AsianScientist (Mar. 3, 2015) – Organic molecules have been found concentrated in a region surrounding a supermassive black hole, possibly shielded from strong X-rays and UV photons by dust and gas. These findings have been reported in the Publications of the Astronomical Society of Japan.
Interstellar gas contains a wide variety of molecules and its chemical composition differs widely depending on the environment. For example, an active star forming region with a temperature higher than the surrounding environment stimulates the production of certain types of molecules by chemical reactions which are difficult to take place in a cold temperature region.
The difference in chemical composition enables us to probe the environment (e.g. temperature and density) of a target regio. Since each molecule has its own frequency spectrum, we can identify the chemical composition and the environment of a remote target object through observations with a radio telescope.
From this perspective, astronomers have been actively working on the starburst regions of galaxies and the active galactic nuclei at the center of galaxies, which are called circumnuclear disks. These regions are very important in understanding the evolution of galaxies, and radio observations of molecular emissions are essential to explore the mechanism and environment of these regions. However, the weak radio emission from molecules makes observations difficult using conventional radio telescopes.
A research team led by Shuro Takano at the National Astronomical Observatory of Japan (NAOJ) and Taku Nakajima at Nagoya University observed the spiral galaxy M77 in the direction of the constellation Cetus (the Whale) about 47 million light years away. M77 is known to have an active galactic nucleus at its center which is surrounded by a starburst ring with a radius of 3,500 light years.
They used the Atacama Large Millimeter/submillimeter Array (ALMA) telescope, which is suitable for analyzing molecules in galaxies due to its high sensitivity, high fidelity imaging capability to image actual gas distributions, the ability to observe wideband multiple wavelengths simultaneously, and high spatial resolution.
ALMA observations revealed clearly the distributions of nine types of molecules in the circumnuclear disks and in the starburst ring. The observational results show that the molecular distribution varies according to the type of molecule. While carbon monoxide (CO) is distributed mainly in the starburst ring, five types of molecules, including complex organic molecules such as cyanoacetylene (HC3N) and acetonitrile (CH3CN), are concentrated in the circumnuclear disks.
In addition, carbon monosulfide (CS) and methanol (CH3OH) are distributed both in the starburst ring and the circumnuclear disks. ALMA provided the first high resolution observation of the five types of molecules in M77, and revealed that they are concentrated in the circumnuclear disks.
“It was quite unexpected that acetonitrile and cyanoacetylene, which have a large number of atoms, are concentrated in the circumnuclear disks,” says Nakajima.
The supermassive black hole devours surrounding materials by its strong gravity and generates a disk around the black hole. The disk will be heated to a high temperature and emit intense X-rays or UV photons. When an organic molecule with multiple atomic linkages is exposed to strong X-rays or UV photons, the atomic bonding will be broken and the molecule will be destroyed. This is why the circumnuclear disks was thought to be a very difficult environment for organic molecules to survive. However, this ALMA observation proved the contrary; organic molecules are abundant in the circumnuclear disks.
The research team assumes that organic molecules remain intact in the circumnuclear disks due to a large amount of gas which is shielded from X-rays and UV photons, while organic molecules cannot survive the exposure to the strong UV photons in the starburst region where the gas density is lower compared with the circumnuclear disks.
Researchers have been actively engaged in observational research and the establishment of theoretical models of active galactic nucleis, but are just beginning to study the shielding effect on molecules. These results were a significant first step in understanding the structure, temperature and density of gas surrounding the active galactic nuclei.
“We expect that future observations with wider bandwidth and higher resolution will show us the whole picture of our target object in further detail and achieve even more remarkable results,” says Takano.
The articles can be found at:
Takano et al. (2014) Distributions of Molecules in the Circumnuclear Disk and Surrounding Starburst Ring in the Seyfert galaxy NGC 1068 Observed with ALMA.
Nakajima et al. (2015) A Multi-Transition Study of Molecules Toward NGC 1068 Based on High-Resolution Imaging Observations with ALMA.
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Source: National Astronomical Observatory of Japan.
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