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

TL;DR

This study presents 146 spectroscopically confirmed Type Ia supernovae from the first 1.5 years of Pan-STARRS1 MDF observations, focusing on precise photometric calibration (1.2% systematics) and robust light-curve fitting to constrain dark energy. Using SALT2 fits and a three-stage sample-cut process, the authors derive SN distances for 113 high-quality PS1 SNe Ia and 222 low-z SNe, then combine with BAO, Planck CMB, and H_0 to constrain the cosmological parameters. They find w = -1.166^{+0.072}_{-0.069} and Ω_M = 0.280^{+0.013}_{-0.012} when including external data, with a mild tension (∼2.3σ) against a cosmological constant in a flat universe. The results emphasize calibration as the dominant systematic and anticipate substantial improvements with the full PS1 dataset and future HST/NIR observations to better understand dust, intrinsic SN colors, and potential new physics.

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 $Ω_{\rm M}=0.280^{+0.013}_{-0.012}$ 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$σ$ 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 $<2σ$. We cannot conclude whether the tension with flat $Λ$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.

Figures (21)

• Figure 1: PS1 MDF survey characteristics for all spectroscopically confirmed PS1 SN Ia. Upper panel: Histogram of the number of SN Ia with respect to the phase of discovery in the rest-frame. Middle panel: Histogram of the redshift distribution. Lower panel: Histogram of SN Ia as a function of distance to the MDF center. The observed SN rate per constant area is shown with the red symbols and line. The rate is constant within the uncertainties to a distance of $\sim\!\!$ 1.3 degrees from center.
• Figure 2: Histogram of $r_{\rm P1}$ peak magnitudes of spectroscopically confirmed SN Ia (red) and events classified as highly likely SN Ia (black) based on light-curve fitting with PSNID Sako11.
• Figure 3: Comparison between deep and nightly stack photometry for stars in the MDFs. The red circles indicate the weighted average of the magnitude difference $\Delta m = m_{\mathrm{deep}} - m_{\mathrm{nightly}}$ from 100 randomly selected images for each of $griz_{\rm P1}$ from top to bottom, respectively. They are consistent with zero within the errors at all magnitudes except at the very brightest and faintest ends. In particular $i_{\rm P1}$ and $z_{\rm P1}$ show significant deviation from zero for $m < 17$ mag. Malmquist bias is likely the cause of the deviation at faint magnitudes. The black dots show a small subset of $\Delta m$ values for illustration.
• Figure 4: Comparison between $r_{\rm P1}$ and $r_{\mathrm{SDSS}}$, where $r_{\mathrm{SDSS}}$ is the SDSS $r$ band magnitude converted into Pan-STARRS1 natural system magnitudes using Equation 6 and Table 6 from T12b. The red circles indicate the average of the magnitude difference.
• Figure 5: Comparison of forced and regular photometry for PS1 SN Ia as a function of SNR. The red circles indicate the weighted average of the magnitude difference $\Delta m = m_{\mathrm{forced}} - m_{\mathrm{regular}}$ of SN detections for different SNR bins. The error bars indicate the standard deviation. The black dots show a small subset of $\Delta m$ values for illustration. At high SNR, the average magnitude difference is an insignificant 0.5 mmag different from zero, indicating that there are no issues with the astrometric calibration, centroiding, or differences between forced and unforced PSF fitting.