GalSBI-SPS: a stellar population synthesis-based galaxy population model for cosmology and galaxy evolution applications

TL;DR

GalSBI-SPS presents a physically motivated, SPS-based forward-model of the galaxy population designed for cosmology and galaxy evolution applications. It integrates ProSpect-generated SEDs with a two-population GSMF, flexible SFHs, chemically evolving metallicity, dust, gas ionisation, and AGN components, and forwards the intrinsic properties through UFig image simulations to emulate HSC COSMOS data consistently. The model reproduces observed magnitudes, colours, and sizes down to i<23 and qualitatively recovers the stellar mass–SFR and size–mass relations, while redshift distributions show modest mean-shifts relative to COSMOS photo-zs; discrepancies highlight SFH and size modelling limitations. The work demonstrates GalSBI-SPS as a realistic, survey-independent baseline that will gain predictive power once SBI constrains the parameters against rich photometric and spectroscopic datasets, enabling accurate, Stage IV–compliant redshift distributions and robust scaling-relations studies.

Abstract

Next generation photometric and spectroscopic surveys will enable unprecedented tests of the concordance cosmological model and of galaxy formation and evolution. Fully exploiting their potential requires a precise understanding of the selection effects on galaxies and biases on measurements of their properties, required, above all, for accurate estimates of redshift distributions n(z). Forward-modelling offers a powerful framework to simultaneously recover galaxy $n(z)$s and characterise the observed galaxy population. We present GalSBI-SPS, a new SPS-based galaxy population model that generates realistic galaxy catalogues, which we use to forward-model HSC data in the COSMOS field. GalSBI-SPS samples galaxy physical properties, computes magnitudes with ProSpect, and simulates HSC images in the COSMOS field with UFig. We measure photometric properties consistently in real data and simulations. We compare $n(z)$s, photometric and physical properties to observations and to GalSBI. GalSBI-SPS reproduces the observed grizy magnitude, colour, and size distributions down to i<23. Median differences in magnitudes and colours remain below 0.14 mag, with the model covering the full colour space spanned by HSC. Galaxy sizes are overestimated by 0.2 arcsec on average and some tension exists in the g-r colour, but the latter is comparable to that seen in GalSBI. $n(z)$s show a mild positive offset (0.01-0.08) in the mean. GalSBI-SPS qualitatively reproduces the stellar mass-SFR and size-stellar mass relations seen in COSMOS2020. GalSBI-SPS provides a realistic, survey-independent galaxy population description at a Stage-III depth using only literature-based parameters. Its predictive power will improve significantly when constrained against observed data using SBI, thereby providing accurate $n(z)$s satisfying the stringent requirements set by Stage IV surveys.

Figures (20)

• Figure 1: Flowchart describing the main components of the GalSBI-SPS galaxy population model. Galaxy physical properties are sampled hierarchically from analytical distributions and conditional relations, as detailed in Sect. \ref{['sect:gal_pop_model']}. The box colours reflect the hierarchy of dependencies. Blue boxes represent properties that are independently sampled, with the GSMF being the starting point of the sampling. Yellow boxes represent properties conditioned only on galaxy stellar mass and redshift (namely, the galaxy SFH, velocity dispersion, morphology, and AGN). Green boxes represent properties conditioned simultaneously on galaxy stellar mass, redshift, and SFH (galaxy metallicity history, dust attenuation, and gas ionisation). All sampled properties are then passed to ProSpect (red box) to generate galaxy SEDs and magnitudes. Together with the morphological properties they form the input catalogue for the forward-modelling. Each model component is described in its corresponding subsection of Sect. \ref{['sect:gal_pop_model']}.
• Figure 2: Redshift evolution of the characteristic stellar mass $\log{\mathcal{M^*}}$ for blue and red galaxies. Blue and red points represent the measurements from Weaver2023. The blue and red dashed lines are built by fitting the measurements with equation \ref{['eq:log_mstar_z_evo']}. The blue and red bands represent the $1 \sigma$ uncertainty band on the fit.
• Figure 3: Redshift evolution of the characteristic density $\phi^*$ for blue and red galaxies. Blue, deep sky blue, magenta and red points represent the measurements from Weaver2023. The Blue, deep sky blue, magenta and red dashed lines are built by fitting the measurements with equation \ref{['eq:phi_star_z_evo']}. The bands represent the $1 \sigma$ uncertainty band on the fit.
• Figure 4: Dependence of the SFH shape parameters on galaxy redshifts and stellar mass. $\log{(\mathrm{mpeak}')}$ linearly depends on the galaxy stellar mass and its coefficients in turns depend on the galaxy redshift. The upper panels show the dependence of the slope $\log{(\mathrm{mpeak}')}_{\mathrm{mass,evo,slope}}$ and the intercept $\log{(\mathrm{mpeak}')}_{\mathrm{mass,evo,intcpt}}$ on redshift. Red error bars refer to quiescent galaxies from the mass-complete GAMA and DEVILS sample, while blue error bars to star-forming galaxies. The dashed lines represent the best-fitting linear relations for the redshift evolution of these parameters, while the red and blue bands represent the $1 \sigma$ uncertainty on the fit. Lower panels show instead the dependence of $\log{(\mathrm{mperiod})}$ and $\mathrm{mskew}$ on the galaxy stellar mass. The error bars refer to the median and standard deviation of these quantities computed in bins of $0.1 \ \mathrm{dex}$ in stellar mass, overlayed on top of the overall distribution. Dashed lines and colour bands represent best-fitting linear relations and $1 \sigma$ uncertainty on the fit for the populations of red and blue galaxies.
• Figure 5: Correlation between $\log{(\tau_\mathrm{ISM})}$ and redshift, stellar mass and $sSFR$ for star-forming (left panel) and quiescent (right panel) galaxies from DEVILS data. The solid red (blue) line shows the best-fitting relation expressing this dependence for star-forming (quiescent) galaxies. The dotted lines show the observed rms scatter of the relation. Each sample contour encloses $50\%,84\%,99\%$ of the values.