Supermassive Black Holes Hide in Dust Cocoons
February 15, 2006
New observations with the Subaru telescope show that dusty cocoons in galaxies exceptionally bright in infrared light hide
supermassive black holes actively ingesting matter. Large amounts of material spiraling into a supermassive black hole more
massive than a million suns produces strong radiation. However, if the black hole is buried in dust from all directions, this
radiation may not easily be detected. Although theories suggest that actively radiating supermassive black holes deeply
buried in dust outnumber those surrounded by a doughnut-shaped region of dust, so far most black holes detected have been the
kind surrounded by a doughnut of dust. A research group led by an astronomer at the National Astronomical Observatory of
Japan used the Subaru telescope to perform infrared spectroscopy on galaxies exceptionally bright in the infrared, and found
evidence for actively mass accreting super-massive black holes completely surrounded by dust.
Figure 1: A schematic picture of energy generation by accretion of material (gas) onto a supermassive black hole...(more)
In the Universe, there are large numbers of galaxies which are not so bright in visible (optical) light, but radiate strongly in the infrared (Note 1). The brightest of such galaxies are called "ultraluminous infrared galaxies" (ULIRGs). When taking into account total energy output, ultraluminous infrared galaxies are some of the brightest objects in the Universe. Interestingly, most ultraluminous infrared galaxies appear to be two gas-rich spiral (M63;M84) galaxies that have collided and merged [Reference 1] (Note 2). If colliding galaxies contain a lot of gas, as spiral galaxies typically do, the collision can trigger star formation and funnel material into any existing supermassive black hole, both processes that generate large amounts of energy. Although this energy may originate as ultraviolet or visible light, gas is usually accompanied by dust, and the dust absorbs this light (Note 3) and re-emits as heat in the infrared. Once the ultraviolet and visible light is converted to heat, identifying the original energy source becomes an observational challenge.
Although both star formation and the feeding of a black hole can generate large amounts of energy, the source of the energy is very different. Stars generate energy in their cores by nuclear fusion and radiate it into space from their surfaces. When material spirals into a supermassive black hole, material looses gravitational energy, and the lost energy is converted into radiation. This process is called "accretion", and a black hole experiencing accretion is called "active". Since the sources of energy are different, active star-formation and active supermassive black holes are distinguished relatively easily through optical (visible light) spectroscopy, observations which disperse visible light into many wavelengths or colors [Reference 2], if radiation from the active supermassive blackhole can escape for a large anglar range (namely, totally unobscured or obscured by doughnut-shaped dust; see Figure 2 Left).
Previous observations show that supermassive black holes with one to ten million solar masses exist at the center of many galaxies (Note 4). Many of these black holes are active, and are thought to be surrounded by gas and dust in the shape of a doughnut [Reference 3] (Figure 2, Left). However, since ultraluminous infrared galaxies contain a large amount of dust and gas, active supermassive black holes are likely to be obscured in virtually all directions (Figure 2, Right). Such "buried" active supermassive black holes are elusive and have seldom been found observationally, despite theoretical predictions that their number in the Universe is much larger than active supermassive black holes surrounded by doughnut-shaped dust from which ultraviolet and visible light can escape [Reference 4].
An effective way to detect radiation from buried active supermassive black holes is to observe them at wavelengths of light that can penetrate barriers of dust better than ultraviolet or visible light. Infrared light with wavelengths longer than 3 micrometers (Note 5) is such an example, but infrared light from stars and galaxies is usually absorbed by Earth's atmosphere when observing from Earth's surface. However, the summit of Mauna Kea, where the Subaru telescope is located, is so high in elevation (about 4200 meters) that the absorption by Earth's atmosphere of infrared light is minimal at 3-4 micrometers. Mauna Kea is one of the best places in the world to observe faint objects in this wavelength range.
A research group led by Dr. Matatoshi Imanishi from the National Astronomical Observatory of Japan took advantage of this unique opportunity to disentangle a buried active supermassive black hole from active star-formation as the primary energy source of ultraluminous infrared galaxies using infrared 3-4 micrometer spectroscopy (Figure 3). The research group spectroscopically observed nearby (less than two billion light years away from Earth) ultraluminous infrared galaxies at 3-4 micrometers using the instrument IRCS (http://www.naoj.org/Observing/Instruments/IRCS/index.html) with the Subaru telescope (Figure 4). Thanks to the high sensitivity achieved by the combination of Subaru and IRCS, the research group was able to apply a new energy diagnostic method to reveal signatures of deeply buried active supermassive black holes in a significant fraction of the observed galaxies for the first time (Note 6). The new observations also confirmed that the active supermassive black holes could account for the bulk of the large infrared luminosities of these galaxies.
