New Measurements of the Lyα Forest Continuum and Effective Optical Depth with LyCAN and DESI Y1 Data

Journal Article · 19 November 2024 · The Astrophysical Journal

DOI: https://doi.org/10.3847/1538-4357/ad8239 · OSTI ID:2478031

Turner, Wynne ; Martini, Paul ; Karaçaylı, Naim Göksel ; Aguilar, J.; Ahlen, S. ; Brooks, D. ; Claybaugh, T.; de la Macorra, A. ; Dey, A. ; Doel, P.; Fanning, K. ; Forero-Romero, J. .. E. ; Gontcho A Gontcho, S. ; Gonzalez-Morales, A. X. ; Gutierrez, G.; Guy, J. ; Herrera-Alcantar, H. K. ; Honscheid, K.; Juneau, S. ; Kisner, T. ; Kremin, A. ; Lambert, A.; Landriau, M. ; Le Guillou, L. ; Meisner, A. ; Miquel, R. ; Moustakas, J. ; Mueller, E.; Muñoz-Gutiérrez, A.; Myers, A. D.; Nie, J. ; Niz, G. ; Poppett, C. ; Prada, F. ; Rezaie, M. ; Rossi, G.; Sanchez, E. ; Schlafly, E. F. ; Schlegel, D. ; Schubnell, Michael F.; Seo, H. ; Sprayberry, D. ; Tarlé, G. ; Weaver, B. A.; Zou, H.

Abstract We present the Ly α 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 Ly α 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 $\tau {\left(z\right)={\tau }_0(1+z)}^{\gamma }$ to our measurements and find τ ₀ = (2.46 ± 0.14) × 10 ⁻³ and γ = 3.62 ± 0.04. Our results show particular agreement with high-resolution, ground-based observations around z = 2, indicating that LyCAN is able to predict the quasar continuum in the forest region with only spectral information outside the forest.

• Sponsoring Organization: USDOE
• Grant/Contract Number: NONE; SC0011726
• OSTI ID: 2478031
• Journal Information: The Astrophysical Journal, Journal Name: The Astrophysical Journal Journal Issue: 1 Vol. 976; ISSN 0004-637X
• Publisher: American Astronomical SocietyCopyright Statement
• Country of Publication: United States
• Language: English