The farthest known supernova: Support for an accelerating universe and a glimpse of the epoch of deceleration

We present photometric observations of an apparent Type Ia supernova (SN Ia ) at a redshift of similar to1.7, the farthest SN observed to date. The sup ernova, SN 1997ff, was discovered in a repeat observation by the Hubble Spa ce Telescope (HST) of the Hubble Deep Field-North (HDF-N) and serendipitous ly monitored with NICMOS on HST throughout the Thompson et al. Guaranteed-T ime Observer (GTO) campaign. The SN type can be determined from the host ga laxy type : an evolved, red elliptical lacking enough recent star formation to provide a significant population of core-collapse supernovae. The class ification is further supported by diagnostics available from the observed c olors and temporal behavior of the SN, both of which match a typical SN Ia. The photometric 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 der ived from the SN photometry, z = 1.7 +/- 0.1, is in excellent agreement wit h the redshift estimate of z = 1.65 +/- 0.15 derived from 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 re dshift 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 ac celerating universe hypothesis. The apparent SN brightness is consistent wi th that expected in the decelerating phase of the preferred cosmological mo del, Omega (M) approximate to 1/3, Omega (Lambda) approximate to 2/3. It is inconsistent with gray dust or simple luminosity evolution, candidate astr ophysical effects that could mimic previous evidence for an accelerating un iverse from SNe Ia at z approximate to 0.5. We consider several sources of potential systematic error, including gravitational lensing, supernova misc lassification, sample selection bias, and luminosity calibration errors. Cu rrently, none of these effects alone appears likely to challenge our conclu sions. Additional SNe Ia at z > 1 will be required to test more exotic alte rnatives to the accelerating universe hypothesis and to probe the nature of dark energy.