Discovery of "Hot Gas" May Be a New Way for Finding Hidden Black Holes in the Early Universe 

An international research team led by Professor Ken-ichi Tadaki of Hokkai-Gakuen University has made a groundbreaking discovery using the Atacama Large Millimeter/submillimeter Array (ALMA). The team has captured unprecedented high-resolution radio signals from hot gas surrounding a supermassive black hole located 12.9 billion light-years away. This innovative observation technique promises to reveal hidden black holes that formed during the Universe's early stages.

The research team observed a supermassive black hole that existed in the Universe 12.9 billion years ago, with a mass exceeding one billion times that of the Sun. Quasars are among the brightest objects in the Universe, powered by supermassive black holes that generate intense energy as they consume surrounding matter. Examining the innermost regions of distant quasars has remained challenging despite their brightness. In this breakthrough study, researchers focused on radio signals emitted by highly energized carbon monoxide (CO) molecules. Their ultra-high-resolution observations revealed, for the first time, the heating mechanisms affecting gas within just a few hundred light-years of the black hole¹. 

Detecting strong CO emissions at high energy levels indicates extraordinarily hot gas conditions around the black hole. While ultraviolet radiation from newly formed stars typically heats gas in star-forming regions, the extreme conditions observed cannot be explained by stellar activity alone. The research points to powerful X-rays emanating from the black hole's accretion disk and corona as the primary heating source. These X-rays can elevate gas temperatures far beyond levels observed in typical star-forming regions. Additionally, the team found evidence that powerful quasar winds and shock waves further contribute to this extreme heating, demonstrating that the central quasar region represents one of the most dynamic environments in the cosmos. 

This discovery has significant implications for our understanding of black hole populations in the early Universe. Quasars oriented with relatively clear sightlines appear exceptionally bright in visible light and X-rays. However, if the quasar is viewed through a much thicker layer of cosmic dust, visible light and X-rays can be blocked, causing it to remain "hidden." In other words, many supermassive black holes may lie concealed within dusty regions of the early Universe, simply undetected. Because the radio waves observed by ALMA are not easily absorbed by dust, this technique becomes a powerful tool for discovering such "hidden supermassive black holes." By applying similar high-resolution observations of energetic CO emissions to other objects, researchers expect to develop a more comprehensive census of early supermassive black holes and gain crucial insights into their formation and evolution.  

Additional information 

These research findings were published online in the Nature Astronomy: Tadaki et al. "Warm Gas in the Vicinity of a Supermassive Black hole 13 Billion Years Ago" (DOI: 10.1038/s41550-025-02505-x).

The original press release was published by National Radioastronomy Observatory of Japan (NAOJ), an ALMA partner on behalf of East Asia.   

Research Team Ken-ichi Tadaki (Hokkai-Gakuen University), Federico Esposito (Università degli Studi di Bologna), Livia Vallini (Osservatorio di Astrofisica e Scienza dello Spazio, INAF–OAS), Takafumi Tsukui (Australian National University), Toshiki Saito (Shizuoka University), Daisuke Iono (National Astronomical Observatory of Japan/ SOKENDAI), Tomonari Michiyama (Shunan University) 

This work was supported by JSPS KAKENHI grants (JP 23K03466 and 23K20870). 

The Atacama Large Millimeter/submillimeter Array (ALMA), an international astronomy facility, is a partnership of the European Organization for Astronomical Research in the Southern Hemisphere (ESO), the U.S. National Science Foundation (NSF) and the National Institutes of Natural Sciences (NINS) of Japan in cooperation with the Republic of Chile. ALMA is funded by ESO on behalf of its Member States, by NSF in cooperation with the National Research Council of Canada (NRC) and the National Science and Technology Council (NSTC) in Taiwan, and by NINS in cooperation with the Academia Sinica (AS) in Taiwan and the Korea Astronomy and Space Science Institute (KASI).  

ALMA construction and operations are led by ESO on behalf of its Member States; by the National Radio Astronomy Observatory (NRAO), managed by Associated Universities, Inc. (AUI), on behalf of North America; and by the National Astronomical Observatory of Japan (NAOJ) on behalf of East Asia. The Joint ALMA Observatory (JAO) provides the unified leadership and management of the construction, commissioning, and operation of ALMA.

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1. In other high-resolution studies of quasars, researchers have observed radio emissions from ionized atomic carbon and dust particles. However, atomic carbon emission mainly traces cold gas distributed throughout the galaxy, providing limited insight into conditions near the black hole itself. The breakthrough in this research comes from specifically targeting radio emissions from CO molecules in higher energy states, which uniquely reveals the hot gas conditions in the immediate vicinity of the supermassive black hole.  ↩︎