1301.3010 (Alexia Gorecki et al.)

A new method to improve photometric redshift reconstruction. Applications to the Large Synoptic Survey Telescope    [PDF]

Alexia Gorecki, Alexandra Abate, Réza Ansari, Aurélien Barrau, Sylvain Baumont, Marc Moniez, Jean-Stéphane Ricol

In the next decade the Large Synoptic Survey Telescope 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. Reconstruction of the redshifts with high resolution and well-understood uncertainties is mandatory for many science goals, including the study of baryonic acoustic oscillations. We investigate different approaches to establish the accuracy that can be reached by the LSST six-band photometry. We construct a realistic mock galaxy catalog, based on the GOODS survey 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 methods is tested on real CFHTLS data and also on simulated catalogs. We describe a new technique to efficiently remove catastrophic outliers via a likelihood ratio statistical test that uses the posterior probability functions of the fit parameters and the colors. We show that the photometric redshift accuracy will meet the stringent LSST requirements up to redshift $\sim2.5$ after a selection based on the likelihood ratio test or a selection based on the apparent magnitude, for galaxies with $S/N>5$ 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. We conclude that the photometric redshifts will be accurately estimated with the LSST if a Bayesian prior probability and a calibration sample are used.

View original: http://arxiv.org/abs/1301.3010