arXiv subjects

Kazuhisa Kamegai

Publications and source records attributed to Kazuhisa Kamegai.

23 records · Page 2Linked to original sources

arXiv2007

A Search for CO(J=3-2) Emission from the Host Galaxy of GRB 980425 with the Atacama Submillimeter Telescope Experiment

We report on a deep search for CO(J=3-2) line emission from the host galaxy of GRB 980425 with the Atacama Submillimeter Telescope Experiment (ASTE). We observed five points of the galaxy covering the entire region. After combining all of the spectra, we obtained a global spectrum with the rms noise level of 3.3 mK in T_mb scale at a velocity resolution of 10 km s^-1. No significant emission was detected, though we find a marginal emission feature in the velocity range corresponding to the redshift of the galaxy. We derive 3 sigma upper limits on the global properties: the velocity-integrated CO(3-2) intensity of I_CO(3-2) < 0.26 K km s^-1 by adopting a velocity width of 67 km s^-1; the H_2 column density of N(H_2) < 3 x 10^20 cm^-2; the molecular gas mass of M(H_2) < 3 x 10^8 M_sun, by assuming a CO line luminosity to H_2 molecular gas mass conversion factor of X_CO = 5.0 x 10^20 cm^-2 (K km s^-1)^-1; and the star formation rate of SFR < 0.1 M_sun yr^-1, based on the Schmidt law. The SFR is consistent with the previous results of H_alpha and mid-IR observations, thereby suggesting that there is no significant obscured star formation in the host galaxy of GRB 980425. This result implies that there is a variety of GRB hosts with regard to the presence of obscured star formation.

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arXiv2007

High Resolution Mappings of the L=1.3 deg Complex in Molecular Lines : Discovery of a Proto-Superbubble

We report the results of molecular line observations toward the l=1.3 deg complex, ananomalous cloud complex in the central molecular zone of the Galaxy. We have taken high resolution maps of the CO J=1-0, HCN J=1-0, HCO+ J=1-0, SiO J=1-0 and J=2-1 lines. The complex is found to be rich in shells and arcs of dense molecular gas. We have identified 9 expanding shells in HCN maps and compact SiO features associated to the shells. The intensity ratios of HCN/CO, HCO+/CO and CO J=3-2/J=1-0 are coherently enhanced by a factor of a few in gas with an LSR velocity higher than 110 kms^-1. The high-velocity gas has a high density [n_H ~ 10^4.5 cm^-3] and high SiO/13CO intensity ratio indicating that the gas was shocked. The typical HCN/HCO+ intensity ratio is found to be 2.3, being higher by an factor of a few than those in the Galactic disk clouds. The typical kinetic energy and expansion time of the shells are estimated to be 10^(50.9 - 52.5) erg and 10^(4.6 - 5.3) yr, respectively. The kinetic energy could be furnished by multiple supernova and/or hypernova explosions with a rate of 10^(-3 - -4) yr^-1. These estimates suggest that the expanding shells as a whole may be in the early stage of superbubble formation. This proto-superbubble may be originated by a massive cluster formation which took place 10^(6.8 - 7.6) yr ago.

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arXiv2007

A CO J=3-2 Survey of the Galactic Center

We have surveyed the central molecular zone (CMZ) of our Galaxy in the CO J=3-2 line with the Atacama Submillimeter-wave Telescope Experiment (ASTE). Molecular gas in the Galactic center shows high J=3-2/J=1-0 intensity ratio (~ 0.9) while gas in the spiral arms in the Galactic disk shows the lower ratio (~ 0.5). The high-velocity compact cloud CO 0.02-0.02 and the hyperenergetic shell CO 1.27+0.01, as well as gas in the Sgr A region exhibit high J=3-2/J=1-0 intensity ratio exceeding 1.5. We also found a number of small spots of high ratio gas. Some of these high ratio spots have large velocity widths and some seem to associate with nonthermal threads or filaments. These could be spots of hot molecular gas shocked by unidentified supernovae which may be abundant in the CMZ.

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arXiv2007

Physical Conditions of Molecular Gas in the Galactic Center

We estimated physical conditions of molecular gas in the central molecular zone (CMZ) of the Galaxy, using our CO J=3-2 data obtained with the Atacama Submillimeter Telescope Experiment (ASTE) in conjunction with J=1-0 12CO and 13CO data previously observed with the NRO 45m telescope. The large velocity gradient (LVG) approximation was employed. Distributions of gas density, kinetic temperature, and CO column density are derived as functions of position and velocity for the entire coverage of the CO J=3-2 data. We fairly determined physical conditions for 69 % of data points in the CMZ with >= 1 sigma CO detections. Kinetic temperature was found to be roughly uniform in the CMZ, while gas density is higher in the 120-pc star forming ring than in the outer dust lanes. Physical conditions of high J=3-2/J=1-0 features are also discussed.

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arXiv2007

ASTE CO(3-2) Observations of the Barred Spiral Galaxy M 83: I. Correlation between CO(3-2)/CO(1-0) Ratios and Star Formation Efficiencies

We present CO(J=3-2) emission observations with the Atacama Submillimeter Telescope Experiment (ASTE) toward the 5' x 5' (or 6.6 x 6.6 kpc at the distance D = 4.5 Mpc) region of the nearby barred spiral galaxy M 83. We successfully resolved the major structures, i.e., the nuclear starburst region, bar, and inner spiral arms in CO(J=3-2) emission at a resolution of 22'' (or 480 pc), showing a good spatial coincidence between CO(J=3-2) and 6 cm continuum emissions. We found a global CO(J=3-2) luminosity L'_CO(3-2) of 5.1 x 10^8 K km s^-1 pc^2 within the observed region. We also found L'_CO(3-2) in the disk region (0.5 < r < 3.5 kpc) of 4.2 x 10^8 K km s^-1 pc^2, indicating that CO(J=3-2) emission in the disk region significantly contributes to the global L'_CO(3-2). From a comparison of a CO(J=3-2) data with CO(J=1-0) intensities measured with Nobeyama 45-m telescope, we found that the radial profile of CO(J=3-2)/CO(J=1-0) integrated intensity ratio R_3-2/1-0 is almost unity in the central region (r < 0.25 kpc), whereas it drops to a constant value, 0.6--0.7, in the disk region. The radial profile of star formation efficiencies (SFEs), determined from 6 cm radio continuum and CO(J=1-0) emission, shows the same trend as that of R_3-2/1-0. At the bar-end (r ~ 2.4 kpc), the amounts of molecular gas and the massive stars are enhanced when compared with other disk regions, whereas there is no excess of R_3-2/1-0 and SFE in that region. This means that a simple summation of the star forming regions at the bar-end and the disk cannot reproduce the nuclear starburst of M 83, implying that the spatial variation of the dense gas fraction traced by R_3-2/1-0 governs the spatial variation of SFE in M 83.

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