arXiv subjects

K. Sakamoto

Publications and source records attributed to K. Sakamoto.

45 records · Page 3Linked to original sources

arXiv2004

SMA CO(J=3-2) interferometric observations of the central region of M51

We present the first interferometric CO(J=3-2) observations (beam size of 3.9"x1.6" or 160pc x 65pc) 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 CO(J=3-2) is almost twice as high as that in CO(J=1-0), indicating that the molecular gas within an ~80 pc radius of the nucleus is warm (>~100 K) and dense (~10^4 cm^-3). Similar intensity ratios are seen in shocked regions in our Galaxy, suggesting that these gas properties may be related to AGN or starburst activity. The central component shows a linear velocity gradient (~1.4 km/s/pc) 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/pc) 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.

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arXiv2001

Experimental Limit to Interstellar 244Pu Abundance

Short-lived nuclides, now extinct in the solar system, are expected to be present in the interstellar medium (ISM). Grains of ISM origin were recently discovered in the inner solar system and at Earth orbit and may accrete onto Earth after ablation in the atmosphere. A favorable matrix for detection of such extraterrestrial material is presented by deep open-sea sediments with very low sedimentation rates (0.8-3 mm/kyr). We report here on the measurement of Pu isotopic abundances in a 1-kg deep-sea dry sediment collected in 1992 in the North Pacific. Our measured value of (3+-3)x10^5 244Pu atoms in the Pu-separated fraction of the sample shows no excess over the expected stratospheric nuclear fallout content and under reasonable assumptions we derive a limit of 2x10^-11 g-244Pu/g-ISM for the abundance of 244Pu in ISM.

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arXiv1999

Effective Vector Meson Masses in a Cutoff Field Theory

Based on quantum hadrodynamics with a finite cutoff, the effective masses of vector mesons(ω, ρ) in nuclear medium are calculated. We use a low-energy effective Lagrangian which is obtained by integrating high-energy quantum fluctuations. Although we use an artificial cutoff, the cutoff-dependence can be removed order by order. It is shown that there is a strong correlation between the effective ω-meson mass and the effective nucleon mass at the normal density. It is also found that the effective ρ-meson mass m_ρ^* decreases as density increases. The rate of the decrease becomes smaller at high density. As a result, at the normal density, the m^*_ρ/m_ρis 0.85 \sim 0.95.

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arXiv1999

Bar-driven Transport of Molecular Gas to Galactic Centers and Its Consequences

We study the characteristics of molecular gas in the central regions of spiral galaxies on the basis of our CO(J=1-0) imaging survey of 20 nearby spiral galaxies using the NRO and OVRO millimeter arrays. Condensations of molecular gas at galactic centers with sizescales < 1 kpc and CO-derived masses M_gas(R<500pc) = 10^8 - 10^9 M_sun are found to be prevalent in the gas-rich L^* galaxies. Moreover, the degree of gas concentration to the central kpc is found to be higher in barred systems than in unbarred galaxies. This is the first statistical evidence for the higher central concentration of molecular gas in barred galaxies, and it strongly supports the theory of bar-driven gas transport. It is most likely that more than half of molecular gas within the central kpc of a barred galaxy was transported there from outside by the bar. The supply of gas has exceeded the consumption of gas by star formation in the central kpc, resulting in the excess gas in the centers of barred systems. The mean rate of gas inflow is statistically estimated to be larger than 0.1 - 1 M_sun/yr. The correlation between gas properties in the central kpc and the type of nuclear spectrum (HII, LINER, or Seyfert) is investigated. A correlation is found in which galaxies with larger gas-to-dynamical mass ratios tend to have HII nuclear spectra, while galaxies with smaller ratios show spectra indicating AGN. Also, the theoretical prediction of bar-dissolution by condensation of gas to galactic centers is observationally tested. It is suggested that the timescale for bar dissolution is larger than 10^8 - 10^10 yr, or a bar in a L^* galaxy is not destroyed by a condensation of 10^8 - 10^9 M_sun gas in the central kpc.

