Tamara M. Davis

University of Queensland , Brisbane, Queensland, Australia

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Publications (17)107.68 Total impact

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    ABSTRACT: We present the first scale-dependent measurements of the normalised growth rate of structure $f\sigma_{8}(k, z=0)$ using only the peculiar motions of galaxies. We use data from the 6-degree Field Galaxy Survey velocity sample (6dFGSv) together with a newly-compiled sample of low-redshift $(z < 0.07)$ type Ia supernovae. We constrain the growth rate in a series of $\Delta k \sim 0.03 h{\rm Mpc^{-1}}$ bins to $\sim35%$ precision, including a measurement on scales $>300 h^{-1}{\rm Mpc}$, which represents the largest-scale growth rate measurement to date. We find no evidence for a scale dependence in the growth rate, or any statistically significant variation from the growth rate as predicted by the Planck cosmology. Bringing all the scales together, we determine the normalised growth rate at $z=0$ to $\sim15%$ in a manner independent of galaxy bias and in excellent agreement with the constraint from the measurements of redshift-space distortions from 6dFGS. We pay particular attention to systematic errors. We point out that the intrinsic scatter present in Fundamental-Plane and Tully-Fisher relations is only Gaussian in logarithmic distance units; wrongly assuming it is Gaussian in linear (velocity) units can bias cosmological constraints. We also analytically marginalise over zero-point errors in distance indicators, validate the accuracy of all our constraints using numerical simulations, and we demonstrate how to combine different (correlated) velocity surveys using a matrix `hyper-parameter' analysis. Current and forthcoming peculiar velocity surveys will allow us to understand in detail the growth of structure in the low-redshift universe, providing strong constraints on the nature of dark energy.
    04/2014;
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    ABSTRACT: We measure the strongest current cosmological upper limit on the neutrino mass, by adding observations of the large-scale matter power spectrum from the WiggleZ Dark Energy Survey to observations of the cosmic microwave background data from the Planck surveyor, and measurements of the baryon acoustic oscillation scale. Together with the lower limit from neutrino oscillation experiments, the allowed range for the sum of neutrino masses is now 0.05 eV to 0.15 eV (95% confidence).
    06/2013;
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    ABSTRACT: We present the ultraviolet (UV) luminosity function of galaxies from the GALEX Medium Imaging Survey with measured spectroscopic redshifts from the first data release of the WiggleZ Dark Energy Survey. This sample selects galaxies with high star formation rates: at 0.6 < z < 0.9 the median star formation rate is at the upper 95th percentile of optically-selected (r<22.5) galaxies and the sample contains about 50 per cent of all NUV < 22.8, 0.6 < z < 0.9 starburst galaxies within the volume sampled. The most luminous galaxies in our sample (-21.0>M_NUV>-22.5) evolve very rapidly with a number density declining as (1+z)^{5\pm 1} from redshift z = 0.9 to z = 0.6. These starburst galaxies (M_NUV<-21 is approximately a star formation rate of 30 \msuny) contribute about 1 per cent of cosmic star formation over the redshift range z=0.6 to z=0.9. The star formation rate density of these very luminous galaxies evolves rapidly, as (1+z)^{4\pm 1}. Such a rapid evolution implies the majority of star formation in these large galaxies must have occurred before z = 0.9. We measure the UV luminosity function in 0.05 redshift intervals spanning 0.1<z<0.9, and provide analytic fits to the results. At all redshifts greater than z=0.55 we find that the bright end of the luminosity function is not well described by a pure Schechter function due to an excess of very luminous (M_NUV<-22) galaxies. These luminosity functions can be used to create a radial selection function for the WiggleZ survey or test models of galaxy formation and evolution. Here we test the AGN feedback model in Scannapieco et al. (2005), and find that this AGN feedback model requires AGN feedback efficiency to vary with one or more of the following: stellar mass, star formation rate and redshift.
