RADIO RADIATION FROM THE GALACTIC NUCLEAR REGIONI

By BERNARD F. BURKE Department of Terrestrial Magnetism, Carnegie Institution of Washington,

Washington, D.C.

INTRODUCTION In the course of his pioneering studies of cosmic radio noise, Karl Jansky

had shown, by 1935 (1, 2, 3), not only that the continuum radio noise emis­sion was concentrated in the sky toward the Milky Way, but also that the greatest intensity appeared to be reached in the direction of the galactic center. His conclusions were confirmed by Reber's early maps (4, 5) which showed the prominent maximum in the direction of Sagittarius very clearly. The special nature of the galactic nucleus was not obvious for some time, probably because of the exciting discoveries that followed one another in such rapid succession during the growth period in radio astronomy that started in the late 1940's, although its discrete nature was recognized in 1951 by Piddington & Minnett (6), who gave it the designation Sagittarius A, a term that has continued to be used, although with modifications as will be seen below. The most fruitful advances have generally occurred when optical and radio observations complement each other, but the heavily obscuring dust clouds that lie in the plane of the Milky Way have effectively blocked the view of the optical observer.

Under such circumstances, the only recourse is to compare observations of external galactic systems, and in particular to study those galaxies that most closely resemble our own. Most spiral systems, when photographed with short exposures, display quasi-stellar nuclei that presumably are densely packed systems of stars. The central regions of spiral galaxies are composed mainly of older, well evolved stars, as emphasized by Baade (7), and the nu­cleus would presumably be a concentration of such stars. Not all galactic nuclei are passive concentrations of stars, however, for Seyfert (8) pointed out that there are galaxies whose nuclei do not exhibit typical stellar spectra, but instead show strong emission lines characteristic of gaseous nebulae. The [0 II] doublet 3727 is particularly strong in the Seyfert galaxies, and shows broadening corresponding to motions of several thousand kilometers per second in some cases. Such galaxies are often unusually strong radio galaxies (unlike our own galaxy, which is a "normal" galaxy in terms of its total radio noise production) . The presence of strong emission lines, however, does not guarantee that a Seyfert galaxy is a radio galaxy, for there are those, such as NGC 4593, which are normal radio galaxies. Since we have, as yet, no way of

1 The survey of literature for this review was concluded in November 1964. 275

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276 BURKE telling whether or not our galaxy is of the Seyfert type, astrophysical condi­tions in the galactic nucleus must be deduced from radio observations alone.

The principal astrophysical measurements that can be made by radio techniques fall into two categories, continuum studies and radio spectros­copy. The continuum radiation was, as noted, the first area of study and consists, in principle, of the determination of the radio noise intensity as a function of frequency and of position in the sky. The detection of the radio source Sagittarius A is not difficult at low frequencies, because it is one of the most intense radio sources in the sky, but the detailed structure of the radio source is not easy to study. The overall size of the source is of the order of a degree of arc, and consequently a resolving power of a few minutes of arc is required to show meaningful detail. Such resolution is only obtainable with large paraboloids at centimeter wavelengths, where detection becomes diffi­cult; and it was not until 1958 that Drake (9), using the newly completed 85-ft Tatel telescope of the National Radio Astronomy Observatory at Green Bank, West Virginia, was able to show that the radio source Sagittarius A is actually a complex of at least five sources. Indeed, this raised the question of whether the positional agreement with the galactic center was merely fortuitous, with Sagittarius A being a chance foreground or background ob­ject. The next section of this review is concerned primarily with the con­tinuum observations, and discusses the possible mechanisms of radio emis­sion.

The first application of radio spectroscopy to the study of the galactic nucleus was begun by the Leiden group under Oort (10). In the course of their survey of 21-cm hydrogen line radiation from the Galaxy, they noted that in the central regions of the Galaxy large Doppler shifts could be seen, implying hydrogen gas motions of several hundred kilometers per second. Hagen, Lilley & McClain (11), at the Naval Research Laboratory, first saw hydrogen gas in absorption against the continuum source Sagittarius A; and in 1958 Rougoor & Oort (12), as part of a careful study of both the emission and absorption lines, showed that the complexity and velocity of the gas motions in the central regions of our galaxy are remarkable. Their conclusions were threefold: firstly, much if not all of the radio source Sagittarius A is located at the center of the Galaxy; secondly, out to a distance of about 3.5 kpc, the hydrogen gas is not only rotating but expanding at high velocity, carrying matter away from the center at a rate of approximately one solar mass per year; thirdly, within a few hundred parsecs of the center there is hydrogen rotating at a circular velocity much greater than one would expect from a simple extrapolation of the velocity law determined for the hydrogen in the outer parts of the Galaxy.

