- 4590 - GALAXIES - what are the earliest found? - The James Webb Space Telescope (JWST) is the largest and most powerful space telescope built to date. Since it was launched in December 2021 it has provided groundbreaking insights. These include discovering the earliest and most distant known galaxies, which existed just 300 million years after the Big Bang.
---------------------------------- 4590 - GALAXIES - what are the earliest found?
- Distant objects are also very ancient
because it takes a long time for the light from these objects to reach
telescopes. JWST has now found a number of these very early galaxies. We're
effectively looking back in time at these objects, seeing them as they looked
shortly after the birth of the universe.
-
- These observations from JWST agree with our
current understanding of cosmology and of galaxy formation. But they also
reveal aspects we didn't expect. Many of these early galaxies shine much more
brightly than we would expect given that they existed just a short time after
the Big Bang.
-
- Brighter galaxies are thought to have more
stars and more mass. It was thought that much more time was needed for this
level of star formation to take place. These galaxies also have actively
growing black holes at their centers.
This is a sign that these objects matured quickly after the Big Bang.
-
- Scientists have been able to study these
early galaxies by combining JWST's detailed images with its powerful
capabilities for spectroscopy.
Spectroscopy is a method for interpreting the electromagnetic radiation
that's emitted or absorbed by objects in space. This in turn can tell you about
the properties of an object.
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- Our understanding of cosmology and galaxy
formation rests on a few fundamental ideas. One of these is the “cosmological
principle”, which states that, on a large scale, the universe is homogeneous
(the same everywhere) and isotropic (the same in all directions). Combined with
Einstein's theory of general relativity, this principle allows us to connect
the evolution of the universe to its energy and mass content.
-
- The James Webb telescope has brought
cosmology to a tipping point. Will it soon reveal new physics? The standard cosmological model, known as the
"Hot Big Bang" theory, includes three main components. One is the
ordinary matter that we can see with our eyes in galaxies, stars and planets. A
second ingredient is cold dark matter (CDM), slow-moving matter particles that
do not emit, absorb or reflect light.
-
- The third component is what's known the
“cosmological constant” (Λ, or lambda). This is linked to something called
“dark energy” and is a way of explaining the fact that the expansion of the
universe is accelerating. Together, these components form what is called the
ΛCDM model of cosmology.
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- “Dark energy” makes up about 68% of the
total energy content of today's universe.
Despite not being directly observable with scientific instruments, “dark
matter” is thought to make up most of the matter and comprises about 27% of the
universe's total mass and energy content.
-
- While dark matter and dark energy remain
mysterious, the ΛCDM model of cosmology is supported by a wide range of
detailed observations. These include the measurement of the universe's
expansion, the cosmic microwave background, or CMB (the "afterglow"
of the Big Bang) and the development of galaxies and their large-scale
distribution and the way that galaxies cluster together.
-
- The ΛCDM model lays the groundwork for our
understanding of how galaxies form and evolve.
The CMB, which was emitted about 380,000 years after the Big Bang,
provides a snapshot of early fluctuations in density that occurred in the early
universe. These fluctuations, particularly in dark matter, eventually developed
into the structures we observe, galaxies and stars.
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- How does gas in galaxies cool and condense
to form stars? The effects of
supernovae, stellar winds and black holes that emit significant amounts of
energy (sometimes called active galactic nuclei, or AGN) can all heat or expel
gas from galaxies. This in turn can boost or curtail star formation and
therefore influence the growth of galaxies.
-
- The efficiency and scale of these
"feedback processes", as well as their cumulative impact over time,
are poorly understood. They are a significant source of uncertainty in
mathematical models, or simulations, of galaxy formation.
-
- Significant advances in complex numerical
simulations of galaxy formation have been made over the past ten years. They
relate star formation to the evolution of dark matter halos. These halos are
massive, invisible structures made from dark matter that effectively anchor
galaxies within them.
-
- One model of galaxy formation assumes that
the rate at which stars form in a galaxy is directly tied to gas flowing into
those galaxies. This model also proposes that the star formation rate in a
galaxy is proportional to the rate at which dark matter halos grow. It assumes
a fixed efficiency at converting gas into stars.
-
- This "constant star formation
efficiency" model is consistent with star formation increasing
dramatically in the first billion years after the Big Bang. The rapid growth of
dark matter halos during this period would have provided the necessary
conditions for galaxies to form stars efficiently. Despite its simplicity, this
model has successfully predicted a wide range of real observations, including
the overall rate of star formation.
-
- JWST has ushered in a new era of discovery.