    Monthly Notices of the Royal Astronomical Society 06/2013; 434(1). · 5.52 Impact Factor
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    ABSTRACT: We characterise the stellar masses and star formation rates in a sample of almost 40000 spectroscopically confirmed UV luminous galaxies at 0.3<z<1.0 selected from within the WiggleZ Dark Energy Survey. In particular, we match this UV bright population to wide-field infrared surveys such as the near infrared UKIDSS LAS and the mid infrared WISE All-Sky Survey. We find that ~30% of the UV luminous WiggleZ galaxies are detected at >5sigma in the UKIDSS-LAS at all redshifts. An even more luminous subset of 15% are also detected in the WISE 3.4 and 4.6um bands. We compute stellar masses for this very large sample of extremely blue galaxies and quantify the sensitivity of the stellar mass estimates to various assumptions made during the SED fitting. The median stellar masses are log10(M*/M0)=9.6\pm0.7, 10.2\pm0.5 and 10.4\pm0.4 for the IR-undetected, UKIDSS detected and UKIDSS+WISE detected galaxies respectively. We demonstrate that the inclusion of NIR photometry can lead to tighter constraints on the stellar masses. The mass estimates are found to be most sensitive to the inclusion of secondary bursts of star formation as well as changes in the stellar population synthesis models, both of which can lead to median discrepancies of the order of 0.3dex in the stellar masses. We find that the best-fit M/LK is significantly lower (by ~0.4 dex) than that predicted by simple optical colour based estimators, in particular for the bluer galaxies with younger best-fit ages. The WiggleZ galaxies have star formation rates of 3-10 M0/yr and mostly lie at the upper end of the main sequence of star-forming galaxies at these redshifts. Their rest-frame UV luminosities and stellar masses are comparable to both local compact UV-luminous galaxies as well as Lyman break galaxies at z~2-3.(abridged)
    Monthly Notices of the Royal Astronomical Society 02/2013; 431(3). · 5.52 Impact Factor
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    ABSTRACT: The growth history of large-scale structure in the Universe is a powerful probe of the cosmological model, including the nature of dark energy. We study the growth rate of cosmic structure to redshift $z = 0.9$ using more than $162{,}000$ galaxy redshifts from the WiggleZ Dark Energy Survey. We divide the data into four redshift slices with effective redshifts $z = [0.2,0.4,0.6,0.76]$ and in each of the samples measure and model the 2-point galaxy correlation function in parallel and transverse directions to the line-of-sight. After simultaneously fitting for the galaxy bias factor we recover values for the cosmic growth rate which are consistent with our assumed $\Lambda$CDM input cosmological model, with an accuracy of around 20% in each redshift slice. We investigate the sensitivity of our results to the details of the assumed model and the range of physical scales fitted, making close comparison with a set of N-body simulations for calibration. Our measurements are consistent with an independent power-spectrum analysis of a similar dataset, demonstrating that the results are not driven by systematic errors. We determine the pairwise velocity dispersion of the sample in a non-parametric manner, showing that it systematically increases with decreasing redshift, and investigate the Alcock-Paczynski effects of changing the assumed fiducial model on the results. Our techniques should prove useful for current and future galaxy surveys mapping the growth rate of structure using the 2-dimensional correlation function.
    Monthly Notices of the Royal Astronomical Society 02/2013; 430(2). · 5.52 Impact Factor
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    ABSTRACT: Neutrinos are one of the major puzzles in modern physics. Despite measurements of mass differences, the Standard Model of particle physics describes them as exactly massless. Additionally, recent measurements from both particle physics experiments and cosmology indicate the existence of more than the three Standard Model species. Here we review the cosmological evidence and its possible interpretations.
    Publications of the Astronomical Society of Australia 01/2013; · 3.12 Impact Factor
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    ABSTRACT: This paper presents cosmological results from the final data release of the WiggleZ Dark Energy Survey. We perform full analyses of different cosmological models using the WiggleZ power spectra measured at z=0.22, 0.41, 0.60, and 0.78, combined with other cosmological data sets. The limiting factor in this analysis is the theoretical modeling of the galaxy power spectrum, including nonlinearities, galaxy bias, and redshift-space distortions. In this paper we assess several different methods for modeling the theoretical power spectrum, testing them against the Gigaparsec WiggleZ simulations (GiggleZ). We fit for a base set of six cosmological parameters, {Ωbh2,ΩCDMh2,H0,τ,As,ns}, and five supplementary parameters {nrun,r,w,Ωk,∑mν}. In combination with the cosmic microwave background, our results are consistent with the ΛCDM concordance cosmology, with a measurement of the matter density of Ωm=0.29±0.016 and amplitude of fluctuations σ8=0.825±0.017. Using WiggleZ data with cosmic microwave background and other distance and matter power spectra data, we find no evidence for any of the extension parameters being inconsistent with their ΛCDM model values. The power spectra data and theoretical modeling tools are available for use as a module for CosmoMC, which we here make publicly available at http://smp.uq.edu.au/wigglez-data. We also release the data and random catalogs used to construct the baryon acoustic oscillation correlation function.
