Euclid preparation. Spatially resolved stellar populations of local galaxies with Euclid: a proof of concept using synthetic images with the TNG50 simulation

TL;DR

This study validates a spatially resolved SED-fitting pipeline for local galaxies using synthetic Euclid, LSST, and GALEX images derived from the high-resolution TNG50-SKIRT Atlas. It shows that stellar-mass surface density $\Sigma_{*}$ is robustly recovered with Euclid-alone data, while age $t_M$ and metallicity $Z$ benefit substantially from additional UV/optical coverage and from informative priors based on mass-$Z$-age relations. The mass-$Z$-age priors mitigate degeneracies inherent in Bayesian SED fitting, but their influence grows when data are limited, highlighting the data-prior interplay in low-S/N, narrow-wavelength regimes. Overall, Euclid's imaging will enable deep, wide-field maps of local stellar masses, and combining Euclid with ancillary data will yield the most reliable maps of star-formation history and chemical enrichment. The work paves the way for large-scale mapping of local galaxies with Euclid, complemented by LSST/GALEX data and informed priors, to advance galaxy evolution studies.

Abstract

The European Space Agency's Euclid mission will observe approximately 14,000 $\rm{deg}^{2}$ of the extragalactic sky and deliver high-quality imaging for many galaxies. The depth and high spatial resolution of the data will enable a detailed analysis of stellar population properties of local galaxies. In this study, we test our pipeline for spatially resolved SED fitting using synthetic images of Euclid, LSST, and GALEX generated from the TNG50 simulation. We apply our pipeline to 25 local simulated galaxies to recover their resolved stellar population properties. We produce 3 types of data cubes: GALEX + LSST + Euclid, LSST + Euclid, and Euclid-only. We perform the SED fitting tests with two SPS models in a Bayesian framework. Because the age, metallicity, and dust attenuation estimates are biased when applying only classical formulations of flat priors, we examine the effects of additional priors in the forms of mass-age-$Z$ relations, constructed using a combination of empirical and simulated data. Stellar-mass surface densities can be recovered well using any of the 3 data cubes, regardless of the SPS model and prior variations. The new priors then significantly improve the measurements of mass-weighted age and $Z$ compared to results obtained without priors, but they may play an excessive role compared to the data in determining the outcome when no UV data is available. The spatially resolved SED fitting method is powerful for mapping the stellar populations of galaxies with the current abundance of high-quality imaging data. Our study re-emphasizes the gain added by including multiwavelength data from ancillary surveys and the roles of priors in Bayesian SED fitting. With the Euclid data alone, we will be able to generate complete and deep stellar mass maps of galaxies in the local Universe, thus exploiting the telescope's wide field, NIR sensitivity, and high spatial resolution.

Figures (18)

• Figure 1: The Sample of 25 simulated galaxies used for the analyses in this paper. The selected sample and the parent sample from the TNG50-SKIRT Atlas are shown with red squares and blue dots, respectively. The sample is selected to cover the entire mass range in the TNG50-SKIRT catalogue and a wide range of sSFR.
• Figure 2: Examples of simulated images of GALEX, LSST, and Euclid to which observational effects have been added, including spatial resampling (i.e. regridding), simulated noise, and convolution with the PSF of the corresponding cameras. The galaxies here are TNG501725 with O1 orientation index and TNG414917 with O4 orientation index. The original synthetic images are taken from the TNG50-SKIRT Atlas database and are generated from the TNG50 simulation using the SKIRT radiative transfer code. The variation in spatial resolution and depth can be seen in the images with GALEX images being the most blurred and shallowest, and $I_{\rm E}$ images being the sharpest and deepest. The pixel size difference is demonstrated with the red horizontal line (;;60 long). The PSF FWHM and pixel size of the images are summarized in Table \ref{['tab:mock_images']}. The colour images are made from the composite of LSST $g$, $r$, and $i$ images.
• Figure 3: Examples of the maps of flux densities produced from the image processing on the sets of three data cubes: G + L + E (a), L + E (b), and E (c). This example of the galaxy is TNG501725 with the O1 orientation index. The final PSF size and pixel size of the 3 data cubes are summarized in Table \ref{['tab:image_processing']}. The G + L + E data cube has the lowest spatial resolution, trading the high spatial resolution of Euclid and LSST with the wider wavelength coverage from UV to NIR.
• Figure 4: Surface brightness radial profiles of TNG501725 derived from the stamp images of G + L + E data. The vertical dashed grey line indicates the radial extent of the data cube after the galaxy's RoI is defined, roughly corresponding to a detection threshold of $2$ above the background RMS of Euclid images.
• Figure 5: Examples of the pixel binning maps of TNG501725 obtained with the three data cubes: G + L + E (a), L + E (b), and Euclid only (c). In each row, the three panels from left to right show the binning map, $\rm{S}/\rm{N}$ radial profile of pixels, and $\rm{S}/\rm{N}$ radial profile of spatial bins. The pixel binning scheme of piXedfit can achieve the $\rm{S}/\rm{N}$ threshold of $5$ (indicated by the dashed black lines) for the $g$, $r$, $i$, $z$, $I_{\rm E}$, $Y_{\rm E}$, $J_{\rm E}$, and $H_{\rm E}$ filters.