259 research outputs found
Famine Review of The IPC Acute Food Insecurity Analysis: Conclusions and Recommendations For Tigray Region, Ethiopia
Standardizing Type Ia Supernova Absolute Magnitudes Using Gaussian Process Data Regression
We present a novel class of models for Type Ia supernova time-evolving
spectral energy distributions (SED) and absolute magnitudes: they are each
modeled as stochastic functions described by Gaussian processes. The values of
the SED and absolute magnitudes are defined through well-defined regression
prescriptions, so that data directly inform the models. As a proof of concept,
we implement a model for synthetic photometry built from the spectrophotometric
time series from the Nearby Supernova Factory. Absolute magnitudes at peak
brightness are calibrated to 0.13 mag in the -band and to as low as 0.09 mag
in the blueshifted -band, where the dispersion includes
contributions from measurement uncertainties and peculiar velocities. The
methodology can be applied to spectrophotometric time series of supernovae that
span a range of redshifts to simultaneously standardize supernovae together
with fitting cosmological parameters.Comment: 47 pages, 15 figures, accepted for publication by Astrophysical
Journa
Robotic flow shop scheduling with parallel machines and no-wait constraints in an aluminium anodising plant with the CMAES algorithm
Testing Models of Intrinsic Brightness Variations in Type Ia Supernovae, and their Impact on Measuring Cosmological Parameters
For spectroscopically confirmed Type Ia supernovae we evaluate models of
intrinsic brightness variations with detailed data/Monte Carlo comparisons of
the dispersion in the following quantities: Hubble-diagram scatter, color
difference (B-V-c) between the true B-V color and the fitted color (c) from the
SALT-II light curve model, and photometric redshift residual. The data sample
includes 251 ugriz light curves from the 3-season Sloan Digital Sky Survey-II,
and 191 griz light curves from the Supernova Legacy Survey 3-year data release.
We find that the simplest model of a wavelength-independent (coherent) scatter
is not adequate, and that to describe the data the intrinsic scatter model must
have wavelength-dependent variations. We use Monte Carlo simulations to examine
the standard approach of adding a coherent scatter term in quadrature to the
distance-modulus uncertainty in order to bring the reduced chi2 to unity when
fitting a Hubble diagram. If the light curve fits include model uncertainties
with the correct wavelength dependence of the scatter, we find that the bias on
the dark energy equation of state parameter is negligible. However,
incorrect model uncertainties can lead to a significant bias on the distance
moduli, with up to ~0.05 mag redshift-dependent variation. For the recent SNLS3
cosmology results we estimate that this effect introduces an additional
systematic uncertainty on of ~0.02, well below the total uncertainty.
However, this uncertainty depends on the samples used, and thus this small
-uncertainty is not guaranteed in future cosmology results.Comment: accepted by Ap
Host Galaxy Properties and Hubble Residuals of Type Ia Supernovae from the Nearby Supernova Factory
We examine the relationship between Type Ia Supernova (SN Ia) Hubble
residuals and the properties of their host galaxies using a sample of 115 SNe
Ia from the Nearby Supernova Factory (SNfactory). We use host galaxy stellar
masses and specific star-formation rates fitted from photometry for all hosts,
as well as gas-phase metallicities for a subset of 69 star-forming (non-AGN)
hosts, to show that the SN Ia Hubble residuals correlate with each of these
host properties. With these data we find new evidence for a correlation between
SN Ia intrinsic color and host metallicity. When we combine our data with those
of other published SN Ia surveys, we find the difference between mean SN Ia
brightnesses in low and high mass hosts is 0.077 +- 0.014 mag. When viewed in
narrow (0.2 dex) bins of host stellar mass, the data reveal apparent plateaus
of Hubble residuals at high and low host masses with a rapid transition over a
short mass range (9.8 <= log(M_*/M_Sun) <= 10.4). Although metallicity has been
a favored interpretation for the origin of the Hubble residual trend with host
mass, we illustrate how dust in star-forming galaxies and mean SN Ia progenitor
age both evolve along the galaxy mass sequence, thereby presenting equally
viable explanations for some or all of the observed SN Ia host bias.Comment: 20 pages, 11 figures, accepted for publication in Ap
Measuring cosmic bulk flows with Type Ia Supernovae from the Nearby Supernova Factory
Context. Our Local Group of galaxies appears to be moving relative to the
cosmic microwave background with the source of the peculiar motion still
uncertain. While in the past this has been studied mostly using galaxies as
distance indicators, the weight of type Ia supernovae (SNe Ia) has increased
recently with the continuously improving statistics of available low-redshift
supernovae.
Aims. We measured the bulk flow in the nearby universe ()
using 117 SNe Ia observed by the Nearby Supernova Factory, as well as the
Union2 compilation of SN Ia data already in the literature.
Methods. The bulk flow velocity was determined from SN data binned in
redshift shells by including a coherent motion (dipole) in a cosmological fit.
Additionally, a method of spatially smoothing the Hubble residuals was used to
verify the results of the dipole fit. To constrain the location and mass of a
potential mass concentration (e.g., the Shapley supercluster) responsible for
the peculiar motion, we fit a Hubble law modified by adding an additional mass
concentration.
Results. The analysis shows a bulk flow that is consistent with the direction
of the CMB dipole up to , thereby doubling the volume over which
conventional distance measures are sensitive to a bulk flow. We see no
significant turnover behind the center of the Shapley supercluster. A simple
attractor model in the proximity of the Shapley supercluster is only marginally
consistent with our data, suggesting the need for another, more distant source.
