The Farthest Known Supernova: Support for an Accelerating Universe and a Glimpse of the Epoch of Deceleration

TL;DR

Riess et al. report photometric and (tentative) spectroscopic analyses of SN 1997ff, the farthest known Type Ia SN at $z \approx 1.7$, discovered in the Hubble Deep Field-North and monitored with NICMOS. They integrate host photometric redshifts, SN color evolution, and SN light-curve fitting within a Bayesian-like framework to derive the SN's redshift, distance modulus $m-M = 42.15 \pm 0.34$ mag, and age relative to $B$-maximum, highlighting a robust constraint on the redshift via multiple independent indicators. The resulting redshift–distance relation extends SN tests of cosmology to $z>1$, and the data favor a decelerating epoch prior to the current acceleration, arguing against simple grey-dust or luminosity-evolution mimics. Nonetheless, the authors carefully evaluate systematics, including gravitational lensing and misclassification, and call for a larger sample of high-$z$ SNe to tighten constraints on dark-energy models and the cosmic expansion history.

Abstract

We present photometric observations of an apparent Type Ia supernova (SN Ia) at a redshift of ~1.7, the farthest SN observed to date. SN 1997ff, was discovered in a repeat observation by the HST of the HDF-), and serendipitously monitored with NICMOS on HST throughout the GTO campaign. The SN type can be determined from the host galaxy type:an evolved, red elliptical lacking enough recent star formation to provide a significant population of core-collapse SNe. The class- ification is further supported by diagnostics available from the observed colors and temporal behavior of the SN, both of which match a typical SN Ia. The photo- metric record of the SN includes a dozen flux measurements in the I, J, and H bands spanning 35 days in the observed frame. The redshift derived from the SN photometry, z=1.7+/-0.1, is in excellent agreement with the redshift estimate of z=1.65+/-0.15 derived from the U_300,B_450,V_606,I_814,J_110,J_125,H_160, H_165,K_s photometry of the galaxy. Optical and near-infrared spectra of the host provide a very tentative spectroscopic redshift of 1.755. Fits to observations of the SN provide constraints for the redshift-distance relation of SNe~Ia and a powerful test of the current accelerating Universe hypothesis. The apparent SN brightness is consistent with that expected in the decelerating phase of the preferred cosmological model, Omega_M~1/3, Omega_Lambda~2/3. It is inconsistent with grey dust or simple luminosity evolution, candidate astro- physical effects which could mimic past evidence for an accelerating Universe from SNe Ia at z~0.5.We consider several sources of possible systematic error including lensing, SN misclassification, selection bias, and calibration errors. Currently, none of these effects appears likely to challenge our conclusions.