Using W. M. Keck Observatory on Maunakea in Hawaiʻi, astrophysicists have discovered a galaxy protocluster called COSTCO-I in the early universe. This galaxy protocluster dates back to when the gas that filled most of the space outside of visible galaxies- called intergalactic medium, was significantly 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 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.
COSTCO-I was observed when the universe was 11 billion years younger. Galaxies in the universe were at the height 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 flourish, with temperatures of roughly 10,000 degrees Celsius.
The intergalactic medium of today, which swelters 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 to be ahead of its time and roasting in a hot, complex state.
Khee-Gan Lee, an assistant professor at Kavli IPMU and co-author of the paper, said, “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 had observed, during an era in the distant past when most of the pot was still cold.”
While such far-off galaxy protoclusters are now frequently observed by astronomers, the team noticed something peculiar when they examined the ultraviolet spectra encompassing the COSTCO-I region using the Low-Resolution Imaging Spectrometer at Keck Observatory (LRIS). Typically, when observed in wavelengths odd to neutral hydrogen associated with the protocluster gas, galaxy protoclusters would throw a shadow due to their vast mass and size.
Chenze Dong, a Master’s degree 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 search 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 make hydrogen gas maps with high confidence, and the signature of COSTCO-I simply wasn’t there.”
“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.”
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. 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.”
“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.”
- Chenze Dong et al. Observational Evidence for Large-scale Gas Heating in a Galaxy Protocluster at z = 2.30. The Astrophysical Journal Letters. DOI: 10.3847/2041-8213/acba89