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Spectra and Light Curves of Six Type Ia Supernovae at 0.511 < z < 1.12 and the Union2 Compilation

R. Amanullah, C. Lidman, D. Rubin, G. Aldering, P. Astier, K. Barbary, M. S. Burns, A. Conley, K. S. Dawson, S. E. Deustua, M. Doi, S. Fabbro, L. Faccioli, H. K. Fakhouri, G. Folatelli, A. S. Fruchter, H. Furusawa, G. Garavini, G. Goldhaber, A. Goobar, D. E. Groom, I. Hook, D. A. Howell, N. Kashikawa, A. G. Kim, R. A. Knop, M. Kowalski, E. Linder, J. Meyers, T. Morokuma, S. Nobili, J. Nordin, P. E. Nugent, L. Ostman, R. Pain, N. Panagia, S. Perlmutter, J. Raux, P. Ruiz-Lapuente, A. L. Spadafora, M. Strovink, N. Suzuki, L. Wang, W. M. Wood-Vasey, N. Yasuda

TL;DR

We extend the public SN Ia cosmology datasets by adding six high-$z$ SNe discovered in 2001, including two at $z>1$, with deep HST optical/IR light curves and ground-based near-IR photometry. All light curves are re-fitted with the SALT2 model, and a comprehensive covariance-based treatment of systematic errors is implemented in the Union2 framework. The resulting cosmological constraints reaffirm a flat $\Lambda$CDM model with $w\approx -1$ and show improved precision in $0<z<1$; however, constraints on dark energy evolution at $z>1$ remain weak without significantly larger high-$z$ SN samples. The work also updates HST calibrations and demonstrates the value of near-IR data for high-$z$ SN colors, highlighting the ongoing need for high-quality, high-$z$ SN measurements.

Abstract

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 HST 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 VLT. 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 (Kowalski et al. 2008). 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 supernovae, the Union2 compilation. The flat concordance LambdaCDM 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=-1.035^{+0.055}_{-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 depend strongly on redshift. In particular, at z > 1, the existence and nature of dark energy are only weakly constrained by the data.

Spectra and Light Curves of Six Type Ia Supernovae at 0.511 < z < 1.12 and the Union2 Compilation

TL;DR

We extend the public SN Ia cosmology datasets by adding six high- SNe discovered in 2001, including two at , with deep HST optical/IR light curves and ground-based near-IR photometry. All light curves are re-fitted with the SALT2 model, and a comprehensive covariance-based treatment of systematic errors is implemented in the Union2 framework. The resulting cosmological constraints reaffirm a flat CDM model with and show improved precision in ; however, constraints on dark energy evolution at remain weak without significantly larger high- SN samples. The work also updates HST calibrations and demonstrates the value of near-IR data for high- SN colors, highlighting the ongoing need for high-quality, high- SN measurements.

Abstract

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 HST 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 VLT. 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 (Kowalski et al. 2008). 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 supernovae, the Union2 compilation. The flat concordance LambdaCDM 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=-1.035^{+0.055}_{-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 depend strongly on redshift. In particular, at z > 1, the existence and nature of dark energy are only weakly constrained by the data.

Paper Structure

This paper contains 40 sections, 22 equations, 17 figures, 13 tables.

Table of Contents

  1. Introduction
  2. Search, discovery and spectroscopic confirmation
  3. ESI
  4. LRIS
  5. Spectral fitting and supernova typing
  6. Photometric observations
  7. Ground-based optical observations and reductions
  8. Ground-based IR observations and reduction
  9. HST observations and reduction
  10. Photometry of the ground-based data
  11. Calibration of the ground-based data
  12. Photometry of the HST data
  13. Light curve fitting
  14. SALT2
  15. The Union2 Compilation
  16. ...and 25 more sections

Figures (17)

  • Figure 1: SN finding charts. North is up and East is to the left. The SNe are marked with red cross hairs. The images have been created by stacking all $I$-band data taken with the search instrument and all F814W data obtained with WFPC2. The patch widths are $1'$ and $0.5'$ for the ground-based and HST images respectively.
  • Figure 2: Illustration of filters (colored) used in the SN campaign together with the rest frame Bessel filters (gray). The red (ground-based) and blue (HST) filters were used for the observations of the three SNe below ( upper panel) and above ( lower panel) $z=0.7$ respectively. The filters have been blueshifted to the mean of the two sub samples. Note that the Bessel filters shown here differ slightly from the corresponding filters used at the different telescopes. Not shown in the figure is the SuprimeCam $i$ filter, which is close to the $I$ filter and was only used for 2001cw. We show the $J$ band filter curve for NIRI which is very similar to the ISAAC Js filter. The atmospheric transmission for 2 mm of precipitable water vapor, typical for Paranal and Mauna Kea, is also plotted between $10000-16000$Å in the observer frame. The transmission curve was provided by Alain Smette (private communication).
  • Figure 4: VLT/ISAAC $J$ band observations of the distant SN Ia, 2001gn (North is up and East is to the left). On the left, we show a $27"$ wide and $51"$ high image with the SN; in the middle we show the reference image, which was taken 2 years later. The images on the right is the subtraction between the two and shows the SN with a signal-to-noise ratio of 10. Each image is the result of 10 hours of integration in good conditions. The image quality in the left hand image is FWHM=$0.39"$.
  • Figure 5: Stars observed both by Peter Stetson 2000PASP..112..925S and SDSS for which $-0.5 < r_\mathrm{SDSS}-i_\mathrm{SDSS} < 1.0$ together with fitted linear relations between the SDSS $r_\mathrm{SDSS\xspace},i_\mathrm{SDSS\xspace}$ and the Landolt $R, I$ magnitudes. The left column shows the relation as a function of color, while the middle column shows the residuals once the fitted relation has been subtracted, and a histogram of the residuals are presented in the right column.
  • Figure 6: SALT2 light curve fits together with the data for the six SNe presented here. Light curves for different bands/instruments, indicated by the color coding, have been offset. The dotted lines following the curves represents the model errors.
  • ...and 12 more figures