Type Ia Supernova Distances at z > 1.5 from the Hubble Space Telescope Multi-Cycle Treasury Programs: The Early Expansion Rate

TL;DR

This work integrates 9 high-$z$ SNe Ia from the CANDELS and CLASH HST programs into the Pantheon+MCT compilation to achieve a model-independent reconstruction of the expansion history $E(z)=H(z)/H_0$ up to $z\approx1.5$, with ~20% precision at $z=1.5$. It introduces a six-anchor interpolation framework for $E(z)^{-1}$ that yields unbiased constraints and effectively compresses the SN Ia information into six measurements, nearly matching the full dataset's cosmological impact for smooth expansion histories. The high-$z$ SN sample also enables tests of SN evolution versus cosmology, disfavoring simple evolving-power-law explanations, and the study provides forecasted, high-precision $E(z)$ constraints for future missions like WFIRST that will extend measurements to $z\sim2.5$. The results demonstrate the value of high-redshift SNe for direct expansion-history probes and for informing design and expectations of next-generation SN surveys.

Abstract

We present an analysis of 15 Type Ia supernovae (SNe Ia) at redshift z > 1 (9 at 1.5 < z < 2.3) recently discovered in the CANDELS and CLASH Multi-Cycle Treasury programs using WFC3 on the Hubble Space Telescope. We combine these SNe Ia with a new compilation of 1050 SNe Ia, jointly calibrated and corrected for simulated survey biases to produce accurate distance measurements. We present unbiased constraints on the expansion rate at six redshifts in the range 0.07 < z < 1.5 based only on this combined SN Ia sample. The added leverage of our new sample at z > 1.5 leads to a factor of ~3 improvement in the determination of the expansion rate at z = 1.5, reducing its uncertainty to ~20%, a measurement of H(z=1.5)/H0=2.67 (+0.83,-0.52). We then demonstrate that these six measurements alone provide a nearly identical characterization of dark energy as the full SN sample, making them an efficient compression of the SN Ia data. The new sample of SNe Ia at z > 1 usefully distinguishes between alternative cosmological models and unmodeled evolution of the SN Ia distance indicators, placing empirical limits on the latter. Finally, employing a realistic simulation of a potential WFIRST SN survey observing strategy, we forecast optimistic future constraints on the expansion rate from SNe Ia.

Figures (5)

• Figure 1: Constraints on $E(z) \equiv H(z)/H_0$, relative to $E(z)$ for a fiducial $\Lambda$CDM model ($\Omega_m = 0.3$). We compare the constraints with (blue points) and without (red points) the high-redshift CANDELS and CLASH (MCT) SNe Ia. Note that these $E(z)$ measurements are correlated and have non-Gaussian distributions (the error bars enclose 68.3% of the likelihood). For comparison, we also show the three (correlated) measurements of $E(z)$ from combined BOSS DR12 BAO data Alam:2016hwk after calibration with Planck$\Lambda$CDM constraints on $H_0 \, r_d$ (green points).
• Figure 2: For the same data as in Figure \ref{['fig:Hz']}, we show constraints on the time derivative of the scale factor $\dot{a}(z)$ relative to its present value, obtained by scaling the $E(z)$ values by $(1 + z)^{-1}$. We compare the fiducial $\Lambda$CDM model to alternative models with a constant deceleration parameter $q_0 = 0$ (coasting cosmology), $q_0 = -0.5$ (pure acceleration), and $q_0 = 0.5$ (pure deceleration), all assuming a flat universe.
• Figure 3: Constraints on $\Omega_m$ and a constant equation-of-state parameter $w$ in a flat universe (left panel) and for the $w_0$--$w_a$ model Chevallier:2000qyLinder:2002et, marginalized over $\Omega_m$ and also assuming a flat universe (right panel). We compare the constraints when using the full SN Ia likelihood with individual distance moduli (filled blue contours) with the constraints from the six moderately correlated $E(z)$ measurements (open red contours). Contours contain 68.3%, 95.4%, and 99.7% of the likelihood, and for the $w_0$--$w_a$ constraints we have also included distance priors derived from Planck data Ade:2015xua.
• Figure 4: Comparison of $\Lambda$CDM and power-law cosmology ($a(t) \propto t^n$) fits to our SN Ia data, where in each case we allow the intrinsic luminosity to evolve as $\Delta M(z) = \epsilon z^\delta$, corresponding to Model B from Tutusaus:2017ibk, where we fix $\delta = 0.3$. The SN data are binned for clarity, and $\Delta \chi^2_{\Lambda \text{CDM}} \equiv \chi^2 - \chi^2_{\Lambda \text{CDM}}$.
• Figure 5: Simulated WFIRST constraints on $E(z) \equiv H(z)/H_0$, relative to $E(z)$ for a fiducial $\Lambda$CDM model ($\Omega_m = 0.3$). We compare the constraints from current data (blue points) with simulated constraints from the WFIRSTImaging All-z observing strategy (green points). We overlay the same dark energy models as in Figure \ref{['fig:Hz']}.