Reconstructing Quasar Spectra and Measuring the Ly$α$ Forest with ${\rm S{\scriptsize pender}Q}$
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2025
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| author | Hahn, ChangHoon Gontcho, Satya Gontcho A Melchior, Peter Herrera-Alcantar, Hiram K. Aguilar, Jessica Nicole Ahlen, Steven Bianchi, Davide Brooks, David Claybaugh, Todd de la Macorra, Axel Dey, Arjun Doel, Peter Forero-Romero, Jaime E. Gutierrez, Gaston Ishak, Mustapha Juneau, Stephanie Kirkby, David Kisner, Theodore Kremin, Anthony Lambert, Andrew Landriau, Martin Guillou, Laurent Le Manera, Marc Miquel, Ramon Moustakas, John Myers, Adam D. Niz, Gustavo Palanque-Delabrouille, Nathalie Poppett, Claire Prada, Francisco Pérez-Ràfols, Ignasi Rossi, Graziano Sanchez, Eusebio Schlegel, David Schubnell, Michael Seo, Hee-Jong Sprayberry, David Tarlé, Gregory Weaver, Benjamin A. Zou, Hu |
| author_facet | Hahn, ChangHoon Gontcho, Satya Gontcho A Melchior, Peter Herrera-Alcantar, Hiram K. Aguilar, Jessica Nicole Ahlen, Steven Bianchi, Davide Brooks, David Claybaugh, Todd de la Macorra, Axel Dey, Arjun Doel, Peter Forero-Romero, Jaime E. Gutierrez, Gaston Ishak, Mustapha Juneau, Stephanie Kirkby, David Kisner, Theodore Kremin, Anthony Lambert, Andrew Landriau, Martin Guillou, Laurent Le Manera, Marc Miquel, Ramon Moustakas, John Myers, Adam D. Niz, Gustavo Palanque-Delabrouille, Nathalie Poppett, Claire Prada, Francisco Pérez-Ràfols, Ignasi Rossi, Graziano Sanchez, Eusebio Schlegel, David Schubnell, Michael Seo, Hee-Jong Sprayberry, David Tarlé, Gregory Weaver, Benjamin A. Zou, Hu |
| contents | Quasar spectra carry the imprint of foreground intergalactic medium (IGM) through absorption features. In particular, absorption caused by neutral hydrogen gas, the ``Ly$α$ forest,'' is a key spectroscopic tracer for cosmological analyses used to measure cosmic expansion and test physics beyond the standard model. Despite their importance, current methods for measuring LyA absorption cannot directly derive the intrinsic quasar continuum and make strong assumptions on its shape, thus distorting the measured LyA clustering. We present SpenderQ, a ML-based approach for directly reconstructing the intrinsic quasar spectra and measuring the LyA forest from observations. SpenderQ uses the Spender spectrum autoencoder to learn a compact and redshift-invariant latent encoding of quasar spectra, combined with an iterative procedure to identify and mask absorption regions. To demonstrate its performance, we apply SpenderQ to 400,000 synthetic quasar spectra created to validate the Dark Energy Spectroscopic Instrument Year 1 LyA cosmological analyses. SpenderQ accurately reconstructs the true intrinsic quasar spectra, including the broad LyB, LyA, SiIV, CIV, and CIII emission lines. Redward of LyA, SpenderQ provides percent-level reconstructions of the true quasar spectra. Blueward of LyA, SpenderQ reconstructs the true spectra to < 5\%. SpenderQ reproduces the shapes of individual quasar spectra more robustly than the current state-of-the-art. We, thus, expect it will significantly reduce biases in LyA clustering measurements and enable studies of quasars and their physical properties. SpenderQ also provides informative latent variable encodings that can be used to, e.g., classify quasars with Broad Absorption Lines. Overall, SpenderQ provides a new data-driven approach for unbiased LyA forest measurements in cosmological, quasar, and IGM studies. |
| format | Preprint |
| id |
arxiv_https___arxiv_org_abs_2506_18986 |
| institution | arXiv |
| publishDate | 2025 |
| record_format | arxiv |
| spellingShingle | Reconstructing Quasar Spectra and Measuring the Ly$α$ Forest with ${\rm S{\scriptsize pender}Q}$ Hahn, ChangHoon Gontcho, Satya Gontcho A Melchior, Peter Herrera-Alcantar, Hiram K. Aguilar, Jessica Nicole Ahlen, Steven Bianchi, Davide Brooks, David Claybaugh, Todd de la Macorra, Axel Dey, Arjun Doel, Peter Forero-Romero, Jaime E. Gutierrez, Gaston Ishak, Mustapha Juneau, Stephanie Kirkby, David Kisner, Theodore Kremin, Anthony Lambert, Andrew Landriau, Martin Guillou, Laurent Le Manera, Marc Miquel, Ramon Moustakas, John Myers, Adam D. Niz, Gustavo Palanque-Delabrouille, Nathalie Poppett, Claire Prada, Francisco Pérez-Ràfols, Ignasi Rossi, Graziano Sanchez, Eusebio Schlegel, David Schubnell, Michael Seo, Hee-Jong Sprayberry, David Tarlé, Gregory Weaver, Benjamin A. Zou, Hu Cosmology and Nongalactic Astrophysics Astrophysics of Galaxies Quasar spectra carry the imprint of foreground intergalactic medium (IGM) through absorption features. In particular, absorption caused by neutral hydrogen gas, the ``Ly$α$ forest,'' is a key spectroscopic tracer for cosmological analyses used to measure cosmic expansion and test physics beyond the standard model. Despite their importance, current methods for measuring LyA absorption cannot directly derive the intrinsic quasar continuum and make strong assumptions on its shape, thus distorting the measured LyA clustering. We present SpenderQ, a ML-based approach for directly reconstructing the intrinsic quasar spectra and measuring the LyA forest from observations. SpenderQ uses the Spender spectrum autoencoder to learn a compact and redshift-invariant latent encoding of quasar spectra, combined with an iterative procedure to identify and mask absorption regions. To demonstrate its performance, we apply SpenderQ to 400,000 synthetic quasar spectra created to validate the Dark Energy Spectroscopic Instrument Year 1 LyA cosmological analyses. SpenderQ accurately reconstructs the true intrinsic quasar spectra, including the broad LyB, LyA, SiIV, CIV, and CIII emission lines. Redward of LyA, SpenderQ provides percent-level reconstructions of the true quasar spectra. Blueward of LyA, SpenderQ reconstructs the true spectra to < 5\%. SpenderQ reproduces the shapes of individual quasar spectra more robustly than the current state-of-the-art. We, thus, expect it will significantly reduce biases in LyA clustering measurements and enable studies of quasars and their physical properties. SpenderQ also provides informative latent variable encodings that can be used to, e.g., classify quasars with Broad Absorption Lines. Overall, SpenderQ provides a new data-driven approach for unbiased LyA forest measurements in cosmological, quasar, and IGM studies. |
| title | Reconstructing Quasar Spectra and Measuring the Ly$α$ Forest with ${\rm S{\scriptsize pender}Q}$ |
| topic | Cosmology and Nongalactic Astrophysics Astrophysics of Galaxies |
| url | https://arxiv.org/abs/2506.18986 |