Social Sciences Chile , Atacama, Friday, February 27 of 2015, 10:03

ALMA revealed calm pockets protecting organic molecules

Such complex carbon-based molecules are thought to be easily obliterated by the strong X-rays and ultraviolet (UV) photons that permeate the environment surrounding supermassive black holes

ALMA OBSERVATORY/DICYT Researchers using the Atacama Large Millimeter/submillimeter Array (ALMA) have discovered regions where certain organic molecules somehow endure the intense radiation near the supermassive black hole at the center of galaxy NGC 1068, also known to amateur stargazers as M77.


Such complex carbon-based molecules are thought to be easily obliterated by the strong X-rays and ultraviolet (UV) photons that permeate the environment surrounding supermassive black holes. The new ALMA data indicate, however, that pockets of calm exist even in this tumultuous region, most likely due to dense areas of dust and gas that shield molecules from otherwise lethal radiation.


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 researchers to probe the environment (temperature and density) of a target region by studying the molecular chemical compositions in it. 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 surrounding region of the active galactic nuclei (AGN) at the center of galaxies, called circumnuclear disk (CND). These regions are very important in understanding the evolution of galaxies, and radio observations of molecular emissions are essential to explore its mechanism and environment. However, the weak radio emission from molecules often made the observations difficult and took many days for signal detection using conventional radio telescopes.


ALMA Observations Trace Molecules


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 of Cetus (the Whale) about 47 million light years away with ALMA. M77 is known to have an active galactic nucleus 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 meters telescope at the Nobeyama Radio Observatory of NAOJ, they aimed to develop their research further with ALMA's extreme sensitivity, high-fidelity and ability to observe wideband in multiple wavelenght along with a high spatial resolution; and identify the difference in chemical composition between AGNs and starburst regions.


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


Shielding Complex Organics around a Balck Hole


The supermassive black hole devours surrounding materials by its strong gravity and generates such a hot disk around him that it emits intense X-rays or UV photons. When complex organic molecules are exposed to strong X-rays or UV photons, their multiple atomic bonds are broken and the molecules destroyed. This is why the circumnuclear disk was thought to be a very difficult environment for organic molecules to survive. ALMA observations, however, proved the contrary: Complex organic molecules are abundant in the circumnuclear disk.


"It was quite unexpected that acetonitrile (CH3CN) and cyanoacetylene (HC3N), which have a large number of atoms, are concentrated in the circumnuclear disk," said Nakajima.


The research team assumes that organic molecules remain intact in the circumnuclear disk due to a large amount of gas, which act as a shield 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.


The researchers point out that these results are a significant first step in understanding the structure, temperature, and density of gas surrounding the active black hole in M77. "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.


"ALMA has launched an entirely new era in astrochemistry," said Eric Herbst of the University of Virginia in Charlottesville and a member of the research team. "Detecting and tracing molecules throughout the cosmos enables us to learn so much more about otherwise hidden areas, like the regions surrounding the black hole in M77."


These observation results were published as Takano et al. "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 as 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).