Tutorial: Postprocessing of Stellar Spectra

Stellar luminosities are computed by convolving a library of simple stellar populations with the star formation history of each galaxy. Galacticus allows the spectra of those simple stellar populations to be postprocessed (after being read from file or internally generated for example) before they are utilized in the convolution integral. This postprocessing can modify the spectra in arbitrary ways that depend on wavelength, redshift, and age of stellar population. Furthermore, Galacticus allows you to chain together stellar spectra postprocessors into a set to allow multiple postprocessings to be applied. Furthermore again, you can define an arbitrary number of sets and apply different sets to different luminosities.

Typical uses of stellar spectra postprocessors include accounting for absorption of galaxy light by the intervening , or capturing only the light from recent star formation¹. A full list of the available postprocessors can be found in here.

If you don’t specify a postprocessing set, the “default” set (consisting of the inoue2014 postprocessor is applied to each luminosity calculation. To specify other postprocessing sets add the following to your parameter file:

     <luminosityPostprocessSet value="default recent unabsorbed recentUnabsorbed"/>

where one set must be specified for each luminosity specified in the luminosityFilter parameter. Note that set names can be reused in order to apply the same postprocessor set to multiple luminosities.

The chain of postprocessors to apply for each set is then specified as follows:

     <stellarPopulationSpectraPostprocessorBuilder value="lookup">
       <names value="default recent unabsorbed recentUnabsorbed"/>
       <stellarPopulationSpectraPostprocessor      value="inoue2014"/>
       <stellarPopulationSpectraPostprocessor      value="sequence"  >
         <stellarPopulationSpectraPostprocessor    value="inoue2014"/>
         <stellarPopulationSpectraPostprocessor    value="recent"    >
     	   <timeLimit value="1.0e-2"/>
         </stellarPopulationSpectraPostprocessor>
       </stellarPopulationSpectraPostprocessor>
       <stellarPopulationSpectraPostprocessor      value="identity" />
       <stellarPopulationSpectraPostprocessor      value="recent"    >
         <timeLimit value="1.0e-2"/>
       </stellarPopulationSpectraPostprocessor>
     </stellarPopulationSpectraPostprocessorBuilder>

In this case we’ve constructed four postprocessor sets using the lookup stellarPopulationSpectraPostprocessorBuilder. This builder is responsible for constructing a suitable postprocessor from the names in the luminosityPostprocessSet parameter. The lookup implementation simply takes a list of postprocessor set names, and a corresponding list of stellarPopulationSpectraPostprocessors and select the relevant one based on the name.

In the above the default set applies the inoue2014 absorption postprocessor, while the recent set applies both the inoue2014 absorption postprocessor, followed by the recent postprocessor to retain only recently emitted light. The unabsorbed set ignores absorption entirely—it does this by using the identity postprocessor which leaves the spectrum unaffected. Finally, the recentUnabsorbed set applies only the recent filter while ignoring absorption.

In this way it is relatively easy to extract multiple different measures of luminosity from a Galacticus model. For example, you could construct four postprocessor sets, each corresponding to one of the four different absorption models (lycSuppress, madau1995, meiksin2006, and inoue2014) and apply these to the same luminosity filter to assess how luminosity depends on the model used.

[1]: Perhaps so that additional dust extinction can be applied to the light of recently formed stars.