Supermassive Black Holes Hide in Dust Cocoons (Forwarded)

National Astronomical Observatory of Japan
Tokyo, Japan

February 15 , 2006

Supermassive Black Holes Hide in Dust Cocoons

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 detectable. 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.

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 galaxies
(http://www.naoj.org/Pressrelease/2000/06/M63.jpg and
http://www.naoj.org/Pressrelease/200...3/image_84.jpg) 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 distinguishable 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 black hole 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/Instru...RCS/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)

NOTES:

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/ngc4038s.jpg
http://www.nao.ac.jp/Subaru/hdtv/ngc4567s.jpg
show 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/press-re.../pr-17-02.html

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".

REFERENCE:

[1] Murphy, Jr. et al. 2001, Astrophysical Journal, 559, 201
Cui et al. 2001, Astronomical Journal, 122, 63
Veilleux et al. 2002, Astrophysical Journal Supplement Series, 143, 315
[2] Antonucci, R. 1993, Anual Review of Astronomy & Astrophysics, 31, 473
[3] Veilleux, S., and Osterbrock, D. E. 1987, Astrophysical Journal
Supplement Series, 63, 295
[4] Fabian, A. C. et al. 2002, Monthly Notices of the Royal Astronomical
Society, 329, L18
[5] Taniguchi, Y. et al. 1999, Astrophysical Journal, 514, L9
[6] Sanders, D. B. et al. 1988, Astrophysical Journal, 325, 74
[7] Imanishi et al. 2006, Astrophyiscal Journal, 637, 114

IMAGE CAPTIONS:

[Figure 1:
http://www.naoj.org/Pressrelease/2006/02/15/fig1_l.jpg (241KB)]
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.

[Figure 2:
http://www.naoj.org/Pressrelease/2006/02/15/fig2_l.jpg (44KB)]
(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).

[Figure 3]
A method to distinguish whether the primary energy source of a galaxy is
a buried active supermasive black hole (denoted as "AGN" in the figure)
or active star-formation, through infrared 3-4 micrometer spectroscopy.

[Figure 3a:
http://www.naoj.org/Pressrelease/2006/02/15/fig3a_l.jpg (72KB)]
An actively star-forming galaxy, such as M82,
(http://www.naoj.org/Pressrelease/2000/03/j_index.html) shows strong 3.3
micrometer olycyclic Aromatic Hydrocarbon emission (PAH). PAHs consist
of benzene-like carbonaceous olecules, and are known to be widely
distributed in the interstellar medium of galaxies. Image credit of M82:
National Astronomical Observatory of Japan.

[Figure 3b:
http://www.naoj.org/Pressrelease/2006/02/15/fig3b_l.jpg (61KB)]
A buried active supermassive black hole displays no PAH emission. Only
smooth continuum emission and strong dust absorption features are found.
The absence of the PAH emission is caused by the destruction of PAH
molecules by radiation from the active supermassive black which is more
energetic than radiation from active star-formation regions. The
observed absorption feature at 3.4 micrometers in this figure originates
in carbonaceous dust grains.

[Figure 4]
Examples of observed ultraluminous infrared galaxies.

[Figure 4a:
http://www.naoj.org/Pressrelease/2006/02/15/fig4a_l.jpg (81KB)]
An infrared K-band (2.2 micrometers) image (Left; 10 arcsec field of
view, North is up and East is to the left) of the ultraluminous infrared
galaxy IRAS 14060+2919, and its infrared 3-4 micrometer spectrum. Strong
PAH emission is observed, suggesting that active star-formation is
responsible for the infrared luminosity of this galaxy. Although the
observed wavelength of the PAH emission is shifted to longer (redder)
wavelength because of the redshift (Note 8) of this galaxy, the spectrum
is shown with the effect of the redshift removed.

[Figure 4b:
http://www.naoj.org/Pressrelease/2006/02/15/fig4b_l.jpg (78KB)]
A K-band image and 3-4 micrometer spectrum of another ultraluminous
infrared galaxy IRAS 12127-1412. No PAH emission and strong dust
absorption are detected, indicating that an active supermassive black
hole dominates the luminosity of this galaxy. An absorption feature at
3.05 micrometers is found, because some fraction of absorbing dust is
covered with ice.

[Figure 4c:
http://www.naoj.org/Pressrelease/2006/02/15/fig4c_l.jpg (83KB)]
A K-band image and 3-4 micrometer spectrum of another ultraluminous
infrared galaxy IRAS 17044+6720. Both active star-formation and buried
active supermassive black hole are thought to contribute to the observed
infrared emission from this galaxy.