Title: Deep learning Approach for Classifying, Detecting and Predicting Photometric Redshifts of Quasars in the Sloan Digital Sky Survey Stripe 82,
Abstract: We apply a convolutional neural network (CNN) to classify and detect quasars
in the Sloan Digital Sky Survey Stripe 82 and also to predict the photometric
redshifts of quasars. The network takes the variability of objects into account
by converting light curves into images. The width of the images, noted w,
corresponds to the five magnitudes ugriz and the height of the images, noted h,
represents the date of the observation. The CNN provides good results since its
precision is 0.988 for a recall of 0.90, compared to a precision of 0.985 for
the same recall with a random forest classifier. Moreover 175 new quasar
candidates are found with the CNN considering a fixed recall of 0.97. The
combination of probabilities given by the CNN and the random forest makes good
performance even better with a precision of 0.99 for a recall of 0.90.
For the redshift predictions, the CNN presents excellent results which are
higher than those obtained with a feature extraction step and different
classifiers (a K-nearest-neighbors, a support vector machine, a random forest
and a gaussian process classifier). Indeed, the accuracy of the CNN within
|\Delta z|<0.1 can reach 78.09%, within |\Delta z|<0.2 reaches 86.15%, within
|\Delta z|<0.3 reaches 91.2% and the value of rms is 0.359. The performance of
the KNN decreases for the three |\Delta z| regions, since within the accuracy
of |\Delta z|<0.1, |\Delta z|<0.2 and |\Delta z|<0.3 is 73.72%, 82.46% and
90.09% respectively, and the value of rms amounts to 0.395. So the CNN
successfully reduces the dispersion and the catastrophic redshifts of quasars.
This new method is very promising for the future of big databases like the
Large Synoptic Survey Telescope.