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CIV Absorber Catalog from DESI DR1 Quasars

Overview

This catalog contains the details of C IV absorbers (a doublet at rest-frame wavelengths 1548 and 1550 Å) detected in DESI DR1 quasar spectra.

Data Access

Data URL: https://data.desi.lbl.gov/public/dr1/vac/dr1/civ-absorber

NERSC Access: 

/global/cfs/cdirs/desi/public/dr1/vac/dr1/civ-absorber

Documentation

Notes on Absorber Catalog

  • This catalog is based on DESI DR1 Quasars and contains CIV absorbers identified in the spectra.
  • Rest-frame equivalent widths (EWs) are measured in Angstroms (Å).
  • Velocity dispersions (VDISP) are corrected for instrumental resolution and given in km/s.
  • In version qsoabsfind/v1.0.3 (used to construct the original catalog), a small bug affected the estimation of velocity dispersion; specifically, the instrumental resolution was underestimated by a factor of 2.355. All VDISP values have been recomputed in the current catalog using the correct resolution. As a result, <3.5% of systems now have VDISP = 0, indicating unresolved lines. Most of these are visually confirmed to be genuine absorbers and are therefore retained in the catalog and analysis.
  • This bug has been fixed in qsoabsfind/v1.0.5, and any future catalogs built from version >= v1.0.5 will automatically have correct velocity dispersions for the lines.
  • Column densities are measured using the Apparent Optical Depth Method (AODM; Savage & Sembach 1991).

Citation

If you use this catalog in your work, please cite Anand et al. (2025) and Anand, Nelson & Kauffmann (2021). These two papers describe the methodology, completeness, and limitations.

Code

The pipeline used to generate this catalog is open source and available at: GitHub/qsoabsfind.

Example Notebook

Two example notebooks with preliminary visualizations of the parent quasar and absorber properties are available.

Files and Data Model

  • CIV-Absorbers-dr1-v1.0.fits: This is the CIV absorber catalog file. It contains 32,321 CIV absorber systems detected in 94,986 Quasars from DESI DR1. Each entry corresponds to a detected absorber, with columns described in the data model below. Note that TARGETID is not a unique identifier, as multiple absorbers can be found along the line of sight to a single target. To uniquely specify an absorber, use the combination of TARGETID and Z_ABS.
  • CIV-Absorbers-parent-QSO-dr1-v1.0.fits: This is the DESI DR1 QSO catalog file. It contains 94,986 unique Quasars that were searched for CIV absorbers. The quasars were selected from the healpix-based DESI DR1 quasar catalog. A full description of the selection procedure is provided in the catalog paper.

Data Model for CIV-Absorbers-dr1-v1.0.fits

HDU1: ABSORBER

This HDU contains information on detected CIV absorbers.

Column Type Units Description
Z_ABS float64 Redshift of the absorber.
CIV_1548_EW float64 Å Rest-frame equivalent width (EW) of CIV 1548 (Å).
CIV_1550_EW float64 Å Rest-frame equivalent width (EW) of CIV 1550 (Å).
CIV_EW_TOTAL float64 Å Total rest-frame equivalent width (EW) of CIV doublet (Å).
CIV_1548_EW_ERROR float64 Å 1σ error on CIV 1548 EW (Å).
CIV_1550_EW_ERROR float64 Å 1σ error on CIV 1550 EW (Å).
CIV_EW_TOTAL_ERROR float64 Å 1σ error on total CIV doublet EW (Å).
Z_ABS_ERR float64 Measured error in the absorber redshift.
GAUSS_FIT float64[6] Rest-frame double Gaussian fitting parameters for the absorber doublet. (Amp1, line_center1, sigma1, Amp2, line_center2, sigma2). These correspond to the amplitudes, line centers, and Gaussian widths (σ) for the 1548 Å and 1550 Å components, respectively.
GAUSS_FIT_STD float64[6] Uncertainties in double Gaussian fitting parameters.
SN_CIV_1548 float64 SNR of 1548 line from residual and errors.
SN_CIV_1550 float64 SNR of 1550 line from residual and errors.
CIV_1548_VDISP float64 km/s Rest-frame intrinsic velocity dispersion (corrected for instrumental resolution) for CIV 1548 (km/s).
CIV_1550_VDISP float64 km/s Rest-frame intrinsic velocity dispersion (corrected for instrumental resolution) for CIV 1550 (km/s).
CORRECTED_N float64 Completeness-corrected absorber count.
LOG10N float64 cm⁻² (log10) log of total column density (in cm-2), calculated from apparent optical depth method.
SIG_LOG10N float64 cm⁻² (log10) uncertainty on log of total column density (in cm-2), calculated from apparent optical depth method.
SATURATION int64 saturation flag, 1: saturated, 0: unsaturated
fN int64 Column density measurement method, 1: WEIGHTED MEAN, 2: FIRST, 3: SECOND, 4: Corrected λ1550 (partial saturation), 5: Lower limit from λ1550 (strong saturation), 6: Lower limit from λ1548 (strong saturation and 1550 is not available) -1: FAIL.
HDU2: METADATA

This HDU contains metadata related to the observed quasars.

Column Type Units Description
LOS_ID int64 Line-of-sight (LOS) ID in picca continuum modeling (same as TARGETID).
RA float64 deg Right Ascension of quasars (degrees).
DEC float64 deg Declination of quasars (degrees).
Z_QSO float64 Redshift of quasars.
MEANSNR float64 Mean signal-to-noise ratio of the quasar spectrum from picca.
TARGETID int64 DESI target identifier for each object.
SURVEY char[4] Survey name in which the object was observed.
PROGRAM char[4] Program name in which the object was observed.
HPXPIXEL int64 Healpix ID for the spectrum, do HPXPIXEL//100 to get the healpix directory name.
ZQSO_ERR float64 Error on quasar redshift.
TSNR2_LRG float32 Template signal-to-noise ratio for each quasar. Can be used to calculate effective exposure time, Teff = 12.15 x TSNR2_LRG.
EFFTIME float32 s Effective exposure time in seconds, Teff = 12.15 x TSNR2_LRG.

Data Model for CIV-Absorbers-parent-QSO-dr1-v1.0.fits

HDU1: METADATA

This HDU contains information on parent quasars.

Column Type Units Description
LOS_ID int64 Line-of-sight (LOS) ID in picca continuum modeling (same as TARGETID).
RA float64 deg Right Ascension of quasars (degrees).
DEC float64 deg Declination of quasars (degrees).
Z_QSO float64 Redshift of quasars.
MEANSNR float64 Mean signal-to-noise ratio of the quasar spectrum from picca.
TARGETID int64 DESI target identifier for each object.
SURVEY char[4] Survey name in which the object was observed.
PROGRAM char[4] Program name in which the object was observed.
HPXPIXEL int64 Healpix ID for the spectrum, do HPXPIXEL//100 to get the healpix directory name.
ZQSO_ERR float64 Error on quasar redshift.
TSNR2_LRG float32 Template signal-to-noise ratio for each quasar. Can be used to calculate effective exposure time, Teff = 12.15 x TSNR2_LRG.
EFFTIME float32 s Effective exposure time in seconds, Teff = 12.15 x TSNR2_LRG.

Contact

Contact Abhijeet Anand for questions about this catalog.