A summary of the present state of radio spectroscopic observations is given in the final section, which also includes a discussion of the recent ob­servations of the OR radical.

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RADIO RADIATION FROM GALACTIC NUCLEAR REGION 277 THE CONTINUUM RADIATION

Theory.-The fundamental measurements of radio brightness distribu­tion have usually been made with large paraboloids, except at the longest wavelengths where adequate angular resolution can only be achieved by array techniques. The receiving equipment measures the surface brightness B, averaged over the antenna directivity G (8, rfJ) to give a measured power per unit bandwidth, at frequency I, B(j):

B(j) = f B(f, El, t/»G(El, cf»dw

where dw is the element of solid angle. The surface brightness is not usually specified in units of power flux per steradian per frequency interval, however, since the received power is most commonly determined by comparing its magnitude with the power received from a resistor at temperature T. It is thus more convenient to express the surface brightness in terms of the tem­perature Tb, of an equivalent black body, whose surface brightness would be (in the Rayleigh-Jeans limit, which is an adequate approximation):

B(f) = 2kTbfN where k is Boltzmann's constant and c is the velocity of light. It follows that the received power P(f) can be instead expressed as apparent antenna tem­perature TA(f). If the surface brightness temperature Tb varies slowly in comparison with the antenna beamwidth, then TA is equal to Th• In the limit of very rapid variations of Tb, as in the case of a source whose angular extent is much smaller than the antenna beamwidth, surface brightness is no longer determined and one can only measure the source flux. In the vicinity of the galactic center both conditions occur.

The interpretation of the brightness temperature, when it can be mea­sured, is straightforward in the case of thermal sources, Le., those sources whose radio emission can be understood in terms of quasi-equilibrium pro­cesses, such as occur in H II regions. If the object is optically thick, the surface brightness is the actual temperature T of the emitting gas; while if the object is optically thin, the surface brightness will be given by the usual relation

Tb(f) = T(1 - e-T(f) where T is the optical depth. For the thermal radio sources the only important continuum process is electron bremsstrahlung, or free-free emission in the hot gas comprising an H II region. The review of Mills (13) summarizes the basic references and results; for a thermal electron velocity distribution the linear absorption coefficient K is given by [a misprint in (13) has been cor­rected] : N 2 [ (P/2)]

" = 9.8 X 10-3 P:2j2 19.8 + In f

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278 . BURKE

In the limit of great optical depth, the surface brightness temperature will of course be To. In the limit of small optical depth the surface brightness will have the same frequency dependence as the emissivity, which is very nearly constant, implying that the brightness temperature will vary asJ2. Thus we can predict the spectrum of an H II region. At low frequencies its brightness temperature will be constant, curving downward at intermediate frequencies and in the limit of very high frequencies asymptotically approaching the in­verse square law. If one wishes to describe the spectrum in terms of source flux instead of brightness temperature, the source flux will increase as the square of the frequency at low frequencies and will approach a flat spectrum, i.e., constant flux, in the high frequency limit.

It has become apparent, however, that there are many sources of radio radiation which cannot be understood in terms of thermal processes. Such sources are classified as "non thermal" and in such cases Tb(f) is only a con­venient measure of surface brightness and does not refer to a physical tem­perature. At present, only one nonthermal mechanism, the synchrotron mechanism, need be discussed for radio sources other than the Sun. The salient features of this mechanism will only be summarized here; the reader may refer to Mills (13) for a more complete summary and to Westfold (14) for a complete analysis.

A highly relativistic electron, of energy 1'moC2, will circulate in a magnetic

field H at an angular frequency of wah, where Wa is the cyclotron frequency of a low energy electron. In contrast to the nonrelativistic case, the radiation is not primarily at the gyrofrequency, but occurs instead in harmonics of the gyrofrequency, peaking sharply at a frequency of 1'3WO. The radiation is strongly linearly polarized, with the E vector perpendicular to the magnetic field, and is highly directional, being strongly confined in the direction of the velocity vector within an angle of approximately 1'-2. Normally, one would expect to find the electrons with a distribution of energies. For an electron energy spectrum NCE) given by a power law relation, of exponent -/3, i.e.:

N(E)dE = (const)�dE the resulting radio spectrum is particularly simple. The volume emissivity '1J/ is given by a power law

rif = f( -y, tl)H L 11[2(1+/l)lj-l/[2(P-l)] where HL is the perpendicular component of magnetic field. Most of the ob­served radio sources have spectra that are adequately represented by such a power law over wide ranges of frequency. The theory is so successful in ex­plaining the observed polarization and intensity of the Crab nebula from the radio region of the spectrum to the ultraviolet that the extrapolation to all radio sources seems to be a reasonable postulate. This hypothesis has the virtue of simplicity, since it only requires magnetic fields and cosmic ray electrons, both of which are known to exist in the stellar and galactic plasmas.

