We report on work to increase the number of well-measured Type Ia
supernovae (SNe Ia) at high redshifts. Light curves, including high
signal-to-noise Hubble Space Telescope data, and spectra of six SNe Ia
that were discovered during 2001, are presented. Additionally, for the
two SNe with z > 1, we present ground-based J-band photometry from
Gemini and the Very Large Telescope. These are among the most distant
SNe Ia for which ground-based near-IR observations have been obtained.
We add these six SNe Ia together with other data sets that have recently
become available in the literature to the Union compilation. We have
made a number of refinements to the Union analysis chain, the most
important ones being the refitting of all light curves with the SALT2
fitter and an improved handling of systematic errors. We call this new
compilation, consisting of 557 SNe, the Union2 compilation. The flat
concordance ΛCDM model remains an excellent fit to the Union2
data with the best-fit constant equation-of-state parameter w =
–0.997
+0.050–0.054(stat)
+0.077–0.082(stat + sys together) for a flat universe, or w =
–1.038
+0.056–0.059(stat)
+0.093–0.097(stat + sys together) with curvature. We also
present improved constraints on w(z). While no significant change in w
with redshift is detected, there is still considerable room for
evolution in w. The strength of the constraints depends strongly on
redshift. In particular, at z >~ 1, the existence and nature of dark
energy are only weakly constrained by the data.
Based in part on observations made with the NASA/ESA Hubble Space
Telescope, obtained from the data archive at the Space Telescope Science
Institute (STScI). STScI is operated by the Association of Universities
for Research in Astronomy (AURA), Inc. under the NASA contract NAS
5-26555. The observations are associated with programs HST-GO-08585 and
HST-GO-09075. Based, in part, on observations obtained at the ESO La
Silla Paranal Observatory (ESO programs 67.A-0361 and 169.A-0382).
Based, in part, on observations obtained at the Cerro-Tololo
Inter-American Observatory (CTIO), National Optical Astronomy
Observatory (NOAO). Based on observations obtained at the
Canada-France-Hawaii Telescope (CFHT). Based, in part, on observations
obtained at the Gemini Observatory (Gemini programs GN-2001A-SV-19 and
GN-2002A-Q-31). Based, in part on observations obtained at the Subaru
Telescope. Based, in part, on data that were obtained at the W. M. Keck
Observatory.
