Division VI gathers astronomers studying the diffuse matter in space between stars, ranging from primordial intergalactic clouds, via dust and neutral and ionized gas in galaxies, to the densest molecular clouds and the processes by which stars are formed.

Commission 34 covers diffuse matter in space on scales ranging from the circumstellar to the galactic and intergalactic. As such it has enormous scope and because of this, it alone forms Division VI. Key aspects include star formation, matter around evolved stars, astrochemistry, nebulae, galactic and intergalactic clouds and the multitude of effects of the interaction of stars with their surroundings. Associated with these areas are a huge range of physical and chemical processes including hydrodynamics and magnetohydrodynamics, radiative processes, molecular physics and chemistry, plasma processes and others too numerous to name. These are complemented by an equally huge range of observational studies using practically all space and ground-based instrumentation at nearly all observable wavelengths. A glance at any data-base of publications over the past few years attests to the vigorous state of these studies. The current membership of the Division is around 800. It also has three separate working groups.

We have discovered a number of nonthermal X-ray features within central 40 pc region of the Galactic center by analysing 600-ksec observations of Chandra archival data. Most of the detected X-ray structures exhibit small-scale knot-like morphologies and their spectra are well reproduced by single hard power-law with photon indices of 1-2. Among them, the most outstanding features are the three X-ray knots which are aligned on a straight line from the potition of Sgr A* to north-northwest direction. The X-ray properties of these knots lead us to suspect that they are X-ray jets ejected from Sgr A* in the recent past. In addition, we have obtained an indication that the summed flux of nonthermal diffuse X-rays within 30 pc of the GC seems to be smoothly connected to the 20-100 keV flux detected with INTEGRAL IBIS/ISGRI. These results suggest that the origin of GC hard X-rays (or High energy Gamma-rays) is not (or partly) from the Galactic nucleus.

Several groups have been successful in the survey of high-redshift (z > 4) quasars. For example, Schneider et al. (1994) and Schmidt et al. (1995) were successful in detecting 90 quasars with 2.75 < z < 4.75, R < 21 for 62deg2 using Palomar Transit Grism. Among them, there are 9 quasars with z > 4.0. Storrie-Lombardi et al. (1996) were detected 31 quasars with z > 4.0, mT < 19 for 2500deg2 by APM photographic multicolor survey using UK Schmidt telescope. Survey observations of quasars up to now have to select one of the following conditions; wide field surveys using photographic Schmidt plate which cover large areas but do not go faint enough, or deep surveys using large telescopes which go faint magnitude but which cover relatively small areas.

The radio emission from the Galactic Center is a mixture of thermal (free-free) and non-thermal (synchrotron) emissions (Fig. 1a). However, the spectral index in the central 3° region is flat almost everywhere (Sofue 1985), even in regions where strong linear polarization is detected. Therefore, a flat spectrum observed near the galactic center can no longer be taken as an indicator of thermal emission.

Analyses of the distribution of far infrared point sources in the Galactic bulge have suggested that from a face-on perspective the bulge has a bar like shape. Here, we investigate how a rotating bar-like bulge affects the global gas dynamics in a disk and compare the longitude-velocity (LV) maps from selfgravitating hydrodynamical simulations with observed maps of neutral hydrogen and carbon monoxide in the Galaxy. We found that the features on the numerical LV maps depend strongly on four factors: the pattern speed of the bar, the position angle of the Sun, the strength of the bar potential and the ratio of the gas mass to total dynamical mass. We conclude that our Galaxy has a rotating, weak, bar-like bulge (a/b ∼ 0.8) observed from nearly end on (θp < 20°). The allowed range of pattern speed of the bar is surprisingly narrow (∼ 20 km s–1 kpc–1) and is consistent with recent observations of bulge stars. Selfgravity of the interstellar matter is needed to account for some of the observations.

By contrast-enhancing and colour-excess analyses of BV RI CCD images of the central 12’.5 × 12’.5 region of M31 taken with the Kiso 105 cm Schmidt telescope, we found a “face-on” spiral feature of dark clouds, which are associated with Hα spirals. The feature can be understood as being due to accretion of gas clouds from companion galaxies and/or from a merger galaxy in the past 108–9 years.

The origin of the straight magnetic filaments in the Radio Arc near the Galactic Center is still controversial: either if they are permanent structure of constant configuration for a galactic-time scale, or they are temporary structure of short life time such as due to a sudden injection of cosmic rays or to sudden shock compression. Measurement of life time of the radio emitting cosmic ray electrons consisting the filaments is one of the crucial methods besides a time variation measurement of the fine structure of the filaments.

In order to investigate possible spatial variation of spectral index at higher frequencies in the filaments, which might give information about the life time of cosmic ray electrons, we have performed high-resolution radio continuum observations at 43 GHz of the GO.18-0.04 region of the radio Arc near the galactic center using the Nobeyama Millimeter-wave Array. A detailed description of this work is given in Sofue et al. (1992).

Most of observational researches on molecular gas in galaxies have focused on the two-dimensional distributions in galactic disks so far. In order to investigate vertical structure of the galactic disk and the disk-halo interaction we need high-resolution observations of edge-on galaxies.

