When NASA's Nancy Grace Roman Space Telescope launches in the mid-2020s, it
will revolutionize astronomy by providing a panoramic field of view at least
100 times greater than Hubble's at similar image sharpness, or resolution.
The Roman Space Telescope will survey the sky up to thousands of times
faster than can be done with Hubble. This combination of wide field, high
resolution, and an efficient survey approach promises new understandings in
many areas, particularly in how galaxies form and evolve over cosmic time.
How did the largest structures in the universe assemble? How did our Milky
Way galaxy come to be in its current form? These are among the questions
that Roman will help answer.
Galaxies are conglomerations of stars, gas, dust, and dark matter. The
largest can span hundreds of thousands of light-years. Many gather together
in clusters containing hundreds of galaxies, while others are relatively
isolated.
How galaxies change over time depends on many factors: for example, their
history of star formation, how rapidly they formed stars over time, and how
each generation of stars influenced the next through supernova explosions
and stellar winds. To tease out these details, astronomers need to study
large numbers of galaxies.
"Roman will give us the ability to see faint objects and to view galaxies
over long intervals of cosmic time. That will allow us to study how galaxies
assembled and transformed," said Swara Ravindranath, an astronomer at the
Space Telescope Science Institute (STScI) in Baltimore, Maryland.
While wide-field imaging will be important for galaxy studies, just as
important are Roman's spectroscopic capabilities. A spectrograph takes light
from an object and spreads it into a rainbow of colors known as a spectrum.
From this range of colors, astronomers can glean many details otherwise
unavailable, like an object's distance or composition. Roman's ability to
provide a spectrum of every object within the field of view, combined with
Roman imaging, will enable astronomers to learn more about the universe than
from either imaging or spectroscopy alone.
Revealing When and Where Stars Were Born
Galaxies don't form stars at a constant rate. They speed up and slow
down—forming more or fewer stars—under the influence of a variety of
factors, from collisions and mergers to supernova shock waves and
galaxy-scale winds powered by supermassive black holes.
By studying a galaxy's spectrum in detail, astronomers can explore the
history of star formation. "Using Roman we can estimate how fast galaxies
are making stars and find the most prolific galaxies that are producing
stars at an enormous rate. More importantly, we can find out not only what's
happening in a galaxy at the moment we observe it, but what its history has
been," stated Lee Armus, an astronomer at IPAC/Caltech in Pasadena,
California.
Some precocious galaxies birthed stars very rapidly for a short time, only
to cease forming stars surprisingly early in the universe's history,
undergoing a rapid transition from lively to "dead."
"We know galaxies shut off star formation, but we don't know why. With
Roman's wide field of view, we stand a better chance of catching these
galaxies in the act," said Kate Whitaker, an astronomer at the University of
Massachusetts in Amherst.
Growing the Cosmic Web
Even as galaxies themselves have grown over time, they also have gathered
together in groups to form intricate structures billions of light-years
across. Galaxies tend to collect into bubbles, sheets, and filaments,
creating a vast cosmic web. By combining high-resolution imaging, which
yields a galaxy's position on the sky, with spectroscopy, which provides a
distance, astronomers can map this web in three dimensions and learn about
the universe's large-scale structure.
The expansion of the universe stretches light from distant galaxies to
longer, redder wavelengths—a phenomenon called redshift. The more distant a
galaxy is, the greater its redshift. Roman's infrared detectors are ideal
for capturing light from those galaxies. More distant galaxies are also
fainter and harder to spot. Combining this with the fact that that some
galaxy types are rare, you have to search a larger area of the sky with a
more sensitive observatory to find the objects that often have the most
interesting stories to tell.
"Right now, with telescopes like Hubble we can sample tens of high-redshift
galaxies. With Roman, we'll be able to sample thousands," explained Russell
Ryan, an astronomer at STScI.
Seeking the Unknown
While astronomers can anticipate many of the discoveries of the Roman Space
Telescope, perhaps most exciting is the possibility of finding things that
no one could have predicted. Typical high-resolution observations from
space-based observatories like Hubble, target specific objects for detailed
investigation. Roman's survey approach will cast a wide net, thereby opening
up a new "discovery space."
"Roman will excel in unknown unknowns. It will certainly find rare, exotic
things that we don't expect," said Ryan.
"Roman's combined imaging and spectroscopy surveys will gather the 'gold
nuggets' that we never would have mined otherwise," added Ravindranath.
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