arXiv subjects

S. K. Okumura

Publications and source records attributed to S. K. Okumura.

12 recordsLinked to original sources

arXiv2011

Digital Spectro-Correlator System for the Atacama Compact Array of the Atacama Large Millimeter/submillimeter Array

We have developed an FX-architecture digital spectro-correlator for the Atacama Compact Array (ACA) of the Atacama Large Millimeter/submillimeter Array. The correlator is able to simultaneously process four pairs of dual polarization signals with the bandwidth of 2 GHz, which are received by up to sixteen antennas. It can calculate auto- and cross-correlation spectra including cross-polarization in all combinations of all the antennas, and output correlation spectra with flexible spectral configuration such as multiple frequency ranges and multiple frequency resolutions. Its spectral dynamic range is estimated to be higher than 10^4 relative to Tsys from processing results of thermal noise for eight hours with a typical correlator configuration. The sensitivity loss is also confirmed to be 0.9 % with the same configuration. In this paper, we report the detailed design of the correlator and the verification results of the developed hardware.

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arXiv2007

Arc-like distribution of high CO(J=3-2)/CO(J=1-0) ratio gas surrounding the central star cluster of the supergiant HII region NGC 604

We report the discovery of a high CO(J=3-2)/CO(J=1-0) ratio gas with an arc-like distribution (``high-ratio gas arc'') surrounding the central star cluster of the supergiant HII region NGC 604 in the nearby spiral galaxy M 33, based on multi-J CO observations of a 5' $\times$ 5' region of NGC 604 conducted using the ASTE 10-m and NRO 45-m telescopes. The discovered ``high-ratio gas arc'' extends to the south-east to north-west direction with a size of $\sim$ 200 pc. The western part of the high-ratio gas arc closely coincides well with the shells of the HII regions traced by H$α$ and radio continuum peaks. The CO(J=3-2)/CO(J=1-0) ratio, R_{3-2/1-0}, ranges between 0.3 and 1.2 in the observed region, and the R_{3-2/1-0} values of the high-ratio gas arc are around or higher than unity, indicating very warm (T_kin > 60 K) and dense (n(H_2) > 10^{3-4} cm^{-3}) conditions of the high-ratio gas arc. We suggest that the dense gas formation and second-generation star formation occur in the surrounding gas compressed by the stellar wind and/or supernova of the first-generation stars of NGC 604, i.e., the central star cluster of NGC 604.

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arXiv2005

Dense and Warm Molecular Gas between Double Nuclei of the Luminous Infrared Galaxy NGC 6240

High spatial resolution observations of the 12CO(1-0), HCN(1-0), HCO+(1-0), and 13CO(1-0) molecular lines toward the luminous infrared merger NGC 6240 have been performed using the Nobeyama Millimeter Array and the RAINBOW Interferometer. All of the observed molecular emission lines are concentrated in the region between the double nuclei of the galaxy. However, the distributions of both HCN and HCO+ emissions are more compact compared with that of 12CO, and they are not coincident with the star-forming regions. The HCN/12CO line intensity ratio is 0.25; this suggests that most of the molecular gas between the double nuclei is dense. A comparison of the observed high HCN/13CO intensity ratio, 5.9, with large velocity gradient calculations suggests that the molecular gas is dense [n(H_2)=10^{4-6} cm^-3] and warm (T_kin>50 K). The observed structure in NGC 6240 may be explained by time evolution of the molecular gas and star formation, which was induced by an almost head-on collision or very close encounter of the two galactic nuclei accompanied with the dense gas and star-forming regions.

