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.
Molecules Reveal Clues to Galactic Environments
Interstellar
gas contains a wide variety of molecules, which differ wildly depending
on the environment. For example, high-temperature, active star forming
regions produce different molecules than would be found in colder
interstellar regions. This enables scientists to probe the temperature
and density of certain regions by studying their chemical composition.
Astronomers
have long been studying the molecular signatures around supermassive
black holes: both nearby starburst regions and surrounding rings of dust
and gas known as a circumnuclear disks (CND) that spiral-in to feed an
active black hole. These regions are important for understanding the
evolution of galaxies. However, weak radio emission from the molecules
there often makes observations difficult.
ALMA Observations Trace Molecules
To
better understand the complex and energetic environs around a
supermassive black hole, the 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, which is located
about 47 million light-years from Earth in the direction of the
constellation Cetus (the Whale).
This galaxy is known to have an
actively feeding central black hole, which indicates it has a
substantial circumnuclear disk. That disk, in turn, is surrounded by a
3,500 light-year wide starburst ring. To probe these areas, the research
team added ALMA’s extreme sensitivity and high-fidelity imaging
capabilities to earlier observations conducted by the 45-meter radio
telescope at the Nobeyama Radio Observatory of the National Astronomical
Observatory of Japan (NAOJ).
The new ALMA observations clearly
reveal the distributions of nine types of molecules in the surrounding
disk and 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,” said Takano, the
leader of the research team.
The results clearly 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 primarily in the CND. In addition, carbon monosulfide (CS) and methanol (CH3OH) are distributed both in the starburst ring and the CND.
Shielding Complex Organics around a Black Hole
As
the supermassive black hole devours the surrounding material, this disk
is heated to such extreme temperatures that it emits intense X-rays and
UV photons. When complex organic molecules are exposed to these
photons, their atomic bonds are broken and the molecules are destroyed.
Astronomers assumed that such regions would therefore be devoid of such
complex organics. The ALMA observations, however, proved the contrary:
Complex organic molecules are abundant in the CND, though not so in the
broader starburst region.
"It was quite unexpected that
complex molecules with a large number of atoms like acetonitrile and
cyanoacetylene are concentrated around the black hole's disk," said
Nakajima.
The research team speculates that organic molecules
remain intact in the CND due to the large amount of gas there, which
acts as a barrier for the 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 comparatively 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 this region," said 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 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 August 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).
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Contacts:
Charles Blue,
NRAO Public Information Officer
(434) 296-0314;
cblue@nrao.edu
Masaaki Hiramatsu,
PhD, Public Outreach Officer at NAOJ
+81-422-34-3630;
hiramatsu.masaaki@nao.ac.jp
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.
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.
