- 4196 -
GALAXIES - after the Big
Bang? When scientists viewed the James
Webb Space Telescope’s (JWST) first images of the universe’s earliest galaxies,
they were shocked. The young galaxies appeared too bright, too massive and too
mature to have formed so soon after the Big Bang.
-
-------------------- 4196 - GALAXIES - after the Big Bang?
- The startling
discovery even caused some physicists to question the standard model of
cosmology, wondering whether or not it should be upended. Astrophysicists now has discovered that these
galaxies likely are not so massive after all. Although a galaxy’s brightness is
typically determined by its mass, the new findings suggest that less massive
galaxies can glow just as brightly from irregular, brilliant bursts of star
formation.
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- Not only does this
finding explain why young galaxies appear deceptively massive, it also fits
within the standard model of cosmology.
Typically, a galaxy is bright because it’s big. But because these
galaxies formed at cosmic dawn, not enough time has passed since the Big Bang.
How could these massive galaxies assemble so quickly?
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- If star formation
happens in bursts, it will emit flashes of light. That is why we see several
very bright galaxies. A period that
lasted from roughly 100 million years to
1 billion years after the Big Bang, cosmic dawn is marked by the
formation of the universe’s first stars and galaxies. Before the JWST launched
into space, astronomers knew very little about this ancient time period.
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- Prior to JWST,
most of our knowledge about the early universe was speculation based on data
from very few sources. With this huge increase in observing power, we can see
physical details about the galaxies and use that solid observational evidence to
study the physics to understand what’s happening.
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- New computer simulations produced cosmic dawn galaxies
that were just as bright as those observed by the JWST. The “FIRE” simulations combine astrophysical
theory and advanced algorithms to model galaxy formation. The models enable
researchers to probe how galaxies form, grow and change shape, while accounting
for energy, mass, momentum and chemical elements returned from stars.
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- The simulations
discovered that stars formed in bursts, a concept known as “bursty star
formation.” In massive galaxies like the Milky Way, stars form at a steady
rate, with the numbers of stars gradually increasing over time. But “bursty
star formation” occurs when stars form in an alternating pattern, many stars at
once, followed by millions of years of very few new stars and then many stars
again.
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- Bursty star
formation is especially common in low-mass galaxies. This happens when a burst of stars form, then
a few million years later, those stars explode as supernovae. The gas gets
kicked out and then falls back in to form new stars, driving the cycle of star
formation.
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- But when galaxies
get massive enough, they have much stronger gravity. When supernovae explode,
they are not strong enough to eject gas from the system. The gravity holds the
galaxy together and brings it into a steady state.
-
- The simulations
also were able to produce the same abundance of bright galaxies as the JWST
revealed. The number of bright galaxies
predicted by simulations matches the number of observed bright galaxies.
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- Because more
massive stars burn at a higher speed, they are shorter lived. They rapidly use
up their fuel in nuclear reactions. So, the brightness of a galaxy is more
directly related to how many stars it has formed in the last few million years
than the mass of the galaxy as a whole.
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- While the night
sky may appear tranquil the cosmos is filled with constant stellar explosions
and collisions. Among the rarest of these transient events are what is known as
“Luminous Fast Blue Optical” (LFBOTs), which shine intensely bright in blue
light and fade after a few days.
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- These transient
events are only detectable by telescopes that continually monitor the sky.
Using the Hubble Space Telescope, an international team of astronomers recently
observed an LFBOT far between two galaxies, the last place they expected to see
one.
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- The first LFBOT
was observed in 2018 (AT2018cow) by the Asteroid Terrestrial-impact Last Alert
System (ATLAS). This event, nicknamed “The Cow,” was 10–100 times brighter than
a normal supernova and took place in a galaxy roughly 200 million light-years
(60 million parsecs) away.
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- Since then,
astronomers have detected LFBOTs at a rate of about one a year, so only a
handful are known, and very little is known about them. While several theories
about their possible causes exist, Hubble’s recent discovery has made this
phenomenon even more mysterious.
-
- After its initial
detection, the latest LFBOT (AT2023fhn, aka. “The Finch”) was observed by
multiple telescopes in various wavelengths from X-rays to radio waves. The
“Zwicky Transient Facility”, with an extremely wide-angle ground-based camera
that scans the entire northern sky every two days, was the first to alert
astronomers about the event on April 10th, 2023.
-
- The Gemini South
telescope in Chile obtained spectroscopic measurements, which revealed that The
Finch has a temperature of about 19,980 °C (36,000 °F). It also helped
determine its distance from Earth, allowing astronomers to calculate its
luminosity.
-
- Combined with
Chandra’s X-ray data and radio data from the VLA telescopes, their findings
confirmed that the explosion had all the characteristics of an LFBOT. It shined
intensely in blue light and evolved rapidly, reaching peak brightness and
fading again in a matter of days whereas supernovae take weeks or months to
dim.
-
- But unlike other
LFBOTs, Hubble found that the Finch was located about 50,000 light-years from a
nearby spiral galaxy and about 15,000 light-years from a smaller galaxy. This
raises serious questions about what is driving these massive explosions.
-
- A popular theory
is that they are a rare and extremely powerful type of core-collapse
supernovae, which occur when massive stars reach the end of their main sequence
and explode brilliantly. However, these stars are short-lived by stellar
standards, lasting 10-20 million years or up to one hundred million years
depending on their overall mass.
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- Therefore, massive
progenitor stars don’t have enough time to travel far from their birthplace
(stellar clusters inside galaxies) before reaching the end of their lifespans.
Whereas all previous LFBOTs have been found in the spiral arms of galaxies
(where star birth is ongoing), the Finch is an outlier.
-
- The more we learn
about LFBOTs, the more they surprise us. We’ve now shown that LFBOTs can occur
a long way from the center of the nearest galaxy, and the location of the Finch
is not what we expect for any kind of supernova.
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- This could be the
result of a collision between two neutron stars that were ejected from their
host galaxy and had been spiraling toward each other for billions of years.
These produce “kilonovae”, which are powerful explosions 1,000 times more
powerful than a standard nova and are also a well-known source of Gravitational
Waves (GWs).
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- Another theory is
that LFBOTs are caused by collisions between neutron stars, where one is a
magnetar (a highly-magnetized neutron star).
This would greatly amplify the power of the explosion to the point where
it would exceed that of a supernova by a factor of 100.
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- Another
possibility is that LFBOTs result from stars being torn apart by an
intermediate-mass black hole (between 100 to 1,000 solar masses).
Intermediate-mass black holes are most likely to be found in globular star
clusters.
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- Several more
LFBOTs must be discovered before the population can be properly characterized.
This will be challenging since transients can happen anywhere, at any time, and
are fleeting in astronomical terms (hence the name).
-
- Much like
Gamma-Ray Bursts (GRBs) and Fast Radio Bursts (FRBs), the only way to detect
them is through wide-field surveys that constantly monitor large areas of the
sky. Once detected, space-based and ground-based observatories can conduct
follow-up observations to learn more about their properties.
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- This will be
assisted greatly when the Vera C. Rubin Observatory is completed in 2024, one of
many next-generation all-sky survey telescopes that will be observing the
cosmos shortly. Among its objectives is the study of “objects that change
position or brightness over time”, transient objects.
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-
October 23, 2023 GALAXIES - after the Big Bang 4196
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Tuesday, October 24, 2023 ---------------------------------
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