Observations.-The first high resolution study of the galactic center was

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RADIO RADIATION FROM GALACTIC NUCLEAR REGION 279

Oo�_ --·-J--...---�""--�-....:::::��--+=--4�=1:::--..rl�=t=======1 R/

6

FIG. 1. Mills Cross survey of the galactic center at 85.5 Mc/s. Brightness tem­perature given in thousands of degrees K [Mills (15)].

that of Mills (15) , at the relatively low frequency of 85.5 Mc/s, using the Mills Cross of the Radiophysics Laboratory near Sydney. The interpretation of Mills Cross observations is not as straightforward as those made with paraboloids, particularly for extended objects, but the brightness contours that can be constructed with such an instrument have a resolution far ex­ceeding that of physically realizable paraboloids at the longer wavelengths. The 3.5-m brightness contours derived by Mills are depicted in Figure 1 , which gives the brightness temperature as seen with an antenna having a resolution of 0.80 K. The most notable feature is that the position of the radio source Sagittarius A, marked by the x at [1=327.8, b1= -1.4, is ac­tually a saddle point between two maxima of nearly equal intensity. The gen­eral background shows a distinct increase above the galactic disk component

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280 BURKE

over a range of 4° or more in galactic longitude, although the exact division is hard to specify. It is interesting to contrast this picture with the 1390-Mc/s map in Figure 2, made at Leiden by Westerhout (16), in which Sagit­tarius A shows as a distinct maximum at the position of the saddle in the 85.5-Mc/s contours. It will be seen that this is an indicator of the relative importance of thermal and nonthermal radiation in the galactic nuclear region, where both processes seem to be operating. The resolution of the 25-m Leiden telescope is 0.6° at 1390 Mc/s, but no fine structure can be seen in contours, which show Sagittarius A as a smoothly rising, elliptical object whose extent along the galactic equator is at least 4°, as was also the case at 85.5 Mc/s. Confusion problems limit our ability to distinguish the total ex­tent of the radiation from the galactic nuclear region; there are discrete thermal objects such as H II regions and nonthermal sources (probably old supernova remnants) occurring frequently near the plane of the Milky Way, whose chance occurrence in the line of sight can lead to confusion in the interpretation of the galactic center contour diagrams.

A major increase in angular resolution was achieved in 1958, when Drake

h------r----�------+_-----r----�----��_+--_+---- +,

��==���--r_----_r------r_----_r------r_----_r--� +2

t:===�t----j.L--�:.._-=[,L...---=::, �---I----I---_4

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332 324 320 t

f�-J FIG. 2. Leiden continuum survey of the galactic center at 1390 Mc/s. Contour

interval is 3.25° K [Westerhout (16)].

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RADIO RADIATION FROM GALACTIC NUCLEAR REGION 281 (17), using the 85-ft Tatel telescope of the National Radio Astronomy Ob­servatory, constructed the SOOO-Mc/s map with an angular resolution of 6 min of arc. The results are shown in Figure 3. The remarkable amount of detail is in striking contrast to the lower-frequency maps, giving clear evi­dence that the structure of the galactic center is not simple. Since Drake's first resolution of the structure, the central regions have been mapped with comparable resolution at frequencies of 3000 Mc/s and 14,500 Mc/s by Cooper & Price (IS) and Hollinger (19), the resulting maps being given in Figures 4 and 5, respectively. The observations of Cooper & Price were made with the Australian 210-ft telescope at Parkes, with a beamwidth of 6'.7, while those of Hollinger were made with the NRL 2.07-cm radiometer­polarimeter on the Carnegie Institution of Washington's 60-ft telescope at Derwood, Maryland. Drake's map has been corrected for beam smoothing; all other maps are of brightness temperature, uncorrected for the antenna beam.

All three high resolution maps bear striking similarities to one another, except for minor details. No common nomenclature has been adopted, but the general outlines of the maps can be described as follows, with the no­menclature of the various authors cited in parenthesis in the following order: Cooper & Price (18), Rougoor (23), and Drake (17).