A more massive black hole can attract a larger amount of material and can produce brighter radiation. Supermassive black holes with more than ten million solar mass are required to account for the bulk of the brightness of ultraluminous infrared galaxies. When a spiral galaxy, which possesses a supermassive black hole with the mass of one to ten million solar mass, merges with another spiral galaxy, not only do stars form very actively through the collision of gas (Note 7), expelling a lot of dust to the surrounding interstellar medium, but also the originally existing supermasive black hole(s) can increase its mass by swallowing a large quantity of gas. This research supports the idea that when gas-rich spiral galaxies merge and become ultraluminous infrared galaxies, supermassive black hole(s) can grow up to more than ten million solar masses [Reference 5] and produce strong radiation through active mass accretion [Reference 6].
This result has been published as Imanishi et al. (2006) [Reference7] in the Astrophysical Journal (volume 637, pages 114-137, 2006 January 20 issue)
Figure 1: A schematic picture of energy generation by accretion of material (gas) onto a supermassive black hole. The black hole itself, indicated as the central black circle, can never be observed by a telescope, because a black hole is, by definition, "black" in radiation, where no light can escape. However, when a black hole attracts a large quantity of gas to its vicinity, the gas loses gravitational energy and begins to move at very high speeds. As a result of collisions and friction, the gas heats up and radiates strongly in ultraviolet to visible light. Since the gas has spin (angular momentum), it forms a disk, called an "accretion disk", as the gas accretes onto the central black hole. Strong emission from a black hole actually comes from this accretion disk. The size of a black hole (known as the "Schwarzschild radius") increases with its mass. The size of a supermassive black hole with ten million solar masses is estimated to be approximately thirty million kilometers (about one-fifth of the distance between the Sun and Earth). The accretion disk can exist at a distance a few to several times larger than the outer edge of the black hole. How close the accretion disk can get to the black hole depends on how fast the black hole is rotating. Figure credit: NASA/CXC/SAO. (Larger Image)
Figure 2:(Left): A supermassive black hole surrounded by a doughnut of dust and gas. Since dust in the close vicinity of an actively mass accreting, and thereby luminous, supermassive black hole evaporates, dust can exist only at a distance roughly a hundred billion kilometers (several hundreds times of the distance between the Sun and Earth) away. In this figure, the actively mass-accreting supermassive black hole can be seen directly from the above and below. The view to the black hole from the horizontal direction is blocked by dust. However, gas which is distributed along the vertical direction, higher than the doughnut-shaped dust, can be illuminated and ionized by the radiation from the accretion disk of the central active supermassive black hole. Since emission from this ionized gas is visible from the horizontal direction, the presence of an active supermassive black hole hidden behind doughnut-shaped dust is detectable through conventional optical spectroscopy. Figure credit: NASA/CXC/SAO. (Right): A supermassive black hole deeply "buried" in dust and gas. The central supermassive black hole is obscured by dust and gas in virtually all directions, and is no longer detectable through optical spectroscopy. Image created by Naomi Ishikawa (National Astronomical Observatory of Japan).(Larger Image)
Note 1: Light which humans can see with the naked eye is called "visible light". Light with longer wavelengths (lower energy) than visible light is called "infrared". Light with shorter wavelengths (higher energy) is called "ultraviolet". X-rays are light with even shorter wavelengths.
Note 2: Mergers of galaxies are thought to occur frequently in the Universe. Pictures at http://www.nao.ac.jp/Subaru
/hdtv/ngc4567s.jpgshow examples of such galaxy-galaxy mergers.
Note 3: When gas is present, dust (solids) is usually present as well. However, in the close vicinity of a luminous energy source, strong radiation causes dust to evaporate and only gas can be present (see the caption of figure 2).
Note 4: The Milky Way galaxy also contains a supermassive black hole with a mass of 2.6 million solar masses. For more information, see: http://www.eso.org/outreach
Note 5: 1 micrometer is 1/1000 of 1 millimeter.
Note 6: Compared to active star-formation, an actively mass-accreting supermassive black hole emits stronger X-rays (Note 1) from the vicinity of the accretion disk. The presence of a buried active supermassive black hole can be recognized if strong (but highly absorbed) X-ray emission is detected, or if signatures of the chemical effects of the X-ray emission on the surrounding gas is found. However, these methods can currently be applied only to a handful of bright ultraluminous infrared galaxies, because of the lack of sensitivity of existing observing facilities. The infrared 3-4 micrometer spectroscopic method using the Subaru telescope can be used to systematically investigate the energy sources of ultraluminous infrared galaxies, by observing a much larger number of sources.
Note 7: When galaxies possessing gas and stars merge, the collision of stars is negligible, but gas collides violently.
Note 8: The Universe is expanding uniformly.A galaxy more distant from the Earth is moving faster away from the Earth because of the Universe's uniform expansion. When we observe a galaxy from Earth, its light is shifted to longer (redder) wavelengths. This phenomenon is called "redshift".
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