700 research outputs found
Brown dwarf census with the Dark Energy Survey year 3 data and the thin disc scale height of early L types
27 pages, 18 figuresIn this paper we present a catalogue of 11 745 brown dwarfs with spectral types ranging from L0 to T9, photometrically classified using data from the Dark Energy Survey (DES) year 3 release matched to the Vista Hemisphere Survey (VHS) DR3 and Wide-field Infrared Survey Explorer (WISE) data, covering ≈2400 deg2 up to iAB = 22. The classification method follows the same phototype method previously applied to SDSS-UKIDSS-WISE data. The most significant difference comes from the use of DES data instead of SDSS, which allow us to classify almost an order of magnitude more brown dwarfs than any previous search and reaching distances beyond 400 pc for the earliest types. Next, we also present and validate the GalmodBD simulation, which produces brown dwarf number counts as a function of structural parameters with realistic photometric properties of a given survey. We use this simulation to estimate the completeness and purity of our photometric LT catalogue down to iAB = 22, as well as to compare to the observed number of LT types. We put constraints on the thin disc scale height for the early L (L0–L3) population to be around 450 pc, in agreement with previous findings. For completeness, we also publish in a separate table a catalogue of 20 863 M dwarfs that passed our colour cut with spectral types greater than M6. Both the LT and the late M catalogues are found at DES release page https://des.ncsa.illinois.edu/releases/other/y3-mlt.Peer reviewedFinal Published versio
Transfer learning for galaxy morphology from one survey to another
© 2018 The Author(s). Published by Oxford University Press on behalf of the Royal Astronomical Society.Deep Learning (DL) algorithms for morphological classification of galaxies have proven very successful, mimicking (or even improving) visual classifications. However, these algorithms rely on large training samples of labelled galaxies (typically thousands of them). A key question for using DL classifications in future Big Data surveys is how much of the knowledge acquired from an existing survey can be exported to a new dataset, i.e. if the features learned by the machines are meaningful for different data. We test the performance of DL models, trained with Sloan Digital Sky Survey (SDSS) data, on Dark Energy survey (DES) using images for a sample of 5000 galaxies with a similar redshift distribution to SDSS. Applying the models directly to DES data provides a reasonable global accuracy ( 90%), but small completeness and purity values. A fast domain adaptation step, consisting in a further training with a small DES sample of galaxies (500-300), is enough for obtaining an accuracy > 95% and a significant improvement in the completeness and purity values. This demonstrates that, once trained with a particular dataset, machines can quickly adapt to new instrument characteristics (e.g., PSF, seeing, depth), reducing by almost one order of magnitude the necessary training sample for morphological classification. Redshift evolution effects or significant depth differences are not taken into account in this study.Peer reviewedFinal Accepted Versio
Evidence for Color Dichotomy in the Primordial Neptunian Trojan Population
In the current model of early Solar System evolution, the stable members of
the Jovian and Neptunian Trojan populations were captured into resonance from
the leftover reservoir of planetesimals during the outward migration of the
giant planets. As a result, both Jovian and Neptunian Trojans share a common
origin with the primordial disk population, whose other surviving members
constitute today's trans-Neptunian object (TNO) populations. The cold classical
TNOs are ultra-red, while the dynamically excited "hot" population of TNOs
contains a mixture of ultra-red and blue objects. In contrast, Jovian and
Neptunian Trojans are observed to be blue. While the absence of ultra-red
Jovian Trojans can be readily explained by the sublimation of volatile material
from their surfaces due to the high flux of solar radiation at 5AU, the lack of
ultra-red Neptunian Trojans presents both a puzzle and a challenge to formation
models. In this work we report the discovery by the Dark Energy Survey (DES) of
two new dynamically stable L4 Neptunian Trojans,2013 VX30 and 2014 UU240, both
with inclinations i >30 degrees, making them the highest-inclination known
stable Neptunian Trojans. We have measured the colors of these and three other
dynamically stable Neptunian Trojans previously observed by DES, and find that
2013 VX30 is ultra-red, the first such Neptunian Trojan in its class. As such,
2013 VX30 may be a "missing link" between the Trojan and TNO populations. Using
a simulation of the DES TNO detection efficiency, we find that there are 162
+/- 73 Trojans with Hr < 10 at the L4 Lagrange point of Neptune. Moreover, the
blue-to-red Neptunian Trojan population ratio should be higher than 17:1. Based
on this result, we discuss the possible origin of the ultra-red Neptunian
