Recent high resolution near infrared (HST-NICMOS) and mm-interferometric
imaging have revealed dense gas and dust accretion disks in nearby
ultra-luminous galactic nuclei. In the best studied ultraluminous IR galaxy,
Arp 220, the 2 micron imaging shows dust disks in both of the merging galactic
nuclei and mm-CO line imaging indicates molecular gas masses approx. 10^9 M_sun
for each disk. The two gas disks in Arp 220 are counterrotating and their
dynamical masses are approx. 2x10^9 M_sun, that is, only slightly larger than
the gas masses. These disks have radii approx 100 pc and thickness 10-50 pc.
The high brightness temperatures of the CO lines indicate that the gas in the
disks has area filling factors of approx. 25-50% and mean densities of >~ 10^4
cm^(-3). Within these nuclear disks, the rate of massive star formation is
undoubtedly prodigious and, given the high viscosity of the gas, there will
also be high radial accretion rates, perhaps >~ 10 M_sun/yr. If this inflow
persists to very small radii, it is enough to feed even the highest luminosity
AGNs.Comment: LaTex, 6 pages with 1 postscript and 1 jpg figure, and 1 postscript
  table, To appear in the proc. of the Ringberg workshop on "Ultraluminous
  Galaxies: Monsters or Babies" (Ringberg castle, Sept. 1998), Ap&SS, in pres

Scoville, N. Z.

English

arXiv

Recent high resolution near infrared (HST-NICMOS) and mm-interferometric imaging have revealed dense gas and dust accretion disks in nearby ultra-luminous galactic nuclei. In the best studied ultraluminousIR galaxy, Arp 220, the 2μm imaging shows dust disks in both of the merging galactic nuclei and mm-CO line imaging indicates molecular gasmasses ∼ 10⁹M⊙ for each disk. The two gas disks in Arp 220 are counterrotating and their dynamical masses are ∼ 2×10⁹ M⊙, that is, only slightly largerthan the gas masses. These disks have radii ∼100 pc and thickness 10-50 pc. The high brightness temperatures of the CO lines indicatethat the gas in the disks has area filling factors ∼25-50% and mean densitiesof ≥ 10⁴ cm⁻³. Within these nuclear disks, the rate of massive star formation is undoubtedly prodigious and, given the high viscosity of the gas, there will also be high radial accretion rates, perhaps ≥ 10 M⊙ yr ⁻¹. If this inflow persists to very small radii, it is enough to feed even the highest luminosity AGNs

