Euclid data: End-to-End Simulations

Computer simulations are a unique laboratory to model the origin and evolution of the universe. These digital laboratories allow us to investigate the origin of the large-scale pattern in the galaxy distribution, i.e, the so-called “cosmic web” we observe from the light emitted by distant galaxies using the most powerful telescopes, like the ESA Euclid satellite mission. In turn, using the information encoded in the galaxy distribution, we can test the standard cosmological model, and understand the nature of the mysterious dark-energy that drives the observed accelerated expansion of the universe.

In order to match the unprecedented quality and resolution of the images that Euclid will obtain, scientists in its consortium have undertaken a massive effort to model the formation and evolution of large-scale structures in the universe, such as galaxies, galaxy clusters, and the filamentary structures they form, using state-of-the art numerical simulations. Modeling the evolution of distant galaxies over cosmic times is closely linked to the dynamics of the underlying dark-matter structures where galaxies live. Using numerical simulations that evolve a large number of dark-matter particles over cosmological volumes under their own gravitational pull, we can predict how the most distant galaxies formed and evolved, and their distribution across the celestial sphere.

Euclid Flagship Simulations

A team of Euclid researchers, have recently created the largest simulated galaxy catalogue ever produced, the Euclid Flagship mock galaxy catalogue. It is based on a record-setting supercomputer simulation of four trillion dark matter particles developed using a highly efficient N-body code, PKDGRAV3, a high performance N-body treecode for self-gravitating astrophysical simulations. PKDGRAV3 is designed to run efficiently in serial and on a wide variety of parallel computers including both shared memory and message passing architectures, with or without GPU acceleration. Aside from the dark-matter particles, this simulation also includes the gravitational effect of massive neutrinos that leave a characteristic imprint in the large-scale distribution of dark-matter and galaxies. Besides, general relativistic effects, such as radiation and gravitational potential fluctuations, that are present in the early evolution of the universe, have also been accurately modeled.  On the other hand, thanks to its very large volume, the Flagship simulation is able to portray very distant galaxies we observe today that emitted their light more than 10 giga-years ago, when the universe was at its very infancy. This will provide us with a unique window to investigate how galaxies form and evolve across their entire lifetime.

But in order to model galaxies from the dark-matter distribution simulated by N-body codes, Euclid scientists have designed a pipeline to assign galaxies to dark-matter structures, called halos, using state-of-the-art observational recipes. The scheme used is based on a combination of the so-called halo occupation distribution (i.e, how many galaxies populate a host dark-matter halo of a given mass) and abundance matching techniques that associate the most massive halos to the brightest galaxies that we observe. The complex code used for the galaxy assignment step is highly modular and has been run in a massively parallel high performance computing platform hosted by the Spanish Euclid Data Center at the “Port d’Informacio Cientifica” in Barcelona.

The resulting simulation reproduces with exquisite precision the emergence of the large-scale structure of the Universe – galaxies and galaxy clusters within the network of the cosmic web that comprises both dark and visible matter: it contains about 150 billion halos across the sky, with masses as small as one hundredth the mass of our Milky Way halo, hosting more than two billion galaxies distributed over the 3D space that Euclid will survey.

Euclid Flagship Simulation mock galaxy catalog
Excerpt of the simulation, spanning from today’s local Universe (left) back to when it was about 3 billion years old (right), when clusters of galaxies were beginning to form. Zooming in (click on image) provides finer and finer detail. Central galaxies, which populate the center of dark matter ‘halos’, are coloured blue. Satellite galaxies, which reside in massive halos orbiting around their central galaxies, are indicated in red.

The simulation also mimics the complex properties that real sources display, such as shapes, sizes, colours, luminosities, and other observational properties relevant for galaxy formation and evolution, including fluxes in many passbands in the optical and infra-red, spectral energy distributions, star formation rate, metallicity, and many different emission lines present in their spectra like the H-alpha lines that will be detected by the spectrophotometer on-board the Euclid satellite. The galaxy mock also incorporates weak lensing distortions in the light emitted by distant galaxies, caused by the intervening matter distribution which, along with the galaxy sky positions and estimated distances from us, constitute the basic cosmological information that will be exploited by the Euclid satellite to understand the nature of dark-energy and place very tight constraints on possible deviations from the General Relativity Theory.

Realistic pixel-level simulations

This huge synthetic galaxy catalogue is the key input for the generation of simulated images of what the Euclid telescope will obtain through its two main instruments. But in order to reproduce the observed universe with high fidelity, sources of emission from our own solar system and milky way galaxy, all the real-world effects coming from the instrument, and other systematic observational effects have to be included in a complex end-to-end pipeline.

Armed with this new virtual universe, scientists will be able to best prepare for the mission and also assess its performance. Moreover, it will be an essential tool to develop the data processing and the science analysis software needed for such a data-heavy mission.

Updated 2023-05-15, KJ

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