Trojan population and its implications for the formation history of Neptunian
Trojans
Semliki Forest virus induced, immune mediated demyelination: the effect of irradiation
International audienceThe Dark Energy Camera has captured a large set of images as part of Science Verification (SV) for the Dark Energy Survey (DES). The SV footprint covers a large portion of the outer Large Magellanic Cloud (LMC), providing photometry 1.5 mag fainter than the main sequence turn-off of the oldest LMC stellar population. We derive geometrical and structural parameters for various stellar populations in the LMC disc. For the distribution of all LMC stars, we find an inclination of i = -38.14° ± 0.08° (near side in the north) and a position angle for the line of nodes of θ0 = 129.51° ± 0.17°. We find that stars younger than ∼4 Gyr are more centrally concentrated than older stars. Fitting a projected exponential disc shows that the scale radius of the old populations is R>4 Gyr = 1.41 ± 0.01 kpc, while the younger population has R = 0.72 ± 0.01 kpc. However, the spatial distribution of the younger population deviates significantly from the projected exponential disc model. The distribution of old stars suggests a large truncation radius of Rt = 13.5 ± 0.8 kpc. If this truncation is dominated by the tidal field of the Galaxy, we find that the LMC is {∼eq } 24^{+9}_{-6} times less massive than the encircled Galactic mass. By measuring the Red Clump peak magnitude and comparing with the best-fitting LMC disc model, we find that the LMC disc is warped and thicker in the outer regions north of the LMC centre. Our findings may either be interpreted as a warped and flared disc in the LMC outskirts, or as evidence of a spheroidal halo component
Evolution of Galaxy Luminosity Function Using Photometric Redshifts
We examine the impact of using photometric redshifts for studying the
evolution of both the global galaxy luminosity function (LF) and that for
different galaxy types. To this end we compare LFs obtained using photometric
redshifts from the CFHT Legacy Survey (CFHTLS) D1 field with those from the
spectroscopic survey VIMOS VLT Deep Survey (VVDS) comprising ~4800 galaxies. We
find that for z<2, in the interval of magnitudes considered by this survey, the
LFs obtained using photometric and spectroscopic redshifts show a remarkable
agreement. This good agreement led us to use all four Deep fields of CFHTLS
comprising ~386000 galaxies to compute the LF of the combined fields and
estimate directly the error in the parameters based on field-to-field
variation. We find that the characteristic absolute magnitude M* of Schechter
fits fades by ~0.7mag from z~1.8 to z~0.3, while the characteristic density
phi* increases by a factor of ~4 in the same redshift bin. We use the galaxy
classification provided by the template fitting program used to compute
photometric redshifts and split the sample into galaxy types. We find that
these Schechter parameters evolve differently for each galaxy type, an
indication that their evolution is a combination of several effects: galaxy
merging, star formation quenching and mass assembly. All these results are
compatible with those obtained by different spectroscopic surveys such as VVDS,
DEEP2 and zCosmos, which reinforces the fact that photometric redshifts can be
used to study galaxy evolution, at least for the redshift bins adopted so far.
This is of great interest since future very large imaging surveys containing
hundreds of millions of galaxies will allow to obtain important precise
measurements to constrain the evolution of the LF and to explore the dependence
of this evolution on morphology and/or color helping constrain the mechanisms
of galaxy evolution.Comment: 29 pages, 10 figures. Approved for publication in The Astronomical
Journa
Spectral variability of a sample of extreme variability quasars and implications for the MgII broad-line region
FINANCIADORA DE ESTUDOS E PROJETOS - FINEPFUNDAÇÃO CARLOS CHAGAS FILHO DE AMPARO À PESQUISA DO ESTADO DO RIO DE JANEIRO - FAPERJCONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICO - CNPQWe present new Gemini/GMOS optical spectroscopy of 16 extreme variability quasars (EVQs) that dimmed bymore than 1.5mag in the g band between the Sloan Digital Sky Survey (SDSS) and the Dark Energy Survey epochs (separated by a few years in the quasar res493457735787FINANCIADORA DE ESTUDOS E PROJETOS - FINEPFUNDAÇÃO CARLOS CHAGAS FILHO DE AMPARO À PESQUISA DO ESTADO DO RIO DE JANEIRO - FAPERJCONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICO - CNPQFINANCIADORA DE ESTUDOS E PROJETOS - FINEPFUNDAÇÃO CARLOS CHAGAS FILHO DE AMPARO À PESQUISA DO ESTADO DO RIO DE JANEIRO - FAPERJCONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICO - CNPQSem informaçãoSem informação465376/2014-2We thank the referee, Andy Lawrence, for useful comments that improved the manuscript. QY and YS acknowledge support from an Alfred P. Sloan Research Fellowship (YS) and NSF grant AST-1715579. We thank Patrick Hall, Tamara Davis, Shu Wang, and Hengxiao G
Forward Global Photometric Calibration of the Dark Energy Survey
