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The Composite Spectrum of QSO Absorption Line Systems in DESI DR2

Lucas Napolitano, Adam D. Myers, Adam Tedeschi, Abhijeet Anand, Hiram K. Herrera-Alcantar, Jessica Aguilar, Steven Ahlen, Stephen Bailey, Segev BenZvi, Davide Bianchi, David Brooks, Todd Claybaugh, Andrei Cuceu, Axel de la Macorra, Arjun Dey, Biprateep Dey, Peter Doel, Andreu Font-Ribera, Jaime E. Forero-Romero, Enrique Gaztanaga, Satya Gontcho A Gontcho, Gaston Gutierrez, Julien Guy, Dick Joyce, Anthony Kremin, Martin Landriau, Laurent Le Guillou, Marc Manera, Aaron Meisner, Ramon Miquel, John Moustakas, Seshadri Nadathur, Nathalie Palanque-Delabrouille, Will Percival, Francisco Prada, Ignasi Perez-Rafols, Graziano Rossi, Eusebio Sanchez, David Schlegel, Michael Schubnell, Joesph Harry Silber, David Sprayberry, Gregory Tarle, Benjamin Alan Weaver, Rongpu Zhou, Hu Zou

TL;DR

This work addresses the challenge of characterizing the circumgalactic gas traced by quasar absorption lines by building an extremely high-SNR median rest-frame composite from $N=238{,}838$ DESI DR2 Mg II absorber spectra and a matched control set. The authors shift spectra to the absorber restframe, resample to a common grid with $0.4$ Å spacing over roughly $[1030,7500]$ Å, and normalize using regions adjacent to the Mg II doublet, then extract a detailed atlas of 72 absorption lines from 26 species and 7 emission lines with robust line-fitting using Gaussian and Voigt profiles. They validate line identifications, discuss several challenging identifications, and demonstrate that detected emission features (notably [O II]) are intrinsically linked to the absorbers, requiring large sample sizes to recover. The resulting atlas and methods advance CGM studies, provide a resource for modeling absorber spectra, and set the stage for DESI DR3-based composite analyses. The work has implications for interpreting the Lyman-$\alpha$ forest and for understanding the physical conditions and metal enrichment of intervening galactic environments.

Abstract

We present details regarding the construction of 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 in any previous study. As the light from a distant quasar travels towards an observer, it may interact with the circumgalactic medium environment of an intervening galaxy, forming absorption lines. In order to maximize the signal of these absorption lines, we have selected a sample of 238,838 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 restframe 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. This atlas should aid in future studies investigating the compositions and physical conditions of these absorbers.

The Composite Spectrum of QSO Absorption Line Systems in DESI DR2

TL;DR

This work addresses the challenge of characterizing the circumgalactic gas traced by quasar absorption lines by building an extremely high-SNR median rest-frame composite from DESI DR2 Mg II absorber spectra and a matched control set. The authors shift spectra to the absorber restframe, resample to a common grid with Å spacing over roughly Å, and normalize using regions adjacent to the Mg II doublet, then extract a detailed atlas of 72 absorption lines from 26 species and 7 emission lines with robust line-fitting using Gaussian and Voigt profiles. They validate line identifications, discuss several challenging identifications, and demonstrate that detected emission features (notably [O II]) are intrinsically linked to the absorbers, requiring large sample sizes to recover. The resulting atlas and methods advance CGM studies, provide a resource for modeling absorber spectra, and set the stage for DESI DR3-based composite analyses. The work has implications for interpreting the Lyman- forest and for understanding the physical conditions and metal enrichment of intervening galactic environments.

Abstract

We present details regarding the construction of 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 in any previous study. As the light from a distant quasar travels towards an observer, it may interact with the circumgalactic medium environment of an intervening galaxy, forming absorption lines. In order to maximize the signal of these absorption lines, we have selected a sample of 238,838 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 restframe 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. This atlas should aid in future studies investigating the compositions and physical conditions of these absorbers.

Paper Structure

This paper contains 10 sections, 1 equation, 9 figures.

Table of Contents

  1. Introduction
  2. Data Methods
  3. Sample Selection
  4. Composite Construction
  5. Composite Reduction & Line Fitting
  6. Results and Discussion
  7. Line Fitting Results
  8. Challenging Line Identifications
  9. Examining Emission Line Detection
  10. Conclusion

Figures (9)

  • Figure 1: Histogram of absorber redshift values for the 238,838 absorption line systems in our sample. Bins are 0.1 wide in redshift space.
  • Figure 2: Histograms displaying the distributions of the redshift (top), TSNR2_QSO (middle) and W2 band flux values for both our absorber (light green) and control (blue) sample. Each histogram is drawn with 500 equally sized bins.
  • Figure 3: The number of spectra, that when resampled into the absorber restframe, cover each pixel of the composite's wavelength grid. The distribution peaks in the range [2795$\textrm{\AA}$, 2804$\textrm{\AA}$], which is the location of the Mg2 doublet, and is available in the restframe of all 238,838 absorption systems.
  • Figure 4: Median composite spectra calculated for both our sample of QSO spectra with a single detected Mg2 absorber, and our matched control sample of QSO spectra with no detected absorbers. Also presented is the error spectrum associated with the absorber sample composite. Note that we do not show the error spectrum associated with the control sample, as the two error spectra would appear nearly identical.
  • Figure 5: Visualization of the absorber sample continuum reduction method. Top: The result of dividing the absorber sample composite by the control sample composite, as seen in Figure \ref{['Initial-Comp']}. Middle: The result of masking the regions around the detected absorption and emission lines as described in the text. The purple line then shows the result of fitting a cubic B-spline with smoothness = 1.1 to the masked data. Bottom: The continuum-reduced absorber composite, calculated by dividing the product of the spline fit from the middle panel and the initial control sample composite.
  • ...and 4 more figures