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Open Access Publications from the University of California

Scholarly Works (6 results)

  • Article
  • Peer Reviewed
UC Santa Cruz Previously Published Works (2005)
We have obtained high-quality Keck optical spectra at three epochs of the Type la supernova 1997ex, whose redshift z is 0.361. The elapsed calendar time between the first two spectra was 24.88 days, and that between the first and third spectra was 30.95 days. In an expanding universe where 1 + z represents the factor by which space has expanded between the emission and detection of light, the amount of aging in the supernova rest frame should be a factor of 1/(1 + z) smaller than the observed-frame aging; thus, we expect SN 1997ex to have aged 18.28 and 22.74 days between the first epoch and the second and third epochs, respectively. The quantitative method for determining the spectral-feature age of an SN la reveals that the corresponding elapsed times in the supernova rest frame were 16.97 ± 2.75 and 18.01 ± 3.14 days, respectively. This result is inconsistent with no time dilation with a significance level of 99.0%, providing evidence against "tired light" and other hypotheses in which no time dilation is expected. Moreover, the observed timescale of spectral evolution is inconsistent with that expected in the "variable mass theory." The result is within ∼1 σ of the aging expected from a universe in which redshift is produced by cosmic expansion. © 2005. The American Astronomical Society. All rights reserved.
    Cover page: A Definitive Measurement of Time Dilation in the Spectral Evolution of the Moderate-Redshift Type Ia Supernova 1997ex
    • Article
    • Peer Reviewed
    UC Berkeley Previously Published Works (1998)
    We present an empirical method that measures the distance to a Type la supernova (SN Ia) with a precision of ∼10% from a single night's data. This method measures the supernova's age and luminosity/light-curve parameter from a spectrum and the extinction and distance from an apparent magnitude and color. We are able to verify the precision of this method from error propagation calculations, Monte Carlo simulations of well-sampled SNe Ia, and the Hubble diagram of sparsely observed supernovae. With the reduction in telescope time needed, this method is 3-4 times more efficient for measuring cosmological parameters than conventional light-curve-based distance estimates. © 1998, The American Astronomical Society, All rights reserved.
      Cover page: Snapshot Distances to Type Ia Supernovae: All in “One” Night's Work
      • Article
      • Peer Reviewed
      Lawrence Berkeley National Laboratory (2001)

      We present photometric observations of an apparent Type Ia supernova (SN Ia) at a redshift of approximately 1.7, the farthest SN observed to date. The supernova, SN 1997, was discovered in a repeat observation by the Hubble Space Telescope (HST) of the Hubble Deep Field{North (HDF-N), and serendipitously monitored with NICMOS on HST throughout the Thompson et al. GTO campaign. The SN type can be determined from the host galaxy type: an evolved, red elliptical lacking enough recent star formation to provide a significant population of core-collapse supernovae. The classification is further supported by diagnostics available from the observed colors and temporal behavior of the SN, both of which match a typical SN Ia. The photometric record of the SN includes a dozen flux measurements in the I, J, and H bands spanning 35 days in the observed frame. The redshift derived from the SN photometry, z = 1:7 plus or minus 0:1, is in excellent agreement with the redshift estimate of z = 1:65 plus or minus 0:15 derived from the U_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 tentative spectroscopic redshift of 1.755. Fits to observations of the SN provide constraints for the redshift-distance relation of SNe Ia and a powerful test of the current accelerating Universe hypothesis. The apparent SN brightness is consistent with that expected in the decelerating phase of the preferred cosmological model, Omega_M approximately equal to 1/3; Omega_Lambda approximately equal to 2/3. It is inconsistent with grey dust or simple luminosity evolution, candidate astrophysical effects which could mimic previous evidence for an accelerating Universe from SNe Ia at z approximately equal to 0:5. We consider several sources of potential systematic error including gravitational lensing, supernova misclassification, sample selection bias, and luminosity calibration errors. Currently, none of these effects alone appears likely to challenge our conclusions. Additional SNe Ia at z > 1 will be required to test more exotic alternatives to the accelerating Universe hypothesis and to probe the nature of dark energy.

