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Session 85 - Cosmology: Observations.
Display session, Friday, January 09
Exhibit Hall,
We have coordinated Hubble Space Telescope photometry with ground-based discovery for three supernovae: two SN Ia near z\sim 0.5 (SN 1997ce, SN 1997cj) and a third event at z=0.97 (SN 1997ck). The superb spatial resolution of HST separates each supernova from its host galaxy and leads to good precision in the light curves. The HST data combined with ground-based photometry provide good temporal coverage. We use these light curves and relations between luminosity, light curve shape, and color calibrated from low-z samples to derive relative luminosity distances which are accurate to 10%\ at z\sim 0.5 and 20%\ at z=1. The redshift-distance relation is used to place constraints on the global mean matter density, Ømega_m, and the normalized cosmological constant, Ømega_\Lambda. When the HST\ sample is combined with the distance to SN 1995K (z=0.48), analyzed by the same precepts, it suggests that matter alone is insufficient to produce a flat Universe. Specifically, for Ømega_m+Ømega_\Lambda =1, Ømega_m is less than 1 with >95%\ confidence, and our best estimate of Ømega_m is -0.1 \pm 0.5 if Ømega_\Lambda =0. Although the present result is based on a very small sample whose systematics remain to be explored, it demonstrates the power of HST\ measurements for high redshift supernovae.
