We present the first results of a high angular resolution-high sensitivity survey of CO (J=1—0) and (2—1) emission in M 51, made with the IRAM 30 m telescope. Of all tracers of the inner spiral structure, CO, i.e. presumably the molecular gas, appears as the tracer with the smoothest in-arm distribution, the highest arm-interarm contrast and the most visible streaming motions. Although observed about everywhere, CO, in the interarm region, is mostly concentrated along 4 radial lanes, or bridges, connecting the two main spiral arms.

We present maps of CO emission in M51 made with the Owens Valley Millimeter Interferometer. We first summarize the results from our mosaic map at 8″ resolution covering the disk of M51. We then present a map of two 1′ fields along the spiral arm south of the nucleus at 2.5″ resolution. The Giant Molecular Associations (GMAs) seen in our 8″ map are found to have substructure at the higher resolution. The interarm GMAs have a particularly patchy structure. The streaming motions found from our 8′ map are confirmed at this resolution. The velocity dispersion along the arms is about 10 km s—1. Possible explanations are given for the poor small-scale correlation of the Hα and CO emission.

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.

We present 6″ (460 pc at a distance of 9.6 Mpc) resolution CO(J=1−0) map of the central region of the grand-design spiral galaxy M51 and very preliminary 4″ (180pc) resolution CO(J=1−0) map of the arm regions, using the Nobeyama Millimeter Array (NMA). The central 3kpc region (1') was mapped with a velocity resolution of 20km/s, and the rest with 10km/s.

We present aperture synthesis CO maps of two 2'-fields of M51. These maps, in contrast to previous interferometer maps, do not have any missing flux. They show directly that most of the CO emission arise from broad spiral arms, whereas previously it was assumed that the missing CO flux (≥ 65% of the total flux) is uniformly distributed. Inferences involving extended CO emission and widths of the CO spiral arms based on interferometer maps with missing flux may need to be reviewed. The “missing short spacings” problem is also discussed.

An intensive study of M81 has shown a number of interesting features of the spiral-arm star formation process there.

We present CO(2-1) and (1-0) maps of the inner disk of the nearby Scd spiral NGC 6946 (D= 5 Mpc). These maps have been obtained using the IRAM 30m telescope (beamsizes = 22″ and 14″, map sampling 10″= 250 pc). The CO(2-1) map contains about 1500 points and covers approximately the inner disk out to R = 3.5′. The overlay of CO(2-1) line area and Hα emission (figure 1) shows that spiral arms are seen in CO and is a very good correspondence between the optical and molecular arms. However there is CO emission in between the arms; the contrast in both lines is about 4. Note also the oval distribution of CO emission in the nucleus, revealing the presence of a bar. The CO(2-1)/CO(1-0) emissivity ratio is low (around 0.5 in the disk) suggesting subthermal excitation.

We show examples of a new technique we have devised to compare star formation efficiencies in the arms and discs of spirals. First results show striking evidence of the presence and influence of density wave systems of star formation in grand design galaxies.

We apply a newly devised quantitative technique to infer the relative arm/interarm star formation efficiency ratio in NGC 4321, revealing a complex but clearly defined density wave structure pattern.

Seventeen giant (170 — 700 pc) star-gas complexes (SGCs) have been detected within 3 kpc from the Sun. These SGCs include stellar groupings younger than (2-3)×107 yrs and molecular clouds with masses 105—106M⊙, embedded into HI superclouds. An investigation of seven large-scale SGCs has shown, within them, an age gradient of stellar groups, equal to (0.3-1.2)×107 yrs for distances 270 − 500 pc. The sequential changing of stellar groupings' ages across the Sgr-Car arm (with the youngest stellar groups, all the young H2O masers and CO clouds located at the inner boundary of the arm) evidences for formation of this arm by a spiral density wave. This wave, propagating in individual HI superclouds with molecular clouds inside, stimulates star formation therein. Crude estimates of the spiral pattern angular velocity yield Ωp ≈ 17 — 25 km s—1 kpc—1 and the corotation radius ≈ 8.8 − 13 kpc. Perhaps the Cygnus arm is lying near the corotation radius, since there is no age gradient across this arm. The direction of the age's changing is different in all the Cas — Per arm's SGCs. It cannot be excluded that in one of these SGCs a “reverse” age gradient is observed.