    Physical review D: Particles and fields 11/2012; 86(10).
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    ABSTRACT: The absolute neutrino mass scale is currently unknown, but can be constrained from cosmology. The WiggleZ high redshift star-forming blue galaxy sample is less sensitive to systematics from non-linear structure formation, redshift-space distortions and galaxy bias than previous surveys. We obtain a upper limit on the sum of neutrino masses of 0.60eV (95% confidence) for WiggleZ+Wilkinson Microwave Anisotropy Probe. Combining with priors on the Hubble Parameter and the baryon acoustic oscillation scale gives an upper limit of 0.29eV, which is the strongest neutrino mass constraint derived from spectroscopic galaxy redshift surveys.
    12/2011;
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    ABSTRACT: We analyze the effect that peculiar velocities have on the cosmological inferences we make using luminosity distance indicators, such as Type Ia supernovae. In particular we study the corrections required to account for (1) our own motion, (2) correlations in galaxy motions, and (3) a possible local under- or overdensity. For all of these effects we present a case study showing the impact on the cosmology derived by the Sloan Digital Sky Survey-II Supernova Survey (SDSS-II SN Survey). Correcting supernova (SN) redshifts for the cosmic microwave background (CMB) dipole slightly overcorrects nearby SNe that share some of our local motion. We show that while neglecting the CMB dipole would cause a shift in the derived equation of state of Δw ~ 0.04 (at fixed Ω m ), the additional local-motion correction is currently negligible (Δw 0.01). We then demonstrate a covariance-matrix approach to statistically account for correlated peculiar velocities. This down-weights nearby SNe and effectively acts as a graduated version of the usual sharp low-redshift cut. Neglecting coherent velocities in the current sample causes a systematic shift of Δw ~ 0.02. This will therefore have to be considered carefully when future surveys aim for percent-level accuracy and we recommend our statistical approach to down-weighting peculiar velocities as a more robust option than a sharp low-redshift cut.
    The Astrophysical Journal 10/2011; 741(1):67. · 6.73 Impact Factor
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    ABSTRACT: Even in a universe that is homogeneous on large scales, local density fluctuations can imprint a systematic signature on the cosmological inferences we make from distant sources. One example is the effect of a local under-density on supernova cosmology. Also known as a Hubble-bubble, it has been suggested that a large enough under-density could account for the supernova magnitude- redshift relation without the need for dark energy or acceleration. Although the size and depth of under-density required for such an extreme result is extremely unlikely to be a random fluctuation in an on-average homogeneous universe, even a small under-density can leave residual effects on our cosmological inferences. In this paper we show that there remain systematic shifts in our cosmological parameter measure- ments, even after excluding local supernovae that are likely to be within any small Hubble-bubble. We study theoretically the low-redshift cutoff typically imposed by supernova cosmology analyses, and show that a low-redshift cut of z0 \sim 0.02 may be too low based on the observed inhomogeneity in our local universe. Neglecting to impose any low-redshift cutoff can have a significant effect on the cosmological pa- rameters derived from supernova data. A slight local under-density, just 30% under-dense with scale 70h^{-1} Mpc, causes an error in the inferred cosmological constant density {\Omega}{\Lambda} of \sim 4%. Imposing a low-redshift cutoff reduces this systematic error but does not remove it entirely. A residual systematic shift of 0.99% remains in the inferred value {\Omega}{\Lambda} even when neglecting all data within the currently pre- ferred low-redshift cutoff of 0.02. Given current measurement uncertainties this shift is not negligible, and will need to be accounted for when future measurements yield higher precision. Comment: 12 pages, 11 figures. Accepted for publication in The Astrophysical Journal
    The Astrophysical Journal 06/2010; · 6.73 Impact Factor
