A supercomputer simulation of a galaxy protocluster similar to
costco-i that is surrounded by hot gas (yellow) boiling amid an
intergalactic medium filled with much cooler gas (blue). Credit: The Three Hundred Collaboration
Maunakea, Hawaiʻi – Astrophysicists
using W. M. Keck Observatory on Maunakea in Hawaiʻi have discovered a
galaxy protocluster in the early universe surrounded by gas that is
surprisingly hot.
This scorching gas hugs a region that consists of a giant collection
of galaxies called COSTCO-I. Observed when the universe was 11 billion
years younger, COSTCO-I dates back to a time when the gas that filled
most of the space outside of visible galaxies, called the intergalactic
medium, was significantly cooler. During this era, known as ‘Cosmic
Noon,’ galaxies in the universe were at the peak of forming stars; their
stable environment was full of the cold gas they needed to form and grow, with temperatures measuring around 10,000 degrees Celsius.
In contrast, the cauldron of gas associated with COSTCO-I seems ahead
of its time, roasting in a hot, complex state; its temperatures
resemble the present-day intergalactic medium, which sear from 100,000
to over 10 million degrees Celsius, often called the ‘Warm-Hot
Intergalactic Medium’ (WHIM).
This discovery marks the first time astrophysicists have identified a
patch of ancient gas showing characteristics of the modern-day
intergalactic medium; it is by far the earliest known part of the
universe that’s boiled up to temperatures of today’s WHIM.
The research, which is led by a team from the Kavli Institute for the
Physics and Mathematics of the Universe (Kavli IPMU, part of the
University of Tokyo), is published in today’s issue of The Astrophysical Journal Letters.
A simulated visualization depicts the scenario of large-scale heating
around a galaxy protocluster, using data from supercomputer simulations.
This is believed to be a similar scenario to that observed in the
COSTCO-I protocluster. The yellow area in the center of the picture
represents a huge, hot gas blob spanning several million light years.
The blue color indicates cooler gas located in the outer regions of the
protocluster and the filaments connecting the hot gas with other
structures. The white points embedded within the gas distribution is
light emitted from stars. Simulation Credit: The THREE HUNDRED
Collaboration
“If we think about the present-day intergalactic medium as a gigantic
cosmic stew that is boiling and frothing, then COSTCO-I is probably the
first bubble that astronomers have observed, during an era in the
distant past when most of the pot was still cold,” said Khee-Gan Lee, an
assistant professor at Kavli IPMU and co-author of the paper.
The team observed COSTCO-I when the universe was only a quarter of
its present age. The galaxy protocluster has a total mass of over 400
trillion times the mass of our Sun and spans several million light
years.
While astronomers are now regularly discovering such distant galaxy
protoclusters, the team found something strange when they checked the
ultraviolet spectra covering COSTCO-I’s region using Keck Observatory’s
Low Resolution Imaging Spectrometer (LRIS). Normally, the large mass and
size of galaxy protoclusters would cast a shadow when viewed in the
wavelengths specific to neutral hydrogen associated with the
protocluster gas.
No such absorption shadow was found at the location of COSTCO-I.
“We were surprised because hydrogen absorption is one of the common
ways to search for galaxy protoclusters, and other protoclusters near
COSTCO-I do show this absorption signal,” said Chenze Dong, a Master’s
degree student at the University of Tokyo and lead author of the study.
“The sensitive ultraviolet capabilities of LRIS on the Keck I Telescope
allowed us to make hydrogen gas maps with high confidence, and the
signature of COSTCO-I simply wasn’t there.”
The absence of neutral hydrogen tracing the protocluster implies the
gas in the protocluster must be heated to possibly million-degree
temperatures, far above the cool state expected for the intergalactic
medium at that distant epoch.
This
figure compares observed hydrogen absorption in vicinity of the
COSTCO-I galaxy protocluster (top panel), compared with the expected
absorption given the presence of the protocluster as computed from
computer simulations. Strong hydrogen absorption is shown in red, lower
while weak absorption is shown in blue, and intermediate absorption is
denoted as green or yellow colors. The black dots in the figure show
where astronomers have detected galaxies in that area. At the position
of COSTCO-I (with its center marked as a star in both panels),
astronomers found that the observed hydrogen absorption is not of much
different from the mean value of the universe at that epoch. This is
surprising because one would expect to find extended hydrogen absorption
spanning millions of light years in that region corresponding to the
high observed concentration of galaxies. This figure is adapted from the
Dong et al. 2023 Astrophysical Journal Letters article. Credit: Dong et
al.
“The properties and origin of the WHIM remains one of the biggest
questions in astrophysics right now. To be able to glimpse at one of the
early heating sites of the WHIM will help reveal the mechanisms that
caused the intergalactic gas to boil up into the present-day froth,”
said Lee. “There are a few possibilities for how this can happen, but it
might be either from gas heating up as they collide with each other
during gravitational collapse, or giant radio jets might be pumping
energy from supermassive black holes within the protocluster.”
The intergalactic medium serves as the gas reservoir that feeds raw
material into galaxies. Hot gas behaves differently from cold gas, which
determines how easily they can stream into galaxies to form stars. As
such, having the ability to directly study the growth of the WHIM in the
early universe enables astronomers to build up a coherent picture of
galaxy formation and the lifecycle of gas that fuels it.
Source: W. M. Keck Observatory
About LRIS
The Low Resolution Imaging Spectrometer (LRIS) is a very versatile
and ultra-sensitive visible-wavelength imager and spectrograph built at
the California Institute of Technology by a team led by Prof. Bev Oke
and Prof. Judy Cohen and commissioned in 1993. Since then it has seen
two major upgrades to further enhance its capabilities: the addition of a
second, blue arm optimized for shorter wavelengths of light and the
installation of detectors that are much more sensitive at the longest
(red) wavelengths. Each arm is optimized for the wavelengths it
covers. This large range of wavelength coverage, combined with the
instrument’s high sensitivity, allows the study of everything from
comets (which have interesting features in the ultraviolet part of the
spectrum), to the blue light from star formation, to the red light of
very distant objects. LRIS also records the spectra of up to 50 objects
simultaneously, especially useful for studies of clusters of galaxies in
the most distant reaches, and earliest times, of the universe. LRIS was
used in observing distant supernovae by astronomers who received the
Nobel Prize in Physics in 2011 for research determining that the
universe was speeding up in its expansion.
About W. M. Keck Observatory
The W. M. Keck Observatory telescopes are among the most
scientifically productive on Earth. The two 10-meter optical/infrared
telescopes atop Maunakea on the Island of Hawaii feature a suite of
advanced instruments including imagers, multi-object spectrographs,
high-resolution spectrographs, integral-field spectrometers, and
world-leading laser guide star adaptive optics systems. Some of the data
presented herein were obtained at Keck Observatory, which is a private
501(c) 3 non-profit organization operated as a scientific partnership
among the California Institute of Technology, the University of
California, and the National Aeronautics and Space Administration. The
Observatory was made possible by the generous financial support of the
W. M. Keck Foundation. The authors wish to recognize and acknowledge the
very significant cultural role and reverence that the summit of
Maunakea has always had within the Native Hawaiian community. We are
most fortunate to have the opportunity to conduct observations from this
mountain..
