Harvesting the Ly α forest with convolutional neural networks

We develop a machine learning based algorithm using a convolutional neural network (CNN) to identify low H i column density Ly α absorption systems (log NH i/cm−2 < 17) in the Ly α forest, and predict their physical properties, such as their H i column density (log NH i/cm−2), redshift (zH i), and Doppler width (bH i). Our CNN models are trained using simulated spectra (S/N ≃ 10), and we test their performance on high quality spectra of quasars at redshift z ∼ 2.5−2.9 observed with the High Resolution Echelle Spectrometer on the Keck I telescope. We find that ${\sim}78{{\ \rm per\ cent}}$ of the systems identified by our algorithm are listed in the manual Voigt profile fitting catalogue. We demonstrate that the performance of our CNN is stable and consistent for all simulated and observed spectra with S/N ≳ 10. Our model can therefore be consistently used to analyse the enormous number of both low and high S/N data available with current and future facilities. Our CNN provides state-of-the-art predictions within the range 12.5 ≤ log NH i/cm−2 < 15.5 with a mean absolute error of Δ(log NH i/cm−2) = 0.13, Δ(zH i) = 2.7 × 10−5, and Δ(bH i) = 4.1 km s−1. The CNN prediction costs < 3 min per model per spectrum with a size of 120 000 pixels using a laptop computer. We demonstrate that CNNs can significantly increase the efficiency of analysing Ly α forest spectra, and thereby greatly increase the statistics of Ly α absorbers.