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ABSTRACT: We have observed the nucleus of the nearby luminous infrared galaxy NGC 4418
with subarcsec resolution at 860 and 450 micron for the first time to
characterize its hidden power source. A ~20 pc (0.1") hot dusty core was found
inside a 100 pc scale concentration of molecular gas at the galactic center.
The 860 micron continuum core has a deconvolved (peak) brightness temperature
of 120-210 K. The CO(3-2) peak brightness temperature there is as high as 90 K
at 50 pc resolution. The core has a bolometric luminosity of about 10^{11}
Lsun, which accounts for most of the galaxy luminosity. It is Compton thick
(N_H >~ 10^{25} cm^{-2}) and has a high luminosity-to-mass ratio ~500 Lsun/Msun
as well as a high luminosity surface density 10^{8.5+-0.5} Lsun pc^{-2}. These
parameters are consistent with an AGN to be the main luminosity source (with an
Eddington ratio about 0.3) while they can be also due to a young starburst near
its maximum L/M. We also found an optical color (reddening) feature that we
attribute to an outflow cone emanating from the nucleus. The hidden hot nucleus
thus shows evidence of both an inflow, previously seen with absorption lines,
and the new outflow reported here in a different direction. The nucleus must be
rapidly evolving with these gas flows.
01/2013;
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ABSTRACT: Infrared pumping and its effect on the excitation of HCN molecules can be important when using rotational lines of HCN to probe dense molecular gas in galaxy nuclei. We report the first extragalactic detection of (sub)millimeter rotational lines of vibrationally excited HCN, in the dust-enshrouded nucleus of the luminous infrared galaxy NGC 4418. We estimate the excitation temperature of T_vib ~ 230 K between the vibrational ground and excited (v_2=1) states. This excitation is most likely due to infrared radiation. At this high vibrational temperature the path through the v_2=1 state must have a strong impact on the rotational excitation in the vibrational ground level, although it may not be dominant for all rotational levels. Our observations also revealed nearly confusion limited lines of CO, HCN, HCO+, H13CN, HC15N, CS, N2H+, and HC3N at lambda ~ 1 mm. Their relative intensities may also be affected by the infrared pumping. Comment: ApJL in press, 8 pages
11/2010;
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Satoki Matsushita,
Daisuke Iono,
Glen R. Petitpas,
Richard C.-Y. Chou,
Mark A. Gurwell,
Todd R. Hunter,
Jeremy Lim Sebastien Muller,
Alison B. Peck,
Kazushi Sakamoto,
Satoko Sawada Satoh, Martina C. Wiedner,
David J. Wilner,
and Christine D. Wilson
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ABSTRACT: We have used the Submillimeter Array (SMA) to make the first interferometric observations (beam size ~1'' or ~400 pc) of the 12CO J = 6 – 5 line and 435 μm (690 GHz) continuum emission toward the central region (half power field of view 17'') of the nearby ultra-luminous infrared galaxy (ULIRG) Arp 220. These observations resolve the eastern and western nuclei from each other, in both the molecular line and dust continuum emission. At 435 μm, the peak intensity of the western nucleus is stronger than the eastern nucleus, and the difference in peak intensities is less than at longer wavelengths. Fitting a simple model to the dust emission observed between 1.3 mm and 435 μm suggests that dust emissivity power law index in the western nucleus is near unity and steeper in the eastern nucleus, about 2, and that the dust emission is optically thick at the shorter wavelength. Comparison with single dish measurements indicate that the interferometer observations are missing ~60% of the dust emission, most likely from a spatially extended component to which these observations are not sensitive. The 12CO J = 6 – 5 line observations clearly resolve kinematically the two nuclei. The distribution and kinematics of the 12CO J = 6 – 5 line appear to be very similar to lower J CO lies observed at similar resolution. Analysis of multiple 12CO line intensities indicates that the molecular gas in both nuclei have similar excitation conditions, although the western nucleus is warmer and denser. The excitation conditions are similar to those found in other extreme environments, including the nearby starburst galaxy M82, the active galactic nucleus (AGN) hosting ULIRG Mrk 231, and the high-z QSO BR 1202-0725. Simultaneous lower resolution observations of the 12CO, 13CO, and C18O J = 2 – 1 lines show that the 13CO and C18O lines have similar intensities, which suggests that both of these lines are optically thick, or possibly that extreme high mass star formation has produced in an overabundance of C18O.
