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The Astrophysical Journal, 666:694 Y715, 2007 September 10
# 2007. The American Astronomical Society. All rights reserved. Printed in U.S.A.

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OBSERVATIONAL CONSTRAINTS ON THE NATURE OF DARK ENERGY: FIRST COSMOLOGICAL RESULTS FROM THE ESSENCE SUPERNOVA SURVEY
W. M. Wood-Vasey,1 G. Miknaitis,2 C. W. Stubbs,1,3 S. Jha,4,5 A. G. Riess,6,7 P. M. Garnavich, R. P. Kirshner,1 C. Aguilera,9 A. C. Becker,10 J. W. Blackman,11 S. Blondin,1 P. Challis,1 A. Clocchiatti,12 A. Conley,13 R. Covarrubias,10 T. M. Davis,14 A. V. Filippenko,4 R. J. Foley,4 A. Garg,1,3 M. Hicken,1,3 K. Krisciunas,8,15 B. Leibundgut,16 W. Li,4 T. Matheson,17 A. Miceli,10 G. Narayan,1,3 G. Pignata,12 J. L. Prieto,18 A. Rest,9 M. E. Salvo,11 B. P. Schmidt,11 R. C. Smith,9 J. Sollerman,14,19 J. Spyromilio,16 J. L. Tonry,20 N. B. Suntzeff,9,15 and A. Zenteno9
Received 2006 November 21; accepted 2007 April 2
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ABSTRACT We present constraints on the dark energy equation-of-state parameter, w ј P/( c 2 ), using60SNe Ia from the ESSENCE supernova survey. We derive a set of constraints on the nature of the dark energy assuming a flat universe. By including constraints on (M, w) from baryon acoustic oscillations, we obtain a value for a static equation-of-state parameter w ј ю1:05Ч0::13 (stat 1 ) ф 0:13 (sys) and M ј 0:274Ч0::033 (stat 1 ) with a bestю0 020 ю0 12 fit 2/dof of 0.96. These results are consistent with those reported by the Supernova Legacy Survey from the first year of a similar program measuring supernova distances and redshifts. We evaluate sources of systematic error that afflict supernova observations and present Monte Carlo simulations that explore these effects. Currently, the largest systematic with the potential to affect our measurements is the treatment of extinction due to dust in the supernova host galaxies. Combining our set of ESSENCE SNe Ia with the first-results Supernova Legacy Survey SNe Ia, we obtain a joint constraint of w ј ю1:07Ч0::09 (stat 1 ) ф 0:13 (sys), M ј 0:267Ч0::028 (stat 1 ) with ю0 09 ю0 018 abest-fit 2/dof of 0.91. The current global SN Ia data alone rule out empty (M ј 0), matter-only M ј 0:3, and M ј 1 universes at >4.5 . The current SN Ia data are fully consistent with a cosmological constant. Subject headings: cosmological parameters -- cosmology: observations -- supernovae: general Online material: color figures

1. INTRODUCTION: SUPERNOVAE AND COSMOLOGY We report on the analysis of 60 Type Ia supernovae (SNe Ia) discovered in the course of the ESSENCE program ( Equation of State: SupErNovae trace Cosmic Expansion; an NOAO Survey Program) from 2002 through 2005. The aim of ESSENCE is to measure the history of cosmic expansion over the past 5 billion years with sufficient precision to distinguish whether the dark energy is different from a cosmological constant at the w ј ф 0:1 level. Here we present our first results and show that we are well on our way toward that goal. Our present data are fully consistent with a w ј ю1, flat universe, and our uncertainty in w, the parameter that describes the cosmic equation
1 Harvard-Smithsonian Center for Astrophysics, Cambridge, MA 02138; wmwood-vasey@cfa.harvard.edu. 2 Fermilab, Batavia, IL 60510-0500. 3 Department of Physics, Harvard University, Cambridge, MA 02138. 4 Department of Astronomy, University of California , Berkeley, CA 94720-3411. 5 Kavli Institute for Particle Astrophysics and Cosmology, Stanford Linear Accelerator Center, MS 29, Menlo Park, CA 94025. 6 Space Telescope Science Institute, Baltimore, MD 21218. 7 Johns Hopkins University, Baltimore, MD 21218. 8 Department of Physics, University of Notre Dame, Notre Dame, IN 46556-5670. 9 Cerro Tololo Inter-American Observatory, National Optical Astronomy Observatory, Casilla 603, La Serena, Chile. 10 Department of Astronomy, University of Washington, Seattle, WA 981951580. 11 Research School of Astronomy and Astrophysics, Australian National University, Mount Stromlo and Siding Spring Observatories, Weston Creek PO 2611, Australia.

