Cosmological Constraints from Measurements of Type Ia Supernovae discovered during the first 1.5 years of the Pan-STARRS1 Survey

Abstract: We present griz light curves of 146 spectroscopically confirmed Type Ia Supernovae ($0.03 < z <0.65$) discovered during the first 1.5 years of the Pan-STARRS1 Medium Deep Survey. The Pan-STARRS1 natural photometric system is determined by a combination of on-site measurements of the instrument response function and observations of spectrophotometric standard stars. We find that the systematic uncertainties in the photometric system are currently 1.2\% without accounting for the uncertainty in the HST Calspec definition of the AB system. A Hubble diagram is constructed with a subset of 113 out of 146 SNe Ia that pass our light curve quality cuts. The cosmological fit to 310 SNe Ia (113 PS1 SNe Ia + 222 light curves from 197 low-z SNe Ia), using only SNe and assuming a constant dark energy equation of state and flatness, yields $w=-1.120^{+0.360}_{-0.206}\textrm{(Stat)} ^{+0.269}_{-0.291}\textrm{(Sys)}$. When combined with BAO+CMB(Planck)+$H_0$, the analysis yields $\Omega_{\rm M}=0.280^{+0.013}_{-0.012}$ and $w=-1.166^{+0.072}_{-0.069}$ including all identified systematics (see also Scolnic et al. 2014). The value of $w$ is inconsistent with the cosmological constant value of $-1$ at the 2.3$\sigma$ level. Tension endures after removing either the BAO or the $H_0$ constraint, though it is strongest when including the $H_0$ constraint. If we include WMAP9 CMB constraints instead of those from Planck, we find $w=-1.124^{+0.083}_{-0.065}$, which diminishes the discord to $<2\sigma$. We cannot conclude whether the tension with flat $\Lambda$CDM is a feature of dark energy, new physics, or a combination of chance and systematic errors. The full Pan-STARRS1 supernova sample with $\sim!!$3 times as many SNe should provide more conclusive results.

• The paper presents refined constraints on dark energy by analyzing 146 spectroscopically confirmed PS1 Type Ia supernovae.
• It employs SALT2 light-curve fitting and rigorous photometric calibration to reduce systematic uncertainties to about 1.2%.
• Integrating PS1 with low-redshift SNe, BAO, and CMB data, the study reveals tension with the cosmological constant model.

Cosmological Constraints from Type Ia Supernovae in the Pan-STARRS1 Survey

The paper investigates cosmological parameters derived from observations of Type Ia supernovae (SNe~Ia), considered reliable standard candles for measuring cosmic distances. The dataset is primarily drawn from the Pan-STARRS1 (PS1) Medium Deep Survey (MDS), covering the first 1.5 years and containing 146 spectroscopically confirmed SNe~Ia. The study focuses on improving the precision of cosmological measurements through careful calibration and analysis of SN~Ia light curves, determining cosmological parameters, and evaluating potential systematic uncertainties.

Data and Methods

The PS1 survey uses the Medium Deep Fields (MDFs), which allow for the detection of transients including SNe~Ia up to a redshift of 0.65. The analysis begins with detailed photometric calibrations that involve on-site measurements of the instrument response and comparison with spectrophotometric standards. A critical aspect is the reconstruction of accurate light curves from the difference images produced by subtracting template images from nightly observations. A subset of 113 out of 146 SNe~Ia passing strict quality thresholds forms the basis for cosmological analysis, in conjunction with 222 light curves from 197 low-redshift SNe~Ia.

Results

The cosmological analysis assumes a flat Universe and utilizes the Tripp formula to derive distances based on peak brightness and corrected light-curve parameters. The study employs the SALT2 light curve fitter, adjusting for biases due to detection efficiency and spectroscopic follow-up. A focus on precise calibration shows systematic uncertainties at about 1.2% (excluding the unknowns in the HST Calspec definition), and identifies that intrinsic color and dust extinction corrections represent key areas of systematic uncertainty.

Using both the PS1 data and an aggregate low-redshift SN sample, the paper estimates the dark energy equation of state parameter (commonly denoted as $w$) assuming a constant form and a flat Universe ($\Omega_k = 0$). The combined SN data imply $w = -1.124^{+0.083}_{-0.063}$, indicating tension with the cosmological constant ($w = -1$) at the 2.3σ level.

Integration with External Datasets

The constraints are refined through combination with external datasets such as Baryon Acoustic Oscillations (BAO), Cosmic Microwave Background (CMB), primarily from Planck measurements, and a local measurement of $H_0$. This combined analysis yields values of $\Omega_m = 0.308^{+0.033}_{-0.027}$ and $w = -1.136^{+0.077}_{-0.079}$, further highlighting the tension with $w = -1$. The analysis notes that substituting WMAP9 CMB data for Planck data reduces this discord.

Implications

This study suggests a marginal deviation from the cosmological constant model but highlights the limitations imposed by systematic uncertainties, particularly from photometric calibration and color corrections. The results underscore the potential for the full PS1 dataset to provide more stringent constraints on dark energy parameters. Future work is expected to address calibration improvements and incorporate a more extensive photometrically classified SN sample to refine these cosmological parameters.

The exploratory nature of this study, particularly about emerging discrepancies and systematic uncertainties, positions it well as a springboard for subsequent analyses with further data from PS1 and other surveys. The findings have significant implications for understanding the fundamental nature of dark energy and the expansion history of the Universe.