The study of how individual stars are born and die in galaxies, how new stars are born from remnants of old stars, and how galaxies themselves grow are important themes in astronomy, as they provide insight into our roots in the universe. Galaxy clusters, one of the largest structures in the universe, are the assembly of more than 100 galaxies which are bound together through mutual gravitational force.
Observations of nearby galaxies have shown that the growth
of a galaxy depends on its environment in the sense that mature stellar
populations are commonly seen in regions where galaxies are densely collected.
This is referred to as the "environment effect." Although the
environment effect has been considered an important piece to understand galaxy
formation and evolution, it is not well known when the effect initiated in the
history of the universe.
One of the keys to understanding this is to observe the
ancestors of galaxy clusters shortly after the birth of the universe; known as
galaxy protoclusters (hereafter protoclusters), these are assemblies of about
10 distant galaxies. Fortunately, astronomy allows us to observe the distant
universe as it was in the past. For example, light from a galaxy 13 billion
light-years away takes 13 billion years to reach Earth, so what we observe now
is what that galaxy looked like 13 billion years ago.
However, light that travels 13 billion light-years becomes
fainter, so the telescopes that observe it must have high sensitivity and spatial
resolution.
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(Left) Artist's impression of the "metropolitan
area" of the protocluster A2744ODz7p9 revealed by the James Webb Space
Telescope and ALMA. (Right) Artist's impression of what the "metropolitan
area" will become tens of millions of years after the observed time.
Credit: NAOJ |
An international research team led by Assistant Professor
Takuya Hashimoto (University of Tsukuba, Japan) and researcher Javier
Álvarez-Márquez (Spanish Center for Astrobiology) has used the James Webb Space
Telescope (JWST, observing visible and infrared light) and the Atacama Large
Millimeter/submillimeter Array (ALMA, observing radio waves) to study the
"core region" of the protocluster A2744z7p9OD.
The protocluster A2744z7p9OD had been announced as the most
distant proto-cluster at 13.14 billion light-years away based on observations
with JWST by another research group. "However, we have not been able to
observe the entire core region, the metropolitan area, with the largest number
of galaxy candidates in this protocluster. It was unclear whether the
environmental effects of galaxies had begun in this protocluster. So we decided
to focus our research on the core region," says Hashimoto.
The research team first observed the core region of this
protocluster using JWST. Using NIRSpec, an instrument that observes spectra at
wavelengths ranging from visible to near-infrared, the team made integral field
spectroscopy observations that can simultaneously acquire spectra from all
locations within the field of view.
The team has successfully detected ionized oxygen-ion light
([OIII] 5008 Å) from four galaxies in a quadrangle region measuring 36,000
light-years along a side, which is equivalent to half the radius of the Milky
Way galaxy. Based on the redshift of this light (the elongation of the
wavelength due to the cosmic expansion), the distance of the four galaxies from
the Earth was identified as 13.14 billion light years.
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The background color image shows a map of the light
intensity (redder color shows stronger emission) in the core region of the
protogalactic cluster A2744ODz7p9, acquired with the NIRCam onboard JWST. The
size of the image corresponds to about half of the radius of the Milky Way
Galaxy. (Left) Contours show the distribution of light emitted by ionized
oxygen, obtained with the NIRSpec instrument onboard JWST. 4 galaxies were
identified at 13.14 billion light-years away. (Right) Contours show the
distribution of dust emission from three of the four galaxies. The white circle
in the lower left of the figure indicates the beam size of the ALMA
data.Credit: JWST (NASA, ESA, CSA), ALMA (ESO/NOAJ/NRAO), T. Hashimoto et al. |
"I was surprised when we identified four galaxies by detecting oxygen-ion emission at almost the same distance. The 'candidate galaxies' in the core region were indeed members of the most distant protocluster," says Yuma Sugahara (Waseda/NAOJ), who led the JWST data analysis.
In addition, the research team paid attention to the
archival ALMA data, which had already been acquired for this region. The data
captures radio emission from cosmic dust in these distant galaxies. As a result
of analyses, they detected dust emissions from three of the four galaxies.
This is the first detection of dust emission in member
galaxies of a protocluster this far back in time. Cosmic dust in galaxies is
thought to be supplied by supernova explosions at the end of the evolution of
massive stars in the galaxies, which provide the material for new stars.
Therefore, the presence of large amounts of dust in a galaxy
indicates that many of the first-generation stars in the galaxy have already
completed their lives and that the galaxy is growing. Professor Luis Colina (El
Centro de Astrobiología (CAB, CSIC-INTA)) describes the significance of the
results: "Emission from cosmic dust was not detected in member galaxies of
the protocluster outside the core region. The results indicate that many
galaxies are clustered in a small region and that galaxy growth is accelerated,
suggesting that environmental effects existed only ~700 million years after the
Big Bang."
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Galaxy formation simulations of the future of the core of
A2744z7p9OD. (a) Gas density in a region similar to the proto-cluster
A2744z7p9OD at a cosmological age of 689 million years. (b) A zoomed-in view of
the core region in (a), corresponding to the region observed by JWST. The color
map indicates the light distribution of oxygen ions. (b) to (d) show the
evolution of the simulated object: the four galaxies gradually merge and evolve
into a larger object. Credit: T. Hashimoto et al. |
Furthermore, the research team conducted a galaxy formation
simulation to theoretically test how the four galaxies in the core region
formed and evolved. The results showed that a region of dense gas particles
existed around 680 million years after the Big Bang. In the middle four
galaxies are formed, similar to the observed core region. To follow the
evolution of these four galaxies, the simulation calculated physical processes
such as the kinematics of stars and gas, chemical reactions, star formation,
and supernovae.
The simulations showed that the four galaxies merge and
evolve into a single larger galaxy within a few tens of millions of years,
which is a short time scale in the evolution of the universe.
"We successfully reproduced the properties of the
galaxies in the core region owing to the high spatial resolution of our
simulations and the large number of galaxy samples we have. In the future, we
would like to explore the formation mechanism of the core region and its
dynamical properties in more detail," says Yurina Nakazato, a graduate
student at the University of Tokyo, who analyzed the simulation data.
Javier Álvarez-Márquez (Spanish Center for Astrobiology)
says, "We will conduct more sensitive observations of the proto-cluster
A2744z7p9OD with ALMA to see if there are any galaxies that were not visible
with the previous sensitivity. We will also apply the JWST and ALMA
observations, which have proven to be very powerful, to more protoclusters to
elucidate the growth mechanism of galaxies, and to explore our roots in the
universe."