The central part of the galaxy M77, also known as NGC1068, observed by ALMA and the NASA/ESA Hubble Space Telescope. Yellow: cyanoacetylene (HC₃N), Red: carbon monosulfide (CS), Blue: carbon monoxide (CO), which are observed with ALMA. While HC₃N is abundant in the central part of the galaxy (CND), CO is mainly distributed in the starburst ring. CS is distributed both in the CND and the starburst ring.
Credit: ALMA(ESO/NAOJ/NRAO), S. Takano et al., NASA/ESA Hubble Space Telescope

Research Background

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. This enables us to probe the environment (e.g. temperature and density) of a target region by studying the molecular chemical compositions in the region. 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 (*1) and the active galactic nuclei (AGN) at the center of galaxies, which are called circumnuclear disks (CND) (*2). 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 (*3). However, the weak radio emission from molecules often made the observations difficult and took us many days for signal detection using conventional radio telescopes.

ALMA Observations

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 with ALMA. M77 is known to have an active galactic nucleus (AGN) at its center which is surrounded by a starburst ring with a radius of 3500 light years. Since the research team had already conducted radio observations of various molecular emissions in this galaxy with the 45-m telescope at the Nobeyama Radio Observatory of NAOJ, they aimed to develop their research further with ALMA and identify the difference in chemical composition between AGNs and starburst regions. ALMA is a telescope suitable for analyzing molecules in galaxies because of: 1) a high sensitivity to detect faint radio signals; 2) a high fidelity imaging capability to image actual gas distributions; 3) 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 CND and in the starburst ring. “In this observation, we used only 16 antennas, which are about one-fourth of the complete number of ALMA antennas, but it was really surprising that we could get so many molecular distribution maps in less than two hours. We have never obtained such a quantity of maps in one observation,” says Takano, the leader of the research team.

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 CND. And In addition, carbon monosulfide (CS) and methanol (CH₃OH) are distributed both in the starburst ring and the CND. ALMA provided the first high resolution observation of the five types of molecules in M77, and revealed that they are concentrated in the CND.

“It was quite unexpected that acetonitrile (CH₃CN) and cyanoacetylene (HC₃N), which have a large number of atoms, are concentrated in the CND,” says Nakajima. The supermassive black hole in the AGN 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 CND 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 CND.

The research team assumes that organic molecules remain intact in the CND 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 CND.

Researchers have been actively engaged in observational research and the establishment of theoretical models of AGNs, but it is just the beginning of the study on the shielding effect on molecules, which was discovered by these ALMA observations. These results were a significant first step in understanding the structure, temperature and density of gas surrounding the AGN. “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.

Note
*1) In the Milky Way Galaxy, in which we live, one sun-like star is generated per year on average, while several hundred sun-like stars are churned out each year in a starburst region .

*2) It is believed that most galaxies have in their center a supermassive black hole of millions to hundreds of millions of solar mass. Among them, Active Galactic Nuclei (AGN) represents a type of supermassive black hole which are gulping down surrounding gas very actively and emitting some amount of gas as a high-speed gas flow (jet).

*3) For example, a research team led by Takuma Izumi and Kotaro Kohno at the University of Tokyo, both of whom are engaged in this research, suggests that there is enhanced emission of hydrogen cyanide (HCN) from the supermassive black hole in the barred spiral galaxy NGC1097 by past ALMA observations.
Reference: October 24, 2013, Press release “Unique Chemical Composition Surrounding Supermassive Black Hole—A Step toward Development of New Black Hole Exploration Method”

Paper and Research Team
These observational results were published by Takano et al. as “Distributions of molecules in the circumnuclear disk and surrounding starburst ring in the Seyfert galaxy NGC 1068 observed with ALMA” (in the astronomical journal Publications of the Astronomical Society of Japan (PASJ), issued in July 2014) and by Nakajima et al. “A Multi-Transition Study of Molecules toward NGC 1068 based on High-Resolution Imaging Observations with ALMA” (in PASJ issued in February 2015).

This research was conducted by:
• Shuro TAKANO (NAOJ Nobeyama Radio Observatory/SOKENDAI)
• Taku NAKAJIMA (Solar-Terrestrial Environment Laboratory, Nagoya University)
• Kotaro KOHNO (Institute of Astronomy/Research Center for the Early Universe, The University of Tokyo)
• Nanase HARADA (Academia Sinica Institute of Astronomy and Astrophysics [At the time of writing: Max Planck Institute for Radio Astronomy])
• Eric HERBST (University of Virginia)
• Yoichi TAMURA (Institute of Astronomy, The University of Tokyo)
• Takuma IZUMI (Institute of Astronomy, The University of Tokyo)
• Akio TANIGUCHI (Institute of Astronomy, The University of Tokyo)
• Tomoka TOSAKI (Joetsu University of Education)

Eric Herbst gratefully acknowledges the support of the National Science Foundation for his astrochemistry program. He also acknowledges support from the NASA Exo-biology and Evolutionary Biology program through a subcontract from Rensselaer Polytechnic Institute.

ALMA
The Atacama Large Millimeter/submillimeter Array (ALMA), an international astronomy facility, is a partnership of the European Organisation 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 Council of Taiwan (NSC) 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.