First-year Sloan Digital Sky Survey-II (SDSS-II) Supernova Results: Hubble Diagram and Cosmological Parameters

TL;DR

This paper presents a first-year SDSS-II Type Ia supernova Hubble diagram comprising 103 SNe in the redshift range 0.04 < z < 0.42, aimed at filling the intermediate redshift gap and enabling robust cosmological inferences when combined with BAO and CMB data. It employs two independent light-curve fitters, mlcs2k2 and SALT-II, to derive distances and explores extensive Monte Carlo simulations to model selection effects and systematic uncertainties. The joint SN+BAO+CMB analysis yields constraints on the dark-energy equation of state parameter w and matter density Ω_M in flat and non-flat cosmologies, with w ≈ -0.76 ± 0.07(stat) ± 0.11(syst) for mlcs2k2 and w ≈ -0.96 ± 0.06(stat) ± 0.12(syst) for SALT-II when all SN datasets are included; discrepancies between the methods are traced to differences in rest-frame UV modeling and color variation implementations, with the rest-frame U-band presenting the dominant systematic. The study emphasizes the U-band anomaly as the major limiting systematic and demonstrates that dust properties (R_V and A_V) and underlying distributions are essential priors for distance estimation, while the SDSS-II dust sample provides a robust basis for these inferences. The work highlights the potential of the full three-season SDSS-II data to further tighten constraints and reduce systematic uncertainties, and it makes the analysis software publicly available for reproducibility and further refinement by the community.

Abstract

We present measurements of the Hubble diagram for 103 Type Ia supernovae (SNe) with redshifts 0.04 < z < 0.42, discovered during the first season (Fall 2005) of the Sloan Digital Sky Survey-II (SDSS-II) Supernova Survey. These data fill in the redshift "desert" between low- and high-redshift SN Ia surveys. We combine the SDSS-II measurements with new distance estimates for published SN data from the ESSENCE survey, the Supernova Legacy Survey, the Hubble Space Telescope, and a compilation of nearby SN Ia measurements. Combining the SN Hubble diagram with measurements of Baryon Acoustic Oscillations from the SDSS Luminous Red Galaxy sample and with CMB temperature anisotropy measurements from WMAP, we estimate the cosmological parameters w and Omega_M, assuming a spatially flat cosmological model (FwCDM) with constant dark energy equation of state parameter, w. For the FwCDM model and the combined sample of 288 SNe Ia, we find w = -0.76 +- 0.07(stat) +- 0.11(syst), Omega_M = 0.306 +- 0.019(stat) +- 0.023(syst) using MLCS2k2 and w = -0.96 +- 0.06(stat) +- 0.12(syst), Omega_M = 0.265 +- 0.016(stat) +- 0.025(syst) using the SALT-II fitter. We trace the discrepancy between these results to a difference in the rest-frame UV model combined with a different luminosity correction from color variations; these differences mostly affect the distance estimates for the SNLS and HST supernovae. We present detailed discussions of systematic errors for both light-curve methods and find that they both show data-model discrepancies in rest-frame $U$-band. For the SALT-II approach, we also see strong evidence for redshift-dependence of the color-luminosity parameter (beta). Restricting the analysis to the 136 SNe Ia in the Nearby+SDSS-II samples, we find much better agreement between the two analysis methods but with larger uncertainties.

Figures (46)

• Figure 1: Light curves for four SDSS-II SNe Ia at different redshifts: SN 2005ff at $z = 0.09$, SN 2005fb at $z = 0.18$, SN 2005fr at $z = 0.29$, and SN 2005gq at $z = 0.39$. The passbands are SDSS $g$ (top), $r$ (middle), and $i$ (bottom). Points are the SMP flux measurements (${\rm flux} = 10^{(11-0.4m)}$, where $m$ is the SN magnitude) with $\pm 1~\sigma$ photometric errors indicated. Solid curves show the best-fit mlcs2k2 model fits (see § \ref{['subsec:MLCS2k2']}), and dashed curves give the $\pm 1~\sigma$ error bands on the model fits. The Modified Julian Date (MJD) under each set of light curves is the fitted time of peak brightness for rest-frame $B$-band.
• Figure 2: For the spectroscopically confirmed SN Ia sample from the SDSS-II 2005 season, distributions are shown for: (a) number of $gri$ measurements with $-15 < T_{\rm rest} < +60$ days as a function of day in the survey season when the SN reached peak luminosity. Vertical arrows show the start (Sept. 1) and end dates (Nov. 30) of the survey season. SNe that lie in the overlap region of strips 82N and 82S (solid dots) tend to have more measurements; (b) $\log_{10}$ of maximum $g$-band SNR versus redshift; (c) time of first measurement relative to peak light in rest-frame $B$; (d) time of last measurement (not necessarily detection) relative to peak light -- the pile-up near 60 days is from SNe that have measurements past 60 days; (e) redshifts before (130, thin line) and after (103, thick line) selection cuts are applied. The arrows in panels (b), (c), and (d) indicate the selection cuts.
• Figure 3: Top panel: summed redshift distribution for the five SN Ia samples indicated in the legend. Bottom panel: maximum observed signal-to-noise ratio (among all passbands) as a function of redshift, averaged in bins of width $\Delta z=0.05$; error bars indicate the rms spread within each bin. All selection requirements have been applied.
• Figure 4: For the mlcs2k2 model, correlation coefficient $\rho_{\Delta t,0}$ between $B$-band epoch at peak brightness ($t_0$) and time $\Delta t = t - t_0$, where $\rho_{\Delta t,0} \equiv {\rm cov}(\Delta t,0)/\sigma_{\Delta t}\sigma_0$, as a function of $\Delta t$, and correlation coefficient $\rho_{t-t_{10},t_{10}}$ between epoch at 10 days past peak ($t_{10}$) and time $t-t_{10}$. The spikes at 0 and 10 days correspond to the requirements $\rho_{0,0} = 1$ and $\rho_{10,10} = 1$.
• Figure 5: Data-model fractional residuals as a function of rest-frame epoch in 5-day bins for mlcs2k2 light-curve fits. The rest-frame passband and SN sample are indicated on each plot. Measurements with SNR$<6$ are excluded, and error bars indicate the rms spread. For SNLS, the residuals are shown only for SNe with $z<0.5$ as explained later in § \ref{['subsec:Uanom_SALT2']}. ${\rm F}_{\rm data}$ (${\rm F}_{\rm model}$) is the SN flux from the data (best-fit mlcs2k2 model). Vertical dashed lines indicate epoch of peak brightness ($T_{\rm rest}=0$); horizontal dashed lines indicate ${\rm F}_{\rm data} = {\rm F}_{\rm model}$.