The Composite Spectrum of QSO Absorption Line Systems in DESI DR2

@article{napolitano2026composite,
  title = {{The Composite Spectrum of QSO Absorption Line Systems in DESI DR2}},
  author = {Lucas Napolitano and Adam D. Myers and Adam Tedeschi and Abhijeet Anand and Hiram K. Herrera-Alcantar and Jessica Aguilar and Steven Ahlen and Stephen Bailey and Segev BenZvi and Davide Bianchi and David Brooks and Todd Claybaugh and Andrei Cuceu and Axel de la Macorra and Arjun Dey and Biprateep Dey and Peter Doel and Andreu Font-Ribera and Jaime E. Forero-Romero and Enrique Gaztañaga and Satya Gontcho A Gontcho and Gaston Gutierrez and Julien Guy and Dick Joyce and Anthony Kremin and Martin Landriau and Laurent Le Guillou and Marc Manera and Aaron Meisner and Ramon Miquel and John Moustakas and Seshadri Nadathur and Nathalie Palanque-Delabrouille and Will Percival and Francisco Prada and Ignasi Pérez-Ràfols and Graziano Rossi and Eusebio Sanchez and David Schlegel and Michael Schubnell and Joesph Harry Silber and David Sprayberry and Gregory Tarlé and Benjamin Alan Weaver and Rongpu Zhou and Hu Zou},
  journal = {The Astronomical Journal},
  year = {2026},
  month = oct,
  doi = {10.3847/1538-3881/aea6da},
  url = {https://iopscience.iop.org/article/10.3847/1538-3881/aea6da},
  note = {CC BY 4.0},
}

TY  - JOUR
TI  - The Composite Spectrum of QSO Absorption Line Systems in DESI DR2
AU  - Lucas Napolitano
AU  - Adam D. Myers
AU  - Adam Tedeschi
AU  - Abhijeet Anand
AU  - Hiram K. Herrera-Alcantar
AU  - Jessica Aguilar
AU  - Steven Ahlen
AU  - Stephen Bailey
AU  - Segev BenZvi
AU  - Davide Bianchi
AU  - David Brooks
AU  - Todd Claybaugh
AU  - Andrei Cuceu
AU  - Axel de la Macorra
AU  - Arjun Dey
AU  - Biprateep Dey
AU  - Peter Doel
AU  - Andreu Font-Ribera
AU  - Jaime E. Forero-Romero
AU  - Enrique Gaztañaga
AU  - Satya Gontcho A Gontcho
AU  - Gaston Gutierrez
AU  - Julien Guy
AU  - Dick Joyce
AU  - Anthony Kremin
AU  - Martin Landriau
AU  - Laurent Le Guillou
AU  - Marc Manera
AU  - Aaron Meisner
AU  - Ramon Miquel
AU  - John Moustakas
AU  - Seshadri Nadathur
AU  - Nathalie Palanque-Delabrouille
AU  - Will Percival
AU  - Francisco Prada
AU  - Ignasi Pérez-Ràfols
AU  - Graziano Rossi
AU  - Eusebio Sanchez
AU  - David Schlegel
AU  - Michael Schubnell
AU  - Joesph Harry Silber
AU  - David Sprayberry
AU  - Gregory Tarlé
AU  - Benjamin Alan Weaver
AU  - Rongpu Zhou
AU  - Hu Zou
PY  - 2026
DA  - 2026/10/07
JO  - The Astronomical Journal
DO  - 10.3847/1538-3881/aea6da
UR  - https://iopscience.iop.org/article/10.3847/1538-3881/aea6da
AB  - We present details regarding a composite spectrum of quasar (QSO) absorption line systems. In this composite spectrum, we identify more than 70 absorption lines and observe oxygen and hydrogen emission features at a higher signal-to-noise ratio than any previous study. As the light from a distant quasar travels towards an observer, it may interact with the circumgalactic medium of an intervening galaxy, forming absorption lines. In order to maximize the signal of these absorption lines, we have selected a sample of 239,174 quasar spectra from the second data release of the Dark Energy Spectroscopic Instrument (DESI), each identified to have absorption lines resulting from such an interaction. By stacking these spectra in the rest frame of the absorption and calculating a median composite spectrum, we are able to isolate and enhance these absorption lines. We provide a full atlas of all detected absorption and emission lines as well as their fit centroids and equivalent width values. We additionally provide analysis of composites constructed using subsamples of absorbers in both redshift and Mg ii equivalent width bins. We find that there is minimal redshift evolution across the considered absorber samples. Additionally, we find that dense gas signatures, such as the presence of Na I absorption, are strongest in the highest equivalent width absorber sample, and that the strengths of the Si IV and C IV absorption lines show a relatively weak correlation with Mg II equivalent width. This atlas should aid in future studies investigating the compositions and physical conditions of these absorbers.
SN  - 0004-6256
SN  - 1538-3881
ER  -