The cold interstellar medium plays a dominant role in determining the observed spiral structure in galaxies not only because it is the site of the generation of young and bright objects, but also because it is both a source of excitation (Jeans instability) and of regulation (since it can dissipate) for the overall dynamics of the disk. Here I review a physical picture of spiral galaxies developed in the last few years and present recent results on the mechanism of self-regulation of the disk, on the modeling of individual objects, and on the modeling of categories of spiral galaxies (morphological classification).

It is suggested that the asymmetric structure of lopsided galaxies can be explained by nonlinear coupling of m=1 and m=2 spirals. These structures cooperate to transfer energy and angular momentum outwards, much farther than they individually could.

The secular evolution of galactic discs, of which the increase of the stellar velocity dispersion with age is the most striking expression from a kinematical point of view, is closely related to their stability properties because of the collective nature of such systems. In this context, however, the crucial role of collective effects is often underestimated or not properly taken into account.

We would like to draw attention to some observational data which should not be neglected, in our opinion, in developing spiral structure theories. Some of these results demonstrate Le Chatelier principle well-known in physics. According to this principle, any physical process is proceeding to cancel its cause. Localization of galactic disks near the marginal curves of various instabilities shown below indicates the important role that these instabilities play in evolutionary processes of disks.

The simplest analytical theory of bars made of stars in excentric orbits is suggested below (such a possibility was mentioned in [1]). Approximately, the subsystem of particles with elongated orbits may be described as consisting of hard “needles” elongated along radii, slowly (compared with radial oscillations of particles) rotating due to orbit precession. As a first approximation, we shall consider the needles mentioned to be infinitesimally thin ones. Besides, let us assume for simplicity that the radial energy of stars is fixed: E=Eo. Then we may introduce the distribution function of such needles, f=f(ϕ, Ω), so that dn = f(ϕ, Ω)dϕdΩ is the number of needles within the angles (ϕ, (ϕ+dϕ), rotating with the angular velocities Ω = ϕ, in the range (Ω, Ω+dΩ). Let us write now the collisionless kinetic equation in the form ∂f/∂t + Ω. ∂f/∂ϕ+P∂f/∂Ω = 0, where P=dΩ/dt=d2ϕ/dt2=Mtot/I; Mtot is the total torque acting on the needle, I is the needle's inertia momentum relative to the disk center.

Spiral structure in galaxies and its relation to interstellar gas are reviewed. The evidence for spiral modes and resonance features in two-arm grand design galaxies is summarized. Gas observations of the corotation regions of these galaxies are needed. Multiple-arm galaxies could have several modes simultaneously, or they could lack modes altogether because of insufficient wave reflection in the central regions; then the spirals could result from transient shear instabilities. Flocculent galaxies may have similar shear instabilities but only in the gas. The importance of variable boundary conditions for clouds that flow through spirals is emphasized. These boundary conditions, such as pressure, radiation field, tidal force, and so on, affect the internal structure and molecular fraction of a cloud, and they could influence the calibration coefficient for determining mass from the CO luminosity. The importance of gas observations beyond the optical disk is also emphasized, to elucidate the role of the outer Lindblad resonance on wave propagation, and to study the proposed low-density cutoff for star formation.

We present a numerical survey of interactions of galaxies. For the considered disc model there is an almost one dimensional sequence of forms in the time evolution of the perturbed (main) system.

We present an N-body simulation of a galaxy with a long-lived two-armed spiral pattern. The model consists of a static bulge and halo and an active disk of 60,000 particles. The disk contains 41 % of the total mass of the system. The resulting rotation curve has a steeply rising central part and is falling at larger radii. Half of the disk particles represents an old stellar population, half of them gas clouds that can collide inelastically with each other.

This paper discuss that a particular type of bar, with some characteristic features, is the result of galaxy interactions. These features are yet not distinguished by morphological classification schemes. The distinguishing characteristics of this transient (one rotation) phase of an interaction are:

∗ A bright oval approximately one-half the size of the galaxy centered on the nucleus with a right angled vertex at each end of the major axis.

∗ Spiral arms extend smoothly from each of the flatter sides of the oval, sometimes showing a double-parallell structure with a large pitch angle.

We develop the idea that star formation is a propagating process in galaxies. The basic mechanism of propagation is the accumulation of mass in expanding cold shock-waves. The expansion energy can be inserted by massive stars of an OB-association or it may originate from collissions of high velocity clouds with the galactic disk.