arXiv subjects

N. Kuno

Publications and source records attributed to N. Kuno.

27 records · Page 2Linked to original sources

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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arXiv2004

Study of the Bipolar Nebula IRAS 19312+1950. I. Mapping Observations

IRAS 19312+1950 is an SiO maser source that exhibits a prominent bipolar nebulosity. Mapping observations of this object were made in the CO J=1--0, 13CO J=1--0, C18O J=1--0, CS J=2--1, and HCN J=1--0 lines and in the 150 GHz continuum band. Near-infrared imaging observations were also made in the J, H, and K-bands. The line profiles of the 12CO and HCN spectra consist of a weak broad component with a line width of about 50 km/s and a strong narrow component of the width of about 3 km/s. The profiles of the 13CO, C18O, and CS lines have only the narrow component. Both of the components have an intensity peak at the IRAS position. The narrow component was clearly resolved with a 15'' telescope beam. The spectral energy distribution of this object exhibits a doubly peaked profile between 1 and 25 micron. The 150 GHz continuum flux density was found to be 0.07 Jy, which is consistent with the flux density predicted by the expanding envelope model with a mass loss rate of ~10^{-4} M_sun/y at a distance of 2.5 kpc. We argue that the broad component originates from the expanding envelope of this object, and that the hot dust cloud, which is the source of the narrow component, is also physically associated with this object. Though the present observations do not preclude the possibility of a young stellar object, we argue that it is less plausible. We conclude that IRAS 19312+1950 is an AGB/post-AGB star that is evolved from a massive progenitor.

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arXiv2004

Radio Observations of the Afterglow of GRB 030329

We present the results of the radio observations of the afterglow of GRB 030329 with the Nobeyama 45-m telescope. The observations were made at 23.5 GHz, 43 GHz, and 90 GHz. The light curves show steep decline after constant phase. The start time of the decline depends on frequency. Namely, the decline started earlier at higher frequency. The spectrum has a peak at mm wavelength range. The peak frequency and the peak flux decreased with time. These results are consistent with the expectation from fireball model.

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arXiv2003

Deep Searches for High Redshift Molecular Absorption

Millimetre-band scans of the frequency space towards optically dim quasars is potentially a highly efficient method for detecting new high redshift molecular absorption systems. Here we describe scans towards 7 quasars over wide bandwidths (up to 23 GHz) with sensitivity limits sufficient to detect the 4 redshifted absorbers already known. With wider frequency bands, highly efficient searches of large numbers of possibly obscured objects will yield many new molecular absorbers.

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arXiv2002

Dense Molecular Gas in Lenticular Galaxies

We made CO and HCN simultaneous observations of lenticular galaxies, NGC 404, NGC 3593 and NGC 4293, and detected HCN emission in NGC 3593 and NGC 4293 as well as CO in all the galaxies. The I(HCN)/I(CO) ratios were 0.025+-0.006 and 0.066+-0.005 in NGC 3593 and NGC 4293, respectively, which are comparable to the late-type spiral galaxies. The average of the I(HCN)/I(CO) ratios at the center of 12 nearby spiral galaxies including late-type was 0.055+-0.028. The line profiles of CO and HCN emission showed different shape in both galaxies. The HCN peaks were not at the systemic velocity of these galaxies, while the CO peaks were near the systemic velocity. These results suggest that the fraction of the dense molecular gas is high around the center in these galaxies.

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arXiv2002

SZ and X-ray combined analysis of a distant galaxy cluster, RX J2228+2037

We have performed a combined analysis of X-ray and Sunyaev-Zel'dovich data in the direction of the distant galaxy cluster, RX J2228+2037. Fitting a $β$-model to the high-resolution HRI data gives $r_c = 103 \pm 12 h_{70}^{-1}$ kpc and $β=0.54 \pm 0.03$. The dependency of the Sunyaev-Zel'dovich effect with respect to the gas temperature allows us, through the additional use of the 21 GHz data of the cluster, to determine $k_B T_e=10.4 \pm 1.8 h_{70}^{1/2}$ keV. Extrapolating the gas density profile out to the virial radius ($R_v=r_{178}=2.9$ Mpc), we derived a gas mass of $M_{g}(r<R_v)=(4.0\pm 0.2)\times 10^{14} h_{70}^{-5/2} \rm{M}_\odot$. Within the hypothesis of hydrostatic equilibrium, the corresponding extrapolated total mass for this source is: $M_{tot}(r<R_v)=(1.8 \pm 0.4)\times 10^{15} h^{-1} \rm{M}_\odot$, which corresponds to a gas fraction of $f_{gas}=0.22\pm 0.06 h_{70}^{-3/2}$. Our results on the temperature and on the cluster mass classify RX J2228+2037 among the distant, hot and very massive galaxy clusters. Our work highlights the power of the association of galaxy cluster mapping observations in X-ray and the SZ effect to derive the cluster's physical properties, even without X-ray spectroscopy.

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arXiv2002

Variation of Molecular Cloud Properties across the Spiral Arm in M 51

We present the results of high-resolution 13CO(1-0) mapping observations with the NRO 45m telescope of the area toward the southern bright arm region of M51, including the galactic center. The obtained map shows the central depression of the the circumnuclear ring and the spiral arm structure.The arm-to-interarm ratio of the 13CO(1-0) integrated intensity is 2-4. We also have found a feature different from that found in the 12CO results. The 12CO/13CO ratio spatially varies, and shows high values (~20) for the interarm and the central region, but low values(~10) for the arm. These indicate that there is a denser gas in the spiral arm than in the interarm. The distribution of the 13CO shows a better correspondence with that of the Hαemission than with the 12CO in the disk region, except for the central region. We found that the 13CO emission is located on the downstream side of the 12CO arm, namely there is an offset between the 12CO and the 13CO as well as the Hαemission. This suggests that there is a time delay between the accumulation of gas caused by the density wave and dense gas formation, accordingly star formation. This time delay is estimated to be ~10^7 yr based on the assumption of galactic rotation derived by the rotation curve and the pattern speed of M51. It is similar to the growth timescale of a gravitational instability in the spiral arm of M51, suggesting that the gravitational instability plays an important role for dense gas formation.

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arXiv2000

Distribution and Kinematics of Molecular Gas in Barred Spiral Galaxies. I. NGC 3504

We present the results of the CO mapping observations of the barred spiral galaxy NGC3504 with the Nobeyama 45-m telescope. The distribution of the molecular gas shows offset ridges which correspond to the distribution of HII regions along the bar. The velocity perpendicular to the bar decreases abruptly at the ridge. The velocity change implies that the molecular gas changes the direction of its motion to parallel to the bar at the ridge. Since the position angle of the major axis of the bar and the line of nodes are almost the same in NGC 3504, an upper limit to the pattern speed of the bar can be derived directly from the radial velocity. The resultant upper limit is 41 km/s/kpc which is much smaller than that derived with an assumption that the corotation radius is located at the end of the bar (77 km/s/kpc). The corotaion radius derived from our upper limit is more than two times larger than the length of the semi-major axis of the bar in NGC 3504.

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