(a) An extended source (called such by all authors) envelops the whole region, extending at least 2° along the galactic equator, and having a center of symmetry that lies close to the galactic plane and within a few minutes of the galactic center, assuming that the brightest source marks the actual loca­tion of the galactic nucleus. There is, of course, no assurance that this is strictly true, and ultimately the proof must rest with the hydrogen-line ob­servations that are discussed in the following section. The extended source itself is almost certainly associated with the galactic nucleus, for at centi­meter wavelengths it is the most prominent extended feature along the plane of the Milky Way, and its centroid does agree closely with the location of the galactic center as determined dynamically from hydrogen-line measurements (lIl =0, bIl =0).

(b) The principal central source of small angular size (the central source, source A, the bright inner source) lies close to the dynamic center and is the component having the highest surface brightness, as shown by the tempera­ture contours. In present usage, it is this source that is usually called Sagit­tarius A. Since the source has a smaller angular size than the antenna beam­width in all three wavelengths, the actual surface brightness is higher still.

(c) A secondary central source (the spur source, source Bl, the faint inner source) lies 12' to the NE of the principal central source. This source is complex, since incipient resolution can be seen in Figures 4 and 5. In a 6-cm survey now in preparation at the CSIRO using the Parkes 210-ft telescope, two components are clearly resolved.

(d) The northeast source (northeast source 1, source B2, the northeast ring source) is located 42' NE of the principal central source, and is partly resolved in all three maps.

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12'

24' -

36' -

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48' -IC1 0\ -

'--- 10

--..:::::-- 025 0.'

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- - - - Optical H II regions

48' ' )l 48 47 46 45 44 43 17h42nL 41 40 39 38 37

Right ascension, 1950.0

FIG. 3. NRAO 8000-Mc/s map of galactic center. Optically observed H II regions are shown by dotted circles [Drake, as reproduced by Steinberg & Lequeux (32)].

N 00 N

ttl c:::: � � trl

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RADIO RADIATION FROM GALACTIC NUCLEAR REGION 283

49'" 4Sm 47m 46m 45'" 44"' 431ft 42m 41m 40" 39m 3Sm 37'" 36m

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-3d'0r:! 49'" 4Sm 47'" 46m 45'" 44'" 43m 421ft 41m 40'" 39m 3S'" 37m 36'"

17" R.A. 1950.0 17" 17"

G. 4. 3000 Mc/s map of the galactic center, as observed at CSIRO with the Parkes telescope [Cooper & Price (18)].

(e) The southwest source (southwest source, source Bs, the southwest ring source) is situated between 32' and 38' SW of the principal central source, depending upon the survey. The source brightness is markedly lower than that of the three previously described discrete sources, and is also less well defined in its location and extent.

(J) In the survey of Cooper & Price, a larger area of sky is included, and an additional discrete source can be seen, at Declination -28°00' and R.A. 17h4S.ms. This they call northeast source 2, although its relationship to the galactic nucleus is not clear. There is an appreciable probability that this is a foreground H II region, although there is no proof that this is so. This source will not be discussed, since no spectrum information is available.

In addition, there is a discrete source visible in Drake's map approxi­mately 2m.S east of the northeast source. It shows in no other survey, and is probably spurious.

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REGION OF GALACTIC CENTER AT 2.07 CM WAVELENGTH

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RADIO RADIATION FROM GALACTIC NUCLEAR REGION 285 TABLE I

BRIGHTNESS TEMPERATURES IN GALACTIC CENTER REGION

Tb(3000 Tb(8000 Tb(14, 000 Spectral Spectral

Mc/s) Mc/s) Mc/s) index index

Feature (Cooper and Price) (Drake) (Hollinger ) (3000-8000 (3000�14, 000

(OK) (OK) (OK) Mc/s) Mc/s)

Principal central source 192 22. 5 3.5 -2.2 -2. 55

Secondary central source 42 8.0 1.3 -1. 7 -2.0

Northeast source 36 8.0 1.1 -1. 55 -2. 0 Southwest source 10 1.5 0.44 -1.95 -2.0 Extended source 15.5 2.0 0.66 -2.1 -2.0

For a complicated object like the galactic center, pencil-beam surveys provide the most reliable information. Fan-beam surveys give only a one­dimensional relation but often give resolutions that can be achieved in no other way. The surveys of Parijsky (20) using the Pulkova instrument showed that the central source has an angular extent of approximately 3' of arc, an angular size too small to be resolved in any of the pencil-beam sur­veys to date. The 21-cm surveys of Biraud, Lequeux & Leroux (21) at Meudon, and the 7S-cm of Burke & Firor (22) at Washington were useful primarily in giving relative spectral information on the discrete sources.