of state, analyzed in the way we outline here, will shrink below 0.1 for models of constant w as the ESSENCE program is completed. Other approaches to using the luminosity distances have been suggested to constrain possible cosmological models. We here provide the ESSENCE observations in a convenient form suitable for testing a variety of models.21 As reported in a companion paper ( Miknaitis et al. 2007), ESSENCE is based on a supernova search carried out with the 4 m Blanco Telescope at the Cerro Tololo Inter-American Observatory (CTIO) with the prime-focus MOSAIC II 64 Megapixel CCD camera. Our search produces densely sampled R-band and I-band light curves for supernovae in our fields. As described by Miknaitis et al. (2007), we optimized the search to provide the best constraints on w, given fixed observing time and the properties of both the MOSAIC II camera and the CTIO 4 m
12 ґ Pontificia Universidad Catolica de Chile, Departamento de Astronomґa y i Astrofґsica, Casilla 306, Santiago 22, Chile. i 13 Department of Astronomy and Astrophysics, University of Toronto, Toronto, ON M5S 3H4, Canada. 14 Dark Cosmology Centre, Niels Bohr Institute, University of Copenhagen, DK-2100 Copenhagen , Denmark. 15 Department of Physics, Texas A&M University, College Station, TX 77843- 4242. 16 European Southern Observatory, D-85748 Garching, Germany. 17 National Optical Astronomy Observatory, Tucson, AZ 85719- 4933. 18 Department of Astronomy, Ohio State University, Columbus, OH 43210. 19 Department of Astronomy, Stockholm University, AlbaNova, 10691 Stockholm, Sweden. 20 Institute for Astronomy, University of Hawaii, Honolulu, HI 96822. 21 See http:// www.ctio.noao.edu /essence /.

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DARK ENERGY FROM ESSENCE SURVEY telescope. Spectra from a variety of large telescopes, including Keck, VLT, Gemini, and Magellan, allow us to determine supernova types and redshifts. We have paid particular attention to the central problems of calibration and systematic errors that, on completion of the survey in 2008, will be more important to the final precision of our cosmological inferences than statistical sampling errors for about 200 objects. This first cosmological report from the ESSENCE survey derives some properties of dark energy from the sample presently in hand, which is still small enough that the statistics of the sample size make a noticeable contribution to the uncertainty in dark energy properties. But our goal is to set out the systematic uncertainties in a clear way so that these are exposed to view and so that we can concentrate our efforts where they will have the most significant effect. To infer luminosity distances to the ESSENCE supernovae over the redshift interval 0.15Y0.70, we employ the relations developed for SNe Ia at low redshift (Jha et al. 2007 and references therein) among their light-curve shapes, colors, and intrinsic luminosities. The expansion history from z % 0:7tothe present provides leverage to constrain the equation-of-state parameter for the dark energy as described below. In x 1 we sketch the context of the ESSENCE program. In x 2 we show from a set of simulated light curves that this particular implementation of light-curve analysis is consistent, with the same cosmology emerging from the analysis as was used to construct the samples, and that the statistical uncertainty we ascribe to the inference of the dark energy properties is also correctly measured. This modeling of our analysis chain gives us confidence that the analysis of