Nebular Emission

Nebular emission in prospector is based on the implementation in FSPS, which uses cloudy photoionization predictions for emission lines and nebular continuum with FSPS stellar populations as the ionization sources, as described in byler17.

FSPS Nebular Emission Parameters

The fundamental parameters of the nebular emission model are the ionization parameter U ("gas_logu") and the gas-phase metallicity ("gas_logz"). In FSPS is possible to turn on or off nebular emission (line and continuum) using the "add_neb_emission" switch. It is also possible to turn off only the nebular continuum with the "add_neb_continuum" switch. By default FSPS will add the emission lines to the model spectrum internally. In some situations (see below) this is not desired, and so the "nebemlineinspec" switch can be used to keep the lines from being added – though their luminosities will still be computed by FSPS – in which case prospector can be used to add the lines to the predicted spectra and photometry.

The nebular emission grids were computed for ionization sources (stellar populations) with the same metallicity as the gas-phase, so to keep perfect consistency it is necessary to tie these parameters together.

Fitting Emission Line Fluxes

The nebular emission line grids available in FSPS may not be flexible enough to fit all the emission line ratios observed in nature (e.g. due to AGN actvitiy, shocks, or radiative transfer, photoionization, and abundance effects not captured in the cloudy modeling). Nevertheless, it may be desirable to have accurate models for the emission lines, e.g. to account for their presence in the center of an absorption line of interest. For this reason it is possible with prospector, when fitting spectroscopic data, to fit for the true emission line fluxes and compute model likelihoods marginalized over the possible emission line fluxes. The methodology is described in johnson21, and is enabled via the "marginalize_elines" model parameter.

Briefly, explicit parameters are introduced to account for the emission line widths ("eline_sigma") and redshift offset from the stellar model ("eline_delta_zred"). These default to 100 km/s and the systemic redshift respectively, but can be adjusted or sampled and can be the same for every line (scalar) or give the value separately for every line (vector). The maximum likelihood emission line fluxes and the uncertainties thereon can then be determined from linear least-squares. Priors based on the cloudy-FSPS predictions can also be incorporated.

Note that this yields emission line ratios that are no longer tied to a physical model. It is possible to use the cloudy-FSPS predictions of the line luminosities for some lines while fitting for others, and this is described below.

Choosing Lines to Fit, Fix, or Ignore

Several additional parameters can be used to decide which lines to fit and marginalize over and which lines to include with luminosities set to the cloudy-FSPS values. It is also possible to completely ignore the contribution of specific lines (i.e. to not include them in the modeled spectrum or photometry). These parameters are:

• "elines_to_fit" A list of lines to fit via linear-least-squares and

  marginalize over. If this parameter is not given but "marginalize_elines" is True, all lines within the observed spectral range will be fit.

• "elines_to_fix" A list of lines to fix to their cloudy+FSPS predicted

  luminosities.

• "elines_to_ignore" A list of lines to ignore in the spectral and

  photometric models. Thier luminosities are still computed, and can be accessed through the prospect.models.sedmodel.SpecModel object at each likelihood call. This parameter takes effect regardless of the presence of spectral data or the value of "marginalize_elines"

In all cases the line names to use in these lists are those given in the FSPS emission line line information table, $SPS_HOME/data/emlines_info.dat, e.g. "Ly-alpha 1215" for the Lyman-alpha 1216 Angstrom line.

Nebular Parameter Templates

Several default model parameter templates are available in prospect.models.templates.TemplateLibrary to easily include different emission line treatments in prospector modeling including properly setting the values of the various switches described above.

• "nebular" A basic parameter set in which the nebular emission is based

  on the cloudy-FSPS grids, the lines are added to the model within FSPS, and the gas-phase metallicity is tied to the stellar metallicity. The only free parameter introduced by this template is "gas_logu".

• "nebular_marginalization" A parameter set for fitting and

  marginalizing over the emission line luminosities, with a prior based on the cloudy-fsps predictions. By default all lines will be fit as long as FSPS is installed, and the line widths are included as a free parameter with uniform prior. This parameter set adds one new free parameter, "eline_sigma".

• "fit_eline_redshift" This template can be used with

  "nebular_marginaliztion" above to also fit for the redshift offset of the emission lines from the stellar model. It adds one free parameter, "eline_delta_zred".