Satoki Matsushita

High-Density and High-Temperature Circumnuclear Molecular Disk in M51

We performed high-resolution (59 × 51)13CO(1-0) imaging observations of the central region (1.5 kpc in radius) of M51 with the Nobeyama Millimeter Array in order to investigate the physical conditions of the circumnuclear dense molecular clouds. The obtained 13CO image looks very similar to the 12CO image in the sense that the emission is distributed along the inner part of the spiral arms, and there is a conspicuous deficiency of emission at the nucleus, in contrast with the centrally peaked HCN image. The intensity ratio of HCN/13CO obtained in the inner 200 pc is higher than 3 and is almost comparable to those in the circumnuclear molecular gas in the Seyfert galaxy NGC 1068 and in the nuclear region of the prototypical starburst galaxy M82. On the other hand, the 12CO/13CO intensity ratio is ~7 ± 2 in almost all of the central region, with no enhancement in the nuclear region. The obtained HCN/13CO ratio in the nucleus of M51 was used to derive the physical conditions of the dense molecular gas using the large velocity gradient approximation. Assuming the usual abundance ratio and the velocity gradient of HCN and 13CO, the inverted HCN/13CO intensity ratio suggests that the density and the temperature of the circumnuclear molecular gas are as high as 105 cm-3 and greater than 100 K, respectively. As possible origins for these physical conditions, we discuss the interactions between nuclear winds or jets and the interstellar material and compression of interstellar material by supernova driven winds.

FTS measurements of submillimeter opacity and other site testing at Pampa la Bola

We have carried out Fourier Transform Spectrometer (FTS) measurements of the millimeter and submillimeter-wave (150 - 1500 GHz or 2 mm - 200 micrometer) atmospheric opacity at Pampa la Bola, 4800 m above sea level in northern Chile on September 1997 and June 1998. One of the best transmission spectra show up to approximately 67% transmission at well- known submillimeter-wave windows. Supra-terahertz windows (located around 1035 GHz, 1350 GHz, and 1500 GHz) were identified in the same spectrum. The observed spectra can be well modeled by newly developed radiative-transfer calculations. Correlations between 220 GHz opacities and those of the center of submillimeter-wave windows or even those of the supra-terahertz windows are obtained using the entire data set. Good correlations were obtained except for the periods affected by the liquid water opacity component. We succeeded to separate the total opacity in two parts: the water vapor opacity and the liquid water opacity, using two frequencies, one in the millimeter domain and another one in the submillimeter. The separated water vapor opacity component shows good correlation with the 183 GHz pure water vapor line opacity which is also covered in the measured spectra, but the liquid water opacity component shows no correlation. The liquid water opacity component also shows no correlation with the phase fluctuation measured with the 11 GHz radio seeing monitor. Modeling of this component is currently under way. Combined with a statistical study of the 225 GHz opacity data of the Chajnantor site (approximately 7 km apart from Pampa la Bola), it is estimated that submillimeter-wave observations can be done with zenith opacity less than 1.0 (at the most transparent frequency in those windows) for about 50% of the winter season, assuming no presence of liquid water absorption.

High-density and high-temperature molecular gas around the AGN in M51

Abstract We investigated the physical properties of molecular gas in the nuclear region of M51 (Seyfert 2). We obtained an aperture synthesis 13 CO(J = 1 − 0) image using the Nobeyama Millimeter Array (NMA), and compared it with NMA 12 CO(J = 1 − 0) and HCN(J = 1 − 0) maps at similar spatial resolutions. Within a radius of 180 pc from the center, the 13 CO(1 − 0) integrated intensity was found to be 3 times weaker than that of HCN(1 − 0). Large-Velocity-Gradient (LVG) calculations suggest that the observed high HCN(1 − 0)/ 13 CO(1 − 0) intensity ratio would arise from dense ( n H 2 ∼ 10 5 cm −3 ) and hot ( T kin ≳ 300 K) molecular clouds in the nuclear molecular disk. We also observed in the 12 CO(1 − 0), (3 − 2), 13 CO(1 − 0), and (3 − 2) lines using the Nobeyama 45m and JCMT 15m telescopes. We detected weak 13 CO lines as well as strong 12 CO lines. The LVG calculations assuming a two-component model suggest that there is a large amount of low-density ( n H 2 ∼ 3 − 6 × 10 2 cm −3 ), low-temperature ( T kin ∼ 20 – 50 K) gas, and a small amount of high-density ( n H 2 ≳ 10 4 cm −3 ), high-temperature ( T kin ≳ 500 K) gas. The existence of the high-density and high-temperature component, although having a quite small beam filling factor, supports the aperture synthesis observation results mentioned above. Since this dense, hot gas is located in the nuclear molecular disk around the Active Galactic Nucleus (AGN), it may be heated by the strong X-ray radiation and/or by the shock induced by the radio jet.