GalSyn I: A Forward-Modeling Framework for Synthetic Galaxy Observations from Hydrodynamical Simulations and First Data Release from IllustrisTNG

Abstract

We present GalSyn (Galaxy Synthesizer), a modular and flexible Python package for generating synthetic spectrophotometric observations from hydrodynamical galaxy simulations. GalSyn employs a particle-by-particle spectral modeling approach that enables the rapid production of large synthetic datasets required for statistical population studies, offering a computationally efficient alternative to full radiative transfer codes. Users have full control over the spectral modeling choices, including the choice of stellar population synthesis engine, stellar isochrones, spectral libraries, and initial mass functions. Dust attenuation is modeled at the spatially resolved level via a line-of-sight column density method, with a comprehensive suite of fixed and adaptive attenuation laws. A decoupled kinematics model independently Doppler-shifts the stellar and nebular components, enabling realistic synthetic IFU data cubes. It also provides features to add observational realism, including PSF convolution and multi-component noise simulation. Beyond imaging and spectroscopic data cubes, GalSyn reconstructs spatially resolved physical property maps and star formation histories. Alongside this paper, we present the first public data release of synthetic imaging observations and spatially resolved star formation histories generated from the IllustrisTNG simulation suites, comprising four mock extragalactic survey fields (with areas of $5$, $8$, $137$, $365$ arcmin$^{2}$), progenitor histories of 290 local massive galaxies ($\log(M_{*,z=0}/M_{\odot}) > 10.5$) tracked across $0<z<5$, and 259 major-merger systems. Each galaxy data cube contains imaging in 47 filters spanning HST, JWST, Euclid, Rubin/LSST, and the Roman Space Telescope. GalSyn is publicly available at https://github.com/aabdurrouf/GalSyn.

Figures (20)

• Figure 1: Comparisons of SSP spectra available in GalSyn. Top left: an example of SSP spectrum broken down into its components: stellar continuum, nebular continuum, and nebular emission. Top right: comparison of SSP spectra for various isochrones using the MILES spectral library and 2003Chabrier IMF, calculated using the FSPS code. A BC03 spectrum generated with the Bagpipes code is also shown for reference. Bottom left: comparison of SSP spectra for different spectral libraries, but the same isochrone (MIST) and IMF (2003Chabrier). Bottom right: SSP spectra with various IMFs, but the same isochrone (MIST) and spectral library (MILES).
• Figure 2: Visualization of two-dimensional projection from three-dimensional coordinates given a viewing angle specified by the polar ($\theta$) and azimuth angles ($\phi$). The new coordinate system (x', y', z') obtained from the transformation is shown by the red, green, and blue arrows. In this new frame, the observer's LOS is aligned with the z'-axis, and the x'-y' plane represents the plane of the sky.
• Figure 3: Options for the dust attenuation curves of the diffuse ISM in GalSyn. (a) The fixed dust attenuation law options: Salim2018, Calzetti2000, SMC by Gordon2003, LMC by Gordon2003, MW by Cardelli1989, and MW by Fitzpatrick1999. These dust curves have a fixed shape, independent of the local $A_{V}$. (b) The modified Calzetti law (option 0), as defined in Equations \ref{['eq:modif_calzetti']}-\ref{['eq:drude']}. This model is shown with a varying slope ($\delta$) and fixed UV bump parameters ($B=2.0$ and $\gamma=0.035$). (c) The modified Calzetti law shown with a varying bump amplitude but fixed slope ($\delta=0$) and bump width ($\gamma=0.035\,\mu$m). (d) The modified Calzetti law shown with a varying bump width but fixed slope ($\delta=0$) and bump amplitude ($B=5$). (e) The modified Calzetti law is applied in an adaptive mode with slope and bump amplitude changes depending on $A_{V}$. The slope varies with $A_{V}$ following the empirical relation from Salim2018, and the bump amplitude then varies depending on slope following the relation from Kriek2013. (f) Dust attenuation curve obtained by fitting modified Calzetti law to the integrated $A_{\lambda}$ curves from synthetic IFS cubes of simulated galaxies from the NIHAO-SKIRT catalog Faucher2023. Details about this will be presented in Abdurro'uf et al. (in prep.).
• Figure 4: Example of synthetic images of a simulated galaxy from TNG50 (subhalo ID=107965) at $z=1.53$ across multiple filters from various telescopes, including HST, JWST, Rubin/LSST, and Roman Space Telescope. The top-left panel shows a color composite image generated from JWST filters: F115W (blue), F150W (green), and F200W (red). These images have a pixel size of $0.03$ arcsec$^{-1}$ and have no observational effects being added.
• Figure 5: Spatially resolved physical property maps of the galaxy TNG50-107965, the same galaxy shown in Figure \ref{['fig:idealized_images1']}. These maps have the same spatial sampling as those of the synthetic images ($0.03"$ pixel$^{-1}$). This illustrates the comprehensive set of physical property maps generated by GalSyn.