4.13. Physical Parameters

Physical parameters of input sources (such as age, stellar mass, star-formation rate, etc.) can be estimated by Phosphoros during the redshift computation if the relation between physical parameters and SED templates is firstly defined by users.

In Phosphoros physical parameters are assumed to be a linear function of the source luminosity (in solar luminosity unit):

(4.4)\[pp = A*L/L_{\odot} + B\]

where A and B depend on the SED template and are fixed by users, while \(L\) is the luminosity in the filter used for the SED template normalization.

In the redshift computation, Phosphoros determines the best-fit value of physical parameters applying the above relation to the source best-fit template. In addition, the 1D (or multi-dimensional) PDF of physical parameters can be easily recovered from the multi-dimensional posterior distribution, if it is computed with the Sampling option (see GUI: Computing Redshifts). In this case, in fact, along with the parameters of the sampled models, Phosphoros stores the related luminosity and physical parameters (see Results Format).

4.13.1. Physical Parameters in the GUI

First of all, for any physical parameter, users must provide the A and B values (Eq. (4.4)) for each SED template of the parameter space. This can be done in the Configuration-->Auxiliary Data-->SEDs sub-panel by clicking on each SED template (see Fig. 4.12): a window opens showing the physical parameters that have been already defined (if any). A new one can be defined at the bottom of the window by filling the name, the unit, and the A, B values.

Fig. 4.12 How to define new physical parameters with the GUI

Note

Information on physical parameters is saved in the header of SED template files. For example, in the case shown in Fig. 4.12, the mass-luminosity relation will be found in the first line of the Ell1_A_0 SED file, as following:

# PARAMETER : mass=0.1*L+1.3[solar mass]

Users can use their favorite editor to modify the A and B values or add new physical parameters inside SED files.

Warning

A physical parameter must be defined with the same unit in all the SEDs used in the parameter space, otherwise it is not considered in the redshift computation.

Once this operation has been completed, users only need to select the physical parameters they want to include in the output of the redshift computation. This is done in the Compute Redshifts-->Input/Output sub-panel by clicking on the Select Parameters (see Fig. 4.13). A window opens with the list of available physical parameters.

Fig. 4.13 How to select physical parameters to be computed

4.13.2. Physical Parameters in the CLI

Here we assume that any physical parameter of interest is already included in the header of SED template files, for example, through the GUI.

Physical parameters can be estimated with the CLI in two simple steps. The first step is to generate a FITS table that includes all the desired physical parameters, their units, and the A and B values for all the SED templates present in the parameter space. The build_pp_config (or BPPC) action in Phosphoros performs this task: it reads the header of the SED template files and writes a FITS table with the required information. An example of configuration file for the BPPC action is:

physical-parameter=mass
physical-parameter=age

output-file=IntermediateProducts/<Catalog Type>/<name>

sed-group=CosmosEll
sed-group=CosmosSp
sed-group=CosmosSB

Here Phosphoros reads two physical parameters (mass and age) from the SED template files located in the directories given by the sed-group options. More options for the template specification can be found in the BPPC help (similarly to the compute_redshift action). The output FITS table will be located in the <Catalog Type> intermediate products directory.

Finally, Phosphoros computes physical parameters if the path to the physical parameters FITS table is provided in the compute_redshift configuration file, through the option:

physical_parameter_config_file=IntermediateProducts/<Catalog Type>/<name>