Systematic rotational properties of field spiral galaxies are presented, as a function of Hubble type and of luminosity. Within a Hubble type, radius, nuclear velocity gradient, rotation velocity, mass, and mass density increase with luminosity, while m/L is constant. At fixed luminosity, V(max), mass, density, and m/L decrease from Sa through Sc. These variations are so systematic that it is possible to display them on a single diagram.

Recent results on 21-cm line velocity fields of spiral galaxies are reviewed. Attention is drawn to the sometimes discrepant results on the spatial orientation of galaxies inferred from various methods. Major classes of deviations from circular, planar, motion of the neutral gas are discussed. Some comments are made about HI rotation curves.

The first results of a new high resolution 21-cm HI line survey of M31 made with the Westerbork Synthesis Radio Telescope are presented. Five areas were mapped, covering the galaxy except for the extreme northern and southern parts, at a resolution of δα × δδ × δV = 24″ × 36″ × 3.2 km s−1. The spatial resolution corresponds to 30 × 120 pc at the distance of M31. This is of the same order as the resolution at the distance of the center or our own galaxy given by a 25-m dish. Consequently the M31 survey is comparable to surveys of the Milky Way galaxy in wealth of detail as well as in amount of data (∼ 1 Gigabyte).

New observations made with the Westerbork Synthesis Radio Telescope of M31 in the line of neutral hydrogen (Brinks, this volume) show the presence of high-velocity HI in the inner 4–5 kpc of the galaxy. There were indications from previous 21-cm line studies that HI gas is moving with peculiar velocities (Whitehurst and Roberts, 1972; Unwin, 1979) but the new maps, made with a resolution of δα × δδ × δV = 24″ × 36″ × 8.2 km s−1 and sufficient sensitivity show for the first time the distribution of this material.

21 cm profiles are compared with high resolution long slit Hα spectra. Asymmetries in intensity and velocity gradient in the spectra are apparent in the global profiles. When measured at 0.5 of peak intensity, the 21cm profile width and optical rotation curve match best.

TAURUS, an imaging Fabry-Perot system, was developed as a collaborative project by the Royal Greenwich Observatory and Imperial College London and is capable of obtaining seeing-limited velocity field information over a 9 arcminute field on a 4 m telescope. At the detector (the IPCS) the image of the source is modified by the fringe pattern of the capacitatively stabilized servo-controlled Fabry-Perot. As the Fabry-Perot is scanned, this fringe pattern tracks radially across the field and each pixel of the detector maps out a spectral line profile within the bandpass of the “blocking” interference filter. At each F-P spacing a picture (Typically 256 × 256 pixels) is recorded on computer disk. 100 pictures make up a complete scan, covering for example 1,200 km sec−1 free spectral range (this range in practice depends on the particular etalon). In this manner a 3 D data cube of the field is built up where (X, Y, Z) are typically (256 pixels, 256 pixels, 100 steps).

IC 10 is seen optically as a patchy nebulosity, approximately 3 × 4 arcmin in size, possibly heavily obscured due to its low (−3°) Galactic latitude. De Vaucouleurs and Freeman (1972) have classified it as a Magellanic barred irregular. With a heliocentric radial velocity of −350 km/sec, its distance is uncertain, and various investigators have placed it as far away as 3, and as near as 1 Mpc (Bottinelli et al., 1972; Roberts, 1962). For the latter value, IC 10 would be a Local Group member.

The use of millimeter wave CO emission as a tracer of molecular hydrogen in the Galaxy (Scoville and Solomon 1975) showed that most of the H2 unlike HI is concentrated in the inner part of the Galaxy in a “ring” between 4–8 kpc and in the inner 1 kpc. Subsequent surveys (Gordon and Burton 1976, Cohen and Thaddeus 1977, Solomon etal. 1979) confirmed this picture with more extensive data. The molecular interstellar medium was shown to be dominated by Giant Molecular Clouds with individual masses between 105 and 3·106M⊙ (Solomon etal. 1979, Solomon and Sanders 1980). The GMC's confined to a layer with a half thickness of only 60 pc are an important component of the galactic disk, and the most massive objects in the galaxy. They affect the dynamics of the disk by contributing significantly to the surface density and through their individual gravitational interactions with stars.

The CO rotation curve exhibits an increase in circular velocity beyond R = 12 kpc (Blitz, Fich, and Stark 1980) which may be as much as 50 km s−1 at R = 18 kpc. Recently, Blitz and Fich (1983) have examined the uncertainties in the rotation curve at large R and have concluded that : 1) Of all the uncertainties, small changes in Ro and θo have the most serious effect on the rise at large R. However, unless ωo < 20 km s−1 kpc−1, a value smaller than that accepted by all observers, the rotation curve rises beyond R = 12 kpc. 2) Systematic errors in stellar distances and non-circular motions might have an effect on the magnitude of the rise. Both effects are thought to be small, but probably work to make the rotation curve even steeper. 3) The global value of the Oort A constant is < 12.5 km s−1 kpc−1, and the local value of 15 km s−1 kpc−1 is most likely due to a local velocity perturbation.

The kinetic temperatures in molecular clouds are usually considered to range upward from about 10 K (e.g., Dickman 1975). These temperatures are generally measured by observing the CO J = 1 → 0 transition and assuming that this line is optically thick and thermalized. This assumption also underlies estimates of the total mass and distribution of molecular material in our galaxy based on CO surveys. Because a significant amount of molecular material could in principle be missed by galactic CO surveys, a search was undertaken for “ultra-cold” molecular gas, by which is meant an excitation temperature, Tex < 5 K. No evidence was found for a large amount of such material (Evans, Rubin, and Zuckerman 1980), but many clouds with Tex between 5 and 10 K were found. To determine if this low Tex is due to low kinetic temperature, low density, or low CO abundance, we have undertaken observations of a large number of clouds in the J = 2 → 1 CO line and the J = 1 → 0 13CO and CO lines. These observations will be analyzed to determine the properties of these clouds.