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arXiv2004

Starbursting Nuclear CO Disks of Early-type Spiral Galaxies

We have initiated the first CO interferometer survey of early-type spiral galaxies (S0-Sab). We observed five early-type spiral galaxies with HII nuclei (indicating circumnuclear starburst activities). These observations indicate gas masses for the central kiloparsec of \sim 1-5% of the dynamical masses. Such low gas mass fractions suggest that large-scale gravitational instability in the gas is unlikely to be the driving cause for the starburst activities. The Toomre Q values were >1 (mostly >3) within the central kiloparsec, indicating that the gas disks are globally gravitationally stable. The area filling factor of the gas disks is estimated to be about 0.05. This small value indicates the existence of lumpy structure, i.e. molecular clouds, in the globally-gravitationally stable disks. The typical surface density of the molecular clouds is as high as \sim 3000 Msun pc^{-2}. We reconsider the nature of the Toomre Q criterion, and conclude that the Q derived from CO observations indicates neither star formation nor molecular cloud formation. This argument should be valid not only for the circumnuclear disks but also for any region in galactic disks. We tentatively explore an alternative model, i.e. cloud-cloud collisions, as an initiating mechanism of star formation.

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arXiv2004

Nobeyama Millimeter Array Observations of GRB 030329: a Decay of Afterglow with Bumps and Molecular Gas in the Host Galaxy

Nobeyama Millimeter Array was used to observe millimeter-wave afterglow of GRB 030329 at 93 GHz and 141 GHz from 2003 April 6 to 2003 May 30. A sensitive search for CO(J=1-0) emission/absorption from the host galaxy of GRB 030329 was also carried out. Unresolved millimeter continuum emission at the position of GRB 030329 was detected until 2003 April 21. We found a steep decline of continuum flux (\propto t^{-2.0}) during this period, in accord with a previous report. Moreover, our data implies that the decay was accompanied by possible plateaus phases, or bumps, on a time scale of several days. From an integrated spectrum, produced by summing up the data from 2003 April 10 to 2003 May 30, we found a possible emission feature, which could be a redshifted CO(J=1-0) line. Its position and redshift coincide well with those of GRB 030329, though further observations are required to confirm the detection. If the emission feature is real, the observed CO flux is 1.4 +/- 0.52 Jy km/s, corresponding to a large molecular gas mass of M(H_2) > 10^9 Mo. This implies that the host galaxy, which is optically faint, is highly obscured due to a rich interstellar medium.

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arXiv2003

The Virgo High-Resolution CO Survey I. CO Atlas

We present the results of the Virgo high-resolution CO survey (ViCS) obtained with the Nobeyama Millimeter-wave Array (NMA). This survey was made in the course of a long-term project at Nobeyama from 1999 December through 2002 April. The objects were selected from Virgo cluster members, considering CO richness from single dish flux, mild inclination, and lack of strong tidal perturbations. The central 1 arc min regions (4.7 kpc) of 15 spiral galaxies were observed with resolutions of 2 to 5 arcsec and 10 to 20 km/s, and sensitivities of 20 mJy/ beam for a 10 km/s channel. The objects lie at the same distance of the Virgo cluster (16.1 Mpc), which is advantageous for comparisons among individual galaxies. We describe the details of observations and data reduction, and present an atlas of integrated CO intensity maps, velocity fields and position-velocity diagrams along the major axes. The molecular gas morphology in the Virgo galaxies shows a wealth of variety, not specifically depending on the Hubble types.

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arXiv2002

NMA CO (J=1-0) Observations of the Halpha/Radio Lobe Galaxy NGC 3079: Gas Dynamics in a Weak Bar Potential and Central Massive Core

