The dark cloud between the North America and Pelican Nebula has been mapped in HI using DRAO synthesis telescope at Penticton, B.C.(Canada) and the 100m telescope at Effelsberg (MPIfR Bonn, F.R.G.) for the short baselines. The angular resolution is 2'= 0.3 pc, assuming 500 pc for the dark cloud distance (Strayzis et al. 1989, Wendker et al. 1983). The spectral resolution was 1.3 km/s. The main part of the dark cloud has been mapped also in the CO (J=1−0) line with the 3m telescope on the Gornergrat (KOSMA, Switzerland) with resolutions of 3.8' and 0.6 km/s. The distribution of HI emission is far from homogenous. The line maps are dominated by irregular patches of up to 10 pc size and systems of filaments up to 20 pc size with substructure down to 1 pc. At radial velocities associated with that of the dark cloud, the HI emission tends to outline the shape of the bulk of the molecular gas seen in the CO data. It could be regarded as the outmost layers of the dark cloud, in which the molecular hydrogen is dissociated by the interstellar radiation field and especially by the ionizing star(s) of the nearby HII region.

1. The HI envelopes were discovered in galaxies of all types. The optical images of galaxies are always seen in the regions where the HI surface density NHI is greater than some critical value N1 ≅ 5·1020 cm−2. The molecular gas is embedded in those parts of galaxies where NHI ≥ N2 − 2·1021 cm−2. As follows from kinetics of molecule formation the molecular hydrogen forms at column densities greater than N1 and CO formation requires N ≥ N2 (Arshutkin and Kolesnik (1981,Astrofizika. 17.359; 1984, ibid.21.147)). These results explain why the observations of external galaxies emphasize the column densities N1 and N2 as critical ones.

Observations of the CS J = 2 → 1 line have been made with SEST toward IRAS point sources associated with the largest molecular cloud complexes in the southern Milky Way. A total of 294 embedded stellar objects have been detected and their distribution in the Galaxy derived.

On the base of the Galaxy survey in OH lines at 1665 and 1667 MHz (Turner (1979,Astron. Astrophys. Suppl. Ser. 37, 1)) the method of distance determination to molecular clouds was proposed. It was shown (Kolesnik and Yurevich (1985,Astrofizika 22,461)) that the Δv - a linewidth (in km·s−1) and TA - a linedepth (in K) of the OH absorption features could be combined into a parameter D(r) = (Δv2/TA)1/3 that is sensitive to the distance r (in kpc) of molecular cloud where this OH absorption is formed. The calibration of (D-r)-relation gives

As part of the Australia Telescope's commissioning astronomy, three antennas of the Compact Array, with spacings of 0.5, 1.5 and 2 km, were used to produce the instrument's first spectral-line images - OH excited-state transitions from the southern spiral galaxy NGC 4945. For the 6030- and 6035-MHz transitions of the 2π3/2, J = 5/2 state, well-defined line absorption was detected towards the 6 × 2 arcsec2 radio continuum nucleus. The line profiles are wide, with heliocentric radial velocities extending from 350 to 770 km s−1, and with overall shapes similar to the corresponding ground-state transitions at 1665 and 1667 MHz (Whiteoak & Gardner 1975). The respective maximum line-to-continuum ratios of 0.04 and 0.07 occur near 650 km s−1. A comparison of the 6035- and 1667-MHz results yields an OH temperature of 36 K. The position of the absorption changes systematically with velocity along a line at position angle 45°. Such behaviour is consistent with that for a rotating molecular cloud which surrounds the nucleus.

Polarized radio continuum emission and degree of polarization are reduced on dust lanes containing molecular clouds, whereas the total power emission is strong there. Faraday effects can explain only part of this depolarization. The remaining part must be due to tangled field lines within the radio beam.

The contributions to the far infrared spectra of galaxies of the ‘cool’ cirrus and ‘warm’ starburst components are reviewed, together with the complications introduced by dust in the narrow-line regions of Seyferts, the destruction of very small grains, and the small percentage of galaxies which are optically thick in the optical band. The role of interactions and mergers in generating ultraluminous infrared galaxies is reviewed.

We have made measurements of the 12 − 25 μm sizes of 17 galaxies with high infrared luminosities. Most of the galaxies are extended on a scale of 0.6 to 1 kpc diameter. Deep silicate absorption features are common. We discuss some of the difficulties in explaining how an extended source can have a deep silicate feature.

The relationship between the far infrared and Hα luminosity has been investigated for a sample of 124 spiral galaxies. We find that the observed far infrared luminosities are comparable to the luminosities expected from the high mass O and B stars which are required to ionize the hydrogen gas. Consequently the far infrared luminosity appears to be a measure of the high mass star formation rate in spiral galaxies. In addition, the all sky IRAS survey has been used to quantify the birthrate of high mass stars for a complete volume limited sample of ∼ 1000 spiral galaxies selected from the Nearby Galaxies Catalog. Our analysis indicates that the median high mass star formation rate is independent of morphology for spirals of types Sa - Scd inclusive. As such, our results challenge the growing perception that the median high mass star formation rate increases along the Hubble sequence Sa - Scd.

Empirical evidence is presented supporting the use of a constant Galactic CO-H2 conversion factor to derive global H2 masses for luminous spirals which are accurate to +/−30%.

