Astrophysicists on Maunakea discover galaxy protocluster COSTCO-1 with surprisingly hot gas

Astrophysicists using W. M. Keck Observatory on Maunakea on the Big Island 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,” also known as 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 “Warm-Hot Intergalatic Medium.”

The research was published March 14, 2023 in the The Astrophysical Journal Letters. The research was led by a team from the Kavli Institute for the Physics and Mathematics of the Universe, which is part of the University of Tokyo.

“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 the paper’s co-author Khee-Gan Lee, an assistant professor at the Kavli Institute for the Physics and Mathematics of the Universe.

The team observed COSTCO-I when the universe was only a quarter of its present age. The galaxy protocluster has a total mass of more than 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.

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 [the spectrometer] 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. 

“The properties and origin of the [Warm-Hot Intergalactic Medium] remains one of the biggest questions in astrophysics right now,” Lee said. “To be able to glimpse at one of the early heating sites of the [Warm-Hot Intergalactic Medium] will help reveal the mechanisms that caused the intergalactic gas to boil up into  the present-day froth.

“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 Warm-Hot Intergalactic Medium in the early universe enables astronomers to build up a coherent picture of galaxy formation and the lifecycle of gas that fuels it.