The discovery of the 21-centimeter line of neutral hydrogen by Ewen and Purcell (1951) and the subsequent confirmation of this dis- covery by Dutch and Australian groups (Muller and Oort, 1951; Pawsey, 1951) marked the birth of a new era in the study of our Milky Way system and of the universe of galaxies.
That radio radiation with a wavelength of 21 cm emanating from neutral atomic interstellar hydrogen might be sufficiently strong to be recorded was first suggested by van de Hulst (1945) and was predicted independently by Shklovsky (1949). But not until the actual discovery of the line did astronomers begin to realize how powerful a tool for future research had become available.
Research in the 21-cm field may at present be divided roughly into three broad areas.
The first concerns the spiral structure of our galaxy, and has yielded spectacular first results.
Really detailed analysis, however, has only begun, and will undoubtedly be fruitful in future studies of spiral structure. The second category is that of related optical and radio studies of the fine structure of the interstellar gaseous medium in our galaxy. We shall list below some of the topics of research most important here, but it is already clear that the
"21-cm astrophysicist" has waiting for him a multitude of specific research problems. The third category is that of extragalactic 21-cm research, one in which the work has barely started but which obviously will attain increas- ing importance as the reach of our equipment increases.
One should bear in mind that up to the present all but one of the five principal radio telescopes that are active in the 21-cm field are paraboloids with apertures of the order of 25 to 30 feet, the one exception being the 50-foot
1 Harvard College Observatory, Cambridge, Mass., and Common- wealth Observatory, Canberra, Australia.
paraboloid at the Naval Research Laboratory in Washington. Also, most of the work until now has been done with electronic equipment with bandwidths of 40 and 15 kc/sec (equivalent to resolutions in radial velocity of 8 and 3 km/sec) and very few observations have been made with bandwidths between 5 and 10 kc/sec (equivalent to radial velocity resolu- tions of 1 to 2 km/sec). Within a year or so there should be in operation at least a half dozen radio telescopes with apertures of 60 to 100 feet, all of them steerable and with surface tolerances sufficiently strict to permit full use at the 21-cm wavelength. Research at 21 cen- timeters has just been started with two large radio telescopes—the 83-foot instrument at Dwingelo in Holland and the 60-foot George R.
Agassiz radio telescope of Harvard Observatory (see the recent article by Heeschen, in press).
Several of these instruments apparently will have electronic equipment capable of record- ing 21-cm profiles with a bandwidth of 5 kc/sec (equivalent to a resolution in radial velocity of 1 km/sec). The group at the California Insti- tute of Technology is planning to use two 90- foot antennas as an interferometer with variable separation of the two component antennas.
Moreover, three very much larger paraboloids intended in part for use in 21-cm research are now either under construction or on the draw- ing board: the 250-foot paraboloid for Jodrell Bank, the 140-foot paraboloid for the Inter- University Radio Observatory now under con- sideration by the National Science Foundation, and the large instrument planned for Australia.
Spiral structure of our galaxy
The Leiden group has contributed most effectively to the study of the spiral structure of our galaxy. The early work by the Leiden group culminated in the famous joint paper by van de Hulst, Muller, and Oort (1954) which
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142 SMITHSONIAN CONTRIBUTIONS TO ASTROPHYSICS
contains the picture of the spiral structure of our galaxy in its broad features as observable from a Northern Hemisphere radio observatory.
For the Southern Hemisphere a preliminary survey has been completed by Christiansen and Hindman (1952), and the most recent results were presented by Carpenter (1955) at the Troy meeting of the American Astro- nomical Society. Since the original Dutch survey was completed, there has been con- siderable activity in the study of the spiral structure for the part of our galaxy between galactic longitudes 40° and 200°; that is, from Cygnus to well beyond Sirius. The most extensive research has been done by Westerhout (in press), who has derived and analyzed 21-cm profiles for the entire section between galactic latitudes —10° and +10°. To supplement the work of Westerhout, we have the published and unpublished results of Matthews (1955) and of Davis at Harvard and of Heifer and Tatel (1955) at the Carnegie Institution. All these researches indicate that the simplified descriptions of three primary spiral arms in this section, the Orion Arm, the Perseus Arm, and the Distant Arm, are only very rough first ap- proximations. The available evidence sug- gests major branching of the principal spiral features; it is evident, furthermore, that all of the spiral features are definitely not precisely in the same plane. Comparable detailed studies for the Southern Hemisphere are still lacking, but the first results by the Australian groups suggest that here, also, complexity rather than simplicity are to be found.
