Mg II Absorber Catalog¶
Overview¶
This value-added catalog contains summarized information regarding the detection and characterization of Mg II absorption systems detected in DESI quasar (QSO) spectra. The parent QSO catalog utilized is the afterburner catalog produced by Edmond Chaussidon. The code used to generate this catalog can be found on GitHub. For more information about the methods by which this catalog has been generated and its statistics verified see Napolitano et al. (2023).
Version 1.0 release notes¶
Adjusted line fitting technique. Lines are now fit using a QSO continuum estimated using NMF code. As a fallback, when a good continuum (\(\chi\)2 > 4.0) cannot be found using the NMF technique we use a wide (85 pixel) median filter. As a result the fit is reduced to 5 parameters from 7. Input QSO catalog has been adjusted to only include QSO targets from dark time observations in interest of higher purity. Additionally adjusted line_snr detection thresholds in effort to improve purity. Previously required candidate doublet lines to have SNR > 3 and > 1.5, respectively. By lowering the first value to > 2.5 we improve catalog completeness by nearly 10% with essentially no change to purity.
Data Access¶
Data URL: https://data.desi.lbl.gov/public/edr/vac/edr/mgii-absorber/v1.0/
NERSC access:
/global/cfs/cdirs/desi/public/edr/vac/edr/mgii-absorber/v1.0/
Documentation¶
Workflow¶
AllSteps_HPfiles are run used scripts stored in/Scripts. This defaults to running over 10 nodes with each handiling 10% of HEALPixel directories.Write_Abs_Catalogswrites individual HEALPixel-based Mg II absorber catalogs. Indiviudal catalogs are then combined into a per-release catalog.Eval_MgII_Catsgenerates a series of plots by which to evaluate the catalogs.
List of files¶
MgII-Absorbers-EDR.fits: Catalog file. One HDU with data namedMGII_ABSORBERS. 23,921 entries, each with the columns described in the data model below. Note that entries are not unique inTARGETID, as multiple absorbers can be found in the line-of-sight to a single target. Because of thisTARGETIDandZ_MGIIare needed to specify a unique entry.MgII-Absorbers-EDR-PI.fits: Catalog file. One HDU with data namedMGII_ABSORBERS. 108 entries, each with the columns described in the data model below. This catalog contains entries that have physically impossible (PI) absorption redshifts, i.e. those with a postive delta_v of greater than 5000 km/s relative to the “background” QSO. This sample has a noticeably lower purity, however does present oppurtunities to improve QSO classifications/redshifts as described in section 4.1 of the paper linked above.
Example of reading the catalog (python using fitsio):
import fitsio
Mgii_fp = '/global/cfs/cdirs/desi/public/edr/mgii-absorber/v1.0/MgII-Absorbers-EDR.fits'
MgII_cat = fitsio.read(Mgii_fp, 'MGII_ABSORBERS')
Data model¶
| Column Name | Format | Units | Description |
|---|---|---|---|
| TARGETID | int64 | – | Unique 64-bit identifier for each object observed by DESI |
| RA | double | deg | Right Ascension in decimal degrees (J2000) |
| DEC | double | deg | Declination in decimal degrees (J2000) |
| SURVEY | str | – | Survey validation stage (sv1, sv2, or sv3) |
| ZWARN | int64 | – | Redshift redshift warning bitmask |
| TSNR2_QSO | double | – | QSO weighted (S/N)2 sum over B, R, Z |
| TSNR2_LYA | double | – | LYA weighted (S/N)2 sum over B, R, Z |
| TSNR2_LRG | double | – | LRG weighted (S/N)2 sum over B, R, Z |
| Z_QSO | double | – | Redshift value from input QSO catalog (following afterburners) |
| EW_2796 | double | Angstrom | Central posterior value for the equivalent width of the 2796Å line of the MgII doublet |
| EW_2803 | double | Angstrom | Central posterior value for the equivalent width of the 2803Å line of the MgII doublet |
| EW_2796_ERR_LOW | double | Angstrom | Low error bar for the equivalent width of the 2796Å line of the Mg II doublet |
| EW_2803_ERR_LOW | double | Angstrom | Low error bar for the equivalent width of the 2803Å line of the Mg II doublet |
| EW_2796_ERR_HIGH | double | Angstrom | High error bar for the equivalent width of the 2796Å line of the Mg II doublet |
| EW_2803_ERR_HIGH | double | Angstrom | High error bar for the equivalent width of the 2803Å line of the Mg II doublet |
| Z_MGII | double | – | Central posterior value for the redshift of the Mg II absorption feature |
| AMP_2796 | double | 1e-17 erg / (Å cm2 s) | Central posterior value for the amplitude of the 2796Å line of the Mg II doublet |
| AMP_2803 | double | 1e-17 erg / (Å cm2 s) | Central posterior value for the amplitude of the 2803Å line of the Mg II doublet |
| STDDEV_2796 | double | Angstrom | Central posterior value for the standard deviation of the 2796Å line of the Mg II doublet |
| STDDEV_2803 | double | Angstrom | Central posterior value for the standard deviation of the 2803Å line of the Mg II doublet |
| Z_MGII_ERR_LOW | double | – | Low error bar for the redshift of the Mg II absorption feature |
| AMP_2796_ERR_LOW | double | 1e-17 erg / (Å cm2 s) | Low error bar for the amplitude of the 2796Å line of the Mg II doublet |
| AMP_2803_ERR_LOW | double | 1e-17 erg / (Å cm2 s) | Low error bar for the amplitude of the 2803Å line of the Mg II doublet |
| STDDEV_2796_ERR_LOW | double | Angstrom | Low error bar for the standard deviation of the 2796Å line of the Mg II doublet |
| STDDEV_2803_ERR_LOW | double | Angstrom | Low error bar for the standard deviation of the 2796Å line of the Mg II doublet |
| Z_MGII_ERR_HIGH | double | – | High error bar for the redshift of the Mg II absorption feature |
| AMP_2796_ERR_HIGH | double | 1e-17 erg / (Å cm2 s) | High error bar for the amplitude of the 2796Å line of the Mg II doublet |
| AMP_2803_ERR_HIGH | double | 1e-17 erg / (Å cm2 s) | High error bar for the amplitude of the 2803Å line of the Mg II doublet |
| STDDEV_2796_ERR_HIGH | double | Angstrom | High error bar for the standard deviation of the 2796Å line of the Mg II doublet |
| STDDEV_2803_ERR_HIGH | double | Angstrom | High error bar for the standard deviation of the 2796Å line of the Mg II doublet |
| CONTINUUM_METHOD | str | – | Method by which QSO continuum was determined while fitting (NMF or Medianfilter) |
| LINE_SNR_MIN | double | – | Minimum SNR value of the two Mg II line (used in initial detection; higher values equate to higher catalog purity) |
| LINE_SNR_MAX | double | – | Maximum SNR value of the two Mg II lines (used in initial detection; higher values equate to higher catalog purity) |