OpenUniverse2024: A shared, simulated view of the sky for the next generation of cosmological surveys

TL;DR

OpenUniverse2024 tackles the need for realistic, joint simulations of LSST and Roman skies to enable cross-survey calibration and science. It presents an end-to-end framework that combines Outer Rim N-body halos, the Diffsky extragalactic model, GalSampler galaxy transfer, a comprehensive transient catalog, and end-to-end image formation through imSim and GalSim, all standardized by SkyCatalog inputs. The result is hundreds of terabytes of synthetic imaging and millions of objects over roughly 70 square degrees, with updated galaxy colors, extended transient models, and updated instrument and survey realism. The data and code are publicly released to empower the community, accelerate pipeline development, and enable joint science such as weak lensing, galaxy evolution, and time-domain astronomy across LSST and Roman, with an eye toward future Euclid-like overlaps.

Abstract

The OpenUniverse2024 simulation suite is a cross-collaboration effort to produce matched simulated imaging for multiple surveys as they would observe a common simulated sky. Both the simulated data and associated tools used to produce it are intended to uniquely enable a wide range of studies to maximize the science potential of the next generation of cosmological surveys. We have produced simulated imaging for approximately 70 deg$^2$ of the Vera C. Rubin Observatory Legacy Survey of Space and Time (LSST) Wide-Fast-Deep survey and the Nancy Grace Roman Space Telescope High-Latitude Wide-Area Survey, as well as overlapping versions of the ELAIS-S1 Deep-Drilling Field for LSST and the High-Latitude Time-Domain Survey for Roman. OpenUniverse2024 includes i) an early version of the updated extragalactic model called Diffsky, which substantially improves the realism of optical and infrared photometry of objects, compared to previous versions of these models; ii) updated transient models that extend through the wavelength range probed by Roman and Rubin; and iii) improved survey, telescope, and instrument realism based on up-to-date survey plans and known properties of the instruments. It is built on a new and updated suite of simulation tools that improves the ease of consistently simulating multiple observatories viewing the same sky. The approximately 400 TB of synthetic survey imaging and simulated universe catalogs are publicly available, and we preview some scientific uses of the simulations.

Figures (17)

• Figure 1: A comparison of color coadd images from the Roman HLWAS (left; Y106/J129/H158 color composite) and LSST WFD (right; g/r/i color composite) overlapping a single LSST patch of the synthetic sky with central coordinates $(\textrm{RA}, \textrm{Dec})=(9.55^{\circ}, 44.1^{\circ})$, $\Delta\textrm{RA}=0.325^{\circ}$, and $\Delta\textrm{Dec}=0.233^{\circ}$. Both coadds are full simulated-survey depth -- the full WAS-depth for Roman and the first five years of the simulated observing sequence for LSST WFD. The LSST coadd matches the native pixel scale of 0.2 arcsec, while the Roman imcom coadd is significantly oversampled from the native Roman pixel scale of 0.11 arcsec to a coadd pixel scale of 0.039 arcsec, in order to achieve a Nyquist sampled coadd image. Note that the images are substantially degraded in quality for rendition in this document.
• Figure 2: A demonstration of how the image scene is built. Each panel is approx. 50 arcsec across. Each unsaturated star, transient, and galaxy is built in the image scene photon-by-photon. Galaxies are built from a composite morphological model consisting of a bulge, disk, and star-forming knot regions. The SED and flux fraction of each component of the galaxy is self-consistently generated in the Diffsky model to represent the appropriate populations of stars present in each component of the galaxy model.
• Figure 3: LSST $r$-band cumulative magnitude distribution for the extragalactic catalog. This is compared to an observed cumulative magnitude distribution from HSC Deep survey data, which is extrapolated to magnitudes fainter than 25. The two begin to diverge in the shaded column at faint magnitudes where the simulation is incomplete. The dark gray band represents uncertainty on the HSC extrapolation.
• Figure 4: LSST redshift distributions and uncertainties for selected ranges of the LSST $r$-band magnitude for the extragalactic catalog. These measurements from the simulation are compared to predicted LSST redshift distributions from coil2004.
• Figure 5: Redshift evolution of broadband optical galaxy colors in the Diffsky mock catalog (green) compared with equivalent samples of galaxy colors from COSMOS20 (black). Each panel contains a different subsample of objects drawn from each limiting-magnitude selection. Figure is broken into: (left to right) u-g, g-r, i-z, and z-y colors, and (top to bottom) several limiting magnitude ranges.