Caltech Authors - Main

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MASSIVE ACCRETION DISKS
N.Z. SCOVILLE
California Institute of Technology
Astronomy Department 105-24
Pasadena, CA 91125, USA
1. Abstract
Recent high resolution near infrared (HST-NICMOS) and mm-interferometric
imaging have revealed dense gas and dust accretion disks in nearby ultra-
luminous galactic nuclei. In the best studied ultraluminous IR galaxy, Arp
220, the 2µm imaging shows dust disks in both of the merging galactic
nuclei and mm-CO line imaging indicates molecular gas masses ∼109M⊙
for each disk. The two gas disks in Arp 220 are counterrotating and their
dynamical masses are ∼ 2x109M⊙, that is, only slightly larger than the
gas masses. These disks have radii ∼100 pc and thickness 10-50 pc. The
high brightness temperatures of the CO lines indicate that the gas in the
disks has area filling factors ∼25-50% and mean densities of ≥ 104 cm−3.
Within these nuclear disks, the rate of massive star formation is undoubt-
edly prodigious and, given the high viscosity of the gas, there will also be
high radial accretion rates, perhaps ≥10 M⊙ yr
−1. If this inflow persists
to very small radii, it is enough to feed even the highest luminosity AGNs.
2. Introduction
In the luminous infrared galaxies, very large masses of interstellar matter
have been concentrated in the galactic nuclei at radii less than 300 pc
as a result of galactic merging. Due to its relatively large volume-filling
factor and ability to cool rapidly, the ISM is much more dissipative than
the stars. This dissipation of rotational energy and angular momentum
will concentrate the ISM in the merging galactic nucleus. The resultant
high density of ISM in the nucleus then becomes the active ingredient for
luminosity generation – via enormously elevated rates of star formation or
by feeding a pre-existing massive black hole. There new evidence for massive
2 N.Z. SCOVILLE
gas and dust accretion disks in a number of these galactic nuclei. Here,
we will concentrate on those structures. The best example is that found
in the ultraluminous infrared galaxy Arp 220. For this object there now
exists sub-arcsecond resolution mm-wave aperture synthesis which probes
both the molecular gas distribution and the kinematics – hence the mass
distribution (Scoville, Yun and Bryant, 1997, Downs and Solomon, 1998,
and Sakamoto et al 1998). Similar, less massive structures are also evident
in millimeter-wave observations of a number of nearby lower level AGN
(Tacconi et al 1998, Baker and Scoville, 1998).
3. Arp 220
Arp 220 (IC 4553/4), with an infrared luminosity of 1.5 × 1012 L⊙ at λ =
8-1000µm, is one of the nearest ultraluminous infrared galaxies (Soifer et
al, 1987). Visual wavelength images reveal two faint tidal tails, indicating a
recent tidal interaction (cf. Joseph and Wright, 1985), and high resolution
ground-based radio and near-infrared imaging show a double nucleus (Baan
et al, 1987, Graham et al, 1990). The radio nuclei are separated by 0.′′98 at
P.A. ∼90◦ (Baan and Haschick, 1995). To power the energy output seen in
the infrared by young stars requires a star formation rate of ∼102 M⊙ yr
−1.
Alternatively, if the luminosity originates from an active galactic nucleus,
this source must be sufficiently obscured by dust that even the mid-infrared
emission lines are highly extincted since spectroscopy with the ISO shows
no evidence of very high ionization lines at wavelengths out to 40µm (Sturm
et al, 1996).
3.1. NEAR INFRARED
The central region of Arp 220 is shown in contour form in Figure 1. Co-
ordinate offsets are measured from the position of peak flux at 2.2µm oc-
curing on the western nucleus. These images clearly show the two nuclear
regions plus several lesser peaks. The morphology of both nuclei changes re-
markably with wavelength. In particular, the bright western nucleus shows
greater extension to the south at 2.2µm and the eastern nucleus has a
southern component which becomes increasingly strong at the longer wave-
lengths. Under the assumption that the background starlight has an in-
trinsic color which is not strongly varying, and that the dust producing
the extinction is in the foreground, ie, not mixed with the stars, we have
