4.3. Galactic absorption correction in the CLI¶

The Milky Way absorption correction is implemented in the CLI through two or three steps, depending on whether Galactic color excess is already included in the input catalog or has to be read from a reddenning map (see the Galactic Absorption section of the Methodology chapter for more details).

First, a grid of correction coefficients is generated. These are the linear coefficients \(a_{{\scriptscriptstyle SED},i}\) of Eq. (5.6) in the Galactic Absorption section, used by Phosphoros to compute the total extinction for a given filter. They depend on the source SED and on the filter transmission curve.

The grid is computed using the compute_galactic_correction_coeff_grid action (or simply CGCCG), which calls the PhosphorosComputeGalacticAbsorptionCoefficientGrid C++ executable. They are generated after building the grid of models.

For this action, Phosphoros reads by default the following configuration file

> $PHOSPHOROS_ROOT/config/PhosphorosComputeGalacticAbsorptionCoefficientGrid.conf

or, if not found, the system one in the installation directory. A different configuration file can be selected through the --config-file option.

An example of the configuration file is:

phosphoros-root=/home/phosphoros/Phosphoros
catalog-type=Challenge2
model-grid-file=Grid_Chalenge2_Parameter_Space_MADAU.dat
igm-absorption-type=MADAU
milky-way-reddening-curve-name=F99/F99_3.1
output-galactic-correction-coefficient-grid=Grid_Challenge2_Parameter_Space_MADAU_MW_Param.dat

The list of action parameters includes the catalog type, the filename of the grid of models, the type of IGM absorption, and the qualified filename of the Milky Way reddening curve (that is searched below the AuxiliaryData/Filters/ directory). The grid of correction coefficients is stored in the IntermediateProducts/<Catalog Type>/GalacticCorrectionCoefficientGrids/ directory.

The Phosphoros CLI gives the possibility to use a generic reddening maps (in HEALPIX 1 format) through the add_galactic_dust_data (or AGDD) action. This action read source coordinates from the input catalog and extract the corresponding color excess values from a reddening map. The \(E_{(B-V)}\) values are then added to the input catalog as a new column.

The main action parameters are input/output data and the source coordinates (in degrees):

planck-dust-map=<path>/<name of reddening map>
input-catalog=<input catalog name>
output-catalog=<output catalog name>
ra=<Right Ascension column in the input catalog>
dec=<Declination column in the input catalog>
galatic-ebv-col=<column name to be added>

The planck-dust-map and galatic-ebv-col parameters are optional and, if not provided, the Planck reddening map will be considered and the column will be named GAL_EBV.

Finally, modeled photometry are corrected by Milky Way absorption. The correction is enabled within the compute_redshift action by setting the dust-column-density-column-name action parameter:

dust-column-density-column-name=<E(B-V) column name>

Other action parameters are:

dust-map-sed-bpc=<bpc value>
galactic-correction-coefficient-grid-file=<coefficient grid name>

The former defines the band-pass correction that is required when Galactic color excess values are derived from sources different from B5 stars. The default value is 1.018 that corresponds to the band pass correction for the Planck reddening map. If the Schlegel et al. [SchlegelFinkbeinerDavis98] reddening map is used the band-pass correction equals to 1.

The filename of the correction coefficients grid, computed in the first step, must be specified through the galactic-correction-coefficient-grid-file parameter (the path is required only if it is not located in the default directory). Its format is by default binary, but it can be stored in ASCII by setting --output-galactic-correction-coefficient-grid-format=TEXT (see Intermediate Products).

Footnotes

1: see https://healpix.sourceforge.io/

SchlegelFinkbeinerDavis98: David J. Schlegel, Douglas P. Finkbeiner, and Marc Davis. Maps of Dust Infrared Emission for Use in Estimation of Reddening and Cosmic Microwave Background Radiation Fo regrounds. ApJ, 500(2):525–553, Jun 1998. arXiv:astro-ph/9710327, doi:10.1086/305772.