Dust Absorption and Emission

Dust absorbs the ultraviolet and optical light of a galaxy—from its stars, from the gas they ionize, and from an active nucleus—and re-radiates that energy in the infrared. This tutorial runs a small model which follows both sides of that exchange, and analyzes its output in a Jupyter notebook.

The model is described by the parameter file parameters/tutorials/dustEmission.xml. It builds a single merger tree for a halo of \(10^{12}\,M_\odot\), and outputs its galaxy at redshift one, when it is actively forming stars and hosts a supermassive black hole. To run it, from the directory where you installed Galacticus:

$ ./Galacticus.exe parameters/tutorials/dustEmission.xml

which writes dustEmissionTutorial.hdf5. The first run takes some time—tens of minutes—because Galacticus tabulates the spectra of stellar populations, and the luminosities of their emission lines, at the output time and spectral resolution of the model. Those tables are cached (in $GALACTICUS_DYNAMIC_DATA_PATH), so later runs with the same settings take only seconds. The parameter file outputs:

  • spectra of the light of the disk and spheroid stars, and of the AGN, and the luminosities of emission lines from HII regions and from the narrow-line region of the AGN;

  • for each of those, attenuated by the dust model of dustAttenuationCharlotFall2000, the light transmitted and the light absorbed by each phase of dust: birth clouds around young stars, and the diffuse interstellar medium;

  • the spectrum of thermal emission from the dust (nodePropertyExtractorSEDDustEmission), in which warm dust in birth clouds emits as a modified blackbody at a fixed temperature, and dust in the diffuse medium emits as PAHs (dustEmissionSpectrumRichieHensley2026), heated by the spectrum of the light they absorb, and larger grains, whose temperature is set by energy balance.

The accompanying notebook, tutorials/models/01-dust-absorption-and-emission.ipynb, reads that output with the dendros package and plots the spectrum of the galaxy before and after dust, the light each phase of dust absorbs, and the infrared emission it produces, and checks that the dust emits exactly the energy it absorbs. It can be viewed, with its output, in the notebook tutorials.