Emission models

Overview

To simplify the calculation of complex emission, with many contributing components and different operations involved in their construction, synthesizer provides EmissionModels. At their simplest, EmissionModels define the generation of a single type of spectra, e.g. the incident emission from a population of stars. Hwoever, EmissionModels can be arbitrarily complex, defining multiple different types of spectra and lines from different components, and defining how they interact. The possible operations that EmissionModels can define are:

Any of these operations can be done in the presence of a mask, to apply the operation to a subset of the components contents (e.g. applying dust attenuation only to young stars). These masks can be applied identically to particle or parametric models.

Once an EmissionModel is constructed it can be used to generate spectra. This is done by passing the EmissionMmodel to the get_spectra or get_lines method on a Galaxy or galaxy component. This will then generate the spectra defined within the EmissionMmodel, given the properties of the Galaxy or component. For more details see Generating spectra.

Named spectra

Synthesizer enables the generation of many different spectra which are associated with Galaxy objects or their components through EmissionModels. These spectra are given standard labels that reflect their origin and the masks that have been applied (though custom labels can be provided). The flowchart also shows how these different spectra are typically generated and related by an emission model.

Flowchart showing the different emission types in synthesizer

Our standard naming system, which is used in the premade EmissionModels, is listed below.

  • incident spectra are the spectra that serve as an input to the photoionisation modelling. In the context of stellar population synthesis these are the spectra that are produced by these codes and equivalent to the “pure stellar” spectra.

  • transmitted spectra is the incident spectra that is transmitted through the gas in the photoionisation modelling. Functionally the main difference between transmitted and incident is that the transmitted has little flux below the Lyman-limit, since this has been absorbed by the gas. This depends on fesc.

  • nebular is the nebular continuum and line emission predicted by the photoionisation model. This depends on fesc.

  • reprocessed is the emission which has been reprocessed by the gas. This is the sum of nebular and transmitted emission.

  • escaped is the incident emission that escapes reprocessing by gas. This is fesc * incident. This is not subsequently affected by dust.

  • intrinsic is the sum of the escaped and reprocessed emission, essentially the emission before dust attenuated.

  • attenuated is the reprocessed emission with attenuation by dust.

  • emergent is the combined emission including dust attenuation and is the sum of reprocessed_attenuated and escaped. NOTE: this does not include thermal dust emission, so is only valid from the UV to near-IR.

  • dust_emission is the thermal dust emission calculated using an energy balance approach, and assuming a dust emission model.

  • total is the sum of attenuated and dust, i.e. it includes both the effect of dust attenuation and dust emission.

  • For two component dust models (e.g. Charlot & Fall 2000 or BimodalPacmanEmission) we also generate the individual spectra of the young and old component. This includes young_incident, young_nebular, young_attenuated etc. young and old are equivalent to total for the young and old components.

All premade models follow these conventions and we encourage the user to employ the same system.

Working with EmissionModels

In the sections linked below we detail the basic functionality of an EmissionModel, the premade stellar and black hole emission models, dust emission generators, and how to customise a model or construct your own.