corrected the 2.2µm image for extinction and use this extinction-corrected
image (shown in the lower right panel of Figure 1) to place the near-infrared
maps relative to the three cm-wave radio continuum sources in Arp 220.
Our registration places one radio nucleus between the two emission peaks
MASSIVE ACCRETION DISKS 3Arp 220
Fig. 1: Central region of Arp 220 imaged at 1.1 (a), 1.6 (b) and 2.2 (c)µm
with NICMOS on HST (Scoville et al, 1998). The lower right panel shows
the ’de-extincted’ 2.2 µm image where the extinction at 2.2µm was deter-
mined from the 1.6/2.2 µm color and the assumption of a foreground dust
screen. (The lower right panel has 0.2′′ resolution since both the 1.6 and 2.2
µm images were smoothed to this resolution to obtain the color). Contours
are spaced logarithmically and the axes are labelled in arcsec offset from
the 2.2 (c)µm peak. The bar indicates 200 pc.
seen at 2.2µm in the east and the western radio nucleus in the area of ex-
tremely high obscuration to the south of the western 2.2µm peak. With this
registration, the third, very weak radio component coincides with the faint
southern peak seen in the images at all three near-infrared wavelengths (cf.
Scoville et al 1998).
The complex near-infrared structure of Arp 220 is undoubtedly due to
4 N.Z. SCOVILLE
there being multiple centers of star formation activity and strongly varying
dust obscuration within the merger nuclei. The crescent or partial ring mor-
phology of the western nucleus might readily arise under two circumstances:
if there is an obscuring disk of dust and gas embedded in a spheroidal nu-
clear star cluster, or if a central starburst ring is partially obscured by its
own dust on one side. In either case, the dust (and dense interstellar gas)
must be confined to a thin disk-like structure, concentric with the western
nucleus of the galaxy. A similar structure probably exists in the eastern nu-
cleus based on the millimeter-wave interferometry (see below) although its
inclination may not be so close to the line of sight (in view of the less sharp
cutoff in the near-infrared light distribution). The suggested geometry for
the western nucleus is a galactic nucleus star cluster cut by an embedded,
opaque dust disk at inclination 20◦ to the line of sight.
3.2. MOLECULAR GAS
The total molecular gas content for Arp 220 is 9 × 109M⊙ based on the
CO (2-1) emission and a CO-to-H2 conversion ratio which is 0.45 times the
Galactic value (cf. Scoville, Yun and Bryant, 1997). This enormous mass
(approximately four times that of the entire Galaxy) is contained entirely
within R <1.5 kpc and approximately 5 × 109M⊙ is apparently concen-
trated in a thin disk in the nuclear region at radii < 250 pc. Within the
last year there have been two studies of the CO (2-1) emission at 0.5′′ (175
pc) resolution (Downs and Solomon, 1998, and Sakamoto et al, 1998). In
Figure 2, the CO emission and 1.3 mm continuum emission are shown from
Sakamoto et al (1998). Figures 2a-c show the CO line emission, mean ve-
locity field and continuum emission obtained from only the high resolution
interferometry data – that is, filtering out any extended structure by not in-
cluding short interferometer baselines. Figure 2d shows the total integrated
CO emission and mean velocity contours, including both high and low res-
olution data. The crosses on all four panels indicate the positions of the
double radio nuclei (Norris, 1985). In both the line (a) and the continuum
(c), there are clear peaks at the positions of the radio nuclei.
The CO peaks coincide with those in the continuum but the line emis-
sion is much more extended than the continuum emission. The diameter
of the CO in Figure 2d, in which most of single-disk flux is recovered, is
about 2 kpc, and the overall emission (2d) is elongated along a direction
parallel to the gradient in the velocity field. The latter provides the basis
for modelling the overall emission as a disk since an inclined disk will have
parallel kinematic and isophotal major axes (see also Scoville, Yun and
Bryant, 1997). The gas disk is, however, not axisymmetric and its velocity
field shows distortions, which suggest non-circular motions or unrelaxed gas
MASSIVE ACCRETION DISKS 5
 