The use of 12CO as a mass tracer of H2 in molecular clouds is based on three independent calibrations, all of which agree. The physical basis for an approximately constant molecular (H2) mass to CO luminosity ratio is the origin of the CO emission from gravitationally bound molecular clouds. We discuss the size-line width and virial mass-CO luminosity relations derived from the Massachusetts-Stony Brook CO Survey. The criticisms of the size-line width relation by the Wolfendale group are wrong. Their tests were not self consistent, and were constrained by the properties of the real clouds, not independent of them, as was implied. We show that the virial masses are a reliable measure of the true cloud masses. The CO to H2 conversion factor depends only weakly on molecular cloud parameters, and its value in other spiral galaxies should not be very different from that in the Milky Way. The H2 masses in the centers of galaxies are not overestimated, since the CO excitation is observed to be normal there, and the brightness temperatures not much different from those in the Milky Way. The effect of metallicity on the conversion factor is also briefly discussed.

Surveys at a variety of wavelengths indicate that galaxy interactions are statistically linked to enhanced rates of star formation. The distributions of star-formation rate (SFR) show typical increases of order 30%, and only a few systems undergoing the order-of-magnitude enhancements typical of strong starbursts (particularly in mergers). Potential advantages and problems of various measures of SFR are discussed, along with issues of sample selection and comparison. Finally, various proposed mechanisms for enhancing the SFR during interactions are listed, with relevant observational tests. Current data suggest that several physical processes may contribute to star formation during interactions.

The CSRG is a comprehensive compilation of diameters, axis ratios, relative position angles, and morphologies of inner, outer, and nuclear rings in 2,300 southern galaxies. This paper describes the motivation for this survey, its present status, and early results and implications from the analysis of its contents.

Outer rings and pseudorings are features commonly observed in disk galaxies in the range of Hubble types S0+ to Sab (see, for example, de Vaucouleurs 1975). As part of a general program to study the nature of rings in disk galaxies, we have begun imaging a large sample of outer rings to study their morphological and photometric properties in detail. Our goal is to evaluate whether these rings are related to the outer Lindblad resonance (OLR), a major 2:1 resonance known to be important for ring formation in n-body models where dissipation in the gaseous component is significant (Schwarz 1981 = S81). The morphology of outer rings is important because S81 predicted that near the OLR two major families of periodic orbits can lead to formation of two morphologically distinct types of outer rings and pseudorings: one type elongated perpendicular to the bar with arms intersecting near the bar axis, and the other elongated parallel to the bar with arms intersecting 90° to the bar axis. These ring types were searched for on the SRC IIIa-J sky survey copy films by Buta (1986), who called them R'1 and R'2, respectively. The full results of Buta's SRC search, which includes other types of rings, is being prepared as the Catalogue of Southern Ringed Galaxies (Buta and Crocker, 1990 = CSRG) and its contents are discussed in Buta (1990, this conference).

Recently many CO distributions in the shape of rings have been uncovered. What dynamically can we learn from these structures? After a brief point on recent observations, I will review the dynamical mechanisms to form rings.

In central regions of many active star forming galaxies, stellar bars and massive gas disks or rings are observed and are supposed to be related with burst star formation (Scoville et al. 1988, Noguchi 1988). We perform the numerical hydrodynamic calculation of self-gravitating, isothermal gas motion in barred spiral galaxies, in order to investigate the effects of self-gravity of gas on the gas accumulation process into galactic center. A weak bar, isothermal gas with T = 104K, and f ∼ 0.1∓0.2 are assumed, where f = Mgas/Mstar. Initial gas is uniformly distributed in the galactic disk. The smoothed particle scheme is used for hydrodynamic calculation and the self-gravity of gas is calculated by FFT. We show that, in order to accumulate gas into the central region of galaxy by the bar potential and to form gaseous ring or disk in the central region, the galaxy model is needed to have the inner Lindblad resonance. Further evolution of the ring or the disk depends on mass of gas, strength of bar and pattern speed of bar. Since the self-gravity of gas ring or disk enhances its density and its elongation, and, as a result, the asphericity of gravitational potential increases, the angular momentum transfer rate and the rate of mass accretion into it increase as shown in Fig. 1. Then, the ring or the disk becomes more massive and shrinks. Finally, central small dense gas ring or disk is formed.

The nature and causes of central star-bursts in S0 galaxies are discussed.

Several different gas dynamical phenomena in barred galaxies are examined with new high resolution CO observations from the OVRO millimeterwave interferometer. In M101, a gas bar is discovered which is offset in position angle by 24° from a weak stellar bar. In M83, the morphology and kinematics of a large molecular gas complex in a bar-spiral arm transition region suggest that it is formed by the combined action of orbit crowding and shocks. CO maps of 2 barred galaxies with nuclear Hα rings show bright CO emission near the ends of the rings, where shockfronts intersect the rings.

NGC 4736 is one of the nearest examples (D ≈ 6.3 Mpc) of spiral galaxies with prominent rings : an inner ring of HII regions (r ≈ 50″ ≈ 1.7 kpc) and an outer stellar and HI ring (r ≈ 5′ ≈ 9 kpc. Atomic gas is present in the inner ring, and throughout the main body of the galaxy, but neither in the nucleus nor in the gap between the main body and the outer ring.

Ringlike structures (in Hα, CO and radio continuum) in central galactic regions have been observed in a number of objects. They seem to be a basic feature of galactic structure (for references see Dickman et al., 1988). These observations can be summarized as follows:

The typical mass of these rings is 108 − 109M⊙.

The gas distribution is smoothly decreasing from the ring into the innermost galactic regions. The rings appear always at the turnover radius where the galactic rotation turns from rigid to differential rotation.