Figure 1: The central region of the spiral galaxy NGC 1068, as observed by ALMA overlay the Hubble Space Telescope image, has a fascinating distribution of hydrogen cyanide isotopes (H13CN) shown in yellow, cyanide radicals (CN) shown in red, and carbon monoxide isotopes (13CO) shown in blue. H13CN is concentrated solely in the center of the active galactic nucleus. However, CN not only appears in the center and the large-scale ring-shaped gas structure, but also exhibits a structure extending from the center towards the northeast (upper left) and southwest (lower right), which is believed to be caused by the jet emanating from the supermassive black hole. Credit: ALMA (ESO/NAOJ/NRAO), NASA/ESA Hubble Space Telescope, T. Nakajima et al.

Some galaxies, across various types, feature an active supermassive black hole at their center, acting as an engine that emits tremendous amounts of energy into the surrounding area, forming what is known as an Active Galactic Nucleus (AGN). Understanding how the activity of the black hole in the galactic nucleus influences the nearby interstellar material, especially whether it accelerates or inhibits the formation of new stars, is crucial for comprehending the process of galaxy evolution. Unfortunately, in many instances, the central regions of AGNs remain obscured by dense gas and interstellar dust, making it challenging, even with large telescopes in the optical and infrared wavelength bands, to answer questions like, ‘What is the structure there?’ and ‘What is happening physically and chemically in that region?’ However, the longer wavelengths like millimeter and submillimeter waves, which ALMA observes, are less susceptible to absorption by dust. This provides a significant advantage, as it allows us to peer into the innermost regions of the galactic nucleus.

Prior efforts have focused on millimeter- and submillimeter-wave observations targeting the central core region of NGC 1068 (M77) in the constellation Cetus, one of the relatively nearby AGNs situated at a distance of approximately 51.4 million light-years from Earth. For instance, between 2007 and 2012, comprehensive observations were conducted using the 45-meter radio telescope at Nobeyama Radio Observatory (NRO) of the National Astronomical Observatory of Japan (NAOJ). During this time, a ‘line survey’ was executed to systematically search for molecular emission lines within the 3 mm wavelength band (84-116 GHz) without frequency bias, focusing on the galactic center direction. The results unveiled the detection of 25 molecular emission lines, providing valuable insights. Nevertheless, the limited spatial resolution of the NRO 45-m radio telescope and the fact that the survey was confined to a single point within the galactic center prevented a detailed determination of the distribution of molecular gas and the structural characteristics surrounding the central core, despite confirming the presence of various molecules.

The international research team, led by Project Assistant Professor Toshiki Saito of NAOJ ALMA Project (at the time of writing this paper, he was a Visiting Researcher at Nihon University’s College of Engineering and a Project Researcher at NAOJ), and Assistant Professor Taku Nakajima of Nagoya University, has conducted a line survey near the central core of NGC 1068 using ALMA, achieving significantly higher spatial resolution. ALMA, a radio interferometer, is capable of conducting line surveys in the 3 mm wavelength band (85-114 GHz) of the central core of NGC 1068. Thanks to its interferometric nature, ALMA can produce high-resolution images within a specific region (field of view) even with observations in a single direction, allowing for the creation of a two-dimensional distribution map of molecular gas. This observation has enabled a clear imaging of two key structures: the circumnuclear disk, located at the center of the galaxy with a radius of approximately 650 light-years (depicted in yellow in Figure 1), and a ring-shaped gas structure (shown in blue in Figure 1) where stars are burstly forming at a radius of approximately 3,300 light-years beyond the disk. Notably, the inner structure of the circumnuclear disk was observed with great clarity. While high-resolution interferometric observations have previously been conducted in the central region of the AGN for specific molecular emission lines, which are easily observable due to their strong radio wave intensity, this study marks the first ‘imaging line survey’ depicting the distribution of all detected molecules through unbiased observations in the frequency domain. The findings from this research constitute an essential catalog of molecular emission lines that contribute to our understanding of the chemical state within AGNs and galaxies.

In this line survey, we detected 23 significant molecular emission lines. Detailed spectral data analysis revealed that the circumnuclear disk, believed to be directly influenced by the central supermassive black hole, exhibits notably higher concentrations of hydrogen cyanide (HCN・H13CN) and silicon monoxide (SiO) molecules compared to the outer ring-shaped gas structure. Conversely, cyanide radical (CN) molecules, previously thought to be abundant in observations with the NRO 45-m telescope, were found to be less prevalent in the circumnuclear disk when observed with the high resolution of ALMA.

Previous observations for astronomical objects have indicated that CN molecules are readily observed in gas clouds exposed to strong X-ray or ultraviolet radiation, while SiO molecules are associated with strong shock waves. Additionally, chemical reaction calculations have shown that the formation reactions of HCN and H13CN molecules become more active in molecular clouds at higher temperatures. Taken together, these observations strongly suggest that the influence of a supermassive black hole on the circumnuclear disk is to heat molecular gas to elevated temperatures through a dynamic mechanism, potentially accompanied by shock waves.