        Cover page: The farthest known supernova: Support for an accelerating universe and a glimpse of the epoch of deceleration
        • Article
        • Peer Reviewed
        UC Davis Previously Published Works (2015)
        In 1964, Refsdal hypothesized that a supernova whose light traversed multiple paths around a strong gravitational lens could be used to measure the rate of cosmic expansion. We report the discovery of such a system. In Hubble Space Telescope imaging, we have found four images of a single supernova forming an Einstein cross configuration around a redshift z = 0.54 elliptical galaxy in the MACS J1149.6+2223 cluster. The cluster's gravitational potential also creates multiple images of the z = 1.49 spiral supernova host galaxy, and a future appearance of the supernova elsewhere in the cluster field is expected. The magnifications and staggered arrivals of the supernova images probe the cosmic expansion rate, as well as the distribution of matter in the galaxy and cluster lenses.
          Cover page: Multiple images of a highly magnified supernova formed by an early-type cluster galaxy lens
          • Article
          • Peer Reviewed
          UC Davis Previously Published Works (2015)
          In 1964, Refsdal hypothesized that a supernova whose light traversed multiple paths around a strong gravitational lens could be used to measure the rate of cosmic expansion. We report the discovery of such a system. In Hubble Space Telescope imaging, we have found four images of a single supernova forming an Einstein cross configuration around a redshift z = 0.54 elliptical galaxy in the MACS J1149.6+2223 cluster. The cluster's gravitational potential also creates multiple images of the z = 1.49 spiral supernova host galaxy, and a future appearance of the supernova elsewhere in the cluster field is expected. The magnifications and staggered arrivals of the supernova images probe the cosmic expansion rate, as well as the distribution of matter in the galaxy and cluster lenses.
            Cover page: Astrophysics. Multiple images of a highly magnified supernova formed by an early-type cluster galaxy lens.
            • Article
            • Peer Reviewed
            UC Irvine Previously Published Works (2014)
            The Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey (CANDELS) was a multi-cycle treasury program on the Hubble Space Telescope (HST) that surveyed a total area of ∼0.25 deg2 with ∼900 HST orbits spread across five fields over three years. Within these survey images we discovered 65 supernovae (SNe) of all types, out to z ∼ 2.5. We classify ∼24 of these as Type Ia SNe (SNe Ia) based on host galaxy redshifts and SN photometry (supplemented by grism spectroscopy of six SNe). Here we present a measurement of the volumetric SN Ia rate as a function of redshift, reaching for the first time beyond z = 2 and putting new constraints on SN Ia progenitor models. Our highest redshift bin includes detections of SNe that exploded when the universe was only ∼3 Gyr old and near the peak of the cosmic star formation history. This gives the CANDELS high redshift sample unique leverage for evaluating the fraction of SNe Ia that explode promptly after formation (<500 Myr). Combining the CANDELS rates with all available SN Ia rate measurements in the literature we find that this prompt SN Ia fraction is , consistent with a delay time distribution that follows a simple t -1 power law for all times t > 40 Myr. However, mild tension is apparent between ground-based low-z surveys and space-based high-z surveys. In both CANDELS and the sister HST program CLASH (Cluster Lensing And Supernova Survey with Hubble), we find a low rate of SNe Ia at z > 1. This could be a hint that prompt progenitors are in fact relatively rare, accounting for only 20% of all SN Ia explosions - though further analysis and larger samples will be needed to examine that suggestion. © 2014. The American Astronomical Society. All rights reserved.
              Cover page: TYPE Ia SUPERNOVA RATE MEASUREMENTS TO REDSHIFT 2.5 FROM CANDELS: SEARCHING FOR PROMPT EXPLOSIONS IN THE EARLY UNIVERSE
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