The Supernova Legacy Survey: Measurement of Omega_M, Omega_Lambda and w from the First Year Data Set

TL;DR

The Supernova Legacy Survey (SNLS) leverages a rolling 4-field MegaCam program plus spectroscopic follow-up to build a homogeneous, multi-band SN Ia sample suitable for precision cosmology. By modeling SN Ia light curves with SALT and calibrating against Landolt/SDSS standards, SNLS delivers a robust Hubble diagram to z ~ 1 and constrains the matter density and dark energy equation of state, even when combined with BAO data. The first-year results yield Omega_M ≈ 0.263 and w ≈ -1.02 (flat cosmology), with systematic uncertainties dominated by photometric calibration and high-z color measurements, and show good agreement with prior SN and external cosmological probes. The study also documents meticulous handling of systematics, from image processing to color evolution checks, and outlines a clear path toward a much larger, high-quality SN Ia dataset to decisively probe dark energy. Practically, SNLS demonstrates the feasibility and advantages of a uniform, multi-band, spectroscopically anchored SN survey for future cosmological inferences.

Abstract

We present distance measurements to 71 high redshift type Ia supernovae discovered during the first year of the 5-year Supernova Legacy Survey (SNLS). These events were detected and their multi-color 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 redshift. With this data set, we have built a Hubble diagram extending to z=1, with all distance measurements involving at least two bands. Systematic uncertainties are evaluated making use of the multi-band photometry obtained at CFHT. Cosmological fits to this first year SNLS Hubble diagram give the following results : Omega_M = 0.263 +/- 0.042(stat) +/- 0.032(sys) for a flat LambdaCDM model; and w = -1.023 +/- 0.090(stat) +/- 0.054(sys) for a flat cosmology with constant equation of state w when combined with the constraint from the recent Sloan Digital Sky Survey measurement of baryon acoustic oscillations.

Figures (13)

• Figure 1: Observed light-curves points of the SN Ia SNLS-04D3fk in $g_M$, $r_M$, $i_M$ and $z_M$ bands, along with the multi-color light-curve model (described in Section \ref{['section:lightcurve_fit_model']}). Note the regular sampling of the observations both before and after maximum light. With a SN redshift of 0.358, the four measured pass-bands lie in the wavelength range of the light-curve model, defined by rest-frame $U$ to $R$ bands, and all light-curves points are therefore fitted simultaneously with only four free parameters (photometric normalization, date of maximum, a stretch and a color parameter).
• Figure 2: Observed light-curves points of the SN Ia SNLS-04D3gx at z=0.91. With a SN redshift of 0.91, only two of the measured pass-bands lie in the wavelength range of the light-curve model, defined by rest-frame $U$ to $R$ bands, and are therefore used in the fit (shown as solid lines). Note the excellent quality of the photometry at this high redshift value. Note also the clear signal observed in $r_M$ and even in $g_M$, which correspond to central wavelength of respectively $\lambda\sim3200\AA$ and $\lambda\sim2500\AA$ in the SN rest-frame.
• Figure 3: The calibration residuals --- i.e. the residuals around the mean magnitude of each Deep field tertiary standard--- in the bands $g_M$, $r_M$, $i_M$ and $z_M$, for all CCDs and fields, with one entry per star and epoch. The dispersion is below 1% in $g_M$, $r_M$ and $i_M$, and about 1.5% in $z_M$.
• Figure 4: Hubble diagram of SNLS and nearby SNe Ia, with various cosmologies superimposed. The bottom plot shows the residuals for the best fit to a flat $\Lambda$ cosmology.
• Figure 5: Contours at 68.3%, 95.5% and 99.7% confidence levels for the fit to an $(\Omega_{\rm M},\Omega_\Lambda)$ cosmology from the SNLS Hubble diagram (solid contours), the SDSS baryon acoustic oscillations (Eisenstein05, dotted lines), and the joint confidence contours (dashed lines).