198 research outputs found
Refined position angle measurements for galaxies of the SDSS Stripe 82 co-added dataset
Position angle measurements of Sloan Digital Sky Survey (SDSS) galaxies, as
measured by the surface brightness profile fitting code of the SDSS photometric
pipeline (Lupton 2001), are known to be strongly biased, especially in the case
of almost face-on and highly inclined galaxies. To address this issue we
developed a reliable algorithm which determines position angles by means of
isophote fitting. In this paper we present our algorithm and a catalogue of
position angles for 26397 SDSS galaxies taken from the deep co-added Stripe 82
(equatorial stripe) images.Comment: 4 pages, 4 figures. Data are published on-line at
http://www.vo.elte.hu/galmorp
Galaxy shape measurement with convolutional neural networks
We present our results from training and evaluating a convolutional neural
network (CNN) to predict galaxy shapes from wide-field survey images of the
first data release of the Dark Energy Survey (DES DR1). We use conventional
shape measurements as ground truth from an overlapping, deeper survey with less
sky coverage, the Canada-France Hawaii Telescope Lensing Survey (CFHTLenS). We
demonstrate that CNN predictions from single band DES images reproduce the
results of CFHTLenS at bright magnitudes and show higher correlation with
CFHTLenS at fainter magnitudes than maximum likelihood model fitting estimates
in the DES Y1 im3shape catalogue. Prediction of shape parameters with a CNN is
also extremely fast, it takes only 0.2 milliseconds per galaxy, improving more
than 4 orders of magnitudes over forward model fitting. The CNN can also
accurately predict shapes when using multiple images of the same galaxy, even
in different color bands, with no additional computational overhead. The CNN is
again more precise for faint objects, and the advantage of the CNN is more
pronounced for blue galaxies than red ones when compared to the DES Y1
metacalibration catalogue, which fits a single Gaussian profile using riz band
images. We demonstrate that CNN shape predictions within the metacalibration
self-calibrating framework yield shear estimates with negligible multiplicative
bias, , and no significant PSF leakage. Our proposed setup is
applicable to current and next generation weak lensing surveys where higher
quality ground truth shapes can be measured in dedicated deep fields
Photo-Met: a non-parametric method for estimating stellar metallicity from photometric observations
Getting spectra at good signal-to-noise ratios takes orders of magnitudes
more time than photometric observations. Building on the technique developed
for photometric redshift estimation of galaxies, we develop and demonstrate a
non-parametric photometric method for estimating the chemical composition of
galactic stars. We investigate the efficiency of our method using
spectroscopically determined stellar metallicities from SDSS DR7. The technique
is generic in the sense that it is not restricted to certain stellar types or
stellar parameter ranges and makes it possible to obtain metallicities and
error estimates for a much larger sample than spectroscopic surveys would
allow. We find that our method performs well, especially for brighter stars and
higher metallicities and, in contrast to many other techniques, we are able to
reliably estimate the error of the predicted metallicities.Comment: 5 pages, 4 figures, accepted for publication in A
Interior Structures and Tidal Heating in the TRAPPIST-1 Planets
With seven planets, the TRAPPIST-1 system has the largest number of
exoplanets discovered in a single system so far. The system is of
astrobiological interest, because three of its planets orbit in the habitable
zone of the ultracool M dwarf. Assuming the planets are composed of
non-compressible iron, rock, and HO, we determine possible interior
structures for each planet. To determine how much tidal heat may be dissipated
within each planet, we construct a tidal heat generation model using a single
uniform viscosity and rigidity for each planet based on the planet's
composition. With the exception of TRAPPIST-1c, all seven of the planets have
densities low enough to indicate the presence of significant HO in some
form. Planets b and c experience enough heating from planetary tides to
maintain magma oceans in their rock mantles; planet c may have eruptions of
silicate magma on its surface, which may be detectable with next-generation
instrumentation. Tidal heat fluxes on planets d, e, and f are lower, but are
still twenty times higher than Earth's mean heat flow. Planets d and e are the
most likely to be habitable. Planet d avoids the runaway greenhouse state if
its albedo is 0.3. Determining the planet's masses within
to 0.5 Earth masses would confirm or rule out the presence of HO and/or
iron in each planet, and permit detailed models of heat production and
transport in each planet. Understanding the geodynamics of ice-rich planets f,
g, and h requires more sophisticated modeling that can self-consistently
balance heat production and transport in both rock and ice layers.Comment: 34 pages, 3 tables, 4 figures. Accepted for publication in Astronomy
& Astrophysics -- final version including corrections made in proof stag
Possibility for albedo estimation of exomoons: Why should we care about M dwarfs?
Occultation light curves of exomoons may give information on their albedo and
hence indicate the presence of ice cover on the surface. Icy moons might have
subsurface oceans thus these may potentially be habitable. The objective of our
paper is to determine whether next generation telescopes will be capable of
albedo estimations for icy exomoons using their occultation light curves. The
success of the measurements depends on the depth of the moon's occultation in
the light curve and on the sensitivity of the used instruments. We applied
simple calculations for different stellar masses in the V and J photometric
bands, and compared the flux drop caused by the moon's occultation and the
estimated photon noise of next generation missions with 5 confidence.
We found that albedo estimation by this method is not feasible for moons of
solar-like stars, but small M dwarfs are better candidates for such
measurements. Our calculations in the J photometric band show that E-ELT
MICADO's photon noise is just about 4 ppm greater than the flux difference
caused by a 2 Earth-radii icy satellite in a circular orbit at the snowline of
an 0.1 stellar mass star. However, considering only photon noise underestimates
the real expected noise, because other noise sources, such as CCD read-out and
dark signal become significant in the near infrared measurements. Hence we
conclude that occultation measurements with next generation missions are far
too challenging, even in the case of large, icy moons at the snowline of small
M dwarfs. We also discuss the role of the parameters that were neglected in the
calculations, e.g. inclination, eccentricity, orbiting direction of the moon.
We predict that the first albedo estimations of exomoons will probably be made
for large icy moons around the snowline of M4 -- M9 type main sequence stars.Comment: 13 pages, 6 figures, accepted for publication in A&
StePS: A Multi-GPU Cosmological N-body Code for Compactified Simulations
We present the multi-GPU realization of the StePS (Stereographically
Projected Cosmological Simulations) algorithm with MPI-OpenMP-CUDA hybrid
parallelization and nearly ideal scale-out to multiple compute nodes. Our new
zoom-in cosmological direct N-body simulation method simulates the infinite
universe with unprecedented dynamic range for a given amount of memory and, in
contrast to traditional periodic simulations, its fundamental geometry and
topology match observations. By using a spherical geometry instead of periodic
boundary conditions, and gradually decreasing the mass resolution with radius,
our code is capable of running simulations with a few gigaparsecs in diameter
and with a mass resolution of in the center in four days
on three compute nodes with four GTX 1080Ti GPUs in each. The code can also be
used to run extremely fast simulations with reasonable resolution for fitting
cosmological parameters. These simulations are useful for prediction needs of
large surveys. The StePS code is publicly available for the research community
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