File formats

Input FITS File Structure

The input fits file must have the following HDU extensions:

  • FLUX: Should ideally contain the residual spectra (usually the flux/continuum, i.e., the continuum normalized spectra).

  • WAVELENGTH: Observed wavelength (in Angstroms).

  • ERROR: Error on residuals.

  • METADATA: Spectral details (such as Z_QSO, RA_QSO, DEC_QSO).

I have also provided two example QSO spectra FITS files. You can use these files to test an example run as described below.

  • data/sdss/qso_test_spectra.fits, which contains 100 continuum-normalized SDSS QSO spectra.

  • data/desi/qso_test_spectra.fits, which contains 100 continuum-normalized DESI DR1 QSO spectra.

Output FITS File Structure

The output fits file will have two (or three, optional) HDUs ABSORBER and METADATA or, (COLUMN_DENSITY, optional):

1) ABSORBER HDU will contain the following structured data:

  • INDEX_SPEC: (int), Index of quasar (can be used to read the RA, DEC, and Z of QSOs).

  • Z_ABS: (float), Redshift of absorber.

  • ${METAL}_${LINE}_EW: (float), Rest-frame equivalent widths (EWs) of absorber lines (e.g., MgII 2796, 2803 or CIV 1548, 1550) in Angstroms.

  • ${METAL}_${LINE}_EW_ERROR: (float), Uncertainties in rest-frame EWs of absorber lines in Angstroms.

  • Z_ABS_ERR: (float), Measured error in the redshift of the absorber.

  • GAUSS_FIT: (float array), Rest-frame fitting parameters of a double Gaussian to the absorber doublet (the width can be used to measure the velocity dispersion).

  • GAUSS_FIT_STD: (float array), Uncertainties in rest-frame fitting parameters of the double Gaussian to the absorber doublet.

  • SN_${METAL}_${LINE}: (float), Signal-to-noise ratio of the lines.

  • ${METAL}_EW_TOTAL: (float), Total EW of the lines in Angstroms.

  • ${METAL}_EW_TOTAL_ERROR: (float), Uncertainties in total EW of the lines in Angstroms.

  • ${METAL}_${LINE}_VDISP: (float), Rest-frame instrumental-resolution-corrected velocity dispersion of each line (e.g., MgII 2796, 2803 or CIV 1548, 1550) in km/s. Can be zero for unresolved lines.

  • DELTA_CHI2: (float), Delta chi2 statistics between Gaussian model and a flat continuum model (null hypothesis).

2) METADATA HDU will contain all the metadata (corresponding to each absorber) available in the input spectra file.

3) COLUMN_DENSITY HDU is optional:

If the --coldens option is provided when running qsoabsfind, the code also calculates the total column density of each detected doublet using the apparent optical depth method. This implementation follows the methodology described by Savage & Sembach (1991).

This optional HDU will contain:

  • LOG10N: (float), log of total column density (in cm-2), calculated from apparent optical depth method.

  • SIG_LOG10N: (float), uncertainty on log of total column density (in cm-2), calculated from apparent optical depth method.

  • SATURATION: (int), saturation flag, 1: saturated, 0: unsaturated

  • fN: (int), Column density measurement method, 1: WEIGHTED MEAN, 2: FIRST, 3: SECOND, 4: Corrected weak line (partial saturation), 5: Lower limit from weak line (strong saturation), 6: Lower limit from strong (strong saturation and weak is not available) -1: FAIL.