The study of the spiral structure in the part of our galaxy interior to our sun offers a variety of complex problems. The principal source of difficulty is that, even on the simple assumption of purely circular galactic rotation, a feature with a given radial velocity may be produced by hydrogen clouds at either one or another distance; or, more generally, that a particular feature may well be the blurred combined impression produced by features at two different distances from the sun. The early results obtained by the Leiden group have been summarized in a paper by Kwee, Muller, and Westerhout (1954); and, more recently, Schmidt (in press) has obtained useful new evidence. At Harvard, Heeschen and Howard (1956) have
begun a study rather similar in scope to that of Schmidt. It seems established that one Inner Arm is present which presumably is identical with the optically discovered Sagittarius Arm, but further evidence exists for the presence of some structure in the internal neutral-hydrogen conglomerate, close to the center of our galaxy.
Two major problems arise in any analysis of spiral features from 21-cm profiles. First, a large group of problems relates to the probable shape of the curve that gives circular velocity of rotation as a function of distance from the galactic center. In 21-cm research we have no direct way of obtaining distances, and we can only assign to a given observed feature a dis- tance based on its observed radial velocity of approach or recession. If our assumed form for the curve relating distance from the galactic center to circular velocity of rotation is in error, then all of our derived distances obvi- ously will be in error also. A second and related cause for difficulties lies in the fact that, on the basis of 21-cm observations alone, we have no way of disentangling effects of peculiar motions from those caused by purely circular galactic rotation.
We should be able to outline on a fairly homogeneous basis the general features of the spiral structure of our galaxy once the southern Milky Way observations are completed and analyzed as have been the profiles for the north- ern Milky Way by the Leiden observers.
Great interest attaches to the future clarifica- tion of the details of spiral structure, but here progress will be slow at best, and, initially at least, somewhat uncertain. We have already indicated that the dynamical study of the law of circular galactic rotation is imperative if we wish to derive meaningful distances for the features found through our 21-cm observations.
To deal conclusively with problems of the branching of spiral arms and with fine structure phenomena, our only hopeful approach appears to be through coordinated optical and radio studies of sections of the Milky Way, one by one. For proper analysis of 21-cm features, we shall need extensive and precise optical data—for example, for OB stars and associations in the area of the sky covered by a particular 21-cm study. For proper interpretation of the 21-cm features, we should at all times attempt
• a t NEW HORIZONS IN ASTRONOMY 143 to obtain radio profiles for a fairly wide range
of galactic latitudes and for points in the area of investigation spaced as close together as appears to be reasonable for a particular radio telescope. Preliminary work in this field by Matthews (1955) has already demonstrated that the results of closely spaced surveys will be well worth the effort, and that correlated radio and optical studies will provide insight into the fine details of spiral structure. Fur- thermore, a great deal of new information should be obtainable regarding the fine structure of the interstellar medium and the association between interstellar hydrogen and certain classes of stars. A related problem of great interest is the relative distribution of neutral hydrogen and cosmic dust inside a spiral arm, or branch of an arm.
Astrophysical studies
The 21-cm astrophysicist is essentially a spectroscopist who has for study only one single spectrum line. Fortunately, the micro- wave spectrographs presently in use are really high-dispersion instruments and thus much can be learned from a single 21-cm profile.
The 21-cm astrophysicist realizes that he is the fortunate fellow who has first achieved access to the all-pervading medium of neutral hydrogen of our galactic system, which means probably as much as 95 percent of all the interstellar hydrogen. In this section, we shall describe first some astrophysical problems that have proved practicable for paraboloid anten- nas with apertures of 20 to 30 feet, then turn to problems whose solution is aided by antennas with apertures of 50 to 80 feet, and conclude with some astrophysical problems that we may attempt to study with antennas of 100-foot and larger apertures.
The work of Lilley (1955) and of Heeschen (1955) at Agassiz Station illustrates the first group—astrophysical problems that may be studied with antennas of relatively small aperture. For the Taurus Complex of dark nebulosity, Lilley found generally far stronger signals for fields well inside the dark complex than for relatively unobscured fields bordering upon the complex. It thus became evident that the large dust complexes have excess neutral hydrogen gas associated with them.
