CFHTLenS: Improving the quality of photometric redshifts with precision photometry

By H. Hildebrandt, T. Erben, K. Kuijken, L. Waerbeke, Catherine Heymans, Jean Coupon, J. Benjamin, C. Bonnett, L. Fu, H. Hoekstra and 9 more. 2011

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Here we present the results of various approaches to measure accurate colours and photometric redshifts (photo-z’s) from wide-field imaging data. We use data from the Canada-France-Hawaii-Telescope Legacy Survey (CFHTLS) which have been re- processed by the CFHT Lensing Survey (CFHTLenS) team in order to carry out a number of weak gravitational lensing studies. An emphasis is put on the correction of systematic effects in the photo-z’s arising from the different Point Spread Functions (PSF) in the five optical bands. Different ways of correcting these effects are discussed and the resulting photo-z accuracies are quantified by comparing the photo-z’s to large spectroscopic redshift (spec-z) data sets. Careful homogenisation of the PSF between bands leads to increased overall accuracy of photo-z’s. The gain is particularly pronounced at fainter magnitudes where galaxies are smaller and flux measurements are affected more by PSF-effects. We also study possible re- calibrations of the photometric zeropoints (ZPs) with the help of galaxies with known spec-z’s. We find that if PSF-effects are properly taken into account, a re-calibration of the ZPs becomes much less important suggesting that previous such re-calibrations described in the literature could in fact be mostly corrections for PSF-effects rather than corrections for real inaccuracies in the ZPs. The implications of this finding for future surveys like KiDS, DES, LSST, or Euclid are mixed. On the one hand, ZP re-calibrations with spec-z’s might not be as accurate as previously thought. On the other hand, careful PSF homogenisation might provide a way out and yield accurate, homogeneous photometry without the need for full spectroscopic coverage. This is the first paper in a series describing the technical aspects of CFHTLenS. (abridged)

Subfields: Astronomy and Astrophysics, Instrumentation

View Paper: https://www.semanticscholar.org/paper/57656c3d1cb907adb17f0512afcef7db66279207

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Citations in the corpus

CFHTLenS: testing the laws of gravity with tomographic weak lensing and redshift-space distortions (2012, methodology): “The CFHTLenS analysis represents the current state-of-the-art in weak lensing data processing (Erben et al. 2009), weak lensing measurement (Miller et al. 2012), photometric redshift measurement (Hildebrandt et al. 2012), and systematic error analysis (Heymans et al. 2012).”
CFHTLenS: the Canada–France–Hawaii Telescope Lensing Survey – imaging data and catalogue products (2012, methodology): “The accuracy of the resulting photometric redshifts for the bright end of our pure galaxy sample can be established by comparing photometric redshift estimates to spectroscopic redshifts in the field (Hildebrandt et al. 2012).”
COSMOS2020: A Panchromatic View of the Universe to z ∼ 10 from Two Complementary Catalogs (2021, methodology): “While this approach is widely applied in the literature (e.g., Hildebrandt et al. 2012; Sawicki & Yee 1998), other approaches avoid this homogenization process in order to preserve the original PSFs.”
Galaxy Alignments: Observations and Impact on Cosmology (2015, methodology): “Therefore the signal is expected to be larger than that of disc galaxies, which is indeed the case, as shown already by Hirata et al. (2007). Joachimi et al. (2011) studied intrinsic alignments in LRGs from the SDSS MegaZ-LRG dataset.”
KiDS-450: testing extensions to the standard cosmological model (2016, background): “Beyond the KiDS–Planck discordance, earlier lensing observations by the Canada–France–Hawaii Telescope Lensing Survey (CFHTLenS; Heymans et al. 2012; Hildebrandt et al. 2012; Erben et al. 2013; Miller et al. 2013) were also found to exhibit a similar tension with Planck (e.g. Planck Collaboration…”
CFHTLenS revisited: assessing concordance with Planck including astrophysical systematics (2016, methodology): “…systematic uncertainties affecting the lensing observables from the Canada–France–Hawaii Telescope Lensing Survey (CFHTLenS; Heymans et al. 2012; Hildebrandt et al. 2012; Erben et al. 2013; Miller et al. 2013) in the form of intrinsic galaxy alignments, baryonic effects in the non-linear…”
KiDS-450: cosmological parameter constraints from tomographic weak gravitational lensing (2016, methodology): “For photometric redshift estimation, we use the BPZ code (Benı́tez 2000), as described in Hildebrandt et al. (2012).”
Star-galaxy Classification Using Deep Convolutional Neural Networks (2016, methodology): “We note that, at these operating conditions (cg = 0.96 or ps = 0.97), both ConvNet and TPCmorph outperform the star-galaxy classification provided by the CFHTLenS pipeline (Hildebrandt et al. 2012) over all magnitudes.”
Photometric redshift analysis in the Dark Energy Survey Science Verification data (2014, methodology): “Of particular interest is the comparison to the CFHTLS and CFHTLenS (which are two different reductions of the same survey), photo-z results (Coupon et al. 2009; Hildebrandt et al. 2012), due to the similitudes between CFHTLS and DES in terms of survey specifications.”
Lensing is Low: Cosmology, Galaxy Formation, or New Physics? (2016, methodology): “Details on the image reduction, weak-lensing shape measurements and photometric redshifts can be found in Erben et al. (2013), Heymans et al. (2012), Miller et al. (2013) and Hildebrandt et al. (2012).”
Gravitational lensing analysis of the Kilo-Degree Survey (2015, methodology): “Compared to the image simulations, which are a good match to the SNR distribution of the CFHTLenS data, we also see a higher proportion of low-SNR galaxies in KiDS.”
THE COSMOS2015 CATALOG: EXPLORING THE 1 < z < 6 UNIVERSE WITH HALF A MILLION GALAXIES (2016, background): “This is because mag auto’s variable aperture is derived from the detection image, which means that fainter objects have can potentially have noisier colors (Hildebrandt et al. 2012; Moutard et al. 2016).”
KiDS-450: The tomographic weak lensing power spectrum and constraints on cosmological parameters (2017, methodology): “For the estimation of individual photometric redshifts for source galaxies, the code BPZ (Benı́tez 2000) is used following the description in Hildebrandt et al. (2012).”
The first and second data releases of the Kilo-Degree Survey (2015, methodology): “…redshifts are derived from the code (Benítez 2000; Coe et al. 2006) applied to the output from a PSF-homogenized photometry pipeline – again an evolution from the CFHTLenS analysis pipeline, see Hildebrandt et al. (2012) – which runs on the calibrated stacked images released in KiDS-ESO-DR1/2.”
Euclid preparation (2021, result): “Such biases are in agreement with the usual values observed in the literature with broadband data (e.g., Hildebrandt et al. 2012).”
Bayesian galaxy shape measurement for weak lensing surveys – III. Application to the Canada–France–Hawaii Telescope Lensing Survey (2012, methodology): “…of 66 762 SDSS discdominated galaxies selected from DR7 (Abazajian et al. 2009) to have g-band absolute magnitude Mg ≤ −19, to correspond in luminosity and waveband to galaxies selected at i′ ≤ 24.5 in CFHTLenS, for which the median redshift is estimated as z 0.66 (Hildebrandt et al. 2012).”