â 26. The Deep layer covers a total of 26âdeg2 in four fields, going roughly a magnitude fainter, while the UltraDeep layer goes almost a magnitude fainter still in two pointings of HSC (a total of 3.5âdeg2). Here we describe the instrument, the science goals of the survey, and the survey strategy and data processing. This paper serves as an introduction to a special issue of the Publications of the Astronomical Society of Japan, which includes a large number of technical and scientific papers describing results from the early phases of this survey
Aankomend debat over het Nederlandstalig, katholiek onderwijs in Brussel. Interview met Johan Leman
Een interview over de plaats van het Nederlandstalig onderwijs te Brussel anno 2009.status: publishe
Mortality and metformin use in patients with advanced chronic kidney disease
International audienc
Metformin and other antidiabetic agents in renal failure patients
International audienceThis review mainly focuses on metformin, and considers oral antidiabetic therapy in kidney transplant patients and the potential benefits and risks of antidiabetic agents other than metformin in patients with chronic kidney disease (CKD). In view of the debate concerning lactic acidosis associated with metformin, this review tries to solve a paradox: metformin should be prescribed more widely because of its beneficial effects, but also less widely because of the increasing prevalence of contraindications to metformin, such as reduced renal function. Lactic acidosis appears either as part of a number of clinical syndromes (i.e., unrelated to metformin), induced by metformin (involving an analysis of the drug's pharmacokinetics and mechanisms of action), or associated with metformin (a more complex situation, as lactic acidosis in a metformin-treated patient is not necessarily accompanied by metformin accumulation, nor does metformin accumulation necessarily lead to lactic acidosis). A critical analysis of guidelines and literature data on metformin therapy in patients with CKD is presented. Following the present focus on metformin, new paradoxical issues can be drawn up, in particular: (i) metformin is rarely the sole cause of lactic acidosis; (ii) lactic acidosis in patients receiving metformin therapy is erroneously still considered a single medical entity, as several different scenarios can be defined, with contrasting prognoses. The prognosis for severe lactic acidosis seems even better in metformin-treated patients than in non-metformin users