Lilley found the ratio between the average densities of gas and of dust to be close to 100:1 inside the Taurus Complex. Lawrence, Menon, and I (Bok et al., 1955) used the 24- foot paraboloid to study some very densely obscured spots inside the Ophiuchus and Taurus Complexes. We found no marked differences in 21-cm signal strength when we compared intensities for the highly obscured fields with those found for neighboring fields still inside the complexes but definitely less obscured. We deduced from our observations that the large dust complexes are regions of markedly high concentration of interstellar atomic neutral hydrogen, but that the dis- tribution of the atomic hydrogen inside the complex does not mirror precisely that of the cosmic dust. Existing 21-cm observations suggest that the neutral hydrogen is more evenly distributed than the cosmic dust, but no really significant conclusions can be drawn until the really large paraboloids become avail- able for studies of this nature. A careful search for neutral hydrogen concentrations by 21-cm interferometer techniques should prove of great interest in connection with this prob- lem; a negative result would be fully as im- portant as a positive one. When the necessary observations have been made, their interpre- tation will still present a good many difficulties.
Our lack of information regarding the presence of molecular hydrogen complicates analysis considerably, and as yet the possible effects of variations in temperature inside the large dust complexes are by no means understood.
The assumption that the interstellar medium of neutral atomic hydrogen possesses one single characteristic temperature is at best a very crude one, and in future research temperature variations undoubtedly will have to be con- sidered along with density variations.
A second area in which 21-cm studies have already made important contributions is that of the study—by combined optical and radio techniques—of regions of special interest, such as the Orion region. The recent work by Menon (in press) illustrates how much we can learn through detailed 21-cm studies. Op- tically, the Orion region is known by its separate features: the Great Nebula in Orion, with a total estimated mass not in excess of
144 SMITHSONIAN CONTRIBUTIONS TO ASTROPHYSICS
1,000 solar masses; the Trapezium Cluster and various OB associations and aggregates;
Barnard's Luminous Arc of very great dimen- sions; and, finally, the large dust complex that is evidently obscuring much of the region. The 21-cm observations by Menon show that all of these features are imbedded in a large "bowl of neutral hydrogen jelly," with a total mass probably of the order of 60 to 100 thousand solar masses. Menon's observations suggest that this entire mass is in slow rotation and that it is, very likely, expanding at the rate of 8 to 10 km/sec. Here the 21-cm approach has already added a dimension to our earlier un- related optical studies. The astrophysical and cosmogonical importance of the new approach is obvious.
The third group of problems that are proving amenable to study by 21-cm techniques deals with the fine structure of the interstellar hydrogen medium. The studies by Lawrence (in press) and by Heeschen and Drake (1956) serve to illustrate two different approaches.
Lawrence, working in close collaboration with Munch, has studied at the Agassiz Station 21-cm profiles for fields centered upon stars for which Munch, at Mount Wilson and Palomar, had found multiple optical interstellar lines.
Two established coincidences between radial velocities of 21-cm features and strong inter- stellar absorption lines indicate that the same interstellar gas clouds may be responsible.
Further study of these phenomena should prove exceedingly rewarding, but we can hope to arrive at a clear understanding through 21-cm studies only with radio telescopes of large apertures with associated electronic equipment of very narrow bandwidth. The research of Heeschen and Drake refers to the general direction of the Pleiades Cluster. They find a double-peaked 21-cm profile—the radial veloc- ity of one peak coinciding almost precisely with that of the Pleiades stars, and that of the other peak being very nearly equal to the mean radial velocity for the strongest components of the interstellar lines. In all these investigations of the fine structure of the interstellar medium, one must insist on high angular resolution ac- companied by narrow bandwidth; without narrow bandwidth, the whole study of the distribution of individual gas-cloud velocities
becomes impracticable. Once we have per- fected our equipment, success in breaking down the interstellar medium into its basic compo- nents should be within our grasp.
The greatest contribution so far to 21-cm research by the 50-foot paraboloid of the Naval Research Laboratory has been in the study of absorption features observed in distant discrete radio sources. Hagen, Haddock, McClain and Lilley (Hagen, et al., 1955; Lilley, 1955) have studied 21-cm profiles and related phenomena in the continuum at wavelengths of 21 cm and less for a number of discrete sources, some of thermal origin, others not. The first relevant 21-cm profiles have exhibited certain amazing absorption features which tend to confirm the cloud or turbulent nature of the interstellar gaseous medium. For the future study of these profiles it will prove profitable to insist on the highest attainable angular resolution ac- companied by very narrow bandwidth of the recording equipment. Almost every one of these sources studied has presented us with a different type of problem. The neatest ab- sorption features are probably shown by the Cassiopeia source, and it seems difficult to escape the conclusion from the 21-cm observa- tions by the Naval Research group and by Davies (in press) and his associates at Jodrell Bank that the source is in the Perseus Arm rather than within 1,000 parsecs of the sun. The study of the source at or near the galactic center—or at least in the direction between our sun and the galactic center—offers a host of possibilities for future research and here very much greater angular resolution than is attain- able to date is urgently required. We shall return in the next section to the Cygnus source, which exhibits absorption features that origi- nate far beyond the bounds of our own galaxy.
