From simulations to observations. Methodology and data release of mock TNG50 galaxies at 0.3 < z < 0.7 for WEAVE-StePS

TL;DR

This work develops a forward-modeling framework to compare WEAVE-StePS observations with state-of-the-art cosmological simulations by generating mock WEAVE-like spectra and images from the TNG50 run using SKIRT radiative transfer and analyzing them with pPXF. The authors produce ~750 noiseless mock galaxies across z = 0.3, 0.5, 0.7, and demonstrate that the recovered mass-weighted ages align with intrinsic values to within about $0.2$–$0.6$ Gyr, while metallicities tend to be biased low by ~0.3 dex. They explore central versus integrated properties and show that cumulative SFHs and key formation timescales are recoverable, especially for galaxies that formed most of their mass early; some biases remain for complex SFHs. The datasets, including spectra and multi-wavelength imaging, are released to support forthcoming WEAVE observations and provide a benchmark for validating stellar-population diagnostics and forward-modeling approaches in galaxy evolution studies.

Abstract

The new generation of optical spectrographs (i.e., WEAVE, 4MOST, DESI, and WST) offer unprecedented opportunities for statistically studying the star formation histories of galaxies. However, these observations are not easily comparable to predictions from cosmological simulations. Our goal is to build a reference framework for comparing spectroscopic observations with simulations and test tools for deriving stellar population properties of galaxies. We focus on the observational strategy of the Stellar Population at Intermediate Redshift Survey (StePS) with the WEAVE instrument. We generate mock datasets of ~750 galaxies at redshifts z = 0.3, 0.5, and 0.7 using the TNG50 simulation, perform radiative transfer with SKIRT, and analyze the spectra with pPXF as if they were real observations. We present the methodology to generate these datasets and provide an initial exploration of stellar population parameters (i.e., mass-weighted ages and metallicities) and star formation histories for three galaxies at z = 0.7 and their descendants at z = 0.5 and 0.3. We find good agreement between the mock spectra and intrinsic ages in TNG50 (average difference $0.2\pm0.3$ Gyr) and successfully recover their star formation histories, especially for galaxies form the bulk of their stars on short timescales and at early epochs. We release these datasets, including multi-wavelength imaging and spectra, to support forthcoming WEAVE observations.

Figures (8)

• Figure 1: $SFR-M_{\star}$ time evolution of the three example galaxies (bigger symbols), from redshift $z=0.7$ to $0.3$. The galaxy ID follows the nomenclature as in TNG50. Galaxy ID 172231 (blue arrows) slowly quenches (ID 218606) until it reaches the red sequence (ID 251598). Galaxy ID 222130 (green arrows) stays in the green valley (ID 234935) and then moves to the blue cloud sequence (ID 277675). Galaxy ID 212087 (red arrows) slowly rejuvenates (ID 253460) and then moves to the green valley (ID 298351). Dashed lines mark the main sequence at z=0.3, 0.5, and 0.7 as defined in Koprowski.M:2024.
• Figure 2: Rest-frame spectra (first column) and RGB images (second to fourth columns) for three example galaxies at redshift $z=0.7$ (top row), $z=0.5$ (middle row), and $z=0.3$ (bottom row), respectively. The first column demonstrates fiber spectra extracted from the central 1.3 arcsec of each galaxy, with arbitrary offset applied in the flux (linear units). In each panel of the first column: green spectra are extracted from galaxy ID 222130, ID 234935, and ID 277675, respectively; blue spectra from galaxy ID 172231, ID 218606, and ID 251598, respectively; red spectra from galaxy ID 212087, ID 253460, and ID 298351, respectively. The fiber size and corresponding physical size are also shown.
• Figure 3: Distribution of the pPXF weights (red colors), indicating the mass fraction of each stellar population of given age and metallicity, derived from fiber noiseless spectra. The mass-weighted age and metallicity measured with pPXF is shown as a red star, while mass-weighted age and metallicity derived from the simulation is marked as a black star. Black lines represent the marginalized distribution of ages and metallicities derived from the simulation, while red curves show those inferred from pPXF. The progenitor galaxies at $z=0.7$ are shown in the top row, while their descendants at $z=0.5$ and $z=0.3$ are shown in the middle and bottom rows, respectively.
• Figure 4: Examples of SFHs (first row) and cumulative SFHs of the target galaxies at redshift $z=0.7$ (second row), $z=0.5$ (third row), and $z=0.3$ (fourth row). The SFHs are derived directly from the cosmological simulation and represent the sum of the $SFR$s of all progenitors at each snapshot. The cumulative SFHs of each galaxy are retrieved by fitting with pPXF both the StePS-like fiber spectra (red solid line) and integrated noiseless spectra (red dashed line). The shaded red regions indicate the 16th–84th percentile range across 100 bootstrap realizations. For comparison, the cumulative SFHs derived directly from the cosmological simulation are shown both for the entire galaxy (black dashed line) and for the central 1.3 arcsec (black solid line).
• Figure 5: Example of the multi-wavelength imaging datasets for galaxy ID 172231.