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    ABSTRACT: The WiggleZ Dark Energy Survey is a survey of 240 000 emission-line galaxies in the distant Universe, measured with the AAOmega spectrograph on the 3.9-m Anglo-Australian Telescope (AAT). The primary aim of the survey is to precisely measure the scale of baryon acoustic oscillations (BAO) imprinted on the spatial distribution of these galaxies at look-back times of 4–8 Gyr.The target galaxies are selected using ultraviolet (UV) photometry from the Galaxy Evolution Explorer satellite, with a flux limit of NUV < 22.8 mag. We also require that the targets are detected at optical wavelengths, specifically in the range 20.0 < r < 22.5 mag. We use the Lyman break method applied to the UV colours, with additional optical colour limits, to select high-redshift galaxies. The galaxies generally have strong emission lines, permitting reliable redshift measurements in relatively short exposure times on the AAT. The median redshift of the galaxies is zmed= 0.6. The redshift range containing 90 per cent of the galaxies is 0.2 < z < 1.0.The survey will sample a volume of ∼1 Gpc3 over a projected area on the sky of 1000 deg2, with an average target density of 350 deg−2. Detailed forecasts indicate that the survey will measure the BAO scale to better than 2 per cent and the tangential and radial acoustic wave scales to approximately 3 and 5 per cent, respectively. Combining the WiggleZ constraints with existing cosmic microwave background measurements and the latest supernova data, the marginalized uncertainties in the cosmological model are expected to be σ(Ωm) = 0.02 and σ(w) = 0.07 (for a constant w model). The WiggleZ measurement of w will constitute a robust, precise and independent test of dark energy models.This paper provides a detailed description of the survey and its design, as well as the spectroscopic observations, data reduction and redshift measurement techniques employed. It also presents an analysis of the properties of the target galaxies, including emission-line diagnostics which show that they are mostly extreme starburst galaxies, and Hubble Space Telescope images, which show that they contain a high fraction of interacting or distorted systems. In conjunction with this paper, we make a public data release of data for the first 100 000 galaxies measured for the project.
    Monthly Notices of the Royal Astronomical Society 11/2009; 401(3):1429 - 1452. · 5.52 Impact Factor
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    ABSTRACT: We present measurements of the Hubble diagram for 103 Type Ia supernovae (SNe) with redshifts 0.04 < z < 0.42, discovered during the first season (Fall 2005) of the Sloan Digital Sky Survey-II (SDSS-II) Supernova Survey. These data fill in the redshift "desert" between low- and high-redshift SN Ia surveys. Within the framework of the MLCS2K2 light-curve fitting method, we use the SDSS-II SN sample to infer the mean reddening parameter for host galaxies, RV = 2.18 ± 0.14stat ± 0.48syst, and find that the intrinsic distribution of host-galaxy extinction is well fitted by an exponential function, P(AV ) = exp(–AV /τV), with τV = 0.334 ± 0.088 mag. We combine the SDSS-II measurements with new distance estimates for published SN data from the ESSENCE survey, the Supernova Legacy Survey (SNLS), the Hubble Space Telescope (HST), and a compilation of Nearby SN Ia measurements. A new feature in our analysis is the use of detailed Monte Carlo simulations of all surveys to account for selection biases, including those from spectroscopic targeting. Combining the SN Hubble diagram with measurements of baryon acoustic oscillations from the SDSS Luminous Red Galaxy sample and with cosmic microwave background temperature anisotropy measurements from the Wilkinson Microwave Anisotropy Probe, we estimate the cosmological parameters w and ΩM, assuming a spatially flat cosmological model (FwCDM) with constant dark energy equation of state parameter, w. We also consider constraints upon ΩM and ΩΛ for a cosmological constant model (ΛCDM) with w = –1 and non-zero spatial curvature. For the FwCDM model and the combined sample of 288 SNe Ia, we find w = –0.76 ± 0.07(stat) ± 0.11(syst), ΩM = 0.307 ± 0.019(stat) ± 0.023(syst) using MLCS2K2 and w = –0.96 ± 0.06(stat) ± 0.12(syst), ΩM = 0.265 ± 0.016(stat) ± 0.025(syst) using the SALT-II fitter. We trace the discrepancy between these results to a difference in the rest-frame UV model combined with a different luminosity correction from color variations; these differences mostly affect the distance estimates for the SNLS and HST SNe. We present detailed discussions of systematic errors for both light-curve methods and find that they both show data-model discrepancies in rest-frame U band. For the SALT-II approach, we also see strong evidence for redshift-dependence of the color-luminosity parameter (β). Restricting the analysis to the 136 SNe Ia in the Nearby+SDSS-II samples, we find much better agreement between the two analysis methods but with larger uncertainties: w = –0.92 ± 0.13(stat)+0.10 –0.33(syst) for MLCS2K2 and w = –0.92 ± 0.11(stat)+0.07 –0.15 (syst) for SALT-II.
