The Supernova Legacy Survey 3-year sample: Type Ia Supernovae photometric distances and cosmological constraints (1010.4743v1)

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.

• The paper presents a detailed analysis of 252 high-redshift SNe Ia using advanced photometric techniques to measure light curves accurately.
• It employs two light curve fitters, SALT2 and SiFTO, to extract key parameters while mitigating systematic calibration uncertainties.
• The study yields cosmological constraints with Ωm = 0.211 ± 0.077, thereby enhancing our understanding of dark energy and cosmic acceleration.

An Analytical Overview of the Supernova Legacy Survey 3-Year Sample: Type Ia Supernovae Photometric Distances and Cosmological Constraints

The paper presents an exhaustive analysis of the Type Ia supernovae (SNe Ia) observations from the first three years of the Supernova Legacy Survey (SNLS), focusing on photometric distances and deriving cosmological constraints. This work represents a significant contribution to our understanding of dark energy and cosmic acceleration.

Key Findings and Methodological Approaches

The SNLS dataset includes 252 high-redshift SNe Ia within the redshift range $0.15 < z < 1.1$, which forms the basis for this analysis. The data collection involved a meticulous process using MegaPrime/MegaCam at the Canada-France-Hawaii Telescope, along with follow-up spectroscopy using VLT, Gemini, and Keck telescopes.

Photometry and Calibration

Two photometry techniques were used to derive light curves:

1. Simultaneous Fit Method (Method A): This involves fitting the SN flux and position while considering host galaxy flux.
2. PSF Photometry on Image Subtractions (Method B): This approach was found to have an over-subtraction issue with the host galaxy flux.

The paper opted for Method A due to its suppression of host subtraction biases at an accuracy better than 1-3 mmag, making it preferable for the subsequent light curve analysis.

Calibration was performed using catalogs of tertiary stars, carefully correcting for systematic errors through careful calibration chains. Detailed assessments were made on magnitude scales and filter transmission functions to ensure precision.

Light Curve Analysis

The analysis relied on two light curve fitters, SALT2 and SiFTO, to extract key parameters (peak magnitude, shape, and color) necessary for deriving photometric distances. Each model handles light curve shape, color law, and training datasets differently, which helps in benchmarking systematic uncertainties.

Results and Comparisons

• SALT2 Results: Derived from a spectral energy density approach capturing temporal and spectral variation in supernova light.
• SiFTO Results: Stretched the light curves and was calibrated using a mix of low-z and SNLS data.

The paper showed consistency checks between the two methods, noting systematic differences in derived distance moduli up to 0.03 magnitudes, attributable to differences in modeling and calibration chains.

Cosmological Constraints

The paper provided a cosmological model fit, focusing on a flat $\Lambda$CDM model, yielding an estimate of $\Omega_m = 0.211 \pm 0.077$. This figure illustrates a significant advancement from the first-year SNLS analysis primarily due to methodological refinements and increased sample size.

Systematic Uncertainties

Considerable attention was given to systematic uncertainties from light curve modeling and photometric calibration. The analysis emphasized the importance of accounting for these uncertainties, particularly at high redshifts where systematic deviations become pronounced.

Theoretical and Practical Implications

The paper corroborates the current understanding that the Universe's expansion is accelerating, largely driven by dark energy with an effective equation of state parameter close to $-1$. Moreover, no evidence was found for evolution in the color-luminosity relation's slope with redshift.

Future Perspectives

The SNLS results enhance our comprehension of SNe Ia as standard candles and support further research into cosmic acceleration mechanisms. Upcoming surveys with enhanced photometric precision and protocol could leverage these methodologies to refine cosmological models. Future works may focus on integrating complementary probes such as baryonic acoustic oscillations and CMB measurements to constrain dark energy more robustly.

This analysis, therefore, not only refines our current cosmological models but also sets the stage for future observational campaigns that can unravel the complexities surrounding dark energy and cosmic evolution.