New Measurements of the Lyα Forest Continuum and Effective Optical Depth with LyCAN and DESI Y1 Data
Juneau, S.; Moustakas, J.; Brooks, D.; Doel, P.; Gutierrez, G.; Honscheid, K.; Miquel, R.; Sanchez, E.; Dey, A.; Martini, Paul; Poppett, C.; Schlegel, D.; Meisner, A.; Aguilar, J.; Ahlen, S.; de la Macorra, A.; Gontcho A Gontcho, S.; Guy, J.; Kisner, T.; Kremin, A.; Landriau, M.; Le Guillou, L.; Prada, F.; Schlafly, E. F.; Zou, H.; Fanning, K.; Weaver, B. A.; Claybaugh, T.; Nie, J.; Rezaie, M.; Rossi, G.; Tarlé, G.; Myers, A. D.; Forero-Romero, J. E.; Lambert, A.; Seo, H.; Muñoz-Gutiérrez, A.; Sprayberry, D.; Mueller, E.; Niz, G.; Gonzalez-Morales, A. X.; Herrera-Alcantar, H. K.; Turner, Wynne; Karaçaylı, Naim Göksel; Schubnell, Michael F.
United States, United Kingdom, Mexico, Colombia, México, France, Spain, China, South Korea
Abstract
We present the Lyα Continuum Analysis Network (LyCAN), a convolutional neural network that predicts the unabsorbed quasar continuum within the rest-frame wavelength range of 1040–1600 Å based on the red side of the Lyα emission line (1216–1600 Å). We developed synthetic spectra based on a Gaussian mixture model representation of nonnegative matrix factorization (NMF) coefficients. These coefficients were derived from high-resolution, low-redshift (z < 0.2) Hubble Space Telescope/Cosmic Origins Spectrograph (COS) quasar spectra. We supplemented this COS-based synthetic sample with an equal number of DESI Year 5 mock spectra. LyCAN performs extremely well on testing sets, achieving a median error in the forest region of 1.5% on the DESI mock sample, 2.0% on the COS-based synthetic sample, and 4.1% on the original COS spectra. LyCAN outperforms principal component analysis (PCA) and NMF-based prediction methods using the same training set by 40% or more. We predict the intrinsic continua of 83,635 DESI Year 1 spectra in the redshift range of 2.1 ≤ z ≤ 4.2 and perform an absolute measurement of the evolution of the effective optical depth. This is the largest sample employed to measure the optical depth evolution to date. We fit a power law of the form