Andrew Yee
February 23rd 06, 04:01 AM
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/2005/10/13/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/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)
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-rel/pr-2002/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.
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/2005/10/13/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/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)
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-rel/pr-2002/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.