  2   1   0  -1  -2
R.A. offset [arcsec]
-1
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0.56"•0.51"
(c) 1.3mm continuum
300pc
 
     
 
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 [a
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0.57"•0.52"
(a) CO(2-1)
300pc
 
  4   3   2   1   0  -1  -2  -3  -4
R.A. offset [arcsec]
-4
-3
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-1
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4
 
5300
5400
5500
5600
0.78"•0.74"
(d) CO(2-1) (LHU)
1 kpc
 
     
 
-1
0
1
 
0.57"•0.52"
(b) CO(2-1)
300pc
5330
Fig. 2: Central region of Arp 220. Crosses in each panel indicate the 1.3
m continuum position of the nuclei. (a) High resolution CO (2-1) emission.
(b) Mean velocity of CO (2-1) with contours spaced 50 km s−1. (c) 1.3 mm
continuum. (d) CO (2-1) integrated intensity map and isovelocity contours
(50 km s−1 step) made from low(L), high(H), and ultra-high(U) resolution
data.
components, probably due to the fact that the system is still an ongoing
merger and has two massive nuclear star clusters embedded in the disk.
About 30% of the total CO emission is associated with the two nuclei,
and a steep velocity gradient is evident across each nucleus in the high
resolution velocity map (Figure 2b). The velocity shift is about 500 km s−1
within 0.′′3 (110 pc) in position-velocity diagrams through each nucleus.
The velocity gradients across the two nuclei are in opposite directions and
neither is aligned with that in the outer molecular disk at P.A. = 25◦
(compare Figures 2b and d). The very steep velocity gradients at the two
nuclei, as well as their directions, exclude the possibility that the gas is
just rotating around the dynamical center of the merger. Instead, the data
suggest that each nucleus has a separate rotating molecular gas disk. These
6 N.Z. SCOVILLE
two disks are counterrotating, and their diameters are ∼200 pc (0.5′′). The
dynamical mass of each nucleus is ∼2 × 109 sin2i M⊙, where i is the
inclination of the disks. The presence of the two dynamically relaxed disks
strongly supports the view that the two peaks (in the millimeter, radio and
NIR) are true nuclei, and that Arp 220 is indeed a merger. The major axis
of the disk inferred from the NICMOS images is consistent with the E-W
velocity gradient at the west nucleus.
The two nuclear disks are embedded in an outer gas disk of kiloparsec
radius. The mean velocity of the two nuclei is different by about 200 km
s−1, with the eastern nucleus more redshifted. This is consistent with NIR
spectroscopy of Brγ (Larkin et al, 1995). The dynamical mass within the
orbit of the two nuclei (ie, R < 250 pc) was estimated to be a 5.4 × 109M⊙
by Scoville, Yun and Bryant (1997). Thus more than half of the mass in
this region belongs to the two nuclei. Note, however, that the dynamical
mass of the central disk has large uncertainty due to its poorly constrained
inclination (i∼ 45 deg).
4. Remarks
In the ultraluminous galaxies the nuclear gas and dust accretion disks are
spectacular: containing up to 109 M⊙ of ISM at radii 100-200 pc but ex-
tremely thin (10-50 pc). The mean gas densities within the disks are typi-
cally 104 cm−3 and, in contrast to our Galactic center where the area-filling
factor is only a few percent, the disk area is substantially filled (area-filling
factors, 20-50%). Our results show that part of the concentrated gas (30%
in CO luminosity in the case of Arp 220) remains intact around the stellar
