Quasars are very bright, distant and active supermassive black holes
that are millions to billions of times the mass of the Sun. Typically located
at the centers of galaxies, they feed on infalling matter and unleash fantastic
torrents of radiation. Among the brightest objects in the universe, a quasar’s
light outshines that of all the stars in its host galaxy combined, and its jets
and winds shape the galaxy in which it resides.
This is an artist's concept of a galaxy with a brilliant quasar at its center. A quasar is a very bright, distant and active supermassive black hole that is millions to billions of times the mass of the Sun. Among the brightest objects in the universe, a quasar’s light outshines that of all the stars in its host galaxy combined. Quasars feed on infalling matter and unleash torrents of winds and radiation, shaping the galaxies in which they reside. Using the unique capabilities of Webb, scientists will study six of the most distant and luminous quasars in the universe. Credits: NASA, ESA and J. Olmsted (STScI)
Shortly after its launch later this year, a team of scientists will train
NASA’s James Webb Space Telescope on six of the most distant and luminous
quasars. They will study the properties of these quasars and their host
galaxies, and how they are interconnected during the first stages of galaxy
evolution in the very early universe. The team will also use the quasars to
examine the gas in the space between galaxies, particularly during the period
of cosmic reionization, which ended when the
universe was very young. They will accomplish this using Webb’s extreme
sensitivity to low levels of light and its superb angular resolution.
Webb: Visiting the Young Universe
As Webb peers deep into the universe, it will actually look back in time.
Light from these distant quasars began its journey to Webb when the universe
was very young and took billions of years to arrive. We will see things as they
were long ago, not as they are today.
“All these quasars we are studying existed very early, when the universe was
less than 800 million years old, or less than 6 percent of its current age. So
these observations give us the opportunity to study galaxy evolution and
supermassive black hole formation and evolution at these very early times,”
explained team member Santiago Arribas, a research professor at the Department
of Astrophysics of the Center for Astrobiology in Madrid, Spain. Arribas is
also a member of Webb’s Near-Infrared Spectrograph (NIRSpec) Instrument Science
Team.
The light from these very distant objects has been stretched by the
expansion of space. This is known as cosmological redshift. The farther the light
has to travel, the more it is redshifted. In fact, the visible light emitted at
the early universe is stretched so dramatically that it is shifted out into the
infrared when it arrives to us. With its suite of infrared-tuned instruments,
Webb is uniquely suited to studying this kind of light.
Studying Quasars, Their Host Galaxies and Environments, and Their Powerful
Outflows
The quasars the team will study are not only among the most distant in the
universe, but also among the brightest. These quasars typically have the
highest black hole masses, and they also have the highest accretion rates — the
rates at which material falls into the black holes.
“We’re interested in observing the most luminous quasars because the very
high amount of energy that they’re generating down at their cores should lead
to the largest impact on the host galaxy by the mechanisms such as quasar
outflow and heating,” said Chris Willott, a research scientist at the Herzberg
Astronomy and Astrophysics Research Centre of the National Research Council of
Canada (NRC) in Victoria, British Columbia. Willott is also the Canadian Space
Agency’s Webb project scientist. “We want to observe these quasars at the
moment when they’re having the largest impact on their host galaxies.”
An enormous amount of energy is liberated when matter is accreted by the
supermassive black hole. This energy heats and pushes the surrounding gas
outward, generating strong outflows that tear across interstellar space like a
tsunami, wreaking havoc on the host galaxy.
Outflows play an
important role in galaxy evolution. Gas fuels the
formation of stars, so when gas is removed due to outflows, the star-formation
rate decreases. In some cases, outflows are so powerful and expel such large
amounts of gas that they can completely halt star formation within the host
galaxy. Scientists also think that outflows are the main mechanism by which
gas, dust and elements are redistributed over large distances within the galaxy
or can even be expelled into the space between galaxies – the intergalactic
medium. This may provoke fundamental changes in the properties of both the host
galaxy and the intergalactic medium.
Examining Properties of Intergalactic Space During the Era of Reionization
More than 13 billion years ago, when the universe was very young, the view
was far from clear. Neutral gas between galaxies made the universe opaque to
some types of light. Over hundreds of millions of years, the neutral gas in the
intergalactic medium became charged or ionized, making it transparent to
ultraviolet light. This period is called the Era of Reionization. But what led
to the reionization that created the “clear” conditions detected in much of the
universe today? Webb will peer deep into space to gather more information about
this major transition in the history of the universe. The observations will
help us understand the Era of Reionization, which is one of the key frontiers in
astrophysics.
The team will use quasars as background light sources to study the gas
between us and the quasar. That gas absorbs the quasar’s light at specific
wavelengths. Through a technique called imaging spectroscopy, they will look for
absorption lines in the intervening gas. The brighter the quasar is, the
stronger those absorption line features will be in the spectrum. By determining
whether the gas is neutral or ionized, scientists will learn how neutral the
universe is and how much of this reionization process has occurred at that
particular point in time.
“If you want to study the universe, you need very bright background
sources. A quasar is the perfect object in the distant universe, because it’s
luminous enough that we can see it very well,” said team member Camilla
Pacifici, who is affiliated with the Canadian Space Agency but works as an
instrument scientist at the Space Telescope Science Institute in Baltimore. “We
want to study the early universe because the universe evolves, and we want to
know how it got started.”
The team will analyze the light coming from the quasars with NIRSpec to
look for what astronomers call “metals,” which are elements heavier than
hydrogen and helium. These elements were formed in the first stars and the
first galaxies and expelled by outflows. The gas moves out of the galaxies it
was originally in and into the intergalactic medium. The team plans to measure
the generation of these first “metals,” as well as the way they’re being pushed
out into the intergalactic medium by these early outflows.
The Power of Webb
Webb is an extremely sensitive telescope able to detect very low levels of
light. This is important, because even though the quasars are intrinsically
very bright, the ones this team is going to observe are among the most distant
objects in the universe. In fact, they are so distant that the signals Webb
will receive are very, very low. Only with Webb’s exquisite sensitivity can
this science be accomplished. Webb also provides excellent angular resolution,
making it possible to disentangle the light of the quasar from its host galaxy.
The quasar programs described here are Guaranteed Time Observations involving the
spectroscopic capabilities of NIRSpec.
The James Webb Space Telescope will be the world's premier space science
observatory when it launches in 2021. Webb will solve mysteries in 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 program led by NASA with its partners, ESA (European Space
Agency) and the Canadian Space Agency.
For more information about Webb, visit www.nasa.gov/webb.