With its advanced instruments, the space telescope can capture both detailed
images and high resolution spectra that charts the intensity of electromagnetic
radiation emitted or absorbed by objects in the sky. For JWST, these spectra
are in the near infrared region of the electromagnetic spectrum. Studying this
region is crucial for observing early galaxies whose optical light has turned
into near infrared (or "redshifted") as the universe has expanded.
-
- Redshift describes how the wavelengths of
light from galaxies become stretched as they travel. The more distant a galaxy
is, the greater its redshift. Over the
past two years, JWST has identified and characterized galaxies at redshifts
with values of between 10 and 15. These galaxies, which formed around 200-500
million years after the Big Bang, are relatively small for galaxies (about 100
parsecs, or 3 quadrillion kilometres, across). They each consist of around 100
million stars, and form new stars at a rate of about one sun-like star per
year.
-
- While this does not sound very impressive,
it implies that these systems double their content of stars within only 100
million years. For comparison, our own Milky Way galaxy takes about 25 billion
years to double its stellar mass.
-
- The surprising findings from JWST of bright
galaxies at high redshifts, or distances, could imply that these galaxies
matured faster than expected after the Big Bang. This is important because it
would challenge existing models of galaxy formation.
-
- The constant star-formation efficiency
model while effective at explaining much of what we see, struggles to account
for the large number of bright and distant galaxies observed with a redshift of
more than ten.
-
- To address this, scientists are exploring
various possibilities. These include changes to their theories of how
efficiently gas is converted into stars over time. They are also reconsidering
the relative importance of the feedback processes how phenomena such as
supernovae and black holes also help regulate star formation.
-
- Some theories suggest that star formation
in the early universe may have been more intense or "bursty" than
previously thought, leading to the rapid growth of these early galaxies and
their apparent brightness.
-
- Others propose that different factors, such
as lower amounts of galactic dust, a top-heavy distribution of star masses, or
contributions from phenomena such as active black holes, could be responsible
for the unexpected brightness of these early galaxies.
-
- These explanations invoke changes to galaxy
formation physics in order to explain JWST's findings. But scientists have also
been considering modifications to broad cosmological theories. For example, the
abundance of early, bright galaxies could be partly explained by a change to
something called the matter power spectrum. This is a way to describe density
differences in the universe.
-
- One possible mechanism for achieving this
change in the matter power spectrum is a theoretical phenomenon called
"early dark energy". This is the idea that a new cosmological energy
source with similarities to dark energy may have existed at early times, at a
redshift of 3,000. This is before the CMB was emitted and just 380,000 years
after the Big Bang.
-
- This early dark energy would have decayed
rapidly after the stage of the universe's evolution known as
“recombination”. Early dark energy
could also alleviate the Hubble tension which is a discrepancy between
different estimates of the universe's age.
-
- However, other phenomena could account for
the bright galaxies. Before JWST's observations are used to invoke changes to
broad ideas of cosmology, a more detailed understanding of the physical
processes in galaxies is essential.
The current record holder for
the most distant galaxy — identified by JWST — is called JADES-GS-z14-0. The
data gathered so far indicate that these galaxies have a large diversity of
different properties.
-
- Some galaxies show signs of hosting black
holes that are emitting energy, while others seem to be consistent with hosting
young, dust-free populations of stars. Because these galaxies are faint and
observing them is expensive (it takes exposure times of many hours), only 20
galaxies for which the redshift is more than ten have been observed with
spectroscopy to date, and it will take years to build a statistical sample.
-
- A different angle of attack could be
observations of galaxies at later cosmic times, when the universe was 1 billion
to 2 billion years old (redshifts of between three and nine). JWST's
capabilities give researchers access to crucial indicators from stars and gas
in these objects that can be used to constrain the overall history of galaxy
formation.
-
- In the first year of JWST's operation, it
was claimed that some of the earliest galaxies had extremely high stellar
masses (the masses of stars contained within them) and a change in cosmology
was needed to accommodate bright galaxies that existed in the very early
universe. They were even dubbed "universe-breaker" galaxies.
-
- Soon after, it was clear that these galaxies
do not break the universe, but their properties can be explained by a range of
different phenomena. Better observational data showed that the distances to
some of the objects were overestimated (which led to an overestimation of their
stellar masses).
-
- The emission of light from these galaxies
can be powered by sources other than stars, such as accreting black holes.
Assumptions in models or simulations can also lead to biases in the total mass
of stars in these galaxies.
-
- As JWST continues its mission, it will help
scientists refine their models and answer some of the most fundamental
questions about our cosmic origins. It should unlock even more secrets about
the universe's earliest days, including the puzzle of these bright, distant
galaxies.
-
-
October 29, 2024 GALAXIES - what
are the earliest found? 4590
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--------------------- --- Tuesday, October 29,
2024
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