Scientists have created a huge synthetic survey that shows what we can expect from future observations of Rome’s Nancy Grace Space Telescope. Although it represents only a small slice of the actual future survey, this simulated version contains a staggering number of galaxies – 33 million of them, along with 200,000 foreground stars in our home galaxy.
Cosmic construction
Roman’s High Latitude Wide Area Survey will consist of imaging — the focus of the new simulation — and spectroscopy across the same vast swath of space. Spectroscopy involves measuring the intensity of light from cosmic objects at different wavelengths, while Roman imaging will reveal the exact positions and shapes of hundreds of millions of faint galaxies, which will be used to map dark matter. Although this mysterious substance is invisible, astronomers can infer its presence by observing its effects on ordinary matter.
Anything with mass warps the fabric of spacetime. The greater the weight, the greater the warp. This creates an effect called gravitational lensing, which occurs when light from a distant source is distorted as it passes by intervening objects. When these lensing objects are massive galaxies or galaxy clusters, background sources can be blurred or appear as multiple images.
Less massive objects can create subtler effects called weak lensing. Roman will be sensitive enough to use a weak lens to see how clumps of dark matter distort the appearance of distant galaxies. By observing these lensing effects, scientists will be able to fill in other gaps in our understanding of dark matter.
“Cosmic structure formation theories predict how seed fluctuations in the early universe grow into the mass distribution that can be seen through gravitational lensing,” said Chris Hirata, a professor of physics at Ohio State University in Columbus and co-author of the paper. “But the predictions are statistical in nature, so we test them by observing large regions of space.” Roman, with its wide field of view, will be optimized for efficient exploration of the sky and will complement observatories such as the James Webb Space Telescope, which are designed for deeper investigation of individual objects.”
Earth and space
A synthetic Roman survey covers 20 square degrees of the sky, roughly equivalent to 95 full moons. The actual survey will be 100 times larger and reveal more than a billion galaxies. Rubin will scan an even larger area — 18,000 square degrees, nearly half the entire sky — but at a lower resolution because it will have to peer through Earth’s turbulent atmosphere.
Pairing the Roman and Rubin simulations offers scientists the first opportunity to try to detect the same objects in both sets of images. This is important because ground-based observations are not always sharp enough to distinguish multiple nearby sources as separate objects. Sometimes they blur together, which affects the poor measurement of the lens. Now, researchers can determine the difficulties and benefits of “unmixing” such objects in Rubin’s images by comparing them to Roman ones.
With Roman’s colossal cosmic view, astronomers will be able to accomplish much more than the primary goals of the survey, which are studying the structure and evolution of the universe, mapping dark matter, and distinguishing between the leading theories that attempt to explain why the expansion of the universe is accelerating. Scientists can comb through the new simulated Roman data for a taste of the bonus science that will come from seeing so much of the universe in such exquisite detail.
“With Roman’s gigantic field of view, we expect many different scientific opportunities, but we will also have to learn to expect the unexpected,” said Julie McEnery, senior project scientist for the Roman mission at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. . “The mission will help answer critical questions in cosmology while potentially revealing entirely new mysteries for us to solve.”
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