Alan Hodgdon Diercks

The High-Z Supernova Search: Measuring Cosmic Deceleration and Global Curvature of the Universe Using Type Ia Supernovae*

The High-Z Supernova Search is an international collaboration to discover and monitor Type Ia supernovae (SNe Ia) at z > 0.2 with the aim of measuring cosmic deceleration and global curvature. Our collaboration has pursued a basic understanding of supernovae in the nearby universe, discovering and observing a large sample of objects and developing methods to measure accurate distances with SNe Ia. This paper describes the extension of this program to z ≥ 0.2, outlining our search techniques and follow-up program. We have devised high-throughput filters that provide accurate two-color rest frame B and V light curves of SNe Ia, enabling us to produce precise, extinction-corrected luminosity distances in the range 0.25 M=-0.2 -->−0.8+1.0 if ΩΛ = 0. For a spatially flat universe composed of normal matter and a cosmological constant, we find Ω -->M=0.4 -->−0.4+0.5, Ω

SUPERNOVA LIMITS ON THE COSMIC EQUATION OF STATE

{Λ}

Constraints on Cosmological Models from Hubble Space Telescope Observations of High-z Supernovae

-->=0.6 -->−0.5+0.4. We demonstrate that with a sample of ~30 objects, we should be able to determine relative luminosity distances over the range 0 < z < 0.5 with sufficient precision to measure ΩM with an uncertainty of ±0.2.

TESTS OF THE ACCELERATING UNIVERSE WITH NEAR-INFRARED OBSERVATIONS OF A HIGH-REDSHIFT TYPE Ia SUPERNOVA

We use Type Ia supernovae studied by the High-z Supernova Search Team to constrain the properties of an energy component that may have contributed to accelerating the cosmic expansion. We find that for a flat geometry the equation-of-state parameter for the unknown component, αx = Px/ρx, must be less than -0.55 (95% confidence) for any value of Ωm, and it is further limited to αx < -0.60 (95% confidence) if Ωm is assumed to be greater than 0.1. These values are inconsistent with the unknown component being topological defects such as domain walls, strings, or textures. The supernova (SN) data are consistent with a cosmological constant (αx = -1) or a scalar field that has had, on average, an equation-of-state parameter similar to the cosmological constant value of -1 over the redshift range of z ≈ 1 to the present. SN and cosmic microwave background observations give complementary constraints on the densities of matter and the unknown component. If only matter and vacuum energy are considered, then the current combined data sets provide direct evidence for a spatially flat universe with Ωtot = Ωm + ΩΛ = 0.94 ± 0.26 (1 σ).

The Optical Afterglow of GRB 971214: [ITAL]R[/ITAL] and [ITAL]J[/ITAL] Photometry

We have coordinated Hubble Space Telescope (HST) photometry with ground-based discovery for three supernovae: Type Ia supernovae near z ≈ 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. We use these light curves and relations between luminosity, light-curve shape, and color calibrated from low-z samples to derive relative luminosity distances that are accurate to 10% at z ≈ 0.5 and 20% at z = 1. When the HST sample is combined with the distance to SN 1995K (z = 0.48), analyzed by the same precepts, we find that matter alone is insufficient to produce a flat universe. Specifically, for Ωm+ΩΛ = 1, Ωm is less than 1 with more than 95% confidence, and our best estimate of Ωm is -0.1±0.5 if ΩΛ = 0. Although this 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.

The Optical Afterglow of GRB 971214: R and J Photometry

We have measured the rest-frame B-, V -, and I-band light curves of a high-redshift type Ia supernova (SN Ia), SN 1999Q (z \ 0.46), using the Hubble Space Telescope (HST ) and ground-based near-infrared detectors. A goal of this study is the measurement of the color excess, a sensitive indicator of E B~I , interstellar or intergalactic dust, which could aUect recent cosmological measurements from high-redshift SNe Ia. Our observations disfavor a 30% opacity of SN Ia visual light by dust as an alternative to an accelerating universe. This statement applies to both Galactic-type dust (rejected at the 3.4 p con—dence level) and grayer dust (grain size ( 0.1 km, rejected at the 2.3¨2.6 p con—dence level) as proposed by Aguirre. The rest-frame I-band light curve shows the secondary maximum 1 month after the B maximum typical of nearby SNe Ia of normal luminosity, providing no indication of evolution as a func- tion of redshift out to z B 0.5. An expanded set of similar observations could improve the constraints on any contribution of extragalactic dust to the dimming of high-redshift SNe Ia. Subject headings: cosmology: observationsdistance scalesupernovae: general

The optical afterglow of GRB 971214: R and J photometry

We present an R-band and J-band photometry of an optical transient which is likely to be associated with the gamma-ray burst event GRB 971214. Our first measurement took place 13 hours after the gamma-ray event. The brightness decayed with a power-law exponent alpha = -1.20 +- 0.02, which is similar to those of GRB 970228 and GRB 970508 which had exponents of alpha = -1.10 +- 0.04 and alpha = -1.141 +- 0.014 respectively. The transient decayed monotonically during the first four days following the gamma-ray event in contrast with the optical transient associated with GRB 970508 which increased in brightness, peaking two days after the burst, before settling to a power-law decay.

Method for extending the dynamic range of CCD instruments

We present an R-band and J-band photometry of an optical transient which is likely to be associated with the gamma-ray burst event GRB 971214. Our first measurement took place 13 hours after the gamma-ray event. The brightness decayed with a power-law exponent alpha = -1.20 +- 0.02, which is similar to those of GRB 970228 and GRB 970508 which had exponents of alpha = -1.10 +- 0.04 and alpha = -1.141 +- 0.014 respectively. The transient decayed monotonically during the first four days following the gamma-ray event in contrast with the optical transient associated with GRB 970508 which increased in brightness, peaking two days after the burst, before settling to a power-law decay.

A 71 Megapixel Mosaic Camera for Weak Lensing at APO

We present an R-band and J-band photometry of an optical transient which is likely to be associated with the gamma-ray burst event GRB 971214. Our first measurement took place 13 hours after the gamma-ray event. The brightness decayed with a power-law exponent alpha = -1.20 +- 0.02, which is similar to those of GRB 970228 and GRB 970508 which had exponents of alpha = -1.10 +- 0.04 and alpha = -1.141 +- 0.014 respectively. The transient decayed monotonically during the first four days following the gamma-ray event in contrast with the optical transient associated with GRB 970508 which increased in brightness, peaking two days after the burst, before settling to a power-law decay.

Observational evidence from supernovae for an accelerating universe and a cosmological constant

Charge coupled devices (CCDs) are the detector of choice for instruments that detect low levels of light for wavelengths 300 nm <EQ (lambda) <EQ 1100 nm. Contemporary devices have read noise level equivalents of a few electrons, and the ability to store over 100,000 electrons per pixel. In order to take full advantage of these characteristics, while ensuring that digital quantization noise will not dominate over readout noise, the dynamic range of the readout system must exceed 18 bits. We present a simple scheme that exploits the fact that Poisson noise dominates the error budget in most contemporary CCD systems in order to achieve an effective dynamic range of over 20 bits by suing dual 16 bit A/D converters of different sensitivity.