    The Astrophysical Journal Supplement Series 10/2009; 185(1):32. · 16.24 Impact Factor
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    ABSTRACT: We present measurements of the Hubble diagram for 103 Type Ia supernovae (SNe) with redshifts 0.04 < z < 0.42, discovered during the first season (Fall 2005) of the Sloan Digital Sky Survey-II (SDSS-II) Supernova Survey. These data fill in the redshift "desert" between low- and high-redshift SN Ia surveys. We combine the SDSS-II measurements with new distance estimates for published SN data from the ESSENCE survey, the Supernova Legacy Survey, the Hubble Space Telescope, and a compilation of nearby SN Ia measurements. Combining the SN Hubble diagram with measurements of Baryon Acoustic Oscillations from the SDSS Luminous Red Galaxy sample and with CMB temperature anisotropy measurements from WMAP, we estimate the cosmological parameters w and Omega_M, assuming a spatially flat cosmological model (FwCDM) with constant dark energy equation of state parameter, w. For the FwCDM model and the combined sample of 288 SNe Ia, we find w = -0.76 +- 0.07(stat) +- 0.11(syst), Omega_M = 0.306 +- 0.019(stat) +- 0.023(syst) using MLCS2k2 and w = -0.96 +- 0.06(stat) +- 0.12(syst), Omega_M = 0.265 +- 0.016(stat) +- 0.025(syst) using the SALT-II fitter. We trace the discrepancy between these results to a difference in the rest-frame UV model combined with a different luminosity correction from color variations; these differences mostly affect the distance estimates for the SNLS and HST supernovae. We present detailed discussions of systematic errors for both light-curve methods and find that they both show data-model discrepancies in rest-frame $U$-band. For the SALT-II approach, we also see strong evidence for redshift-dependence of the color-luminosity parameter (beta). Restricting the analysis to the 136 SNe Ia in the Nearby+SDSS-II samples, we find much better agreement between the two analysis methods but with larger uncertainties. Comment: Accepted for publication by ApJS
    08/2009;
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    ABSTRACT: Using archival data of low-redshift (z < 0.01; Center for Astrophysics and SUSPECT databases) Type Ia supernovae (SNe Ia) and recent observations of high-redshift (0.16 < z < 0.64) SNe Ia, we study the "uniformity" of the spectroscopic properties of nearby and distant SNe Ia. We find no difference in the measurements we describe here. In this paper we base our analysis solely on line-profile morphology, focusing on measurements of the velocity location of maximum absorption (vabs) and peak emission (vpeak). Our measurement technique makes it easier to compare low and high signal-to-noise ratio observations. We also quantify the associated sources of error, assessing the effect of line blending with assistance from the parameterized code SYNOW. We find that the evolution of vabs and vpeak for our sample lines (Ca II λ3945, Si II λ6355, and S II λλ5454, 5640) is similar for both the low- and high-redshift samples. We find that vabs for the weak S II λλ5454, 5640 lines and vpeak for S II λ5454 can be used to identify fast-declining [Δm15(B) > 1.7] SNe Ia, which are also subluminous. In addition, we give the first direct evidence in two high-z SN Ia spectra of a double-absorption feature in Ca II λ3945, an event also observed, although infrequently, in low-redshift SN Ia spectra (6 out of 22 SNe Ia in our local sample). Moreover, echoing the recent studies of Dessart & Hillier in the context of Type II supernovae (SNe II), we see similar P Cygni line profiles in our large sample of SN Ia spectra. First, the magnitude of the velocity location at maximum profile absorption may underestimate that at the continuum photosphere, as observed, for example, in the optically thinner line S II λ5640. Second, we report for the first time the unambiguous and systematic intrinsic blueshift of peak emission of optical P Cygni line profiles in SN Ia spectra, by as much as 8000 km s-1. All the high-z SNe Ia analyzed in this paper were discovered and followed up by the ESSENCE collaboration and are now publicly available.
