Cosmological Results from High-z Supernovae

TL;DR

The paper consolidates an independent SN Ia dataset extending to $z\sim1$, confirming cosmic acceleration and constraining dark energy. It employs multiple distance-fitting techniques (MLCS, $\Delta m_{15}$, BATM) with carefully computed K-corrections and a robust pairwise-difference photometry approach to minimize systematics. The analysis yields $\Omega_M=0.28\pm0.05$, $\Omega_\Lambda=0.72$ for a flat universe and tight constraints on the dark-energy equation of state $-1.48<w<-0.72$ (95% confidence, with 2dF prior), along with $H_0 t_0=0.96\pm0.04$, in agreement with CMB measurements. The results reinforce the standard cosmological model dominated by dark energy while acknowledging residual systematics and outlining future surveys (ESSENCE, GOODS, SNAP) to measure $w$ and its possible evolution with redshift. Overall, the work strengthens evidence for dark energy, tests the cosmological origin of acceleration, and charts a path for precision cosmology with SN Ia.

Abstract

The High-z Supernova Search Team has discovered and observed 8 new supernovae in the redshift interval z=0.3-1.2. These independent observations, confirm the result of Riess et al. (1998a) and Perlmutter et al. (1999) that supernova luminosity distances imply an accelerating universe. More importantly, they extend the redshift range of consistently observed SN Ia to z~1, where the signature of cosmological effects has the opposite sign of some plausible systematic effects. Consequently, these measurements not only provide another quantitative confirmation of the importance of dark energy, but also constitute a powerful qualitative test for the cosmological origin of cosmic acceleration. We find a rate for SN Ia of 1.4+/-0.5E-04 h^3/Mpc^3/yr at a mean redshift of 0.5. We present distances and host extinctions for 230 SN Ia. These place the following constraints on cosmological quantities: if the equation of state parameter of the dark energy is w=-1, then H0 t0 = 0.96+/-0.04, and O_l - 1.4 O_m = 0.35+/-0.14. Including the constraint of a flat Universe, we find O_m = 0.28+/-0.05, independent of any large-scale structure measurements. Adopting a prior based on the 2dF redshift survey constraint on O_m and assuming a flat universe, we find that the equation of state parameter of the dark energy lies in the range -1.48<w<-0.72 at 95% confidence. If we further assume that w>-1, we obtain w<-0.73 at 95% confidence. These constraints are similar in precision and in value to recent results reported using the WMAP satellite, also in combination with the 2dF redshift survey.

Figures (15)

• Figure 1: Host galaxies for the eleven supernovae. Each image is 20$"$ on a side and taken from an average of several I-band frames. A 2$"$ radius circle marks the position of the supernova. North is at the top and east to the left in each image.
• Figure 2: High-$z$ supernova spectra shown at their observed wavelengths, smoothed by a 32-pixel running median and a Gaussian of 4-pixel sigma.
• Figure 3: High-$z$ SN spectra, shifted to rest wavelengths, smoothed by a Savitsky-Golay filter with a width of 100 Å and compared to two low-$z$ SN Ia (SN 1992A and SN 1989B) and a low-$z$ SNIc (SN 1994I). The high-$z$ SN spectra are shown sorted by redshift, with the lowest-$z$ SN at the top. Based on these spectra, we cannot conclude that SN 1999fi, 1999fo, and 1999fu are SN Ia.
• Figure 4: $Z$ bandpass used for these observations. The dashed line is an approximation of the actual transmission, made up of two Gaussians as described in the text.
• Figure 5: MLCS fits to our eight SN Ia. The open diamonds represent K-corrected $V$-band fits, while the filled-in circles represent K-corrected $B$-band fits, offset by +1 mag, except for SN 1999fh, which is $V+1$ and $R$. SN 1999fo, 1999fi, and 1999fu have no light curves because we could not establish from the spectra that these were indeed SN Ia, so they were not followed. Redshifts are indicated in parentheses.