The Astrophysical Journal 02/2009; 693(1):56. · 6.02 Impact Factor
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Satoki Matsushita,
Kazushi Sakamoto,
Cheng-Yu Kuo,
Pei-Ying Hsieh,
Dinh-V- Trung,
Rui-Qing Mao,
Daisuke Iono,
Alison B. Peck, Martina C. Wiedner,
Sheng-Yuan Liu,
Nagayoshi Ohashi,
and Jeremy Lim
[show abstract]
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ABSTRACT: We present the first interferometric 12CO (J = 3-2) observations (beam size of 39 × 16 or 160 × 65 pc) with the Submillimeter Array (SMA) toward the center of the Seyfert 2 galaxy M51. The image shows a strong concentration at the nucleus and weak emission from the spiral arm to the northwest. The integrated intensity of the central component in 12CO (J = 3-2) is almost twice as high as that in 12CO (J = 1-0), indicating that the molecular gas within an ~80 pc radius of the nucleus is warm (100 K) and dense (~104 cm-3). Similar intensity ratios are seen in shocked regions in our Galaxy, suggesting that these gas properties may be related to active galactic nucleus or starburst activity. The central component shows a linear velocity gradient (~1.4 km s-1 pc-1) perpendicular to the radio continuum jet, similar to that seen in previous observations and interpreted as a circumnuclear molecular disk/torus around the Seyfert 2 nucleus. In addition, we identify a linear velocity gradient (0.7 km s-1 pc-1) along the jet. Judging from the energetics, the velocity gradient can be explained by supernova explosions or energy and momentum transfer from the jet to the molecular gas via interaction, which is consistent with the high-intensity ratio.
The Astrophysical Journal 12/2008; 616(1):L55. · 6.02 Impact Factor
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ABSTRACT: The center of M83, a barred starburst galaxy with a double nucleus, has been observed in the CO (J = 2-1) and CO (J = 3-2) lines with the Submillimeter Array. The molecular gas shows a distribution and kinematics typical for barred galaxies at ~1 kpc radii but reveals unusual kinematics around the double nucleus in the central ~300 pc. Our CO velocity data show that the visible nucleus in M83 is at least 3'' (65 pc) away from the galaxy's dynamical center, which most likely coincides with the center of symmetry previously determined in the K band and is suggested to host another nucleus. We discovered high-velocity molecular gas associated with the visible off-center nucleus and also found a steep velocity gradient across it. We attribute these features to a gas disk rotating around the off-center nucleus, which may be the remnant of a small galaxy accreted by M83. The dynamical mass of this component is estimated to be 3 × 108 M☉ within a radius of 40 pc. The dynamical perturbation from the off-center nucleus may have played a key role in shaping the lopsided starburst.
The Astrophysical Journal 12/2008; 616(1):L59. · 6.02 Impact Factor
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ABSTRACT: We used the Submillimeter Array (SMA) to image 860 μm continuum and CO(3-2) line emission in the ultraluminous merging galaxy Arp 220, achieving a resolution of 0.23'' (80 pc) for the continuum and 0.33'' (120 pc) for the line. The CO emission peaks around the two merger nuclei with a velocity signature of gas rotation around each nucleus and is also detected in a kiloparsec-size disk encompassing the binary nucleus. The dust continuum, in contrast, is mostly from the two nuclei. The beam-averaged brightness temperature of both line and continuum emission exceeds 50 K at and around the nuclei, revealing the presence of warm molecular gas and dust. The dust emission morphologically agrees with the distribution of radio supernova features in the east nucleus, as expected when a starburst heats the nucleus. In the brighter west nucleus, however, the submillimeter dust emission is more compact than the supernova distribution. The 860 μm core, after deconvolution, has a size of 50-80 pc, consistent with recent 1.3 mm observations, and a peak brightness temperature of (0.9–1.6) × 102 K. Its bolometric luminosity is at least 2 × 1011 L☉ and could be ~1012 L☉ depending on source structure and 860 μm opacity, which we estimate to be of the order of τ860 ~ 1 (i.e., NH2 ~ 1025 cm −2). The starbursting west nuclear disk must have in its center a dust enshrouded AGN or a very young starburst equivalent to hundreds of super star clusters. Further spatial mapping of bolometric luminosity through submillimeter imaging is a promising way to identify the heavily obscured heating sources in Arp 220 and other luminous infrared galaxies.