nuclei to the late stages of merger evolution – at least until the two nuclei
approach within several hundred parsecs. The rest of the gas is rotating
around the dynamical center of the merger system.
It has been shown in numerical simulations that gas rapidly concen-
trates to the nuclei of merging galaxies at an early phase of interaction.
Subsequently, the concentrated gas quickly merges as the nuclei approach
each other (Barnes and Hernquist, 1991). There is strong consensus that the
activity in the ultraluminous IR galaxies is triggered by strong dynamical
perturbations during a close galactic encounter or galactic merging. The
resultant disruption of the normally circular orbits in a disk system will
lead to extremely rapid dissipation of rotational energy and angular mo-
mentum within the interstellar medium. Once in the nucleus, any vertical
(z) motions of the gas are rapidly damped and dissipated. The resultant
thin disk in the nuclear region, containing a galactic mass of interstellar
medium, will be vertically supported by turbulence and the radiation pres-
sure of young stars formed in this high density medium. In fact, such disks
MASSIVE ACCRETION DISKS 7
may become self-regulating since the stars which form within the disk gen-
erate both turbulence and a high radiation pressure. At the elevated rates
of star formation seen in these galactic nuclei, the resulting turbulent and
radiative pressure can swell the disk, and thus damp the star formation.
The final evolution of such disks to AGNs could proceed either along
the lines suggested by Norman and Scoville (1988) in which mass-loss from
late stellar evolution of the starburst population ultimately accretes into
and builds up a super-massive black hole, or with a pre-existing black hole
being fed directly from the nuclear disk by accretion to very small radii.
In either case, one should not expect the AGN to become immediately
visible, given the extraordinarily large extinctions (AV ≥ 10
3 mag) in the
nuclear disk during these early phases. The discovery that the ISM in the
late merging systems is often concentrated in a thin, rotationally-supported
disk (rather than in a more chaotic spherical distribution) should enable
theoretical studies of their evolution to proceed from a more clearly defined
initial state.
ACKNOWLEDGMENTS
It is a pleasure to acknowledge stimulating discussions with A. Baker, C.
Norman and K. Sakamoto on this work and assistance with the manuscript
from N. Candelin and Z. Turgel.
References
Baan, W.A. and Haschick, A.D. (1995),Astrophys. J., 454, 745 .
Baan, W. A., van Gorkom, J.H., Schmelz, J.T. & Mirabel,I.F. (1987), Astrophys. J., 313,
102.
Barnes, J.E., and Hernquist, L.E., (1991) Astrophys. J., 370, L65.
Downes, D. and Solomon, P.M., (1998) Astrophys. J., in press .
Graham, J.R., Carico, D.P., Matthews, K., Neugebauer, G., Soifer, B.T., & Wilson, T.D.
(1990), Astrophys. J., 354, L5
Joseph, R.D. & Wright, G.S. (1985),MNRAS, 214, 87.
Larkin, J.E., Armus, L., Knop, R.A., Matthews, K., & Soifer, B.T. (1995), Astrophys. J.,
452, 599.
Norman, C.A. and Scoville, N.Z. (1988), Astrophys. J.,332, 124.
Norris, R.P., (1985)MNRAS, 216, 701.
Sakamoto, K., Scoville, N., Yun, M.S., Crosas,M., Genzel, R. and Tacconi, L. (1998),As-
trophys. J., in press .
Scoville, N.Z., et al (1998),Astrophys. J., 492, L107 .
Scoville, N.Z., Yun, M.S., and Bryant, P.M. (1997), Astrophys. J., 484, 702 .
Soifer, B.T., et al (1987), Astrophys. J., 320, 238.
Tacconi, L., et al (1998), Astrophys. J., in press.


