Lyman-alpha forest catalog


This value added catalog (VAC) contains the estimated fluctuations of the transmitted flux fraction in quasars used for the final DR16 cosmological study. Estimates are performed in the Lyman-alpha and Lyman-beta spectral regions of DR16 quasars. More than 210,000 lines-of-sight contain information on absorption in the Lyman-alpha spectral regions and more than 70,000 in the Lyman-beta regions. This VAC contains everything that is needed to compute the 3D auto-correlation of Lyman-alpha absorption in these two spectral regions. When combined with the DR16Q quasar catalog, this VAC also allows the computation of the 3D quasar x Lyman-alpha cross-correlation. These two correlations are used to measure the Baryonic Acoustic Oscillations at z ~ 2.33. The methodology to produce this catalog is largely based on Bautista et al. (2017), du Mas des Bourboux et al. (2017), de Sainte Agathe et al. (2019), and Blomqvist et al. (2019). The final BOSS/eBOSS cosmological study with this sample is found in du Mas des Bourboux et al. (2020).

Catalog content

These two catalogs were produced using picca, the “Package for Igm Cosmological-Correlations Analyses” (tag with a version for the DR16 study). The detailed description of the data model is available here. The demonstration of this sample applied to the clustering analysis can be found in du Mas des Bourboux et al. (2020). The data are located here.

Description of the files

The two catalogs are produced independently. They are used in BAO analyses to extract the fluctuations of Lyman-alpha absorption, but were produced without any assumption on the nature of the absorption. To optimize read time and to simplify the structure, the BOSS/eBOSS footprint is split into HEALPix pixels with the RING ordering. The catalog for the Lyman-alpha region was split into HEALPix pixels with nside=8 and the catalog for the Lyman-beta spectral region was split according to nside=4. All lines-of-sight that lie within a given HEALpix pixel are written to a FITS file named according to the convention delta-<healpix>.fits.gz, where <healpix> is the index of the pixel. For the Lyman-alpha spectral region, the 247 files with the fluctuations of transmitted flux fraction are given in the repository Delta_LYA/Delta/. For the Lyman-beta spectral region, the 76 files are in the repository Delta_LYB/Delta/.

Each file is composed of one HDU per line-of-sight, starting with HDU=1. The external name of this HDU is given by the quasar catalog identification integer: EXTNAME=THING_ID. In the file header, the different quantities RA, DEC, Z, THING_ID, PLATE, MJD, FIBERID are propagated from the quasar redshift catalog, where Z is the PCA redshift of DR16. The keyword PMF corresponds to the PLATE-MJD-FIBERID combination. The keyword ORDER is the order of the log_{10}(\lambda) polynomial used for the continuum fit. The data columns of the HDU give: LOGLAM, the common logarithm of the observed wavelength, log_{10}(\lambda), DELTA, the measured fluctuations of transmitted flux fraction, \delta, and its associated weight WEIGHT,
and finally CONT, the estimated continuum of the quasar spectrum.

Previous data releases

We always recommend that you use the latest version of the catalogs, which is DR16. Older versions of previous data releases are however still available.

The DR11 Lya catalog is described at the DR11 Quasar-Lyman-alpha mock catalogs page. A tutorial for the required MockExpander software can he found at the DR11 Lyman-alpha MockExpander Tutorial pate.

The DR9 Lya catalog is described DR9 Lyman-alpha forest sample page.