Shoichiro Mizukoshi (Doctoral Student, Department of Astronomy)

Takeo Minezaki (Associate Professor, Institute of Astronomy)

Key points of the presentation

• We measured the attenuation of light due to the dust layer surrounding a galactic central black hole for 463 active galactic nuclei and found that there are active galactic nuclei that show attenuation as large as about 1/1 shuttle of their original brightness in visible light, and that there are many dust-free gas clouds around the black hole.
• By analyzing the light alteration phenomena of active galactic nuclei in the infrared, which easily penetrates the dust layer, we have established a new method to easily and massively measure the light attenuation of active galactic nuclei even if they are deeply hidden in the dust layer.
• The method is expected to be applicable to about 100,000 active galactic nuclei, and is expected to provide powerful clues for understanding active galactic nucleus phenomena and the growth of galactic central black holes in the future.

Summary of Presentation

A research group led by Shoichiro Mizukoshi, Graduate Student, Graduate School of Science, The University of Tokyo; Takeo Minezaki, Associate Professor, and Hiroaki Samejima, Assistant Professor, Institute of Astronomy; Mitsuru Kokubo, JSPS Research Fellow, Princeton University; and Hirofumi Noda, Assistant Professor, Graduate School of Science, Osaka University, has analyzed the time variability of infrared radiation from the active galactic nucleus ^{(Note 1)} A research group led by Assistant Professor Noda has developed a new method to measure the attenuation of light from the center of an active galactic nucleus (dust attenuation) due ^{to the dust} layer (dust torus) (Note 2) surrounding the central black hole of the galaxy by analyzing the time variability phenomenon of the infrared radiation intensity of the active galactic nucleus (Note 3). The advantage of this method is that it can be used to measure active galactic nuclei that are deeply hidden by the dust torus by using infrared light, which easily penetrates the dust torus, and can be easily and massively analyzed based on publicly available observational databases. The dust attenuation measurements of 463 active galactic nuclei show that some nuclei are so deeply hidden in the dust torus that their central emission in visible light is fainter by a factor ofabout 1杼( one trillionth of a trillionth of a trillionth ). In contrast to the dust attenuation measured in this study, the amount of gas between the black hole and us measured in previous studies was larger than would be expected from the standard ratio of the two in interstellar space in our Galaxy, and moreover, the values varied from one active galactic nucleus to another. This suggests that there are many dust-free gas clouds inside the dust torus. The method is expected to be applicable to about 100,000 active galactic nuclei in the future, and is expected to provide powerful clues for understanding active galactic nucleus phenomena and the growth of galactic central black holes.

Announcement

The dust torus surrounding a galactic central black hole (CMBH) is like a "dam" that stores a large amount of gas as a fuel source for the enormous amount of radiant energy of the active galactic nucleus. Some of the gas in the dust torus is gravitationally pulled into the black hole, and it becomes "fuel" for the enormous radiation, increasing the mass of the black hole. On the other hand, the dust mixed in with the gas is subjected to pressure from the strong radiation from the active galactic nucleus, and it is thought that a significant amount of the gas is blown out along with the dust. Thus, clarifying the structure and state of the dust torus is very important in the study of active galactic nuclei.

Since dust absorbs and scatters light, the amount of dust between the center of the active galactic nucleus and us can be evaluated by the amount of attenuation of radiation from the center (dust attenuation). Therefore, we can study the structure of the dust torus by measuring the dust attenuation in many active galactic nuclei. Many observations have been made in visible light, but because visible light efficiently attenuates even small amounts of dust, dust attenuation cannot be measured for active galactic nuclei that are deeply hidden in the dust torus. Although observations of active galactic nuclei in the near-infrared (wavelength of about 2 μm), which is less sensitive to dust, are in progress, the number of active galactic nuclei for which dust attenuation has been measured is still not large.

In this study, we developed a new method to measure dust attenuation in active galactic nuclei by analyzing temporal variations in the intensity of near-infrared radiation (variable light). At the inner edge of the dust torus, dust is heated to the point of sublimation by intense ultraviolet and visible light from the active galactic nucleus, and emits near-infrared radiation (wavelengths of about 1-5 μm), which is attenuated by the dust between the dust and us as well as the radiation from the active galactic nucleus center. The absorption and scattering effects of dust on visible and infrared radiation are smaller at longer wavelengths, so the spectrum becomes more "reddish" (i.e., fainter at relatively shorter wavelengths) (Fig. 1). Therefore, the amount of dust attenuation can be estimated by measuring the amount of this "reddening. In this study, we measured the amount of reddening by using the ratio of the intensity variations at two different wavelengths in the near-infrared (Figure 2).

