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Balance of "Fuel" Surrounding Black Holes

Kotaro Kono, Professor, Institute of Astronomy


We have been studying the influx of low-temperature, dense molecular gas in the region of a few light-years around a black hole through observations of electromagnetic waves in the millimeter and submillimeter wavelength bands using the ALMA telescope.
In the region of a few light-years around a black hole, the inflow of low-temperature, high-density molecular gas and the inflow of dilute, high-temperature ionized gas have been observed.
The analysis of the data shows that the inflow of low-temperature molecular gas and the outflow of dilute, high-temperature ionized gas in the region of a few light-years around the black hole were successfully measured.
The analysis indicates that the inflow of low-temperature molecular gas is due to gravitational instability, while the outflow of dilute, high-temperature ionized gas is due to the gravitational instability of the black hole.
The analysis revealed that the inflow of low-temperature molecular gas is due to gravitational instability, and that most of the inflow is blown away like a fountain before reaching the black hole.
This is a significant progress in elucidating the relationship between the growth of a black hole and the galaxy that hosts the black hole.

Many of the galaxies in the universe are believed to harbor mysterious objects called massive or supermassive black holes near their centers. The mass of a black hole is reported to be more than a million times, and sometimes several hundred million times, the mass of the Sun, the most familiar star in our universe. Once inside a black hole (or more precisely, within its event horizon), information cannot be observed from the outside, but massive black holes often emit enormous amounts of energy (1011 to 1013 times the luminosity of the sun, or even brighter in some cases) and are discovered as shining objects. It is the matter around the black hole that is the source of the black hole's light. This is because the matter surrounding the black hole, the so-called interstellar matter, is attracted by the black hole's gravity and releases its potential energy as heat and radiation through a viscous rotating structure called an accretion disk. The accretion disk, or engine, is fueled by the interstellar material and shines. On the other hand, the intense energy release from the black hole engine also has the effect of blowing away the surrounding interstellar matter by exerting radiation pressure, in other words, braking the fuel supply. In order to advance from this qualitative "story" to a quantitative understanding, it is necessary to measure the amount of interstellar matter flowing into the black hole and how much of it is blown outward, and to understand the fate of the interstellar matter that is blown out in this way. We need to understand the fate of the interstellar material that is blown outward.

Left: Spectral lines of molecular hydrogen cyanide obtained in the direction of the galactic nucleus. It shows a characteristic spectral shape called inverse P-Cyg profile, which clearly indicates the existence of low-temperature dense molecular gas approaching (falling into) a black hole. Right] Spatial distribution of hydrogen cyanide molecular gas and ionized hydrogen gas obtained by ALMA.


The key to this is the "multiphase nature" of the interstellar medium. It is necessary to observe and understand the gas in the molecular phase, which is dense at low temperatures of a few tens of Kelvin, the neutral gas phase at higher temperatures, and the ionized plasma phase. Moreover, when approaching a black hole in the vicinity of a few light years, it is covered with a large amount of interstellar matter, and light at visible and infrared wavelengths, for example, is absorbed along the way, making it impossible to see through the interstellar matter. ALMA has been able to overcome this problem by observing in the submillimeter and millimeter wavelength bands, which are much longer than visible and infrared light, and has achieved measurements with a resolution of about one light year in the central core region of Circinus, an active galaxy in the neighborhood. However, this is only the first example. The challenge is to continue these time-consuming and difficult observations in more black holes and to obtain a general picture of the black holes.

The results of this study were published in Science, 382, 554 (2023) .


(Press release, November 3, 2023)

Published in The Rigaku-bu News, March 2024


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