PHAT: PHoto-zAccuracy Testing

Abstract

Context. Photometric redshifts (photo-z's) have become an essential tool in extragalactic astronomy. Many current and upcoming observing programmes require great accuracy of photo-z's to reach their scientific goals. Aims. Here we introduce PHAT, the PHoto-z Accuracy Testing programme, an international initiative to test and compare different methods of photo-z estimation. Methods. Two different test environments are set up, one (PHAT0) based on simulations to test the basic functionality of the different photo-z codes, and another one (PHAT1) based on data from the GOODS survey including 18-band photometry and ∼2000 spectroscopic redshifts.Results. The accuracy of the different methods is expressed and ranked by the global photo-z bias, scatter, and outlier rates. While most methods agree very well on PHAT0 there are differences in the handling of the Lyman-α forest for higher redshifts. Furthermore, different methods produce photo-z scatters that can differ by up to a factor of two even in this idealised case. A larger spread in accuracy is found for PHAT1. Few methods benefit from the addition of mid-IR photometry. The accuracy of the other methods is unaffected or suffers when IRAC data are included. Remaining biases and systematic effects can be explained by shortcomings in the different template sets (especially in the mid-IR) and the use of priors on the one hand and an insufficient training set on the other hand. Some strategies to overcome these problems are identified by comparing the methods in detail. Scatters of 4-8% in Δz/(1 + z) were obtained, consistent with other studies. However, somewhat larger outlier rates (>7.5% with Δz/(1 + z) > 0.15; >4.5% after cleaning) are found for all codes that can only partly be explained by AGN or issues in the photometry or the spec-z catalogue. Some outliers were probably missed in comparisons of photo-z's to other, less complete spectroscopic surveys in the past. There is a general trend that empirical codes produce smaller biases than template-based codes. Conclusions. The systematic, quantitative comparison of different photo-z codes presented here is a snapshot of the current state-ofthe-art of photo-z estimation and sets a standard for the assessment of photo-z accuracy in the future. The rather large outlier rates reported here for PHAT1 on real data should be investigated further since they are most probably also present (and possibly hidden) in many other studies. The test data sets are publicly available and can be used to compare new, upcoming methods to established ones and help in guiding future photo-z method development.

Properties ofz~ 3–6 Lyman break galaxies

“…For the first time Schaerer & de Barros (2009) include nebular emission to fit SEDs of a sample of Lyman break galaxies at z ∼ 6 and show that nebular lines strongly affect age estimation, since some lines can mimick a Balmer break. Ages are strongly reduced, which can lead one to reconsider star formation rate estimations from UV luminosity, since the standard relation used to convert UV luminosity into SFR, as previously explained, assumes a constant star formation activity during 100 Myr (Kennicutt 1998;Madau et al 1998). The analysis of a sample of z ∼ 6-8 LBGs observed with HST and Spitzer further demonstrates the potential impact of nebular emission on the physical parameters as derived from SED fits of high-z galaxies (Schaerer & de Barros 2010).…”

“…Therefore, most studies rely on SFR estimated from UV luminosity (e.g. Bouwens et al 2009) using standard UV-SFR relation (Kennicutt 1998;Madau et al 1998). However, UV photons are strongly affected by dust attenuation, which requires to infer properly dust attenuation to estimate the SFR, which can be done statistically for large samples relying on the relation between dust attenuation and UV continuum slope (β slope, e.g.…”

Joint measurement of lensing–galaxy correlations using SPT and DES SV data

“…Because photometric redshift estimation is challenging, potentially subject to systematic errors, and requires large spectroscopic training sets (e.g. Hildebrandt et al 2010), the possibility of using the joint measurement of w κg (θ) and w γ T g (θ) to constrain galaxy redshift distributions is appealing.…”

A cooperative approach among methods for photometric redshifts estimation: an application to KiDS data

“…A significant advance in comparing different methods was proposed in Hildebrandt et al (2010), through the so-called PHAT (PHoto-z Accuracy Testing) contest, which adopted the "blackbox" approach which is typical of proper benchmarking. They performed a homogeneous comparison of the performances, focusing the analysis on the photo-z methods themselves, and setting an effective standard for the assessment of photo-z accuracy.…”