379 research outputs found
A Calibration of NICMOS Camera 2 for Low Count-Rates
NICMOS 2 observations are crucial for constraining distances to most of the
existing sample of z > 1 SNe Ia. Unlike the conventional calibration programs,
these observations involve long exposure times and low count rates. Reciprocity
failure is known to exist in HgCdTe devices and a correction for this effect
has already been implemented for high and medium count-rates. However
observations at faint count-rates rely on extrapolations. Here instead, we
provide a new zeropoint calibration directly applicable to faint sources. This
is obtained via inter-calibration of NIC2 F110W/F160W with WFC3 in the low
count-rate regime using z ~ 1 elliptical galaxies as tertiary calibrators.
These objects have relatively simple near-IR SEDs, uniform colors, and their
extended nature gives superior signal-to-noise at the same count rate than
would stars. The use of extended objects also allows greater tolerances on PSF
profiles. We find ST magnitude zeropoints (after the installation of the NICMOS
cooling system, NCS) of 25.296 +- 0.022 for F110W and 25.803 +- 0.023 for
F160W, both in agreement with the calibration extrapolated from count-rates
1,000 times larger (25.262 and 25.799). Before the installation of the NCS, we
find 24.843 +- 0.025 for F110W and 25.498 +- 0.021 for F160W, also in agreement
with the high-count-rate calibration (24.815 and 25.470). We also check the
standard bandpasses of WFC3 and NICMOS 2 using a range of stars and galaxies at
different colors and find mild tension for WFC3, limiting the accuracy of the
zeropoints. To avoid human bias, our cross-calibration was "blinded" in that
the fitted zeropoint differences were hidden until the analysis was finalized.Comment: Accepted for Publication in the Astronomical Journal. New version
contains added referenc
The Rise Time of Type Ia Supernovae from the Supernova Legacy Survey
We compare the rise times of nearby and distant Type Ia supernovae (SNe Ia)
as a test for evolution using 73 high-redshift spectroscopically-confirmed SNe
Ia from the first two years of the five year Supernova Legacy Survey (SNLS) and
published observations of nearby SN. Because of the ``rolling'' search nature
of the SNLS, our measurement is approximately 6 times more precise than
previous studies, allowing for a more sensitive test of evolution between
nearby and distant supernovae. Adopting a simple early-time model (as in
previous studies), we find that the rest-frame rise times for a fiducial SN
Ia at high and low redshift are consistent, with values
and
days, respectively; the statistical significance of this difference is only 1.4
\sg . The errors represent the uncertainty in the mean rather than any
variation between individual SN. We also compare subsets of our high-redshift
data set based on decline rate, host galaxy star formation rate, and redshift,
finding no substantive evidence for any subsample dependence.Comment: Accepted for publication in AJ; minor changes (spelling and
grammatical) to conform with published versio
Type Ia Supernovae Rates and Galaxy Clustering from the CFHT Supernova Legacy Survey
The Canada-France-Hawaii Telescope Supernova Legacy Survey (SNLS) has created
a large homogeneous database of intermediate redshift (0.2 < z < 1.0) type Ia
supernovae (SNe Ia). The SNLS team has shown that correlations exist between SN
Ia rates, properties, and host galaxy star formation rates. The SNLS SN Ia
database has now been combined with a photometric redshift galaxy catalog and
an optical galaxy cluster catalog to investigate the possible influence of
galaxy clustering on the SN Ia rate, over and above the expected effect due to
the dependence of SFR on clustering through the morphology-density relation. We
identify three cluster SNe Ia, plus three additional possible cluster SNe Ia,
and find the SN Ia rate per unit mass in clusters at intermediate redshifts is
consistent with the rate per unit mass in field early-type galaxies and the SN
Ia cluster rate from low redshift cluster targeted surveys. We also find the
number of SNe Ia in cluster environments to be within a factor of two of
expectations from the two component SNIa rate model.Comment: 21 pages, 2 figures, 6 tables, accepted for publication in A
Correction to : Blood platelets and sepsis pathophysiology : A new therapeutic prospect in critically ill patients ?
