s3fit 2.2.6

S3Fit: a Simultaneous Spectrum and photometric-SED Fitting code for observation of galaxies

S³Fit

S³Fit: a Simultaneous Spectrum and photometric-SED Fitting code for observation of galaxies

S³Fit is a Python-based tool for analyzing observational data of galaxies. It offers powerful capabilities for decomposing spectroscopic data by supporting multiple continuum and emission line models with multiple components, making it well-suited for complex systems with mixed contributions from Active Galactic Nuclei (AGNs) and their host galaxies. By simultaneously fitting the spectrum and multi-band photometric Spectral Energy Distribution (SED), S³Fit improves constraints on continuum model properties, which may be poorly determined when fitting spectral data alone due to its limited wavelength coverage. With an optimized fitting strategy, S³Fit efficiently derives the best-fit solution for dozens of model parameters. Additionally, it provides an extensible and user-friendly framework, allowing users to modify model configurations and incorporate new model components as needed.

Features of S³Fit

• Easy switch between pure spectral fitting and joint spectrum+SED fitting modes.
• Support for flexible combination of multiple stellar populations with different star formation histories (SFH).
• Support for flexible combination of multiple emission line components.
• Support for AGN continuum models across UV/optical and IR wavelength ranges.
• User-friendly functions for outputting and visualizing fitting results.
• Highly extensible framework, allowing users to add new features such as new SFH functions, emission lines, and custom model types.

Fitting strategy

The full fitting pipeline of S³Fit is shown in the following flowchart, with a detailed description of the fitting strategy in manuals.

An example of the fitting result of S³Fit is shown in the following plots (and details in this paper).

Usage

Please find guides in manuals for basic and advanced usages of this code. Examples of the usage of S³Fit are provided in example1 and example2

Installation

You can install S³Fit with pip (or any pip-compatible installer):

pip install s3fit

S³Fit mainly depends on several most widely utilized repositories for science calculation, scipy, numpy, and astropy. The core requirement of S³Fit is the two functions least_squares and lsq_linear in scipy.optimize (please read the fitting strategy for details). A strong dependency of S³Fit on these repositories is not expected. joblib is required to run fitting in multithreading (from v2.3). It is optional to run S³Fit with PyNeb, which is used to calculate intrinsic flux ratios of emission lines:

pip install s3fit[pyneb]

Dependencies:

python >= 3.10
scipy >= 1.12.0
numpy >= 1.26.4
astropy >= 6.0.0
joblib >= 1.5.2
matplotlib >= 3.9.1
pyneb >= 1.1.23 (optional)

Future updating

• Add ISM dust and synchrotron models (you may also add them or other models by yourself following the advanced usage manual).
• Add iron pseudo continuum templates and Balmer continuum of type-1 AGN (supported in v2.3).
• Absorption lines (supported in v2.3).

Citation

If you would like to use S³Fit, please cite with the following BibTeX code:

@software{2025ascl.soft03024C,
       author = {{Chen}, Xiaoyang},
        title = "{S3Fit: Simultaneous Spectrum and photometric-SED Fitting code for galaxy observations}",
 howpublished = {Astrophysics Source Code Library, record ascl:2503.024},
         year = 2025,
        month = mar,
          eid = {ascl:2503.024},
       adsurl = {https://ui.adsabs.harvard.edu/abs/2025ascl.soft03024C},
      adsnote = {Provided by the SAO/NASA Astrophysics Data System}
}

Reference

S³Fit uses the Single Stellar Population (SSP) library HR-pyPopStar (paper). Please download the HR-pyPopStar library and run the converting code to create the SSP models used for S³Fit. You may also want to download an example of the converted SSP model for test in this link.

S³Fit uses the SKIRTor (paper1, paper2) AGN torus model. Please download the SKIRTor library and run the converting code to create the torus models used for S³Fit. Example of this library is also provided in model libraries for a test of S³Fit, which contains the templates with a fixed dust density gradient in radial (p = 1) and angular direction (q = 0.5). Please refer to SKIRTor website for details of the model parameters.