The galactic nucleus contains the largest concentration of dense molecular clouds in the Galaxy. A new survey of the region has been carried out recently at Jodrell Bank in the 1667 and 1665 MHz lines of OH (Cohen 1981). The OH lines appear primarily in absorption against the galactic continuum background. The survey reveals many new clouds within the previously known nuclear concentration, and also a more extensive outer distribution of clouds extending to projected distances of ±1 kpc from the centre. The outer part of the distribution is tilted with respect to the galactic plane, reaching z-distances of −200 pc at positive longitudes and +200 pc at negative longitudes. The same tilt or warp has been seen in the high-velocity HI “nuclear disk”.

A large observational program investigating the 2.6 mm CO line in spiral galaxies is being conducted by myself and Nick Scoville using the 14 m telescope of the Five College Radio Astronomy Observatory (HPBW = 50″). Thus far we have observed 46 galaxies of types Sa, Sb, Sc and Irr, detected 31, and mapped 16. Our major findings are:

1. (1) In several late type spiral galaxies (IC 342, NGC 6946 and M51) the radial distribution of molecular gas out to 10 kpc follows the exponential blue luminosity profile of the disk within each galaxy (Young and Scoville 1982a, Scoville and Young 1983).

2. (2) From a comparison of the CO and B luminosities of the central 5 kpc in a sample of Sc galaxies, we find that the blue luminosity is proportional to the first power of the CO content (Young and Scoville 1982b). We interpret this to mean that the star formation rate per H2in Sc galaxies (indicated by the B luminosity) is constant.

3. (3) No molecular rings like the one in the Milky Way at radii 4 to 8 kpc were seen in the Sc galaxies.

4. (4) We have found molecular rings in two Sb galaxies, NGC 7331 and NGC 2841, with peaks at radii of 4–5 kpc (Young and Scoville 1982c). The central holes in the CO distributions are possibly related to the presence of large nuclear bulges in these galaxies.

We present observations, which are part of an ongoing investigation, of the CO (J=1–0) emission in the spiral galaxy M51. The spectra were obtained in a beamswitched on-on mode with the Onsala 20 m antenna (beam size ~33″), equipped with a cooled mixer and a 512 ×1 MHz multichannel receiver, and are shown in Figure 1. The inset diagram shows the observed positions superposed on the optical outline of the galaxy. With the present signal-to-noise ratio there is no evidence for an arm-interarm intensity contrast. This is even more apparent in integrated intensity. This result agrees with the lower resolution findings of Rickard et al. (1981). We have observed 13CO in one position (22″ south of the center). The 13CO to 12CO ratio, ∼0.1, agrees with Bell results from observations with a 1:7 beam (Encrenaz et al. 1979).

A considerable fraction of the neutral hydrogen in spiral galaxies is generally found at large radii and beyond the optical image. This outlying gas either forms an extension of the stellar disk and of the inner HI layer, or, is concentrated in a ring. It shows well ordered motion around the galaxy, although the plane of the orbits is generally inclined with respect to the inner disk. The study of this gas is important for at least three reasons: i) although it represents only a very small fraction of the total mass it can be used as a tracer of the kinematics to determine the total mass and the mass distribution of the system, ii) it contributes to the galactic cross-sections needed to explain QSO absorption lines, iii) its physical and chemical properties may throw some light on the formation of galactic disks and their evolution.

The rotation curve of the edge-on disk galaxy NGC 5907 can be interpreted in terms of a two-component model, consisting of a truncated exponential disk and of a spherical halo. About 60% of the total mass inside the edge of the disk turns out to be in the halo. Indicative masses for disk and halo are 9 and 13.5·1010 solar masses respectively.

Deep red Schmidt plates (127−04 + RG630) of the galaxy M83 show the presence of extensive faint optical structure well beyond the Holmberg radius of the galaxy. Digital processing has been used to bring these features out in detail. They consist of arc-like structures and clumps of H II regions, which seem to follow the H I distribution and thus are likely to participate in the warping of the H I disk.

The main plane of a late type galaxy is best determined by its population I components : young stars, gas and dust. In the case of M82 the determination of its principal plane is complicated by its very irregular appearance and the presence of scattered light. We have used three-colour UBV-photometry of this galaxy to determine the stellar population, using the extinction- free parameter Q = (U-B) − 0.65(B-V), and the extinction AB = 4 ((B-V) − (B-V)oo; (B-V)oo being the intrinsic colour of the population, taken, e.g. from population synthesis calculations. Corrections for scattered light were made using the degree of polarization as well as the excess ultraviolet light (see Notni and Bronkalla, 1983; Notni et al., 1981 for details).

Most studies of the mass distribution in spiral galaxies have been based on the observed rotation curves. A serious ambiguity in this approach has always been that the rotation curve contains in itself no information on the mass distribution in the direction perpendicular to the galactic plane. The usual assumption has been that the mass in late type galaxies is distributed as the light, namely outside the central bulge in a highly flattened disk. In recent years it has been found that the rotation curves decline little or not at all, indicating large increases in the local value of M/L with increasing galactocentric radius (e.g. Bosma and van der Kruit, 1979). On the basis of dynamical arguments involving stability it has been suspected that the material giving rise to the large values of M/L - the “dark matter” - is distributed in the halos of these galaxies, so that the assumption of a flat mass distribution would have to be wrong.