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| An artist's representation of what the first stars to light up the universe might have looked like in the Cosmic Dawn. Credit: NASA/WMAP Science Team |
Astronomers have determined that so-called "leaky" galaxies may have
responsible for triggering the last great transformational epoch in our
universe, one which ionized the neutral interstellar gas.
Billions of years ago our universe was a lot smaller and a lot hotter than
it is today. At very early times it was so small and hot that it was in the
state of a plasma, where electrons were separated from atomic nuclei. But
when the universe was roughly 380,000 years old, it cooled to the point that
electrons could recombine onto their nuclei, forming a soup of neutral
atoms.
However, observations of the present-day universe reveal that almost all the
matter in the universe is not neutral at all. Instead it's ionized, once
again in the state of a plasma. Something had to happen in the intervening
billions of years to transform the neutral gas of the cosmos into an ionized
plasma. Astronomers call this event the Epoch of Reionization and suspect
that it happened within the first few hundred million years after the Big
Bang. But they are not sure how this transformational event proceeded.
One of the great debates in modern cosmology is the source of reionization.
One hypothesis is that quasars are responsible. Quasars are the ultra bright
cores surrounding supermassive black holes which pump out enormous amounts
of high energy radiation. This radiation could easily flood the universe and
transform it from neutral to ionized. But the problem with this hypothesis
is that quasars are relatively rare, and so they have difficulty covering
the volume of the universe.
Another hypothesis is that young galaxies rich with star formation are
responsible. In this scenario the process of ionizing the neutral gas is
more spread out throughout the universe. Each individual galaxy is only
capable of ionizing the gas in its nearby vicinity, but since there are so
many galaxies it's possible to reionization the entire universe. But the
only way to do this is if enough high energy radiation leaks out of galaxies
and into the surrounding medium.
One team of astronomers have used the James Webb Space Telescope to
investigate this hypothesis. They can't study the radiation coming out of
the galaxies directly, because that radiation gets absorbed by the billions
of light-years worth of matter between us and those galaxies. So instead
they had to look for other clues. Using the James Webb's ability to study
distant galaxies, they were able to measure how compact the galaxies were,
and how rich in star formation they were. They were then able to compare
these galaxies to similar galaxies found i- the present day universe to
create an estimate of the amount of radiation leaking from them.
They estimate that on average the galaxies in the early universe leaked
roughly 12% of their available high energy photons. This is just enough to
potentially reionization the entire cosmos in a relatively short amount of
time. The findings are published in the journal Astronomy &
Astrophysics.
The results are not conclusive, however, because of the number of
assumptions that the astronomers had to make. But it does point in an
intriguing direction in solving this long-standing cosmic riddle.
Reference:
S. Mascia et al, Closing in on the sources of cosmic reionization: first
results from the GLASS-JWST program, Astronomy & Astrophysics (2023).
DOI: 10.1051/0004-6361/202345866
Tags:
Space & Astrophysics