120 research outputs found
A ground-based 21cm Baryon acoustic oscillation survey
Baryon acoustic oscillations (BAO) provide a robust standard ruler with which
to measure the acceleration of the Universe. The BAO feature has so far been
detected in optical galaxy surveys. Intensity mapping of neutral hydrogen
emission with a ground-based radio telescope provides another promising window
for measuring BAO at redshifts of order unity for relatively low cost. While
the cylindrical radio telescope (CRT) proposed for these measurements will have
excellent redshift resolution, it will suffer from poor angular resolution (a
few arcminutes at best). We investigate the effect of angular resolution on the
standard ruler test with BAO, using the Dark Energy Task Force Figure of Merit
as a benchmark. We then extend the analysis to include variations in the
parameters characterizing the telescope and the underlying physics. Finally, we
optimize the survey parameters (holding total cost fixed) and present an
example of a CRT BAO survey that is competitive with Stage III dark energy
experiments. The tools developed here form the backbone of a publicly available
code that can be used to obtain estimates of cost and Figure of Merit for any
set of parameters.Comment: ApJ accepted version. Important changes in section 2 and 3 - uses a
more realistic instrument response model and removed the discussion of
aliasing effect. The conclusions remain the same. Typos fixed (including eq
5). 11 emulated apj pages with 7 figures and 1 tabl
Stochastic Cooling Overview
The status of stochastic cooling and developments over the years are reviewed
with reference to much of the original work. Both theoretical and technological
subjects are considered.Comment: 10 pages. Includes an extensive bibliograph
The Core Collapse Supernova Rate from the SDSS-II Supernova Survey
We use the Sloan Digital Sky Survey II Supernova Survey (SDSS-II SNS) data to
measure the volumetric core collapse supernova (CCSN) rate in the redshift
range (0.03<z<0.09). Using a sample of 89 CCSN we find a volume-averaged rate
of (1.06 +/- 0.19) x 10**(-4)/(yr Mpc**3) at a mean redshift of 0.072 +/-
0.009. We measure the CCSN luminosity function from the data and consider the
implications on the star formation history.Comment: Minor corrections to references and affiliations to conform with
published versio
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
Photometric Supernova Cosmology with BEAMS and SDSS-II
Supernova cosmology without spectroscopic confirmation is an exciting new
frontier which we address here with the Bayesian Estimation Applied to Multiple
Species (BEAMS) algorithm and the full three years of data from the Sloan
Digital Sky Survey II Supernova Survey (SDSS-II SN). BEAMS is a Bayesian
framework for using data from multiple species in statistical inference when
one has the probability that each data point belongs to a given species,
corresponding in this context to different types of supernovae with their
probabilities derived from their multi-band lightcurves. We run the BEAMS
algorithm on both Gaussian and more realistic SNANA simulations with of order
10^4 supernovae, testing the algorithm against various pitfalls one might
expect in the new and somewhat uncharted territory of photometric supernova
cosmology. We compare the performance of BEAMS to that of both mock
spectroscopic surveys and photometric samples which have been cut using typical
selection criteria. The latter typically are either biased due to contamination
or have significantly larger contours in the cosmological parameters due to
small data-sets. We then apply BEAMS to the 792 SDSS-II photometric supernovae
with host spectroscopic redshifts. In this case, BEAMS reduces the area of the
(\Omega_m,\Omega_\Lambda) contours by a factor of three relative to the case
where only spectroscopically confirmed data are used (297 supernovae). In the
case of flatness, the constraints obtained on the matter density applying BEAMS
to the photometric SDSS-II data are \Omega_m(BEAMS)=0.194\pm0.07. This
illustrates the potential power of BEAMS for future large photometric supernova
surveys such as LSST.Comment: 25 pages, 15 figures, submitted to Ap
Type Ia Supernova Properties as a Function of the Distance to the Host Galaxy in the SDSS-II SN Survey
We use type-Ia supernovae (SNe Ia) discovered by the SDSS-II SN Survey to
search for dependencies between SN Ia properties and the projected distance to
the host galaxy center, using the distance as a proxy for local galaxy
properties (local star-formation rate, local metallicity, etc.). The sample
consists of almost 200 spectroscopically or photometrically confirmed SNe Ia at
redshifts below 0.25. The sample is split into two groups depending on the
morphology of the host galaxy. We fit light-curves using both MLCS2k2 and
SALT2, and determine color (AV, c) and light-curve shape (delta, x1) parameters
for each SN Ia, as well as its residual in the Hubble diagram. We then
correlate these parameters with both the physical and the normalized distances
to the center of the host galaxy and look for trends in the mean values and
scatters of these parameters with increasing distance. The most significant (at
the 4-sigma level) finding is that the average fitted AV from MLCS2k2 and c
from SALT2 decrease with the projected distance for SNe Ia in spiral galaxies.
We also find indications that SNe in elliptical galaxies tend to have narrower
light-curves if they explode at larger distances, although this may be due to
selection effects in our sample. We do not find strong correlations between the
residuals of the distance moduli with respect to the Hubble flow and the
galactocentric distances, which indicates a limited correlation between SN
magnitudes after standardization and local host metallicity.Comment: Accepted for publication in The Astrophysical Journal (33 pages, 5
figures, 8 tables
Type Ia Supernovae Selection and Forecast of Cosmology Constraints for the Dark Energy Survey
We present the results of a study of selection criteria to identify Type Ia
supernovae photometrically in a simulated mixed sample of Type Ia supernovae
and core collapse supernovae. The simulated sample is a mockup of the expected
results of the Dark Energy Survey. Fits to the MLCS2k2 and SALT2 Type Ia
supernova models are compared and used to help separate the Type Ia supernovae
from the core collapse sample. The Dark Energy Task Force Figure of Merit
(modified to include core collapse supernovae systematics) is used to
discriminate among the various selection criteria. This study of varying
selection cuts for Type Ia supernova candidates is the first to evaluate core
collapse contamination using the Figure of Merit. Different factors that
contribute to the Figure of Merit are detailed. With our analysis methods, both
SALT2 and MLCS2k2 Figures of Merit improve with tighter selection cuts and
higher purities, peaking at 98% purity.Comment: submitted to JCAP, 23 pages, 36 picture
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