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Stellar populations

Purpose

The starFormationHistories module derives non-parametric star formation histories from the observed spectra via full spectrum fitting. nGIST is currently equipped with one routine that can readily be used by setting the configuration parameter SFH: METHOD to 'ppxf'. This routine employs the pPXF routine of Cappellari & Emsellem (2004).

There are several options for the treatment of emission lines in this module:

  • To run the SFH module using the cleaned, emission-line subtracted spectra, you simply need to make sure that you ran the emission line module previously (i.e. set GAS: METHOD: 'ppxf')
  • To run the SFH module using the original (un-subtracted) spectrum, run the SFH module with the GAS module turned off (i.e. set GAS: METHOD: False).
  • To run the SFH module and mask all emission lines, use SFH: SPEC_MASK: 'specMask_KIN'
  • To run the SFH module and not mask any emission lines, use SFH: SPEC_MASK: 'specMask_SFH'.

Fitting procedure

This module uses a multi-step fitting approach to determine the best-fit parameters. nGIST first determines an optimal template set (OPT_TEMP, optional), measures and then applies (optional) a dust correction (DUST_CORR), rescales the noise, and cleans (DOCLEAN, optional) the spectrum of outliers (e.g. >3σ noise, sky lines, and emission lines).

To speed up the overall fitting process, the OPT_TEMP keyword can be used to pass a subset of templates to pPXF during the preparation steps (DUST_CORR, noise rescaling, and DOCLEAN). Note that in the final fit (used to determine the best-fit parameters), all templates are always provided. If OPT_TEMP is set to 'galaxy_single' or 'galaxy_set', pPXF is first run once on a single mean spectrum of the entire datacube to identify templates with non-zero weights. For 'galaxy_single', these templates are combined into a single ‘optimal’ template, whereas 'galaxy_set' passes all non-zero-weight templates to pPXF during the preparation steps. If OPT_TEMP is set to 'default', the optimal template set from the dust preparation step is used.

In the dust preparation step, nGIST determines a best-fit dust vector following the procedure described in Cappellari (2023). No additive or multiplicative polynomials are used in this fit. If DUST_CORR: True, all consecutive fits will use this dust vector within pPXF as a fixed component.

Next, a new estimate of the noise is derived by fitting the spectrum using pPXF with a constant noise vector. The input noise vector (either variance or constant) is rescaled using the bi-weight estimate (~standard deviation) of the fit residuals (data minus the best-fit model).

If DOCLEAN: True, we use the clip_outlier function to remove all 3σ outliers based on the best-fit from the noise estimation step.

In the final step, pPXF is run with all templates to determine the best-fit parameters.

Notes - The DEBUG keyword within the module can be used to fit specific bin(s), which can be useful when debugging. This option only works if PARALLEL is set to False in GENERAL. - The PLOT keyword within the module can be used to provide two figures for each spaxel. The first figure (step 1) shows the spectrum (black), the best-fit (red), and residuals (green) from the noise preparation step, whereas the second figure (step 3) shows the results of the final fit. Here, the vertical grey lines are the input mask, and the vertical pink lines show the spectral regions masked by DOCLEAN = True. Using PLOT = True will slow down the fit significantly and will create a large number of files if you are fitting many bins.

Config file input

SFH :
  METHOD : 'ppxf' # Name of the routine in starFormationHistories/ (without .py) to perform the tasks. Set 'False' to turn off module. Set 'ppxf' to use the standard GIST implementation, exploiting the pPXF routine of Cappellari & Emsellem (2004).
  LMIN : 4750 # Rest-frame wavelength range used for the star formation histories analysis [in Angst.]
  LMAX : 5500
  SPEC_MASK : 'specMask_SFH' # File to define wavelength ranges to be masked during the star formation histories analysis. The specified path is relative to the configDir path in defaultDir.
  SPEC_EMICLEAN : False # Keyword (True/False) to enable/disable the use of emission line cleaned spectra from the Emission Line Fitting Module. 
  MOM : 4 # Number of kinematic moments to be extracted. If you use FIXED = True, this should be same number of moments used to extract the stellar kinematics before. Otherwise the parameter can be set independently.
  MDEG : 12 # Degree of the multiplicative Legendre polynomial. Set '0' to not include any multiplicative polynomials. Note that additive Legendre polynomials cannot be used for this module.
  REGUL : 1. # Regularisation for the regularised run of pPXF. Note: the old GIST keyword REGUL_ERR also still works. In that instance, regularisation = 1 / REGUL_ERR
  NOISE : 1. # Set a wavelength independent noise vector to be passed to pPXF.
  FIXED : True # Fix the stellar kinematics to the results obtained with the stellar kinematics module [True / False]. If 'False', please provide an initial guess on the velocity dispersion of the systems [in km/s] by adding the parameter SIGMA.
  MC_PPXF : 0 # Added to add the option of running MC on the SFH module
  LSF_TEMP : 'lsf_MILES' # Path of the file specifying the line-spread-function of the spectral templates. The specified path is relative to the configDir path in defaultDir.
  TEMPLATE_SET : 'miles' # options are 'miles' or 'IndoUS (not recc.)'
  LIBRARY : 'MILES_SFH/' # options are 'MILES/', 'miles_ssp_ch/', or 'IndoUS/'
  NORM_TEMP : 'LIGHT' # Normalise the spectral template library to obtain light- or mass-weighted results [LIGHT / MASS]
  DOCLEAN : True # Keyword to turn on/off the sigma clipping. Set to 'False' for testing.
  OPT_TEMP : 'default' # Keyword to select Optimal Template method. Options are 'default', 'galaxy_single', 'galaxy_set'
  DUST_CORR : True # True/False, fixes the dust in the final pPXF fit 
  NOISE : 'variance' # Keyword to set noise to be used in pPXF run. Option 'constant' and 'variance'
  PLOT : False # produce extra output plots
  DEBUG_BIN : False # Optional Keyword - array of bins [1,2,3] - only works when parallel = False.

Output

Files

  • _sfh.fits:

    • Columns: AGE Age | METAL Metallicity | ALPHA Alpha enhancement | V SIGMA H3 H4 Stellar kinematics | FORM_ERR_* Formal errors on the stellar kinematics

    • Rows: One line per bin.

  • _sfh_weights.fits: The weights assigned to each template during the fit

    • Columns: WEIGHTS The weights

    • Rows: One line per bin.

  • _sfh_bestfit.fits, Extension 1:

    • The best fit to the spectrum

    • Columns: BESTFIT The best fit to the spectrum

    • Rows: One fit per bin.

  • _sfh_bestfit.fits, Extension 2:

    • Columns: LOGLAM The corresponding wavelength array

  • _sfh_bestfit.fits, Extension 3:

    • Columns: GOODPIX The spectral pixels included in the fit

  • _sfh_maps.fits

    • FITS file containing 2D maps of the output stellar population properties, including age, metallicity, and alpha (if varying alpha templates included)

Function returns

The module should end with return(None). If an uncaught exception occurs in the module, all following modules will be skipped for this galaxy. If you intend to manually skip all subsequent modules, simply raise an exception.