It Would Take Hubble 85 Years to Match What Nancy Grace Roman Will See in 63 Days

Less than a year and a half into its primary mission, the James Webb Space Telescope (JWST) has already revolutionized astronomy as we know it. Using its advanced optics, infrared imaging, and spectrometers, the JWST has provided us with the most detailed and breathtaking images of the cosmos to date. But in the coming years, this telescope and its peers will be joined by another next-generation instrument: the Nancy Grace Roman Space Telescope (RST). Appropriately named after “the Mother of Hubble,” Roman will pick up where Hubble left off by peering back to the beginning of time.

Like Hubble, the RST will have a 2.4-meter (7.9 ft) primary mirror and advanced instruments to capture images in different wavelengths. However, the RST will also have a gigantic 300-megapixel camera – the Wide Field Instrument (WFI) – that will enable a field of view two-hundred times greater than Hubble’s. In a recent study, an international team of NASA-led researchers described a simulation they created that previewed what the RST could see. The resulting data set will enable new experiments and opportunities for the RST once it takes to space in 2027.

The team included researchers from the Astrophysics Science Division at NASA’s Goddard Space Flight Center, the Flatiron Institute’s Center for Computational Astrophysics, the National Astronomical Observatory of Japan (NAOJ), the South African Astronomical Observatory (SAAO), the Space Telescope Science Institute (STScI), the European Southern Observatory (ESO), the Mitchell Institute for Fundamental Physics and Astronomy, the Ecole Polytechnique Fédeérale de Lausanne (EPFL), and multiple universities.

Side view of the simulated Universe, each dot represents a galaxy whose size and brightness corresponding to its mass. Credits: NASA/GSFC/A. Yung

The simulation was based on a well-tested theory of galaxy formation that incorporates the most widely accepted cosmological model – the Lambda Cold Dark Matter (LCDM) model. This allowed the team to simulate five light cones measuring two-square-degree in diameter (about ten times the apparent size of a full Moon) that contained over 5 million galaxies each. These galaxies were distributed across the redshift spectrum (z=1-10), corresponding to distances of 1 million and over 13 billion light-years.

The paper describing their results was published in The Monthly Notices of the Royal Astronomical Society in December 2022. Aaron Yung, a postdoctoral fellow at NASA’s Goddard Space Flight Center who led the study, said in a recent NASA press release:

“The Hubble and James Webb Space Telescopes are optimized for studying astronomical objects in depth and up close, so they’re like looking at the universe through pinholes. To solve cosmic mysteries on the biggest scales, we need a space telescope that can provide a far larger view. That’s exactly what Roman is designed to do.”

When it commences operations, these and other simulations will provide a framework for astronomers that can be compared to observational data. This will allow scientists to scrutinize their astrophysical and cosmological models, with implications for everything from the formation and evolution of galaxies to Dark Matter, Dark Energy, and much more. This will be possible thanks to Roman’s ability to combine a field of view two orders of magnitude larger than Hubble (and an angular resolution to match) with advanced spectroscopy.

For instance, by observing how Dark Matter causes light from more distant objects to be warped and amplified (gravitation lensing), Roman will help us see how Dark Matter Haloes developed over time. Whereas it would take other space telescopes close to a century (or more) to map out these vast cosmic structures, Roman could do the same job within 63 days. In addition to its wide field of view, this will be made possible thanks to the observatory’s fast slewing speed and rigid structure. Basically, Roman can move rapidly from one target to the next since its components (like the solar arrays) are fixed in place.

This means that vibrations caused by repositioning will subside quickly, cutting down on the wait time between image acquisition. “Roman will take around 100,000 pictures every year,” said Jeffrey Kruk, a research astrophysicist at NASA Goddard (and a co-author on the paper). “Given Roman’s larger field of view, it would take longer than our lifetimes even for powerful telescopes like Hubble or Webb to cover as much sky.”

Another exciting aspect of the RST is how it will collaborate with other observatories to study the Universe in more detail. This includes identifying targets for follow-up studies using Hubble‘s broader wavelength coverage and Webb‘s more detailed infrared observations. This will provide in-depth studies of cosmic objects ranging from galaxies and galaxy clusters to exoplanets and objects in the Solar System. Said Yung:

“Roman will have the unique ability to match the depth of the Hubble Ultra Deep Field, yet cover several times more sky area than wide surveys such as the CANDELS survey. Such a full view of the early universe will help us understand how representative Hubble and Webb’s snapshots are of what it was like then.” 

“Simulations like these will be crucial in connecting unprecedented large galaxy surveys from Roman to the unseen scaffolding of dark matter that determines the distribution of those galaxies,” added Sangeeta Malhotra, an astrophysicist at Goddard and a co-author of the paper. All told, the simulation provides forecasts on the number density of galaxies, star formation rates (SFR), field-to-field variance, and angular two-point correlation functions. It also demonstrates how future wide-field surveys will be able to improve these measurements relative to current surveys.

In addition to the RST, the team’s simulations also provide photometry for several other instruments on upcoming observatories. This includes the ESA’s Euclid mission and the Vera Rubin Observatory, a space-based telescope that will study Dark Energy and a ground-based observatory that will characterize millions of objects in the Solar System (respectively). Both missions are expected to launch or begin collecting light sometime later this year. The coming years will be an exciting time for astronomers and cosmologists. And, with any luck, revelatory!

Further Reading: NASA, MNRAS