Massive accretion disks

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MASSIVE ACCRETION DISKS
N.Z. SCOVILLE
California Institute of Technology
Astronomy Department 105-24
Pasadena, CA 91125, USA
1. Abstract
Recent high resolution near infrared (HST-NICMOS) and mm-interferometric
imaging have revealed dense gas and dust accretion disks in nearby ultra-
luminous galactic nuclei. In the best studied ultraluminous IR galaxy, Arp
220, the 2µm imaging shows dust disks in both of the merging galactic
nuclei and mm-CO line imaging indicates molecular gas masses ∼109M⊙
for each disk. The two gas disks in Arp 220 are counterrotating and their
dynamical masses are ∼ 2x109M⊙, that is, only slightly larger than the
gas masses. These disks have radii ∼100 pc and thickness 10-50 pc. The
high brightness temperatures of the CO lines indicate that the gas in the
disks has area filling factors ∼25-50% and mean densities of ≥ 104 cm−3.
Within these nuclear disks, the rate of massive star formation is undoubt-
edly prodigious and, given the high viscosity of the gas, there will also be
high radial accretion rates, perhaps ≥10 M⊙ yr
−1. If this inflow persists
to very small radii, it is enough to feed even the highest luminosity AGNs.
2. Introduction
In the luminous infrared galaxies, very large masses of interstellar matter
have been concentrated in the galactic nuclei at radii less than 300 pc
as a result of galactic merging. Due to its relatively large volume-filling
factor and ability to cool rapidly, the ISM is much more dissipative than
the stars. This dissipation of rotational energy and angular momentum
will concentrate the ISM in the merging galactic nucleus. The resultant
high density of ISM in the nucleus then becomes the active ingredient for
luminosity generation – via enormously elevated rates of star formation or
by feeding a pre-existing massive black hole. There new evidence for massive
2 N.Z. SCOVILLE
gas and dust accretion disks in a number of these galactic nuclei. Here,
we will concentrate on those structures. The best example is that found
in the ultraluminous infrared galaxy Arp 220. For this object there now
exists sub-arcsecond resolution mm-wave aperture synthesis which probes
both the molecular gas distribution and the kinematics – hence the mass
distribution (Scoville, Yun and Bryant, 1997, Downs and Solomon, 1998,
and Sakamoto et al 1998). Similar, less massive structures are also evident
in millimeter-wave observations of a number of nearby lower level AGN
(Tacconi et al 1998, Baker and Scoville, 1998).
3. Arp 220
Arp 220 (IC 4553/4), with an infrared luminosity of 1.5 × 1012 L⊙ at λ =
8-1000µm, is one of the nearest ultraluminous infrared galaxies (Soifer et
al, 1987). Visual wavelength images reveal two faint tidal tails, indicating a
recent tidal interaction (cf. Joseph and Wright, 1985), and high resolution
ground-based radio and near-infrared imaging show a double nucleus (Baan
et al, 1987, Graham et al, 1990). The radio nuclei are separated by 0.′′98 at
P.A. ∼90◦ (Baan and Haschick, 1995). To power the energy output seen in
the infrared by young stars requires a star formation rate of ∼102 M⊙ yr
−1.
Alternatively, if the luminosity originates from an active galactic nucleus,
this source must be sufficiently obscured by dust that even the mid-infrared
emission lines are highly extincted since spectroscopy with the ISO shows
no evidence of very high ionization lines at wavelengths out to 40µm (Sturm
et al, 1996).
3.1. NEAR INFRARED
The central region of Arp 220 is shown in contour form in Figure 1. Co-
ordinate offsets are measured from the position of peak flux at 2.2µm oc-
curing on the western nucleus. These images clearly show the two nuclear
regions plus several lesser peaks. The morphology of both nuclei changes re-
markably with wavelength. In particular, the bright western nucleus shows
greater extension to the south at 2.2µm and the eastern nucleus has a
southern component which becomes increasingly strong at the longer wave-
lengths. Under the assumption that the background starlight has an in-
trinsic color which is not strongly varying, and that the dust producing
the extinction is in the foreground, ie, not mixed with the stars, we have
