The Supernova Legacy Survey 3-year sample: Type Ia Supernovae photometric distances and cosmological constraints

Abstract

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.

Figures (18)

• Figure 1: Statistical uncertainty on magnitudes (left panel) and expected bias on PSF photometry with a simultaneous fit of the position (right panel) as a function of supernova redshift. The different $g_M$$r_M$$i_M$$z_M$ MegaCam bands are shown respectively with green solid, red dashed, black dotted, and blue dashed-dotted curves.
• Figure 2: Residuals from the calibration fit ( PSF magnitude minus aperture magnitude of the catalog) for CCD #13, band $r_M$ in field D1. Each point is a tertiary star. Those marked with open symbols were excluded from the fit based on cuts on RMS, magnitude, and number of observations. The fit is performed iteratively with a 2.5 $\sigma$ outlier rejection.
• Figure 3: RMS of the differences between PSF and aperture magnitude of the tertiary stars for the $g_M$$r_M$$i_M$$z_M$ bands. In each histogram, there are $36 \times 4$ entries, each corresponding to a CCD/field combination, for which a zero point is determined.
• Figure 4: Differences between PSF and aperture magnitudes of the tertiary stars in the $g_M$$r_M$$i_M$$z_M$ bands as a function of the colour of stars (gray dots). The black points with error bars represent the average deviation and its uncertainty in bins of colour. Typical SNe colours at maximum light are marked with red dotted vertical lines. The blue curve on the top panel shows the effect of the PSF wavelength dependent correction on synthetic magnitudes obtained with PHOENIX stellar models. The colours of BD+17 4708 are marked by the black vertical lines.
• Figure 5: Average differences between PSF and aperture magnitude of the tertiary stars in $g_M$$r_M$$i_M$$z_M$ bands as a function of the star magnitude (once corrected for the colour dependent terms). The back error bars represent the statistical uncertainty on the average. The shaded areas represent the uncertainty on aperture magnitudes due to a systematic uncertainty on the residual background in the images used for calibration ($0.1$ ADU per pixel for $g_M$$r_M$$z_M$, $0.2$ ADU for the $i_M$-band)