2019A&A...621A..26P

We developed a deep convolutional neural network (CNN), used as a classifier, to estimate photometric redshifts and associated probability distribution functions (PDF) for galaxies in the Main Galaxy Sample of the Sloan Digital Sky Survey at z<0.4. Our method exploits all the information present in the images without any feature extraction. The input data consist of 64 x 64 pixel ugriz images centered on the spectroscopic targets, plus the galactic reddening value on the line-of-sight. For training sets of 100k objects or more (≥20% of the database), we reach a dispersion σ_MAD<0.01, significantly lower than the current best one obtained from another machine learning technique on the same sample. The bias is lower than 10^–4, independent of photometric redshift. The PDFs are shown to have very good predictive power. We also find that the CNN redshifts are unbiased with respect to galaxy inclination, and that σ_MAD decreases with the signal-to-noise ratio (S/N), achieving values below 0.007 for S/N>100, as in the deep stacked region of Stripe 82. We argue that for most galaxies the precision is limited by the S/N of SDSS images rather than by the method. The success of this experiment at low redshift opens promising perspectives for upcoming surveys.