within 24 lines with 80 characters) We present ^12CO (1-0) observations in the central 4.5 kpc (1 arcmin) of the Halpha/Radio lobe galaxy NGC 3079 with the Nobeyama Millimeter Array. The molecular gas shows four components: a main disk, spiral arms, a nuclear disk, and a nuclear core. The main disk extends beyond our spatial coverage. The spiral arms are superimposed on the main disk. The nuclear disk with about 600 pc radius has an intense concentration of molecular gas. Its appearance on PV diagrams is indicative of oval motions of the gas, rather than circular. The nuclear core is more compact than our resolution. Though it is unresolved, the nuclear core shows a very high velocity about 200 km/s even at the radius of about 100 pc on the PV diagram. We propose a model that NGC 3079 contains a weak bar. This model explains the observed features: the main disk and spiral arms result from gaseous x1-orbits and associated crowding respectively. The nuclear disk arises from gaseous x2-orbits. From the appearance of the spiral arms on the PV diagram, the pattern speed of the bar is estimated to be 55+-10 km/s/kpc. The high velocity of the nuclear core cannot be explained by our model for a bar. Thus we attribute it to a central massive core with a dynamical mass of 10^9 Msun within the central 100 pc. This mass is three orders of magnitude more massive than that of a central black hole in this galaxy.

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arXiv2002

Dense Molecular Gas and Star Formation in Nearby Seyfert Galaxies

An imaging survey of CO(1-0), HCN(1-0), and HCO$^+$(1-0) lines in the centers of nearby Seyfert galaxies has been conducted using the Nobeyama Millimeter Array and the RAINBOW interferometer. Preliminary results reveal that 3 Seyferts out of 7 show abnormally high HCN/CO and HCN/HCO$^+$ ratios, which cannot occur even in nuclear starburst galaxies. We suggest that the enhanced HCN emission originated from X-ray irradiated dense obscuring tori, and that these molecular line ratios can be a new diagnostic tool to search for ``pure'' AGNs. According to our HCN diagram, we suggest that NGC 1068, NGC 1097, and NGC 5194 host ``pure'' AGNs, whereas Seyfert nuclei of NGC 3079, NGC 6764, and NGC 7469 may be ``composite'' in nature.

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arXiv2000

Formation of a Massive Black Hole at the Center of the Superbubble in M82

We performed 12CO(1-0), 13CO(1-0), and HCN(1-0) interferometric observations of the central region (about 450 pc in radius) of M82 with the Nobeyama Millimeter Array, and have successfully imaged a molecular superbubble and spurs. The center of the superbubble is clearly shifted from the nucleus by 140 pc. This position is close to that of the massive black hole (BH) of >460 Mo and the 2.2 micron secondary peak (a luminous supergiant dominated cluster), which strongly suggests that these objects may be related to the formation of the superbubble. Consideration of star formation in the cluster based on the infrared data indicates that (1) energy release from supernovae can account for the kinetic energy of the superbubble, (2) the total mass of stellar-mass BHs available for building-up the massive BH may be much higher than 460 Mo, and (3) it is possible to form the middle-mass BH of 100-1000 Mo within the timescale of the superbubble. We suggest that the massive BH was produced and is growing in the intense starburst region.

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arXiv2000

Ultra-High-Density Molecular Core and Warped Nuclear Disk in the Deep Potential of Radio-Lobe Galaxy NGC 3079

We have performed high-resolution synthesis observations of the 12CO (J=1-0) line emission from the radio lobe edge-on spiral NGC 3079 using a 7-element mm-wave interferometer at Nobeyama, which consisted of the 45-m telescope and 6-element Array. The molecular nuclear disk (NMD) of 750 pc radius is found to be inclined by 20 degrees from the optical disk, and the NMD has spiral arms. An ultra-high-density molecular core (UHC) was found at the nucleus. The gaseous mass of the UHC within 125 pc radius is as large as about 3 x 10^8 Msun, an order of magnitude more massive than that in the same area of the Galactic Center, and the mean density is as high as approximately 3 x 10^3 H2 cm^-3. A position-velocity diagram along the major axis indicates that the rotation curve starts at a finite velocity exceeding 300 \kms already from the nucleus. The surface mass density in the central region is estimated to be as high as about 10^5 Msun pc^-2, producing a very deep gravitational potential. We argue that the very large differential rotation in such a deep potential will keep the UHC gravitationally stable from current star formation.

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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

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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