Using WM Keck Observatory on Maunakea in Hawai’i, astrophysicists have discovered a protocluster of galaxies called COSTCO-I in the early universe. This protocluster of galaxies dates back to a time when the gas that filled most of the space outside the visible galaxies – called intergalactic medium – was considerably cooler.

But the surprising fact is that COSTCO-I is surrounded by surprisingly hot gas. This discovery marks the first time astrophysicists have identified a patch of ancient gas that exhibits characteristics of the modern intergalactic medium; it is by far the earliest known part of the universe to have boiled up to temperatures of today’s WHIM.

COSTCO-I was observed when the universe was 11 billion years younger. Galaxies in the universe were at the peak of star formation during this time, known as “Cosmic Noon”, and their stable environment was full of the cold gas they needed to form and bloom, with temperatures around 10,000 degrees Celsius.

Today’s intergalactic medium, which sizzles at temperatures between 100,000 and over 10 million degrees Celsius and is sometimes referred to as the “Warm-Hot Intergalactic Medium” (WHIM), is similar to the cauldron of gas associated with COSTCO-I, which appears ahead of its time and roasts to a hot, complex state.

Khee-Gan Lee, an assistant professor at Kavli IPMU and co-author of the paper, said: “If we think of the current intergalactic medium as a giant cosmic stew that boils and froths, then COSTCO-I is probably the first bubble astronomers have observed, in an era in the distant past when most of the pot was still cold. .”

While such galaxy protoclusters are now commonly observed by astronomers, the team noticed something peculiar when they examined the ultraviolet spectra around the COSTCO-I region using the Keck Observatory’s Low-Resolution Imaging Spectrometer (LRIS). When observed at wavelengths foreign to neutral hydrogen associated with the protocluster gas, protoclusters of galaxies would typically cast a shadow due to their enormous mass and size.

Chenze Dong, a master’s student at the University of Tokyo and lead author of the study, said: “We were surprised because hydrogen absorption is one of the common ways to look for galaxy protoclusters, and other protoclusters near COSTCO-I do show this absorption signal. The sensitive ultraviolet capabilities of LRIS on the Keck I telescope allowed us to we are making hydrogen gas maps with great certainty, and the COSTCO-I signature just wasn’t there.”

“The sensitive ultraviolet capabilities of LRIS on the Keck I telescope allowed us to create hydrogen gas maps with high confidence, and the COSTCO-I signature just wasn’t there.”

Khee-Gan Lee, an assistant professor at Kavli IPMU and co-author of the paper, said: “The properties and origin of the WHIM remains one of the biggest questions in astrophysics right now. Glimpsing one of the WHIM’s early heating sites allows us to reveal the mechanisms that caused the intergalactic gas to boil into its current froth.

“There are a few possibilities for how this could happen, but it could be the gas heating up as they collide during the gravitational collapse, or giant radio jets pumping energy from supermassive black holes in the protocluster.”

Magazine reference:

  1. Chenze Dong et al. Observational evidence for large-scale gas heating in a protocluster of galaxies at z = 2.30. The Astrophysical Journal Letters. DOI: 10.3847/2041-8213/acba89