So we observed the nearest edge-on galaxy, NGC 891, using the Nobeyama Millimeter Array (NMA). Its distance is estimated to be 8.9 Mpc using the Tully-Fisher relation and H-band photometric data. The synthesized beamsize was 4.5” x 4.4” and the field of view due to attenuation of the element antenna (Handa et al. 1992). The field center is 90”-offset from the center of the galaxy along the major axis toward the northern side, where the CO intensity has a local maximum (Sofue et al. 1987). After the standard data reduction procedure in AIPS, including CLEAN, we obtained 10 channel-maps with a 19.5 km s−1 velocity width from 289.8 km s−1 to 465.2 km s−1 with respect to the local standard of rest.

In the integrated intensity map a narrow CO emission ridge is seen (Figure 1). It is an edge-on view of molecular gas disk of the galaxy. The confinement of the CO emission in the thin disk suggests that most of the molecular gas is belonging to the population-I objects like in our Galaxy. The apparent thickness of the ridge is about 7”, which means that the deconvolved width is about 6” or 270 pc (FWHM). The position-velocity diagram averaged along the major axis over the field of view shows no systematic velocity gradient along the minor axis (Figure 2). It means that the deconvolved thickness of the CO disk is intrinsic. This width is broader than that of our Galaxy by factor 2. The geometrically thick CO disk of NGC 891 may be due to active star formation in the disk.

Millimeter-wave continuum sources in NGC 7538 region were observed with the NRO 45-m telescope and Nobeyama Millimeter Array. NRO 45-m telescope observations showed that the compact region which includes IRS1, IRS2, and IRS3 has a strong millimeter-wave intensity excess, cf. figures 1, and 2.

The Tully-Fisher relation makes use of the correlation between the HI linewidth and the luminosity of a galaxy and is one of the most powerful tools to estimate distances of galaxies. Because of the smaller beam the CO line might be an alternative to HI for distant galaxies which so far can not be examined even by the largest telescopes (Dickey and Kazes 1992).

In order to consider the question whether the CO lineprofile could be used as an alternative or a supplement for HI in the Tullly-Fisher relation, we compared the total CO (J=2-1) line profiles of ten galaxies with the corresponding HI profiles. Four almost perfect edge-on galaxies (NGC 891, NGC 4565, NGC 4631 and NGC 3079) and the Seyfert galaxy NGC 1808 have been observed in CO using the Nobeyama 45-m telescope. The HI profiles and the data of the other galaxies has been taken from the literature.

High-spatial-resolution CO observations of galaxies have shown detailed distribution and dynamics of the molecular gas in spiral galaxies. We review observations, and show that the molecular gas distributions in the nuclear regions can be classified into three types: a core type with high concentration toward the nucleus; compact-ring type with ring radius of ∼200 pc; and a broad-ring type with ring radius of several hundred pc to ∼1 kpc. We also discuss their implications.

12CO (J=1−0) emission has been detected in the extremely extinguished galaxy NGC4418 with a 15″ beam on the 45-m telescope at the Nobeyama Radio Observatory. The inferred molecular mass is 4 108 MO at a distance of 25 Mpc. The CO line profile is single peaked with the FWHM of 85 km/s, suggesting that molecular clouds highly concentrate towards the center. For the column density estimated from these observations to be consistent with the extinction value 55 < Av < 120 derived from the 10μm silicate absorption by Roche et al (1986), most of the clouds should be confined within a central region of 3.8″ (500pc) in diameter, through which beam Roche et al detected the 10 and 20μm fluxes comparable to the IRAS ones.

A review is given of large-scale magnetic fields in disks and halos of spiral galaxies. A particular attention is given to vertical field structures, and we discuss their origin and implication on their interaction with halo gas. We point out that the disk-halo magnetic interface plays an important role in circulation of interstellar gas in galaxies, in particular a large-scale circulartion from the galactic center to outer disk regions.

We present the kinematics and dynamics of molecular gas in the almost entire disk (>5′×5') of the grand-design spiral galaxy M51, derived from CO(J=1−0) observations with the Nobeyama 45-m telescope. The spatial resolution of 17″, which corresponds to 790pc at the distance of M51 (9.6 Mpc), is enough to distinguish between a spiral arm and an interarm; therefore a clear spiral structure (cf Nakai et al, this conference) and large streaming motions of the molecular gas were revealed.

A number of starburst galaxies are included in the Atlas of Peculiar Galaxies (Arp 1966) which is the collection of peculiar-morphology galaxies showing mutual tidal interaction. A scenario of starburst can be summarized as follows (e.g. Sofue 1987, Noguchi 1988). Tidal interaction between galaxies causes an oval disturbance in the gravitational potential of a galaxy. The oval disturbance causes a bar in the innermost region of the galaxy. By this bar a galactic shock wave is excited, which results in rapid accretion of gas toward the central region. The accreted gas produces a dense ring or a core of molecular gas in the central region, from which massive stars are born. Intense UV radiation from the stars heats dust in the dense molecular gas core/ring, and the dust emits strong FIR emission.

Evolution of a vertical magnetic field which may have existed in the primeval galaxy is discussed based on a primordial-origin hypothesis for the bisymmetric spiral configuration of magnetic fields in spiral galaxies. The vertical field is accumulated toward the nucleus and forms a strong poloidal field, which may trigger activities like jets. Quasar jets are suggested to be the result of such strong vertical fields in the cores of primeval galaxies.

Radio continuum observations of the galactic center region have revealed a number of vertical structures running across the galactic plane, most of which are reasonably attributed either to vertical magnetic fields or to energy release out of the galactic plane. We review the observed radio structures and discuss their properties and origins with a particular attention to the unusual manifestation of energy release in the galactic center. The relation of the continuum structures to the expanding and/or contracting molecular gas rings is also discussed.