corrected the 2.2µm image for extinction and use this extinction-corrected
image (shown in the lower right panel of Figure 1) to place the near-infrared
maps relative to the three cm-wave radio continuum sources in Arp 220.
Our registration places one radio nucleus between the two emission peaks
MASSIVE ACCRETION DISKS 3Arp 220
Fig. 1: Central region of Arp 220 imaged at 1.1 (a), 1.6 (b) and 2.2 (c)µm
with NICMOS on HST (Scoville et al, 1998). The lower right panel shows
the ’de-extincted’ 2.2 µm image where the extinction at 2.2µm was deter-
mined from the 1.6/2.2 µm color and the assumption of a foreground dust
screen. (The lower right panel has 0.2′′ resolution since both the 1.6 and 2.2
µm images were smoothed to this resolution to obtain the color). Contours
are spaced logarithmically and the axes are labelled in arcsec offset from
the 2.2 (c)µm peak. The bar indicates 200 pc.
seen at 2.2µm in the east and the western radio nucleus in the area of ex-
tremely high obscuration to the south of the western 2.2µm peak. With this
registration, the third, very weak radio component coincides with the faint
southern peak seen in the images at all three near-infrared wavelengths (cf.
Scoville et al 1998).
The complex near-infrared structure of Arp 220 is undoubtedly due to
4 N.Z. SCOVILLE
there being multiple centers of star formation activity and strongly varying
dust obscuration within the merger nuclei. The crescent or partial ring mor-
phology of the western nucleus might readily arise under two circumstances:
if there is an obscuring disk of dust and gas embedded in a spheroidal nu-
clear star cluster, or if a central starburst ring is partially obscured by its
own dust on one side. In either case, the dust (and dense interstellar gas)
must be confined to a thin disk-like structure, concentric with the western
nucleus of the galaxy. A similar structure probably exists in the eastern nu-
cleus based on the millimeter-wave interferometry (see below) although its
inclination may not be so close to the line of sight (in view of the less sharp
cutoff in the near-infrared light distribution). The suggested geometry for
the western nucleus is a galactic nucleus star cluster cut by an embedded,
opaque dust disk at inclination 20◦ to the line of sight.
3.2. MOLECULAR GAS
The total molecular gas content for Arp 220 is 9 × 109M⊙ based on the
CO (2-1) emission and a CO-to-H2 conversion ratio which is 0.45 times the
Galactic value (cf. Scoville, Yun and Bryant, 1997). This enormous mass
(approximately four times that of the entire Galaxy) is contained entirely
within R <1.5 kpc and approximately 5 × 109M⊙ is apparently concen-
trated in a thin disk in the nuclear region at radii < 250 pc. Within the
last year there have been two studies of the CO (2-1) emission at 0.5′′ (175
pc) resolution (Downs and Solomon, 1998, and Sakamoto et al, 1998). In
Figure 2, the CO emission and 1.3 mm continuum emission are shown from
Sakamoto et al (1998). Figures 2a-c show the CO line emission, mean ve-
locity field and continuum emission obtained from only the high resolution
interferometry data – that is, filtering out any extended structure by not in-
cluding short interferometer baselines. Figure 2d shows the total integrated
CO emission and mean velocity contours, including both high and low res-
olution data. The crosses on all four panels indicate the positions of the
double radio nuclei (Norris, 1985). In both the line (a) and the continuum
(c), there are clear peaks at the positions of the radio nuclei.
The CO peaks coincide with those in the continuum but the line emis-
sion is much more extended than the continuum emission. The diameter
of the CO in Figure 2d, in which most of single-disk flux is recovered, is
about 2 kpc, and the overall emission (2d) is elongated along a direction
parallel to the gradient in the velocity field. The latter provides the basis
for modelling the overall emission as a disk since an inclined disk will have
parallel kinematic and isophotal major axes (see also Scoville, Yun and
Bryant, 1997). The gas disk is, however, not axisymmetric and its velocity
field shows distortions, which suggest non-circular motions or unrelaxed gas
MASSIVE ACCRETION DISKS 5
 