Figure 1: Imaginary diagram showing the difference in the visibility of radiation from the center of an active galactic nucleus at different amounts of dust torus attenuation. As a general characteristic of active galactic nuclei, the brightness of radiation from the center changes with time (variable luminosity). Since the effect of dust absorption and scattering on visible and infrared radiation is smaller at longer wavelengths, when the attenuation due to the dust torus is large, the central radiation becomes darker and its spectrum becomes "redder" (relatively darker at shorter wavelengths and brighter at longer wavelengths). Therefore, the amount of dust torus attenuation is measured by measuring how "red" the spectrum of the changing component of the central radiation is.

Figure 2: Overview of the method to evaluate the amount of dust torus attenuation based on the temporal variation of the intensity of near-infrared emission from active galactic nuclei (variable light). In general, the near-infrared emission from active galactic nuclei changes in brightness over an observation period of a decade or more (left graph in the figure). The intensity at two different near-infrared wavelengths (here 3.4 µm and4.6 µ m) is plotted on the vertical and horizontal axes, respectively, for each observation date, and a straight line fit is performed (right graph in the figure). The slope of the fitting straight line indicates the ratio of the amount of dust attenuation at both wavelengths, and is an indicator of the color of the spectrum of the near-infrared radiation that is being attenuated. When dust attenuation is high, radiation at shorter wavelengths is darker than that at longer wavelengths, so the range of the attenuation is also smaller, and the slope of the fitting line changes. The slope of the fitting line for an active galactic nucleus, whose central visible emission is not obscured by dust, is averaged, and the difference in the slope of the fitting line for an active galactic nucleus is measured to evaluate the amount of dust torus attenuation in that active nucleus. Near-infrared (NIR) light at the wavelength of Since the absorption and scattering effects of dust in the near-infrared at this wavelength are much smaller than those of visible light, the amount of dust attenuation can be evaluated by measuring the radiation that has penetrated even active galactic nuclei that are deeply hidden by the dust torus.

Since the emission from stars and other objects in a galaxy with an active galactic nucleus at its center (the parent galaxy) does not change in brightness within an observation period of a few decades at most, this method can measure the amount of reddening of the emission from the active galactic nucleus without the influence of the emission from the parent galaxy. Recently, the infrared astronomical satellite WISE ^{(Note 3)} has been conducting long-term all-sky monitoring observations in the near-infrared at wavelengths from 3 to 5 μm. Thanks to the much smaller absorption and scattering effects of dust in the near-infrared at this wavelength compared to visible light, it is now possible to easily measure the dust attenuation in active galactic nuclei deeply hidden in the dust torus by analyzing publicly available WISE data using this method. The BAT AGN Spectrum

In this study, we applied this method to active galactic nuclei in the BAT AGN Spectroscopic Survey (BASS; Koss et al. 2017) catalog and successfully measured dust attenuation for 463 active nuclei. These active nuclei are broadly classified into those whose central visible emission is not and almost completely hidden by dust (called Type 1 and Type 2, respectively), but the dust attenuation of Type 2 active nuclei is not only larger than that of Type 1 active nuclei, but also ranges from only slightly larger than that to a brightness that, for visible light The dust attenuation of Type 2 active nuclei is not only larger than that of Type 1 active nuclei, but also has a wide range of values, from only slightly larger to extremely large, with a brightness of 1杼(onetrillionth of a trillionth ) in visible light (Figure 3).

Figure 3: Dust attenuation in active galactic nuclei measured in this study compared to the amount of gas between the black hole and us in those active nuclei. The blue circles represent Type 1 active nuclei whose central visible emission is not obscured by dust, and the red circles represent Type 2 active nuclei whose central visible emission is almost completely obscured. The gray bands indicate the position on the diagram for the standard gas-to-dust mixing ratio of the interstellar medium in our Galaxy: Type 2 active galactic nuclei have slightly higher dust attenuation than Type 1 active nuclei (but still have their visible light emission almost completely obscured), while Type 1 active nuclei have a dust attenuation of 1杼 ( 1in a trillion ), which is the brightest in visible light. trillionths ) of a trillionth of a trillionth of a trillionth of a trillionth of a trillionth of a trillionth of a trillionth of a trillionth. There is also a wide distribution of Type 2 active galactic nuclei, from those mostly on the gray belt to those with gas volumes roughly 100 times larger than that.