To further investigate this phenomenon, the research team turned their attention to the region between the circumnuclear disk and the ring-shaped gas structure. Within this region, a distinct structure emerges where the distribution of a specific type of molecular gas extends in two directions, reaching from the circumnuclear disk toward the northeast (upper left of Figure 1) and southwest (lower right of the same figure). To categorize this unique morphology for each molecule, the research group employed Principal Component Analysis (PCA), a form of unsupervised machine learning. Unlike classifications made by humans, which may vary due to subjectivity, machine learning offers objective results. The analysis revealed that the circumnuclear disk and the surrounding region are distinctly categorized in terms of the molecular gas structure (as depicted in Figure 2, left).

This outward-extending region is believed to have captured a bipolar molecular flow, commonly referred to as an ‘outflow.’ Its apparent direction aligns with the bipolar jet emanating from a supermassive black hole, as previously demonstrated in earlier studies. Given the angle of the jet and the resulting outflow relative to the galactic disk, a portion of the outflow grazes the disk surface, leading to shock wave heating (depicted in pink in Figure 3; the right side of Figure 2 shows the same model as seen from Earth). Within the outflow region, fundamental molecular species typically found in galaxies, such as carbon monoxide and methanol, seem to experience destruction, while distinctive molecules, including radicals like the cyanide radical, ethynyl radical, and hydrogen cyanide isomer, experience an increase in concentration. These observations highlight the substantial influence of jets and outflows from supermassive black holes on the central circumnuclear disk, extending their impact far beyond the circumnuclear disk’s boundaries.

These regions of jets and outflows are known to be accompanied by intense shock waves and emit strong radiation in the form of ultraviolet and X-rays, creating hostile environments for common interstellar molecules. Interstellar molecular gas serves as the fundamental building blocks of stars, which, in turn, are the primary constituents of galaxies. The destruction of these molecules near the galactic center, where they play a crucial role in star formation, is believed to stifle the birth of new stars. This study marks the first observational evidence, from a chemical perspective, suggesting that a supermassive black hole residing at a galaxy’s core may impede the evolutionary process of its host galaxy.

Toshiki Saito elaborates on the circumstances leading to the findings, “Initially, observing molecules in the vicinity of such a jet was considered challenging due to their destruction. However, thanks to ALMA’s high sensitivity, high resolution, and the PCA technique, we successfully detected the molecular gas outflow associated with the jet and elucidated its chemical properties. This discovery that the supermassive black hole’s activity at the galaxy’s center hinders its growth is of great significance.” Taku Nakajima further underscores the achievement, summarizing the findings presented in the series of papers, “Using astrochemistry to investigate the properties of celestial objects is a strong point of Japanese research groups. This marks the first imaging line survey of an AGN that provides insights into the extreme environment at the galaxy’s center. We’ve demonstrated that the combination of line survey observations with ALMA and machine learning analysis is highly effective for comprehending the physical and chemical properties of active galaxies.”

Figure 2: (Left) Diagram illustrating the machine learning-based classification of molecular distribution patterns. It reveals a structure (in blue) where a specific type of molecular gas extends in two directions from the circumnuclear disk (approximately represented by the white dot at the center) toward the northeast (upper left) and southwest (lower right). (Right) Schematic representation of the molecular gas distribution structure in the bipolar region, classified as distinct from the circumnuclear disk through machine learning (the same model is shown from a different perspective in Figure 3).
Credit: ALMA (ESO/NAOJ/NRAO), T. Saito et al.

Figure 3: Schematic diagram illustrating the location of the bipolar jet and galactic disk emanating from the supermassive black hole at the galaxy’s center, along with the resulting outflow of molecular gas.
Credit: ALMA (ESO/NAOJ/NRAO), T. Saito et al.

One observational result was published by Saito et al. as “AGN-driven Cold Gas Outflow of NGC 1068 Characterized by Dissociation-sensitive Molecules” in The Astrophysical Journal　on August 23, 2022 (DOI: 10.3847/1538-4357/ac80ff), and another was done as an online paper on September 14, 2023 and will be published by Nakajima et al. “Molecular Abundance of the Circumnuclear Region Surrounding an Active Galactic Nucleus in NGC 1068 based on Imaging Line Survey in the 3-mm Band with ALMA” in The Astrophysical Journal (DOI: 10.3847/1538-4357/ace4c7).

These works were supported by NAOJ ALMA Scientific Research grants No. 2017-06B, 2018-09B, 2020-15A, 2021-18A, and JSPS KAKENHI grants (JP15K05031, JP17H06130, JP18K13577, JP20H00172, JP20H01951, JP21K03632, JP21K03634, JP21K03547, JP22H04939).

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 and Technology Council (NSTC) in Taiwan 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.