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 ~1.7, the farthest SN observed to date. SN 1997ff, wasdiscovered in a repeat observation by the HST of the HDF-), and serendipitouslymonitored with NICMOS on HST throughout the GTO campaign. The SN type can bedetermined from the host galaxy type:an evolved, red elliptical lacking enoughrecent star formation to provide a significant population of core-collapse SNe.The class- ification is further supported by diagnostics available from theobserved colors and temporal behavior of the SN, both of which match a typicalSN Ia. The photo- metric record of the SN includes a dozen flux measurements inthe I, J, and H bands spanning 35 days in the observed frame. The redshiftderived from the SN photometry, z=1.7+/-0.1, is in excellent agreement with theredshift estimate of z=1.65+/-0.15 derived from theU_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 tentativespectroscopic redshift of 1.755. Fits to observations of the SN provideconstraints for the redshift-distance relation of SNe~Ia and a powerful test ofthe current accelerating Universe hypothesis. The apparent SN brightness isconsistent with that expected in the decelerating phase of the preferredcosmological model, Omega_M~1/3, Omega_Lambda~2/3. It is inconsistent with greydust or simple luminosity evolution, candidate astro- physical effects whichcould mimic past evidence for an accelerating Universe from SNe Ia at z~0.5.Weconsider several sources of possible systematic error including lensing, SNmisclassification, selection bias, and calibration errors. Currently, none ofthese effects appears likely to challenge our conclusions.