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arXiv1999

Dispersion relations in quantum hadrodynamics with a finite cutoff

The dispersion relations in the real and imaginary parts of the meson self-energies are studied to check the consistency of the "renormalization" in cutoff field theory. It is shown that the dispersion relations are preserved by the "renormalization"even if the finite cutoff and regulator are introduced in the calculation by hand.

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arXiv1999

Radial Transport of Molecular Gas to the Nuclei of Spiral Galaxies

The NRO/OVRO imaging survey of molecular gas in 20 spiral galaxies is used to test the theoretical predictions on bar-driven gas transport, bar dissolution, and bulge evolution. In most galaxies in the survey we find gas condensations of 10^8-10^9 M_sun within the central kiloparsec, the gas masses being comparable to those needed to destroy bars in numerical models. We also find a statistically significant difference in the degree of gas concentration between barred and unbarred galaxies: molecular gas is more concentrated to the central kiloparsec in barred systems. The latter result supports the theories of bar-driven gas transport. Moreover, it constrains the balance between the rate of gas inflow and that of gas consumption (i.e., star formation, etc.), and also constrains the timescale of the possible bar dissolution. Namely, gas inflow rates to the central kiloparsec, averaged over the ages of the bars, must be larger than the mean rates of gas consumption in the central regions in order to cause and maintain the higher gas concentrations in barred galaxies. Also, the timescale for bar dissolution must be longer than that for gas consumption in the central regions by the same token.

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arXiv1998

Counterrotating Nuclear Disks in Arp 220

The ultraluminous infrared galaxy Arp 220 has been observed at 0.5" resolution in CO(2-1) and 1 mm continuum using the newly expanded Owens Valley Millimeter Array. The CO and continuum peaks at the double nuclei and the surrounding molecular gas disk are clearly resolved. We find steep velocity gradients across each nucleus (dV ~ 500 km/s within r= 0.3") whose directions are not aligned with each other and with that of the outer gas disk. We conclude that the double nuclei have their own gas disks (r ~ 100 pc). They are counterrotating with respect to each other and embedded in the outer gas disk (r ~ 1 kpc) rotating around the dynamical center of the system. The masses of each nucleus are M_dyn > 2* 10^9 M_sun based on the CO kinematics. Although there is no evidence of an old stellar population in the optical or near infrared spectroscopy of the nuclei (probably due to the much brighter young population), it seems likely that these nuclei were 'seeded' from the pre-merger nuclei in view of their counterrotating gas kinematics. The gas disks probably constitute a significant fraction (~ 50 %) of the mass in each nucleus. The CO and continuum brightness temperatures imply that the nuclear gas disks have high area filling factors (~ 0.5-1) and have extremely high visual extinctions (Av ~ 1000 mag). The molecular gas must be hot (>= 40 K) and dense (>= 10^4-5 cm^-3), given the large mass and small scale-height of the nuclear disks. The continuum data suggest that the large luminosity (be it starburst or AGN) must originate within 100 pc of the two nuclear gas disks which were presumably formed through concentration of gas from the progenitor outer galaxy disks.

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arXiv1998

Antiproton Production in p+d Reaction at Subthreshold Energies

An enhancement of antiprotons produced in p+d reaction in comparison with ones in p+p elementary reaction is investigated. In the neighborhood of subthreshold energy the enhancement is caused by the difference of available energies for antiproton production. The cross section in p+d reaction, on the other hand, becomes just twice of the one in elementary p+p reaction at the incident energy far from the threshold energy when non-nucleonic components in deuteron target are not considered.

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arXiv1996

Properties of nuclear matter in cut-off field theory and renormalization group methods

The properties of nuclear matter are studied in the cut-off field theory. It is found that, under the Hartree approximation, the small cut-off makes the equations of state hard, especially at higher densities. The theory is modified in the framework of the renormalization group methods with arbitrary cut-off $Λ^\prime$. It is found that the expansion in terms of the $σ$ meson field is more favorable than the naive expansion of the inverse of $Λ^\prime$, when we do not use very large $Λ^\prime$.

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