Upon publication of the original article [1], it was noticed that the title was incorrect. Instead of 'critical', it should read 'critically', and therefore, the correct title should be
The Supernova Legacy Survey 3-year sample: Type Ia Supernovae photometric distances and cosmological constraints
We present photometric properties and distance measurements of 252 high
redshift Type Ia supernovae (0.15 < z < 1.1) discovered during the first three
years of the Supernova Legacy Survey (SNLS). These events were detected and
their multi-colour light curves measured using the MegaPrime/MegaCam instrument
at the Canada-France-Hawaii Telescope (CFHT), by repeatedly imaging four
one-square degree fields in four bands. Follow-up spectroscopy was performed at
the VLT, Gemini and Keck telescopes to confirm the nature of the supernovae and
to measure their redshifts. Systematic uncertainties arising from light curve
modeling are studied, making use of two techniques to derive the peak
magnitude, shape and colour of the supernovae, and taking advantage of a
precise calibration of the SNLS fields. A flat LambdaCDM cosmological fit to
231 SNLS high redshift Type Ia supernovae alone gives Omega_M = 0.211 +/-
0.034(stat) +/- 0.069(sys). The dominant systematic uncertainty comes from
uncertainties in the photometric calibration. Systematic uncertainties from
light curve fitters come next with a total contribution of +/- 0.026 on
Omega_M. No clear evidence is found for a possible evolution of the slope
(beta) of the colour-luminosity relation with redshift.Comment: (The SNLS Collaboration) 40 pages, 32 figures, Accepted in A&
SALT2: using distant supernovae to improve the use of Type Ia supernovae as distance indicators
We present an empirical model of Type Ia supernovae spectro-photometric
evolution with time. The model is built using a large data set including
light-curves and spectra of both nearby and distant supernovae, the latter
being observed by the SNLS collaboration. We derive the average spectral
sequence of Type Ia supernovae and their main variability components including
a color variation law. The model allows us to measure distance moduli in the
spectral range 2500-8000 A with calculable uncertainties, including those
arising from variability of spectral features. Thanks to the use of
high-redshift SNe to model the rest-frame UV spectral energy distribution, we
are able to derive improved distance estimates for SNe Ia in the redshift range
0.8<z<1.1. The model can also be used to improve spectroscopic identification
algorithms, and derive photometric redshifts of distant Type Ia supernovae.Comment: Accepted for publication in A&A. Data and source code available at :
http://supernovae.in2p3.fr/~guy/salt
Photometric selection of Type Ia supernovae in the Supernova Legacy Survey
We present a sample of 485 photometrically identified Type Ia supernova
candidates mined from the first three years of data of the CFHT SuperNova
Legacy Survey (SNLS). The images were submitted to a deferred processing
independent of the SNLS real-time detection pipeline. Light curves of all
transient events were reconstructed in the g_M, r_M, i_M and z_M filters and
submitted to automated sequential cuts in order to identify possible
supernovae. Pure noise and long-term variable events were rejected by light
curve shape criteria. Type Ia supernova identification relied on event
characteristics fitted to their light curves assuming the events to be normal
SNe Ia. The light curve fitter SALT2 was used for this purpose, assigning host
galaxy photometric redshifts to the tested events. The selected sample of 485
candidates is one magnitude deeper than that allowed by the SNLS spectroscopic
identification. The contamination by supernovae of other types is estimated to
be 4%. Testing Hubble diagram residuals with this enlarged sample allows us to
measure the Malmquist bias due to spectroscopic selections directly. The result
is fully consistent with the precise Monte Carlo based estimate used to correct
SN Ia distance moduli in the SNLS 3-year cosmological analyses. This paper
demonstrates the feasibility of a photometric selection of high redshift
supernovae with known host galaxy redshifts, opening interesting prospects for
cosmological analyses from future large photometric SN Ia surveys.Comment: (The SNLS collaboration) 23 pages, 28 figures, Accepted in A&
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