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| A group of small galaxies, seen almost 13 billion years back in time, likely in the process of forming a massive galaxy. The colors are composed from three different infrared colors. The white, horisontal bar shows the scale of approximately 20,000 lightyears. Credit: Shuowen Jin et al. (2023) |
Astronomers from the Cosmic Dawn Center have unveiled the nature of the
densest region of galaxies seen with the James Webb Space telescope in the
early universe. They find it to be likely the progenitor of a massive, Milky
Way-like galaxy, seen at a time where it is still assembling from smaller
galaxies. The discovery corroborates our understanding of how galaxies form.
According to our current understanding of structure formation in the
universe, galaxies form in a hierarchical manner, with small structures
forming first in the very early universe, later merging to build up larger
structures. This is the prediction of theories and computer simulations, and
is verified by observations of galaxies at various epochs in the history of
the universe.
To observe the very first structures assembling, we have to look as far back
in time, and hence as far away, as possible. But these sources are both very
small and very faint, and their detection requires advanced technologies.
In a new study, the early progenitor of what today will likely have evolved
to a massive, Milky Way-sized galaxy, has been detected. This group of
smaller galaxies, dubbed CGG-z5, was found through the observational program
called "CEERS" with the James Webb Space Telescope, and is seen when the
universe was only 1.1 billion years old, 8% of its current age.
CGG-z5 was discovered using the code GalCluster, which was created by
Nikolaj Sillassen, MSc student at the Cosmic Dawn Center (DAWN).
"I developed the software during my studies to detect this kind of
structures, and now we applied it to data from the CEERS program," says
Nikolaj Sillassen, who already found a similar but more nearby group while
testing the software.
"It's great to see how useful my code is becoming."
Impossible without James Webb
The brightest members of the galaxy group was discovered previously with the
Hubble Space Telescope. But the CEERS program revealed new and smaller
members.
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| Four snapshots of the evolution of a simulated proto-galaxy from the "EAGLE" simulation, chosen to resemble the observed group CGG-z5. The brightness show the density of stars in the galaxies, and the symbols follow individual clumps of matter. In the 1.2 billion years that pass between the upper left and the lower right, the galaxies grows from a total stellar mass of 5 billion suns to 65 billion suns. Credit: A. Vijiayan and S. Jin |
"The other members of the group are both small and faint. Without the
sensitivity and the spatial resolution of James Webb, we simply wouldn't be
able to detect them," explains Shuowen Jin, Marie Curie Fellow at the Cosmic
Dawn Center (DAWN) and lead author of the current study.
Exactly what is the "future" of the galaxy group CGG-z5 will be, is of
course unknown. Rather than forming a single galaxy, it could be that the
group evolves into a large cluster of galaxies at later times. Yet another
possibility is that the members are in reality not so closely packed as it
seems, but instead is a part of a filamentary structure that we just happen
to view from one end to the other.
Help from computer simulations
To distinguish between these scenarios, more precise observations involving
the more time-consuming spectroscopy is needed. But in the meantime, help is
available from computer simulations:
"In order to better understand the nature and evolution of CGG-z5, we
searched for similar structures in large-scale, hydrodynamical simulations,"
says Aswin Vijiayan, Postdoctoral Fellow at the Cosmic Dawn Center who
conducted the simulation analysis in the study. "We found 14 structures that
match closely the physical properties of our observed group CGG-z5, and then
traced the evolution of these structures through time in the simulations,
from the early universe to the present epoch.
Although the exact unfolding of the evolution of these 14 structures are
different, they all shared the same fate: Roughly 0.5 to 1 billion years
later, they merge to form a single galaxy which, by the time the universe is
half its current age, have masses comparable to our own Milky Way.
"Given the predictions of the simulations, it is therefore tempting to
speculate that the CGG-z5 system will also follow a similar evolutionary
path, and that we captured the process of small galaxies assembling into a
single massive galaxy," Shuowen Jin says.
"Interestingly, the number of these early groups like CGG-z5 in a given
volume of space is similar to the number of massive galaxies at later cosmic
times," says Georgios Magdis, associate professor at DAWN and partaker in
the study. "This makes merging groups appealing as the main progenitors of
massive galaxies at later epochs."
The study is published in the journal Astronomy & Astrophysics.
Reference:
S. Jin et al, Massive galaxy formation caught in action at z~5 with JWST,
Astronomy & Astrophysics (2023).
DOI: 10.1051/0004-6361/202245724
Tags:
Space & Astrophysics