A new method to improve photometric redshift reconstruction

Abstract

Context. In the next decade, the Large Synoptic Survey Telescope (LSST) will become a major facility for the astronomical community. However, accurately determining the redshifts of the observed galaxies without using spectroscopy is a major challenge. Aims. Reconstruction of the redshifts with high resolution and well-understood uncertainties is mandatory for many science goals, including the study of baryonic acoustic oscillations (BAO). We investigate different approaches to establish the accuracy that can be reached by the LSST six-band photometry. Methods. We construct a realistic mock galaxy catalog, based on the Great Observatories Origins Deep Survey (GOODS) luminosity function, by simulating the expected apparent magnitude distribution for the LSST. To reconstruct the photometric redshifts (photo-z's), we consider a template-fitting method and a neural network method. The photo-z reconstruction from both of these techniques is tested on real Canada-France-Hawaii Telescope Legacy Survey (CFHTLS) data and also on simulated catalogs. We describe a new method to improve photometric redshift reconstruction that efficiently removes catastrophic outliers via a likelihood ratio statistical test. This test uses the posterior probability functions of the fit parameters and the colors. Results. We show that the photometric redshift accuracy will meet the stringent LSST requirements up to redshift ∼2.5 after a selection that is based on the likelihood ratio test or on the apparent magnitude for galaxies with signal-to-noise ratio S /N > 5 in at least 5 bands. The former selection has the advantage of retaining roughly 35% more galaxies for a similar photo-z performance compared to the latter. Photo-z reconstruction using a neural network algorithm is also described. In addition, we utilize the CFHTLS spectro-photometric catalog to outline the possibility of combining the neural network and template-fitting methods. Conclusions. We demonstrate that the photometric redshifts will be accurately estimated with the LSST if a Bayesian prior probability and a calibration sample are used.

Apples to apples A2– I. Realistic galaxy simulated catalogues and photometric redshift predictions for next-generation surveys

“…Up to date, different studies have been performed to estimate the photometric redshift performance of different surveys (Oyaizu et al 2008;Banerji et al 2008;Benítez et al 2009a;Hildebrandt et al 2010;Bellagamba et al 2012;Dahlen et al 2013;Gorecki et al 2014;Molino et al 2014) with a variety of different techniques. Two approaches have been considered in these works.…”

Impact of photometric redshifts on the galaxy power spectrum and BAO scale in the LSST survey

“…The photo-z reconstruction is performed with a template fitting method presented in details in Gorecki et al (2014). The SED library is made of 51 templates used for the photometry computation.…”

“…The key ingredient for this step is the use of observed galaxy LF to populate each grid cell with a number of galaxies with absolute magnitudes and galaxy classes or spectral types following the LFs. Details of the procedure are provided in Gorecki et al (2014), the main difference being that we have used LF parameters derived by Zucca et al (2009) for the study presented here, instead of the one derived by Dahlen et al (2005) used in our previous work. Both sets of LFs provide the distribution of galaxies divided in broad types, in several redshift ranges.…”

“…We have improved this technique in the present work. The Likelihood Ratio method presented in Gorecki et al (2014) has been indeed replaced by a Boosted Decision Tree (BDT) algorithm based on the same discriminant parameters but more robust and performant. The parameters used in the BDT are:…”

Reconstructing large-scale structure with neutral hydrogen surveys

“…Translating a redshift uncertainty of δz into a comoving distance uncertainty of σ χ = c δz/H(z), to probe k = 0.03 h Mpc −1 requires δz/(1 + z) < 0.01 − 0.015 at z = 2 − 6. Such photo-z precision is in principle achievable, given enough filters, but primarily at lower redshift and for brighter galaxies [67,68]. The more common assumption of δz/(1 + z) = 0.05 corresponds to σ χ 120 h −1 Mpc 1 + z 5 −1/2 (6.1) for 2 ≤ z ≤ 6.…”