    The Astronomical Journal 12/2007; 131(3):1648. · 4.97 Impact Factor
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    ABSTRACT: It is now accepted that long-duration gamma-ray bursts (GRBs) are produced during the collapse of a massive star. The standard 'collapsar' model predicts that a broad-lined and luminous type Ic core-collapse supernova accompanies every long-duration GRB. This association has been confirmed in observations of several nearby GRBs. Here we report that GRB 060505 (ref. 10) and GRB 060614 (ref. 11) were not accompanied by supernova emission down to limits hundreds of times fainter than the archetypal supernova SN 1998bw that accompanied GRB 980425, and fainter than any type Ic supernova ever observed. Multi-band observations of the early afterglows, as well as spectroscopy of the host galaxies, exclude the possibility of significant dust obscuration and show that the bursts originated in actively star-forming regions. The absence of a supernova to such deep limits is qualitatively different from all previous nearby long-duration GRBs and suggests a new phenomenological type of massive stellar death.
    Nature 01/2007; 444(7122):1047-9. · 38.60 Impact Factor
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    ABSTRACT: We use published spectroscopic and photometric data for 8 Type Ia supernovae to construct a dispersion spectrum for this class of object, showing their diversity over the wavelength range 3700A to 7100A. We find that the B and V bands are the spectral regions with the least dispersion, while the U band below 4100A is more diverse. Some spectral features such as the Si line at 6150A are also highly diverse. We then construct two objective measures of 'peculiarity' by (i) using the deviation of individual objects from the average SN Ia spectrum compared to the typical dispersion and (ii) applying principle component analysis. We demonstrate these methods on several SNe Ia that have previously been classified as peculiar. Comment: 8 pages, 8 figures, uses mn2e.cls, accepted for publication by MNRAS
    Monthly Notices of the Royal Astronomical Society 05/2006; · 5.52 Impact Factor
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    Tamara M. Davis, Brian P. Schmidt, Alex G. Kim
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    ABSTRACT: To use type Ia supernovae as standard candles for cosmology we need accurate broadband magnitudes. In practice the observed magnitude may differ from the ideal magnitude-redshift relationship either through intrinsic inhomogeneities in the type Ia supernova population or through observational error. Here we investigate how we can choose filter bandpasses to reduce the error caused by both these effects. We find that bandpasses with large integral fluxes and sloping wings are best able to minimise several sources of observational error, and are also least sensitive to intrinsic differences in type Ia supernovae. The most important feature of a complete filter set for type Ia supernova cosmology is that each bandpass be a redshifted copy of the first. We design practical sets of redshifted bandpasses that are matched to typical high resistivity CCD and HgCdTe infra-red detector sensitivities. These are designed to minimise systematic error in well observed supernovae, final designs for specific missions should also consider signal-to-noise requirements and observing strategy. In addition we calculate how accurately filters need to be calibrated in order to achieve the required photometric accuracy of future supernova cosmology experiments such as the SuperNova-Acceleration-Probe (SNAP), which is one possible realisation of the Joint Dark-Energy mission (JDEM). We consider the effect of possible periodic miscalibrations that may arise from the construction of an interference filter. Comment: Accepted for publication in PASP, 22 pages (preprint format), 17 figures
    Publications of the Astronomical Society of the Pacific 11/2005; · 3.69 Impact Factor