Many scientific goals for the Dark Energy Survey (DES) require calibration of
optical/NIR broadband photometry that is stable in time and uniform
over the celestial sky to one percent or better. It is also necessary to limit
to similar accuracy systematic uncertainty in the calibrated broadband
magnitudes due to uncertainty in the spectrum of the source. Here we present a
"Forward Global Calibration Method (FGCM)" for photometric calibration of the
DES, and we present results of its application to the first three years of the
survey (Y3A1). The FGCM combines data taken with auxiliary instrumentation at
the observatory with data from the broad-band survey imaging itself and models
of the instrument and atmosphere to estimate the spatial- and time-dependence
of the passbands of individual DES survey exposures. "Standard" passbands are
chosen that are typical of the passbands encountered during the survey. The
passband of any individual observation is combined with an estimate of the
source spectral shape to yield a magnitude in the standard
system. This "chromatic correction" to the standard system is necessary to
achieve sub-percent calibrations. The FGCM achieves reproducible and stable
photometric calibration of standard magnitudes of stellar
sources over the multi-year Y3A1 data sample with residual random calibration
errors of per exposure. The accuracy of the
calibration is uniform across the DES footprint to
within . The systematic uncertainties of magnitudes in
the standard system due to the spectra of sources are less than
for main sequence stars with .Comment: 25 pages, submitted to A
Stellar populations of early-type galaxies in different environments I. Line-strength indices
Aims: This paper commences a series devoted to the study of the stellar
content of early-type galaxies. The goal of the series is to set constraints on
the evolutionary status of these objects.
Methods: In this paper we describe the details of the galaxy sample, the
observations, and the data reduction. Line-strength indices and velocity
dispersions sigma are measured in 98 early-type galaxies drawn from different
environments, and the relation of the indices with the velocity dispersion
analysed in detail.
Results: The present sample indicates that some of the index-sigma relations
depend on galaxy environment. In particular, the slope of the relation between
Balmer lines and sigma is steeper for galaxies in the Virgo cluster, small
groups, and in the field than for galaxies in the Coma cluster. In several
indices there is also a significant offset in the zero point between the
relations defined by the different subsamples. The slopes of the index-sigma
relation for the Virgo and low-density environment galaxies are explained by a
variation of both age and metallicity with velocity dispersion, as previously
noted in other studies. For the galaxies in the Coma cluster, however, the
relation of the indices with sigma only requires a variation of the abundance
along the sigma sequence. In agreement with other studies we find that the
models that better reproduce the slopes are those in which the alpha elements
vary more than the Fe-peak elements along the sigma sequence, while, at a given
sigma, older galaxies show an higher alpha/Fe ratio.
Conclusions: The results can be explained assuming that galaxies in the Coma
cluster have experienced a truncated star formation and chemical enrichment
history compared to a more continuous time-extended history for their
counterparts in lower density environments.Comment: accepted for publication in A&
Phenotypic redshifts with self-organizing maps: A novel method to characterize redshift distributions of source galaxies for weak lensing
Wide-field imaging surveys such as the Dark Energy Survey (DES) rely on
coarse measurements of spectral energy distributions in a few filters to
estimate the redshift distribution of source galaxies. In this regime, sample
variance, shot noise, and selection effects limit the attainable accuracy of
redshift calibration and thus of cosmological constraints. We present a new
method to combine wide-field, few-filter measurements with catalogs from deep
fields with additional filters and sufficiently low photometric noise to break
degeneracies in photometric redshifts. The multi-band deep field is used as an
intermediary between wide-field observations and accurate redshifts, greatly
reducing sample variance, shot noise, and selection effects. Our implementation
of the method uses self-organizing maps to group galaxies into phenotypes based
on their observed fluxes, and is tested using a mock DES catalog created from
N-body simulations. It yields a typical uncertainty on the mean redshift in
each of five tomographic bins for an idealized simulation of the DES Year 3
weak-lensing tomographic analysis of , which is a
60% improvement compared to the Year 1 analysis. Although the implementation of
the method is tailored to DES, its formalism can be applied to other large
photometric surveys with a similar observing strategy.Comment: 24 pages, 11 figures; matches version accepted to MNRA
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