We then compared the measured dust attenuation to the amount of gas present between the black hole and us (as measured by attenuation of X-ray emission, as described in the BASS catalog) (Figure 3). We found that in many Type 2 active galactic nuclei, there is more gas than would be expected from dust attenuation assuming the standard gas-to-dust mixing ratio of the interstellar medium in the Galaxy. Moreover, this gas abundance varied for each active galactic nucleus from approximately equal to the value expected from the interstellar medium of the galaxy to nearly 100 times larger than that value.

Although such a trend has been suggested in previous studies, this is the first time that such a large number of active galactic nuclei have been systematically studied. This result can be explained by the depiction of a large number of dust-free gas clouds inside the dust torus, which contribute to this gas excess (Figure 4).

Figure 4: Schematic of the structure of the dust torus suggested by this study. A dust torus surrounds the central giant black hole and accretion disk, and a dust-free gas cloud exists between them. Near-infrared radiation is emitted from the high-temperature dust region at the inner edge of the dust torus and is attenuated when it passes through the dust torus; X-rays are emitted from the high-temperature gas near the giant black hole and are attenuated when they pass through the dust-free gas cloud and gas in the dust torus. Figures (1) to (3) show the different paths taken by near-infrared and X-ray emissions when observing an active galactic nucleus from different directions, the different attenuation patterns, and the positions of the data in Figure 2. In (1), near-infrared radiation is attenuated by the dust torus, while X-ray radiation is attenuated by both the dust torus and the dust-free gas cloud inside the dust torus. In (2), both near-infrared and X-ray emissions are not attenuated. In case (3), near-infrared and X-ray emissions are only attenuated in the dust torus.

A large number of active galactic nuclei have been observed by the WISE satellite, and it is expected that about 100,000 of them are applicable to this method. We hope to estimate the structure and state of the dust torus based on the large amount of dust attenuation data thus obtained, and to obtain clues for understanding the growth of active galactic nuclei and central black holes and their impact on the parent galaxy.

This work was supported by Grant-in-Aid for Scientific Research (Issue No.: 19K21884, 20H01941, 20H01947. 19K03917).

Journals

Journal name	Monthly Notices of the Royal Astronomical Society
Title of paper	Measurement of AGN dust extinction based on the near-infrared flux variability of WISE data
Author(s)	Shoichiro Mizukoshi, Takeo Minezaki, Shoichi Tsunetsugu, Atsuhiro Yoshida, Hiroaki Sameshima, Mitsuru Kokubo, Hirofumi Noda
DOI Number	10.1093/mnras/stac2307

Glossary

1 Active galactic nucleus

An astronomical phenomenon in which a very narrow region at the center of a galaxy radiates enormous electromagnetic waves that match or exceed the brightness of the entire galaxy. The energy source of the enormous radiation is believed to be the gravitational energy released by the fall of matter into the giant black hole at the center of the galaxy. X-rays are emitted from the high-temperature gas near the giant black hole, ultraviolet and visible light from the gas disks (accretion disks) that form around it, and infrared radiation from the dust torus (Note 2) that surrounds it. ↑up

Note 2 Dust torus

Gas is thought to be distributed in a doughnut-like structure surrounding the giant black hole and accretion disk, and the gas is thought to contain dust (solid fine particles ranging in size from a few nm to a few μm). This doughnut-shaped structure is called a dust torus. ↑up

Note 3 Infrared Imaging Satellite WISE

Launched by NASA in 2009, the Wide-field Infrared Survey Explorer (WISE) is an astronomical satellite that conducts infrared observations of all regions of the celestial sphere at semiannual intervals. After an initial period of dormancy, observations resumed in 2014 at wavelengths of 3.4 µm and4.6 µm,and are still ongoing. The data of the observations are available to the public. The data of the observation results are available to the public. ↑up