Since 1964 we have been observing 21-cm line profiles in a new survey of the neutral hydrogen distribution in the neighborhood of the galactic plane with the 300-foot radio telescope of the National Radio Astronomy Observatory in Green Bank, W.Va. This is the largest telescope available for 21-cm line work; it has a beamwidth of 10 min of arc and is equipped with an excellent line receiver. Since it seems unlikely that an extensive hydrogen-line survey will be made with any larger telescope, we felt that for reference purposes a concerted effort should be made to obtain as many 21-cm data as possible pertaining to the structure of the Galaxy with this telescope. The data have been presented in the form of contour maps giving the intensity of the 21-cm line radiation as a function of right-ascension and velocity at constant declination. A series of contour maps was distributed to the astronomical community in 1966 as the first edition of the Maryland–Green Bank Galactic 21-cm Line Survey. The second edition, containing 1200 pages and approximately 1800 maps, was distributed in the summer and fall of 1969. It is expected that additional contour maps, completing the survey as originally planned, covering a latitude range from bII = +1° to −1°, lII = 11° to 235° (bII = +3° to −3° between lII = 100° and 145°), will be finished by the summer of 1970. Scans were made across the galactic equator with a stationary telescope, so that the declination is constant through each scan; the declination intervals varied from 4 to 6 min of arc. Eventually, we plan to cover a strip from bII = +5° to −5° between lII = 11° and 235°, containing 225000 independent points at intervals of 6 min of arc, with an effective beamwidth of 12.5 min of arc, a velocity resolution of 2 km s-1, and a total of 1.2 × 108 individual intensities.

The extensive Hat Creek survey of neutral hydrogen combined with southern observations provides the basis for a new discussion of the spiral structure of the galaxy. The purpose of this investigation is to provide a general picture of the galaxy. It is found that the pitch of the spiral arms is approximately 12°.5 and that there are many spurs and interarm features as we observe in external galaxies.

The sun is not located in a major spiral arm, but rather in a spur or offshoot originating near or at the Sagittarius arm, which is a major structure in the galaxy. The young stars in the general vicinity of the sun delineate this spur, not a major arm structure. The stars and the gas are in agreement in indicating a large pitch angle (20°–25°) for this local structure, which differs from the smaller pitch angle for the arms which form the system as a whole.

In the presentation a computer-produced movie of the galaxy based on Hat Creek hydrogen contour maps similar to those in Figure 1 was shown. It was used to illustrate generally the complexity of the gas structure and, in particular, to show (i) observational aspects of the spur in which the sun is located and (ii) the point of origin of the so-called Perseus arm.

The CSIRO 11 cm continuum survey of the galactic plane from lII = 280° to 37° has recently been finished. I have used this survey to compare the number of sources in various 5° intervals north and south of the galactic center. The NRAO-CSIRO-MIT H 109α line survey (Wilson et al., 1970) found 39 sources lying between 330° and 340°, while the NRAO-MIT (Reifenstein et al., 1970) survey found only 14 between 20° and 30°. If the galaxy were symmetric, one would expect roughly equal numbers. One suspects a selection effect is present, and a rough check is very worthwhile. The CSIRO survey is particularly valuable since it is the highest resolution survey covering large portions of the galaxy on both sides of the center.

Neutral hydrogen concentrations were studied in the region between 48° and 200° longitude (new) and +6° and +20° latitude. Gas associated with individual arms beyond the sun has been found up to distances from 1 to 2 kpc from the galactic plane with an average density between 1 and 2% of that in the arm centres. The gas with the highest negative velocities shows a different behaviour. This has no counterpart in the plane, and it shows little or no concentration towards lower latitudes. Contrary to the hydrogen associated with the arms its distribution is highly asymmetrical. It does not seem to exist at negative latitudes. It may be of a similar origin as the high-velocity gas in high galactic latitudes near the sun.

We have studied the rotation curve of the Galaxy at different heights below and above the equator. In the course of this work we noticed that the maximum brightness temperature of hydrogen oscillates around the galactic plane following a fairly sinusoidal pattern. It is further noticed that the maximum temperature of hydrogen occurs right on the plane in the regions where the rotation curve has a form indicating solid body rotation. A rotation curve based on points of maximum hydrogen temperature does not differ appreciably from a rotation curve measured on the galactic plane.

A survey of the distribution of the intensity of the 21 cm-line of atomic hydrogen in the region with galactic coordinates 270° ≤ lII ≤ 310°, −3° ≤ bII ≤ 2°, was made with the 100-foot radiotelescope and 56-channel receiver of the Radio Astronomy Station of the Instituto Argentino de Radioastronomía and the Department of Terrestrial Magnetism of the Carnegie Institution of Washington. The angular and velocity resolution of the system is 28′ of arc and 2 km s−1.

The observations discussed in this paper were obtained with the 30-m telescope of the Instituto Argentino de Radioastronomía.

A sky survey of the 21 cm hydrogen line has been made with the 100-foot Radiotelescope of the I.A.R.-C.I.W. Radio Astronomy Station in the region 230° ≤ lII ≤ 280°, −15° ≤ bII ≤ −3°.

We study the distribution of the local hydrogen and that in the Orion, Intermediate and Perseus arms. We find a new structure that starts at lII = 265°. We also study the concentrations which lie far below the plane; some of them seem to be related to Lindblad's G arm.

