493 research outputs found
Using negative detections to estimate source finder reliability
We describe a simple method to determine the reliability of source finders
based on the detection of sources with both positive and negative total flux.
Under the assumption that the noise is symmetric and that real sources have
positive total flux, negative detections can be used to assign to each positive
detection a probability of being real. We discuss this method in the context of
upcoming, interferometric HI surveys.Comment: Accepted for publication on the 2012 PASA source finding special
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Yield Curve Shapes and the Asymptotic Short Rate Distribution in Affine One-Factor Models
We consider a model for interest rates, where the short rate is given by a
time-homogenous, one-dimensional affine process in the sense of Duffie,
Filipovic and Schachermayer. We show that in such a model yield curves can only
be normal, inverse or humped (i.e. endowed with a single local maximum). Each
case can be characterized by simple conditions on the present short rate. We
give conditions under which the short rate process will converge to a limit
distribution and describe the limit distribution in terms of its cumulant
generating function. We apply our results to the Vasicek model, the CIR model,
a CIR model with added jumps and a model of Ornstein-Uhlenbeck type
Effect of two-particle correlations on x-ray coherent diffractive imaging studies performed with continuum models
Coherent diffraction imaging (CDI) of single molecules at atomic resolution
is a major goal for the x-ray free electron lasers (XFELs). However, during an
imaging pulse, the fast laser-induced ionization may strongly affect the
recorded diffraction pattern of the irradiated sample. The radiation tolerance
of the imaged molecule should then be investigated 'a priori' with a dedicated
simulation tool. The continuum approach is a powerful tool for modeling the
evolution of irradiated large systems consisting of more than a few hundred
thousand atoms. However, this method follows the evolution of average
single-particle densities, and the experimentally recorded intensities reflect
the spatial two-particle correlations. The information on these correlations is
then inherently not accessible within the continuum approach. In this paper we
analyze this limitation of continuum models and discuss the applicability of
continuum models for imaging studies. We propose a formula to calculate
scattered intensities (including both elastic and inelastic scattering) from
the estimates obtained with a single-particle continuum model. We derive this
formula for systems under conditions typical for CDI studies with XFELs
A Catalog of Spectroscopically Confirmed White Dwarfs from the Sloan Digital Sky Survey Data Release 4
We present a catalog of 9316 spectroscopically confirmed white dwarfs from
the Sloan Digital Sky Survey Data Release 4. We have selected the stars through
photometric cuts and spectroscopic modeling, backed up by a set of visual
inspections. Roughly 6000 of the stars are new discoveries, roughly doubling
the number of spectroscopically confirmed white dwarfs. We analyze the stars by
performing temperature and surface gravity fits to grids of pure hydrogen and
helium atmospheres. Among the rare outliers are a set of presumed helium-core
DA white dwarfs with estimated masses below 0.3 Msun, including two candidates
that may be the lowest masses yet found. We also present a list of 928 hot
subdwarfs.Comment: Accepted by the Astrophysical Journal Supplements, 25 pages, 24
figures, LaTeX. The electronic catalog, as well as diagnostic figures and
links to the spectra, is available at http://das.sdss.org/wdcat/dr4
The completeness and reliability of threshold and false-discovery-rate source extraction algorithms for compact continuum sources
The process of determining the number and characteristics of sources in
astronomical images is so fundamental to a large range of astronomical problems
that it is perhaps surprising that no standard procedure has ever been defined
that has well understood properties with a high degree of statistical rigour on
completeness and reliability. There are now a large number of commonly used
software tools for accomplishing this task, typically with different tools
being used for images acquired using different technologies. Despite this,
there have been relatively few quantitative analyses of the robustness or
reliability of individual tools, or the details of the techniques they
implement. We have an opportunity to redress this omission in the context of
surveys planned with the Australian Square Kilometre Array Pathfinder (ASKAP).
The Evolutionary Map of the Universe (EMU) survey with ASKAP, a continuum
survey of the Southern Hemisphere up to declination +30 deg, aims to utilise an
automated source identification and measurement approach that is demonstrably
optimal, to maximise the reliability, utility and robustness of the resulting
radio source catalogues. A key stage in source extraction methods is the
background estimation (background level and noise level) and the choice of a
threshold high enough to reject false sources yet not so high that the
catalogues are significantly incomplete. In this analysis we present results
from testing such algorithms as implemented in the SExtractor, Selavy
(Duchamp), and sfind tools on simulated data. In particular the effects of
background estimation, threshold and false-discovery rate settings are
explored. For parameters that give similar completeness, the false-discovery
rate method employed by sfind results in a more reliable catalogue compared to
the peak threshold methods of SExtractor and Selavy.Comment: 17 pages, 13 figures, accepted by PAS
Searches for Ultra-Compact Dwarf Galaxies in Galaxy Groups
We present the results of a search for ultra-compact dwarf galaxies (UCDs) in
six different galaxy groups: Dorado, NGC1400, NGC0681, NGC4038, NGC4697 and
NGC5084. We searched in the apparent magnitude range 17.5 < b_j < 20.5 (except
NGC5084: 19.2 < b_j < 21.0). We found 1 definite plus 2 possible UCD candidates
in the Dorado group and 2 possible UCD candidates in the NGC1400 group. No UCDs
were found in the other groups. We compared these results with predicted
luminosities of UCDs in the groups according to the hypothesis that UCDs are
globular clusters formed in galaxies. The theoretical predictions broadly agree
with the observational results, but deeper surveys are needed to fully test the
predictions.Comment: 8 pages, 2 figures, accepted for publication in MNRA
SDSSJ103913.70+533029.7: A Super Star Cluster in the Outskirts of a Galaxy Merger
We describe the serendipitous discovery in the spectroscopic data of the
Sloan Digital Sky Survey of a star-like object, SDSSJ103913.70+533029.7, at a
heliocentric radial velocity of +1012 km/s. Its proximity in position and
velocity to the spiral galaxy NGC 3310 suggests an association with the galaxy.
At this distance, SDSSJ103913.70+533029.7 has the luminosity of a super star
cluster and a projected distance of 17 kpc from NGC 3310. Its spectroscopic and
photometric properties imply a mass of > 10^6 solar masses and an age close to
that of the tidal shells seen around NGC 3310, suggesting that it formed in the
event which formed the shells.Comment: Accepted by AJ: 4 figures (1 color
A Lyman-alpha-only AGN from the Sloan Digital Sky Survey
The Sloan Digital Sky Survey has discovered a z=2.4917 radio-loud active
galactic nucleus (AGN) with a luminous, variable, low-polarization UV
continuum, H I two-photon emission, and a moderately broad Lyman-alpha line
(FWHM = 1430 km/s) but without obvious metal-line emission. SDSS
J113658.36+024220.1 does have associated metal-line absorption in three
distinct, narrow systems spanning a velocity range of 2710 km/s. Despite
certain spectral similarities, SDSS J1136+0242 is not a Lyman-break galaxy.
Instead, the Ly-alpha and two-photon emission can be attributed to an extended,
low-metallicity narrow-line region. The unpolarized continuum argues that we
see SDSS J1136+0242 very close to the axis of any ionization cone present. We
can conceive of two plausible explanations for why we see a strong UV continuum
but no broad-line emission in this `face-on radio galaxy' model for SDSS
J1136+0242: the continuum could be relativistically beamed synchrotron emission
which swamps the broad-line emission; or, more likely, SDSS J1136+0242 could be
similar to PG 1407+265, a quasar in which for some unknown reason the
high-ionization emission lines are very broad, very weak, and highly
blueshifted.Comment: AJ, in press, 10 pages emulateapj forma
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