83 research outputs found
Photometry of supernovae in an image series : methods and application to the Supernova Legacy Survey (SNLS)
We present a technique to measure lightcurves of time-variable point sources
on a spatially structured background from imaging data. The technique was
developed to measure light curves of SNLS supernovae in order to infer their
distances. This photometry technique performs simultaneous PSF photometry at
the same sky position on an image series. We describe two implementations of
the method: one that resamples images before measuring fluxes, and one which
does not. In both instances, we sketch the key algorithms involved and present
the validation using semi-artificial sources introduced in real images in order
to assess the accuracy of the supernova flux measurements relative to that of
surrounding stars. We describe the methods required to anchor these PSF fluxes
to calibrated aperture catalogs, in order to derive SN magnitudes. We find a
marginally significant bias of 2 mmag of the after-resampling method, and no
bias at the mmag accuracy for the non-resampling method. Given surrounding star
magnitudes, we determine the systematic uncertainty of SN magnitudes to be less
than 1.5 mmag, which represents about one third of the current photometric
calibration uncertainty affecting SN measurements. The SN photometry delivers
several by-products: bright star PSF flux mea- surements which have a
repeatability of about 0.6%, as for aperture measurements; we measure relative
astrometric positions with a noise floor of 2.4 mas for a single-image bright
star measurement; we show that in all bands of the MegaCam instrument, stars
exhibit a profile linearly broadening with flux by about 0.5% over the whole
brightness range.Comment: Accepted for publication in A&A. 20 page
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&
Constraining Type Ia Supernovae progenitors from three years of SNLS data
While it is generally accepted that Type Ia supernovae are the result of the
explosion of a carbon-oxygen White Dwarf accreting mass in a binary system, the
details of their genesis still elude us, and the nature of the binary companion
is uncertain. Kasen (2010) points out that the presence of a non-degenerate
companion in the progenitor system could leave an observable trace: a flux
excess in the early rise portion of the lightcurve caused by the ejecta impact
with the companion itself. This excess would be observable only under favorable
viewing angles, and its intensity depends on the nature of the companion. We
searched for the signature of a non-degenerate companion in three years of
Supernova Legacy Survey data by generating synthetic lightcurves accounting for
the effects of shocking and comparing true and synthetic time series with
Kolmogorov-Smirnov tests. Our most constraining result comes from noting that
the shocking effect is more prominent in rest-frame B than V band: we rule out
a contribution from white dwarf-red giant binary systems to Type Ia supernova
explosions greater than 10% at 2 sigma, and than 20% at 3 sigma level.Comment: 14 pages, 15 figures, resubmitted to ApJ, figure 15 modifie
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&
SNLS3: Constraints on Dark Energy Combining the Supernova Legacy Survey Three Year Data with Other Probes
We present observational constraints on the nature of dark energy using the
Supernova Legacy Survey three year sample (SNLS3) of Guy et al. (2010) and
Conley et al. (2011). We use the 472 SNe Ia in this sample, accounting for
recently discovered correlations between SN Ia luminosity and host galaxy
properties, and include the effects of all identified systematic uncertainties
directly in the cosmological fits. Combining the SNLS3 data with the full WMAP7
power spectrum, the Sloan Digital Sky Survey luminous red galaxy power
spectrum, and a prior on the Hubble constant H0 from SHOES, in a flat universe
we find omega_m=0.269+/-0.015 and w=-1.061+0.069-0.068 -- a 6.5% measure of the
dark energy equation-of-state parameter w. The statistical and systematic
uncertainties are approximately equal, with the systematic uncertainties
dominated by the photometric calibration of the SN Ia fluxes -- without these
calibration effects, systematics contribute only a ~2% error in w. When
relaxing the assumption of flatness, we find omega_m=0.271+/-0.015,
omega_k=-0.002+/-0.006, and w=-1.069+0.091-0.092. Parameterizing the time
evolution of w as w(a)=w_0+w_a(1-a), gives w_0=-0.905+/-0.196,
w_a=-0.984+1.094-1.097 in a flat universe. All of our results are consistent
with a flat, w=-1 universe. The size of the SNLS3 sample allows various tests
to be performed with the SNe segregated according to their light curve and host
galaxy properties. We find that the cosmological constraints derived from these
different sub-samples are consistent. There is evidence that the coefficient,
beta, relating SN Ia luminosity and color, varies with host parameters at