An observational material of 21-cm H I emission-line profiles is investigated by a statistical method to derive the kinematical properties of the interstellar gas in the region of the galactic anticenter. A description of the method used as well as the results obtained, concerning deviations from a circular rotation, are given.

The observations were made with the large Pulkovo radiotelescope (beamwidth 7′, bandwidth 20 and 10 kHz). Figure 1 contains our drift curves across the cluster NGC 2264. The details on them may be identified with Raimond's (1966) clouds, but the proofs for the reality of the connection between the cluster and the clouds are not very reliable. The cloud ‘b’ is believed to be connected with NGC 2264 because of their close neighborhood. But on the other side of the galactic plane in this region we have found a bright and narrow (∼1°) detail, whose middle falls exactly on the western border of the H II region around the star λ Orionis (Figure 2). Figure 3 contains the computed drift curves at different velocities for Δf = 20 kHz according to the model of the neutral hydrogen expanding envelope proposed by Wade (1957) around the H II region λ Ori. Figure 4 represents our observations. Although some details, in particular the narrow detail at the velocity −1.4 km s−1, are very similar to the calculated ones, our observations differ from Wade's model in that they do not have any velocity symmetry relative to zero; in fact the details are situated only at the positive side. This fact may eliminate one half of the proposed envelope. There is no symmetry relative to the central star and no coincidence of the velocity of the narrowest detail with the velocities of the central star and nebula.

Young clusters of O type and the corresponding stellar associations, together with the associated hydrogen complexes, make up the spiral arms of our Galaxy. Thus the investigation of the distribution of hydrogen and its relation to the clusters is of definite interest.

I have studied the galactic longitude distribution of low latitude pulsars on the assumption that pulsars are evenly distributed in the galactic disk but that their observability is severely impaired by spiral features, rich in H II regions, in which the high electron density causes a high dispersion measure for a comparatively short optical path. The assumption of even distribution in the galactic disk seems natural as no correlation between positions of pulsars and spiral tracers is known, and since supernovae in external galaxies are not known to confine to spiral arms.

The distances of all pulsars are calculated on the assumption that they are immersed in a uniform medium of average electron density 0.06 cm−3. It then appears that the pulsars are concentrated towards the local and Sagittarius spiral features and that their mean height above the plane is consistent with that of known supernova remnants. The mean distances appear to be approximately correct, but individual distances are uncertain by about a factor of two. Evidence from radio continuum results supports this model of the ionized interstellar medium.

Weighted average values of the ionization ratio ne/nH for the interstellar gas can be obtained from a comparison of pulsar data and 21-cm emission measurements. For high latitude pulsars this procedure is straight forward and reasonably free from assumptions. The resulting ionization ratios are high (ne/nH = 0.39). If this is compared with values as given by the theory of ionization and heating of interstellar gas by subcosmic rays, temperatures for the ‘neutral’ gas above 104 K are obtained. For low latitude pulsars this procedure depends to a much larger extend on the assumed distance of the objects and on the ionization theory. Here the values found are ne/nH = 0.07 and Te = 2000 K.

Aside from the well-known spiral arm tracers such as the OB associations, young galactic clusters, WR stars and possibly the long-period classical cepheids, the more common stars in the neighborhood of the sun within 2 kpc show little or no relationship to the local spiral structure of the galaxy.

Attention is drawn to a few aspects of the state of motion of the local population which may become of importance for the study of local spiral structure. Uncertainties in the present knowledge of the local standard of rest are discussed, (a) with regard to the possible outward or inward motion with respect to the galactic centre, and (b) with regard to the component in the direction of circular motion. We furthermore draw attention to the quiet state of motion of the local interstellar gas and to the contrasting fairly high group velocities among the recently formed OB associations.

The galactic clusters and the exciting stars of H II regions are powerful indicators of the spiral structure since these objects lie on or very close to the zero age main sequence (ZAMS). For clusters this behaviour can be confirmed by direct photometric observation. The exciting stars of H II regions are probably so young as not to have evolved from the ZAMS. The coincidence of these objects with the ZAMS indicates that there is little scattering in the absolute magnitudes and, as a consequence, little uncertainty in the determination of distance. This fact reveals itself very distinctly in the case of H II regions, whose exciting stars define the spiral structure considerably better than the stars of the same early spectral types but not connected with H II regions.

From 6000 optical radial velocities of H II regions a new spiral structure (4 arms of pitch angle 20°) is found. The radial velocities of the observed H II regions are the same with the velocities of the H I regions. The kinematics of H II regions is similar to that of Cepheids and B stars.

The apparent shapes and orientations of optical nebulae in the Cygnus X complex provide possible evidence for the existence of a symmetry in the local spiral arm which may be related to the structure of the local magnetic field within the spiral arm. We have made a preliminary determination of the distances to about 90 nebulae in the Cygnus X complex by use of the values of interstellar absorption as a function of galactic coordinates. These values of absorption were determined from a comparison of optical and radio data for the nebulae. The more prominent nebulae are clumped at a distance of about 1.5 kpc. The total range in distances is from 1 kpc to at least 4 kpc. We have attempted to fit model spiral arms to this three-dimensional distribution of nebulae by approximating the spiral arm with a truncated cylinder. It has been possible to narrow the range of permissible orientations and sizes etc. for this local section of the Orion arm.

It is shown that stars embedded in reflection nebulosity may be used as spiral-arm tracers.