The Complete Light-curve Sample of Spectroscopically Confirmed Type Ia Supernovae from Pan-STARRS1 and Cosmological Constraints from The Combined Pantheon Sample

TL;DR

This work delivers a complete, spectroscopically confirmed Type Ia SN sample from Pan-STARRS1 and combines it with additional SN Ia data to form the Pantheon dataset, achieving unprecedented statistical power and cross-survey calibration. The authors develop and apply improved photometry, calibration (via Supercal), and a bias-corrected distance framework (BBC with SALT2) to produce robust cosmological constraints, including tight Planck+SN results for Ω_m and w. They find no evidence for tension with a cosmological constant when combining SN with CMB, BAO, and H0, and provide the most precise measurements of dark energy to date, while highlighting residual systematics dominated by low-z modeling. The study also emphasizes the importance of addressing low-z calibration and population evolution to further improve SN-based cosmology in the future.

Abstract

We present optical light curves, redshifts, and classifications for 365 spectroscopically confirmed Type Ia supernovae (SNe Ia) discovered by the Pan-STARRS1 (PS1) Medium Deep Survey. We detail improvements to the PS1 SN photometry, astrometry and calibration that reduce the systematic uncertainties in the PS1 SN Ia distances. We combine the subset of 279 PS1 SN Ia ($0.03 < z < 0.68$) with useful distance estimates of SN Ia from SDSS, SNLS, various low-z and HST samples to form the largest combined sample of SN Ia consisting of a total of 1048 SN Ia ranging from $0.01 < z < 2.3$, which we call the `Pantheon Sample'. When combining Planck 2015 CMB measurements with the Pantheon SN sample, we find $Ω_m=0.307\pm0.012$ and $w = -1.026\pm0.041$ for the wCDM model. When the SN and CMB constraints are combined with constraints from BAO and local H0 measurements, the analysis yields the most precise measurement of dark energy to date: $w0 = -1.007\pm 0.089$ and $wa = -0.222 \pm0.407$ for the w0waCDM model. Tension with a cosmological constant previously seen in an analysis of PS1 and low-z SNe has diminished after an increase of $2\times$ in the statistics of the PS1 sample, improved calibration and photometry, and stricter light-curve quality cuts. We find the systematic uncertainties in our measurements of dark energy are almost as large as the statistical uncertainties, primarily due to limitations of modeling the low-redshift sample. This must be addressed for future progress in using SN Ia to measure dark energy.

Figures (23)

• Figure 1: An overview of the various analysis steps in this paper. A common set of steps is done for both the PS1 sample and the combined Pantheon sample.
• Figure 2: Histograms comparing the set of all spectroscopically confirmed SN Ia against the subset that is deemed cosmologically useful. Filled bars indicate the full spectroscopic sample of $365$ SN Ia, while outlined bars indicate the $279$ used for our cosmology analysis. (Top) Distribution of redshift. (Middle) Distribution of radial angular distance from center of focal plane. (Bottom) Distribution of the age at discovery as determined from the date of peak brightness subtracted from the discovery date.
• Figure 3: (Top) A plot of the variance of recovered pixel offsets in one dimension ($y$) versus (FWHM/SNR)$^2$. A similar overestimation of the astrometric error by R14 is seen in the $x$ direction as well. (Bottom) The necessary photometric bias correction versus redshift of the SN due to the expected astrometric uncertainty of the central position of a SN from the combined series of images of that SN in one filter. A best-fit line is overlaid in yellow.
• Figure 4: Agreement between $g$ band nightly photometry and Ubercal photometry of $>1$ million stars and the dependence on magnitude, MJD, airmass and Focal plane position. Different colors of the points represent bins of stellar colors. In the rightmost panel, arrows indicate that the R14 discrepancy with the catalog photometry near the center of the focal plane was $>0.1$ mag.
• Figure 5: Representative light-curves of SNe from the PS1 survey: PS1-520022, PS1-370394, PS1-380040 from top to bottom respectively. These SNe have redshifts, of $z=0.12$, 0.33, and 0.68. The points shown are data from the PS1 survey and the curves shown are fits using SALT2. The Flux units are given for a zeropoint of $27.5$ mag in each band.