  2   1   0  -1  -2
R.A. offset [arcsec]
-1
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300pc
 
     
 
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(d) CO(2-1) (LHU)
1 kpc
 
     
 
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0.57"•0.52"
(b) CO(2-1)
300pc
5330
Fig. 2: Central region of Arp 220. Crosses in each panel indicate the 1.3
m continuum position of the nuclei. (a) High resolution CO (2-1) emission.
(b) Mean velocity of CO (2-1) with contours spaced 50 km s−1. (c) 1.3 mm
continuum. (d) CO (2-1) integrated intensity map and isovelocity contours
(50 km s−1 step) made from low(L), high(H), and ultra-high(U) resolution
data.
components, probably due to the fact that the system is still an ongoing
merger and has two massive nuclear star clusters embedded in the disk.
About 30% of the total CO emission is associated with the two nuclei,
and a steep velocity gradient is evident across each nucleus in the high
resolution velocity map (Figure 2b). The velocity shift is about 500 km s−1
within 0.′′3 (110 pc) in position-velocity diagrams through each nucleus.
The velocity gradients across the two nuclei are in opposite directions and
neither is aligned with that in the outer molecular disk at P.A. = 25◦
(compare Figures 2b and d). The very steep velocity gradients at the two
nuclei, as well as their directions, exclude the possibility that the gas is
just rotating around the dynamical center of the merger. Instead, the data
suggest that each nucleus has a separate rotating molecular gas disk. These
6 N.Z. SCOVILLE
two disks are counterrotating, and their diameters are ∼200 pc (0.5′′). The
dynamical mass of each nucleus is ∼2 × 109 sin2i M⊙, where i is the
inclination of the disks. The presence of the two dynamically relaxed disks
strongly supports the view that the two peaks (in the millimeter, radio and
NIR) are true nuclei, and that Arp 220 is indeed a merger. The major axis
of the disk inferred from the NICMOS images is consistent with the E-W
velocity gradient at the west nucleus.
The two nuclear disks are embedded in an outer gas disk of kiloparsec
radius. The mean velocity of the two nuclei is different by about 200 km
s−1, with the eastern nucleus more redshifted. This is consistent with NIR
spectroscopy of Brγ (Larkin et al, 1995). The dynamical mass within the
orbit of the two nuclei (ie, R < 250 pc) was estimated to be a 5.4 × 109M⊙
by Scoville, Yun and Bryant (1997). Thus more than half of the mass in
this region belongs to the two nuclei. Note, however, that the dynamical
mass of the central disk has large uncertainty due to its poorly constrained
inclination (i∼ 45 deg).
4. Remarks
In the ultraluminous galaxies the nuclear gas and dust accretion disks are
spectacular: containing up to 109 M⊙ of ISM at radii 100-200 pc but ex-
tremely thin (10-50 pc). The mean gas densities within the disks are typi-
cally 104 cm−3 and, in contrast to our Galactic center where the area-filling
factor is only a few percent, the disk area is substantially filled (area-filling
factors, 20-50%). Our results show that part of the concentrated gas (30%
in CO luminosity in the case of Arp 220) remains intact around the stellar
nuclei to the late stages of merger evolution – at least until the two nuclei
approach within several hundred parsecs. The rest of the gas is rotating
around the dynamical center of the merger system.
It has been shown in numerical simulations that gas rapidly concen-
trates to the nuclei of merging galaxies at an early phase of interaction.
Subsequently, the concentrated gas quickly merges as the nuclei approach
each other (Barnes and Hernquist, 1991). There is strong consensus that the
activity in the ultraluminous IR galaxies is triggered by strong dynamical
perturbations during a close galactic encounter or galactic merging. The
resultant disruption of the normally circular orbits in a disk system will
lead to extremely rapid dissipation of rotational energy and angular mo-
mentum within the interstellar medium. Once in the nucleus, any vertical
(z) motions of the gas are rapidly damped and dissipated. The resultant
thin disk in the nuclear region, containing a galactic mass of interstellar
medium, will be vertically supported by turbulence and the radiation pres-
sure of young stars formed in this high density medium. In fact, such disks
MASSIVE ACCRETION DISKS 7
may become self-regulating since the stars which form within the disk gen-
erate both turbulence and a high radiation pressure. At the elevated rates
of star formation seen in these galactic nuclei, the resulting turbulent and
radiative pressure can swell the disk, and thus damp the star formation.
The final evolution of such disks to AGNs could proceed either along
the lines suggested by Norman and Scoville (1988) in which mass-loss from
late stellar evolution of the starburst population ultimately accretes into
and builds up a super-massive black hole, or with a pre-existing black hole
being fed directly from the nuclear disk by accretion to very small radii.
In either case, one should not expect the AGN to become immediately
visible, given the extraordinarily large extinctions (AV ≥ 10
3 mag) in the
nuclear disk during these early phases. The discovery that the ISM in the
late merging systems is often concentrated in a thin, rotationally-supported
disk (rather than in a more chaotic spherical distribution) should enable
theoretical studies of their evolution to proceed from a more clearly defined
initial state.
ACKNOWLEDGMENTS
It is a pleasure to acknowledge stimulating discussions with A. Baker, C.
Norman and K. Sakamoto on this work and assistance with the manuscript
from N. Candelin and Z. Turgel.
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