The Astrophysical Journal 12/2008; 684(2):957. · 6.02 Impact Factor
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Satoki Matsushita,
Daisuke Iono,
Glen R. Petitpas,
Richard C. -Y. Chou,
Mark A. Gurwell,
Todd R. Hunter,
Jeremy Lim,
Sebastien Muller,
Alison B. Peck,
Kazushi Sakamoto,
Satoko Sawada-Satoh, Martina C. Wiedner,
David J. Wilner,
Christine D. Wilson
[show abstract]
[hide abstract]
ABSTRACT: We have used the Submillimeter Array (SMA) to make the first interferometric
observations (beam size ~1") of the 12CO J=6-5 line and 435 micron (690 GHz)
continuum emission toward the central region of the nearby ULIRG Arp 220. These
observations resolve the eastern and western nuclei from each other, in both
the molecular line and dust continuum emission. At 435 micron, the peak
intensity of the western nucleus is stronger than the eastern nucleus, and the
difference in peak intensities is less than at longer wavelengths. Fitting a
simple model to the dust emission observed between 1.3 mm and 435 micron
suggests that dust emissivity power law index in the western nucleus is near
unity and steeper in the eastern nucleus, about 2, and that the dust emission
is optically thick at the shorter wavelength. Comparison with single dish
measurements indicate that the interferometer observations are missing ~60% of
the dust emission, most likely from a spatially extended component to which
these observations are not sensitive. The 12CO J=6-5 line observations clearly
resolve kinematically the two nuclei. The distribution and kinematics of the
12CO J=6-5 line appear to be very similar to lower J CO lies observed at
similar resolution. Analysis of multiple 12CO line intensities indicates that
the molecular gas in both nuclei have similar excitation conditions, although
the western nucleus is warmer and denser. The excitation conditions are similar
to those found in other extreme environments, including M82, Mrk 231, and BR
1202-0725. Simultaneous lower resolution observations of the 12CO, 13CO, and
C18O J=2-1 lines show that the 13CO and C18O lines have similar intensities,
which suggests that both of these lines are optically thick, or possibly that
extreme high mass star formation has produced in an overabundance of C18O.
11/2008;
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[show abstract]
[hide abstract]
ABSTRACT: We used the Submillimeter Array (SMA) to image 860 micron continuum and CO(3-2) line emission in the ultraluminous merging galaxy Arp 220, achieving a resolution of 0.23" (80 pc) for the continuum and 0.33" (120 pc) for the line. The CO emission peaks around the two merger nuclei with a velocity signature of gas rotation around each nucleus, and is also detected in a kpc-size disk encompassing the binary nucleus. The dust continuum, in contrast, is mostly from the two nuclei. The beam-averaged brightness temperature of both line and continuum emission exceeds 50 K at and around the nuclei, revealing the presence of warm molecular gas and dust. The dust emission morphologically agrees with the distribution of radio supernova features in the east nucleus, as expected when a starburst heats the nucleus. In the brighter west nucleus, however, the submillimeter dust emission is more compact than the supernova distribution. The 860 micron core, after deconvolution, has a size of 50-80 pc, consistent with recent 1.3 mm observations, and a peak brightness temperature of (0.9-1.6)x10^2 K. Its bolometric luminosity is at least 2x10^{11} Lsun and could be ~10^{12} Lsun depending on source structure and 860 micron opacity, which we estimate to be of the order of tau_{860} ~ 1 (i.e., N_{H_2} ~ 10^{25} cm^{-2}). The starbursting west nuclear disk must have in its center a dust enshrouded AGN or a very young starburst equivalent to hundreds of super star clusters. Further spatial mapping of bolometric luminosity through submillimeter imaging is a promising way to identify the heavily obscured heating sources in Arp 220 and other luminous infrared galaxies. Comment: ApJ. in press. 26 pages, 10 figures
06/2008;
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ABSTRACT: The central 2x1 kpc of the starburst galaxy NGC 253 has been imaged using the Submillimeter Array at a 60 pc resolution in the J=2-1 transitions of 12CO, 13CO, and C18O as well as in the 1.3 mm continuum. Molecular gas and dust are mainly in the circumnuclear disk of ~500 pc radius, with warm (~40 K) and high area-filling factor gas in its central part. Two gas shells or cavities have been discovered in the circumnuclear disk. They have ~100 pc diameters and have large velocity widths of 80-100 km/s, suggestive of expansion at ~50 km/s. Modeled as an expanding bubble, each shell has an age of ~0.5 Myr and needed kinetic energy of ~1E46 J as well as mean mechanical luminosity of ~1E33 W for its formation. The large energy allows each to be called a superbubble. A ~10^6 Msun super star cluster can provide the luminosity, and could be a building block of the nuclear starburst in NGC 253. Alternatively, a hypernova can also be the main source of energy for each superbubble, not only because it can provide the mechanical energy and luminosity but also because the estimated rate of superbubble formation and that of hypernova explosions are comparable. Our observations indicate that the circumnuclear molecular disk harboring the starburst is highly disturbed on 100 pc or smaller scales, presumably by individual young clusters and stellar explosions, in addition to globally disturbed in the form of the well-known superwind. Comment: Accepted by ApJ. 33 pages, 8 figures
The Astrophysical Journal 09/2005; · 6.02 Impact Factor
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ABSTRACT: The Submillimeter Array (SMA) has been under construction at the 4100 m summit of Mauna Kea, Hawaii. The array is going to allow imaging of lines and continuum at sub-arcsecond resolution in submillimeter wavelengths. The status of the array and the results from recent commissioning observations of nearby galaxies are reported.
04/2004;