But the brief references that we have given above indicate that 21-cm techniques offer great opportunities for future research. The contemplated interferometer for 21-cm re- search, with 2-component 90-foot steerable paraboloids, should make possible an entirely new approach to the study of discrete sources and their absorption features. Large parabo- loid antennas with apertures in excess of 100 feet also promise very significant contributions.
In cases where the 24-foot antenna is useless, the
NEW HORIZONS IN ASTRONOMY 145 50-foot antenna begins to suggest; but the 100-
foot antenna should be able to show the way and the larger antennas should really be able to solve some of the critical galactic astrophysical problems of 21-cm research that are ahead of us.
Extragalactic research
To the Australian group at Sydney (Kerr, Hindman, and Robinson, 1954) and their associates (Kerr and de Vaucouleurs, 1956) goes the credit for having detected and studied the first 21-cm radiation from beyond our own Milky Way system. Their studies of the Magellanic Clouds, which have been blended skillfully with the optical researches carried on at Canberra (Buscombe, Gascoigne, and de Vaucouleurs, 1954), have demonstrated con- clusively that the 21-cm technique offers entirely new possibilities for the study of galaxies beyond our own Milky Way system.
These studies have already made important contributions both to the dynamics of the Magellanic Clouds and to our knowledge of population distributions. Until it was found that the 21-cm signals from the Small and from the Large Magellanic Clouds are comparable in strength, many had doubted that the Small Cloud had anywhere nearly as much neutral hydrogen as the Large Cloud. Through the 21-cm study we are learning a great deal about the dynamics of the two Clouds. Their rota- tion seems well established, and the masses derived from rotational velocities are in good agreement with those estimated in other ways.
Studies of the distribution of random velocities promise to add materially to our knowledge of the dynamics of the gaseous interstellar medium in these two galaxies.
The second big contribution to date to the study of radiation from outside our galaxy was made by Lilley and McClain (1955) in their work on the principal absorption feature in the discrete source in Cygnus. They have established that an absorption feature in the spectrum of the Cygnus source is present at precisely the frequency predicted from the optically known velocity of recession, produced by the red-shift. The absorption feature obviously originates in the galaxy inside which the Cygnus source itself is imbedded. Not only is the work of Lilley and McClain im-
portant in establishing the presence of a 21-cm neutral hydrogen medium surrounding the Cygnus discrete source, but the agreement between the optically predicted red-shift veloc- ity and the radial velocity determined by radio techniques has important consequences for cosmogonical theory.
The 21-cm radiation from the Andromeda Nebula and other nearby galaxies remains as yet undiscovered. Special equipment to aid in its detection and measurement is now under construction in Holland and elsewhere. The importance of 21-cm studies for individual galaxies of the Local Group can hardly be over- rated. If we can attain angular resolution sufficient to separate areas inside spiral arms from those between the arms, very significant contributions can be made to the solution of the whole group of problems related to the dynamics of galactic rotation and of spiral structure. We shall then have also wholly new evidence on the distribution of neutral atomic hydrogen in spiral and other galaxies and new light will be thrown on the problems of stellar birth and evolution. For the future, 21-cm research of nearby galaxies, notably those with spiral structure, has a high priority.
Quite recently Heeschen (in press) has suc- ceeded in detecting 21-cm radiation associated with a cluster of galaxies. At a frequency near 1387 mc/sec, corresponding to a recessional radial velocity of close to 7000 km/sec, he found evi- dence for a weak but consistently present signal (&T=2°K±0.°2) from the direction of the Coma cluster of galaxies. The specifically determined radial velocity of recession for the same cluster is 6680 km/sec, which appears to indicate satisfactory agreement. The disper- sion in radial velocity derived from the 21-cm observations appears to be of the order of 500 km/sec, somewhat smaller than the optical value of about 800 km/sec. The total estimated mass of neutral atomic hydrogen is a little less than 10" solar masses. Heeschen's result was ob- tained with the 24-foot radio telescope at Agassiz Station. Information regarding the distribution of neutral hydrogen inside the cluster can be obtained only with instruments of greater aperture, which afford higher angular resolution.
A potentially very profitable approach to the