Abell 2744, nicknamed Pandora's Cluster, is a giant pile-up of four
smaller galaxy clusters. The cluster is so massive that its powerful
gravity bends the light from galaxies far behind it, making the
background objects appear larger and brighter in a phenomenon called
gravitational lensing. Shown in this Hubble image, the mammoth Abell
2744 cluster is located about 3.5 billion light-years away. Credits: NASA, ESA, and J. Lotz, M. Mountain, A. Koekemoer, and the HFF Team (STScI). Hi-res images
How did the first galaxies in the universe form, and did they make the
universe transparent to light? How did later galaxies produce and
disperse into the universe the heavier elements that are the building
blocks of stars, planets, and even humans? These are questions
astronomers will address in some of the first observations made by
NASA’s James Webb Space Telescope, slated to launch in March 2021.
Astronomers hope the answers will lead to a better understanding of the
origins and evolution of the universe.
Through the combined power of NASA’s James Webb Space Telescope and
gravity creating “natural telescopes” in space, astronomers hope to
answer two science questions that are fundamental to understanding the
origins and evolution of the universe:
- How did the first galaxies in the universe form, and did they make the universe transparent to light?
-
How did later galaxies produce and disperse into the universe the
heavier elements that are the building blocks of stars, planets, and
even humans?
These questions will be addressed in some of the
first observations made by the Webb telescope, slated to launch in March
2021. These observations will be part of the Director’s Discretionary-Early Release Science
program, which provides time to selected projects early in the
telescope’s mission. This program allows the astronomical community to
quickly learn how best to use Webb’s capabilities, while also yielding
robust science.
An international team led by Tommaso Treu of the
University of California, Los Angeles, has been investigating how Webb
can tackle these two key questions about the universe in the Early
Release Science program.
Treu and his team will study the
earliest, most distant galaxies to investigate their origins. After the
big bang, the universe cooled down. As it cooled, protons and electrons
combined to form neutral hydrogen atoms, until the universe became
filled with hydrogen and opaque to light. Then at some point, the first
galaxies formed, and scientists think these first galaxies emitted
enough ultraviolet light to destroy the neutral hydrogen atoms and make
the universe transparent to light. This is called the end of the “dark
ages.”
“We’re not exactly sure when this happens, and we think
it’s galaxies making the universe transparent, but we are not totally
sure,” Treu said. “One of the things our proposal will try to do is
establish whether indeed galaxies are the ones that are making the
universe transparent — ending the cosmic dark ages — and what kind of
galaxies they are, what are their properties, and when this happens.”
Using Gravity as a “Natural Telescope”
To
see the faintest, farthest galaxies, the team will combine the power of
Webb with the magnification of a “natural telescope” in space. The
phenomenon, called gravitational lensing, occurs when a huge amount of
matter, such as a cluster of galaxies, creates a gravitational field
that distorts and magnifies the light from distant galaxies that are
behind it, but in the same line of sight. The effect allows researchers
to study the details of early galaxies too far away to be seen with
current technology and telescopes.
One gravitational lens is Abell
2744, an enormous cluster of four smaller galaxy clusters. Also known
as Pandora’s Cluster, this giant collection of galaxies has been well
studied, including by NASA’s Hubble Space Telescope. Abell 2744 is one
of many clusters that scientists can use in combination with Webb to
peer back into the universe’s distant past.
“It’s a cluster that
we know very well,” Treu said. “The fact that we know it so well means
that we can calculate very precisely the properties of the lens. Using
our models, we can compute very accurately how the background images
have been distorted. Then we can invert that to figure out the intrinsic
properties of the objects as they would look without the lens in
front."
Simultaneously, the team will take deep images in the near
and medium infrared of two fields offset from the cluster. “We will use
those to count galaxies in the very early universe and figure out how
many there are,” explained Treu. “Those are the sources that are
suspected to eventually produce the ionizing photons that end the dark
ages.”
Forming the Universe’s Heavier Elements
The
big bang only formed hydrogen, helium, and traces of other light
elements. Heavier elements like iron, oxygen, and carbon, which are made
in stars, eventually ended up in the universe — but scientists don’t
know exactly how this process happened.
“In astronomy, we think of
hydrogen and helium as the light elements, and everything else we call a
‘metal,’” explained team member Alaina Henry of the Space Telescope
Science Institute in Baltimore, Maryland. “We want to measure the metals
that are produced by the first stars in the first supernovae. This
tells us how the stars evolve, and how many end their lives as
supernovae, where the heaviest elements — such as iron — are made.”
Identifying the “Fingerprints” of Elements in the Light
Answering
both questions requires the unique spectroscopic capabilities of the
Webb telescope. Spectroscopy separates an object’s light into its
component colors, allowing scientists to see the “fingerprints” of
different elements. By analyzing these spectral fingerprints,
astronomers can determine the physical properties of that object,
including its temperature, mass, luminosity, and composition.
Treu
and his team will use two different spectrographs on Webb, each with
different strengths and functions. Comparing and contrasting these
capabilities is an important technical goal of their program.
Webb’s
Near Infrared Imager and Slitless Spectrograph (NIRISS) gives observers
spatial information, so they can determine how a spectrum changes
across the sky. However, it has relatively low spectral resolution,
meaning it is harder to differentiate between very similar colors.
The
telescope’s Near Infrared Spectrograph (NIRSpec) has a quarter of a
million tiny microshutters, each as wide as a human hair. These shutters
can be opened or closed individually to isolate the light from a
particular object. “In exchange for that, you lose spatial information,”
said Treu, “but you get much higher spectral information. You can see
the motion of the gas, both within galaxies and flowing in and out of
them.”
“Webb will effectively be a much more capable spectrograph
than we have ever had in space,” Treu added. “It will have multiple
instruments to disperse the light. We need to understand the strengths
of each one and how they complement each other.”
Expectations
Looking
deep into the cosmos, Treu and his team expect to get a very good idea
of the opacity of the universe, and also learn how ionizing photons —
particles of light — escaped from the very early galaxies. They will
also observe nearer galaxies at later times, when the galaxies are
forming stars very vigorously. “We will get the best view ever of this
process of gas flowing in, forming stars, and then being blown out by
super-winds,” Treu said.
“It would be really fun if we found spectral features that we hadn’t seen very often, or maybe not at all before,” added Henry.
The
James Webb Space Telescope will be the world’s premier space science
observatory when it launches in 2021. Webb will solve mysteries of our
solar system, look beyond to distant worlds around other stars, and
probe the mysterious structures and origins of our universe and our
place in it. Webb is an international project led by NASA with its
partners, ESA (European Space Agency) and the Canadian Space Agency.