In the redshift shell , we constrain the bulk flow velocity to
(68% confidence level) for the direction of the CMB
dipole, in contradiction to recent claims of the existence of a large-amplitude
dark flow.Comment: 12 pages, 5 figures, added corrigendum
(http://adsabs.harvard.edu/abs/2015A%26A...578C...1F
Evidence of Environmental Dependencies of Type Ia Supernovae from the Nearby Supernova Factory indicated by Local H{\alpha}
(Abridged) We study the host galaxy regions in close proximity to Type Ia
supernovae (SNe Ia) to analyze relations between the properties of SN Ia events
and environments most similar to where their progenitors formed. We focus on
local H\alpha\ emission as an indicator of young environments. The Nearby
Supernova Factory has obtained flux-calibrated spectral timeseries for SNe Ia
using integral field spectroscopy, allowing the simultaneous measurement of the
SN and its immediate vicinity. For 89 SNe Ia we measure H\alpha\ emission
tracing ongoing star formation within a 1 kpc radius around each SN. This
constitutes the first direct study of the local environment for a large sample
of SNe Ia also having accurate luminosity, color and stretch measurements. We
find that SNe Ia with local H\alpha\ emission are redder by 0.036+/-0.017 mag,
and that the previously-noted correlation between stretch and host mass is
entirely driven by the SNe Ia coming from passive regions. Most importantly,
the mean standardized brightness for SNe Ia with local H\alpha\ emission is
0.094+/-0.031 mag fainter than for those without. This offset arises from a
bimodal structure in the Hubble residuals, that also explains the
previously-known host-mass bias. We combine this bimodality with the cosmic
star-formation rate to predict changes with redshift in the mean SN Ia
brightness and the host-mass bias. This change is confirmed using high-redshift
SNe Ia from the literature. These environmental dependences point to remaining
systematic errors in SNe Ia standardization. The observed brightness offset is
predicted to cause a significant bias in measurements of the dark energy
equation of state. Recognition of these effects offers new opportunities to
improve SNe Ia as cosmological probes - e.g. SNe Ia having local H\alpha\
emission are more homogeneous, having a brightness dispersion of 0.105+/-0.012
mag.Comment: accepted for publication in Section 3. Cosmology of A&A (The official
date of acceptance is 30/08/2013
Type Ia Supernova Hubble Residuals and Host-Galaxy Properties
Kim et al. (2013) [K13] introduced a new methodology for determining
peak-brightness absolute magnitudes of type Ia supernovae from multi-band light
curves. We examine the relation between their parameterization of light curves
and Hubble residuals, based on photometry synthesized from the Nearby Supernova
Factory spectrophotometric time series, with global host-galaxy properties. The
K13 Hubble residual step with host mass is mag for a supernova
subsample with data coverage corresponding to the K13 training; at , the step is not significant and lower than previous measurements.
Relaxing the data coverage requirement the Hubble residual step with host mass
is mag for the larger sample; a calculation using the modes of
the distributions, less sensitive to outliers, yields a step of 0.019 mag. The
analysis of this article uses K13 inferred luminosities, as distinguished from
previous works that use magnitude corrections as a function of SALT2 color and
stretch parameters: Steps at significance are found in SALT2 Hubble
residuals in samples split by the values of their K13 and
light-curve parameters. affects the light-curve width and color around
peak (similar to the and stretch parameters), and
affects colors, the near-UV light-curve width, and the light-curve decline 20
to 30 days after peak brightness. The novel light-curve analysis, increased
parameter set, and magnitude corrections of K13 may be capturing features of
SN~Ia diversity arising from progenitor stellar evolution.Comment: 17 pages, 6 figures. Accepted by Astrophysical Journa
Host Galaxies of Type Ia Supernovae from the Nearby Supernova Factory
We present photometric and spectroscopic observations of galaxies hosting
Type Ia supernovae (SNe Ia) observed by the Nearby Supernova Factory
(SNfactory). Combining GALEX UV data with optical and near infrared photometry,
we employ stellar population synthesis techniques to measure SN Ia host galaxy
stellar masses, star-formation rates (SFRs), and reddening due to dust. We
reinforce the key role of GALEX UV data in deriving accurate estimates of
galaxy SFRs and dust extinction. Optical spectra of SN Ia host galaxies are
fitted simultaneously for their stellar continua and emission lines fluxes,
from which we derive high precision redshifts, gas-phase metallicities, and
Halpha-based SFRs. With these data we show that SN Ia host galaxies present
tight agreement with the fiducial galaxy mass-metallicity relation from SDSS
for stellar masses log(M_*/M_Sun)>8.5 where the relation is well-defined. The
star-formation activity of SN Ia host galaxies is consistent with a sample of
comparable SDSS field galaxies, though this comparison is limited by systematic
uncertainties in SFR measurements. Our analysis indicates that SN Ia host
galaxies are, on average, typical representatives of normal field galaxies.Comment: 25 pages, 13 figures, accepted for publication in Ap
The Self-Calibrating Hubble Diagram
As an increasing number of well measured type Ia supernovae (SNe Ia) become
available, the statistical uncertainty on w has been reduced to the same size
as the systematic uncertainty. The statistical error will decrease further in
the near future, and hence the improvement of systematic uncertainties needs to
be addressed, if further progress is to be made. We study how uncertainties in
the primary reference spectrum - which are a main contribution to the
systematic uncertainty budget - affect the measurement of the Dark Energy
equation of state parameter w from SNe Ia. The increasing number of SN
observations can be used to reduce the uncertainties by including perturbations
of the reference spectrum as nuisance parameters in a cosmology fit, thus
"self-calibrating" the Hubble diagram.
We employ this method to real SNe data for the first time and find the
perturbations of the reference spectrum consistent with zero at the 1%-level.
For future surveys we estimate that ~3500 SNe will be required for our method
to outperform the standard method of deriving the cosmological parameters.Comment: 17 pages, 8 figures, 1 table. Update to revised version accepted for
publication in JCA
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