>4sigma significance (in addition to the known SN luminosity--host relation);
however this has only a small effect on the cosmological results and is
currently a sub-dominant systematic.Comment: Accepted for publication in ApJ. Data available from
https://tspace.library.utoronto.ca/snl
Euclid: Constraining ensemble photometric redshift distributions with stacked spectroscopy
Context. The ESA Euclid mission will produce photometric galaxy samples over 15 000 square degrees of the sky that will be rich for clustering and weak lensing statistics. The accuracy of the cosmological constraints derived from these measurements will depend on the knowledge of the underlying redshift distributions based on photometric redshift calibrations. Aims. A new approach is proposed to use the stacked spectra from Euclid slitless spectroscopy to augment broad-band photometric information to constrain the redshift distribution with spectral energy distribution fitting. The high spectral resolution available in the stacked spectra complements the photometry and helps to break the colour-redshift degeneracy and constrain the redshift distribution of galaxy samples. Methods. We modelled the stacked spectra as a linear mixture of spectral templates. The mixture may be inverted to infer the underlying redshift distribution using constrained regression algorithms. We demonstrate the method on simulated Vera C. Rubin Observatory and Euclid mock survey data sets based on the Euclid Flagship mock galaxy catalogue. We assess the accuracy of the reconstruction by considering the inference of the baryon acoustic scale from angular two-point correlation function measurements. Results. We selected mock photometric galaxy samples at redshift za>a1 using the self-organising map algorithm. Considering the idealised case without dust attenuation, we find that the redshift distributions of these samples can be recovered with 0.5% accuracy on the baryon acoustic scale. The estimates are not significantly degraded by the spectroscopic measurement noise due to the large sample size. However, the error degrades to 2% when the dust attenuation model is left free. We find that the colour degeneracies introduced by attenuation limit the accuracy considering the wavelength coverage of Euclid near-infrared spectroscopy
Euclid preparation: V. Predicted yield of redshift 7<z<9 quasars from the wide survey
We provide predictions of the yield of 7 < z < 9 quasars from the Euclid wide survey, updating the calculation presented in the
Euclid Red Book in several ways. We account for revisions to the Euclid near-infrared filter wavelengths; we adopt steeper rates
of decline of the quasar luminosity function (QLF; Φ) with redshift, Φ ∝ 10k(z−6)
, k = −0.72, and a further steeper rate of decline,
k = −0.92; we use better models of the contaminating populations (MLT dwarfs and compact early-type galaxies); and we make use
of an improved Bayesian selection method, compared to the colour cuts used for the Red Book calculation, allowing the identification
of fainter quasars, down to JAB ∼ 23. Quasars at z > 8 may be selected from Euclid OY JH photometry alone, but selection over
the redshift interval 7 < z < 8 is greatly improved by the addition of z-band data from, e.g., Pan-STARRS and LSST. We calculate
predicted quasar yields for the assumed values of the rate of decline of the QLF beyond z = 6. If the decline of the QLF accelerates
beyond z = 6, with k = −0.92, Euclid should nevertheless find over 100 quasars with 7.0 < z < 7.5, and ∼ 25 quasars beyond the
current record of z = 7.5, including ∼ 8 beyond z = 8.0. The first Euclid quasars at z > 7.5 should be found in the DR1 data release,
expected in 2024. It will be possible to determine the bright-end slope of the QLF, 7 < z < 8, M1450 < −25, using 8 m class telescopes
to confirm candidates, but follow-up with JWST or E-ELT will be required to measure the faint-end slope. Contamination of the
candidate lists is predicted to be modest even at JAB ∼ 23. The precision with which k can be determined over 7 < z < 8 depends on
the value of k, but assuming k = −0.72 it can be measured to a 1σ uncertainty of 0.07
Euclid preparation: V. Predicted yield of redshift 7 < z < 9 quasars from the wide survey
We provide predictions of the yield of 7 8 may be selected from Euclid OY JH photometry alone, but selection over the redshift interval 7 7.5 should be found in the DR1 data release, expected in 2024. It will be possible to determine the bright-end slope of the QLF, 7 < z < 8, M1450 < −25, using 8 m class telescopes to confirm candidates, but follow-up with JWST or E-ELT will be required to measure the faint-end slope. Contamination of the candidate lists is predicted to be modest even at JAB ∼ 23. The precision with which k can be determined over 7 < z < 8 depends on the value of k, but assuming k = −0.72 it can be measured to a 1σ uncertainty of 0.07
- …