Emulating Galacticus for the Roman Galaxy Redshift Survey

The Roman Galaxy Redshift Survey will measure redshifts from slitless grism spectroscopy, and testing how reliably those redshifts can be extracted requires realistic simulated surveys - mock skies whose galaxy number densities, sizes, and emission-line strengths are right, not merely plausible. We build these by applying the Galacticus semi-analytic galaxy formation model to dark matter halo merger trees from the UNIT N-body simulations, and placing the resulting galaxies on a lightcone. This only works if Galacticus is calibrated against the observables that matter, and calibration is expensive: a single likelihood evaluation involving a stellar mass function requires evolving thousands of halos.

I will describe our approach to this problem. Building on an earlier fast- likelihood method that calibrates against the stellar-to-halo mass relation rather than the mass function directly, we now emulate Galacticus with Gaussian processes. From a 1024-point Latin hypercube spanning 20 model parameters, we train emulators — PCA-compressed, one Gaussian process per component — that predict stellar mass functions, star formation rate functions, sizes, and Hα luminosity functions in milliseconds, with calibrated uncertainties. Held-out tests show accuracies of 0.05–0.13 dex where the observational data lie. This makes simultaneous MCMC calibration against many datasets tractable, and reveals which observables constrain which parameters — including tensions between mass functions and galaxy sizes that point to genuine missing flexibility in the model physics rather than shortcomings of the fit. I will close with the status of the calibrated 16 deg² lightcone being delivered for the team's grism image simulations.

Speaker: Andrew Benson (Carnegie Science)

The Roman Lecture Series is a monthly virtual lecture series focused on the scientific capabilities and technology of the Nancy Grace Roman Space Telescope, organized by Roman mission partners.