Interpretation of the observations.-The effects of obscuration by inter­stellar dust are so great that comparison of the radio observations with optical photographs does not provide further information on the nature of the galactic center. The comparison is useful, however, in helping to elimi­nate the confusion of foreground objects. Optically identified H II regions are shown by the dotted circles in Figure 4, none of which coincide with the major sources. Thus, a chance coincidence of thermal sources, either local or in the Sagittarius arm, with the galactic center can be eliminated. On the basis of symmetry, it will be assumed that all sources listed in the preceding section, except (f) , are associated with the nucleus of the Galaxy.

The separation of thermal and nonthermal components has been dis­cussed by several authors. The most recent discussion is that of Cooper & Price (18) , although the 14,SOO-Mc/s data were not available to them. Table I contains the data tabulated by them, with the addition of Hollinger's 2.07-cm data. The brightness temperature for the extended source is the estimated maximum after removal of the primary and secondary central sources, as well as the northeast and southwest sources. The Tb for the dis­crete sources assumes that the sources are resolved, which is not justified. The fan-beam observations of Parijsky (20) show that the primary central

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286 BURKE

source has an angular diameter that is much smaller than the resolving power of the instruments used, so that source flux is a more meaningful quantity. The secondary central source is resolved in the 2.07 -cm survey, and not quite resol�ed in the remaining surveys, which implies that the quoted surface brightness, while not exact, is not seriously in error. The northeast and southwest sources appear to be partly resolved at the shortest wave­length. At the longer wavelengths, however, the surface brightness will be underestimated. The same will be true for the northeast and southwest sources, which likewise appear to be partly resolved. The spectral informa­tion in Table I does not yield simple power-law relations for the brightness temperatures of the sources, as can be seen by comparing the exponent derived by Cooper & Price from the 10-cm and 3.75-cm data with the expo­nent given in the last column, which is derived from a comparison of the lO-cm data with the 2.07-cm data of Hollinger.

The separation into thermal and nonthermal components depends en­tirely upon the steepness of the spectrum, since a thermal source must have a temperature spectrum whose slope lies between zero and -2. A steeper slope implies that a nonthermal process such as the synchrotron mechanism must be operating. The indices given in column 5 seem to give a clear distinction, since only the primary central source and the extended source appear to have slopes steeper than 2.0. The indices derived in the last column are generally more negative, and would be decidedly non thermal in the case of the north­east and southwest sources if it were not for the large uncertainties. The addi­tional uncertainty caused by resolution effects has not been included, but it is clear that the errors are sufficiently large to admit a thermal origin for all except the principal central source. The fan-beam studies of Biraud, Lequeux & Leroux (21) at 21 cm, and of Burke & Firor (22) at 75 cm, both showed that the upper limits to the fluxes of the northeast and southwest sources implied that both were thermal. The secondary central source is probably thermal also. Cooper & Price concluded that the extended source was at least partly non thermal in origin, even though the slope was very close to -2.0, since the 85.5-Mc/s brightness temperature in the central regions reaches a maximum of 37,000° K, which seems much too high to be explained in terms of a conventional H II region, while it is easily reached by extrapo­lating the 10-cm brightness with a temperature spectrum whose slope is only -2.15. The exponent can be perhaps -2.1, if one allows for a foreground contribution from the disk component of the galactic radiation, which would cause one to overestimate the surface brightness of the extended source.

Some caution is indicated, however, in interpreting the low resolution map of Mills in the light of the high resolution studies. The 85.5-Mc/s con­tours rise from the disk to the extended source so gradually that an exact boundary of the central complex is hard to define. The centimeter-wave ex­tended source seems to be better defined, and confined to a smaller region about the center. In comparing the centimeter-wave and metric-wave bright­ness distributions, one may be comparing two entirely different regions. A

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RADIO RADIATION FROM GALACTIC NUCLEAR REGION 287 TABLE II

FLUX OF SGR A AT VARIOUS FREQUENCIES

Frequency Type of Flux density

Observer 10-26 wm-2 Mc/s observation (Cjs)-l

Hollinger 14, 500 Pencil-beam 140

Drake 8,000 Pencil-beam 175

Howard, Rood, and Boyce 8,000 Pencil-beam 130

Cooper and Price 3, 000 Pencil-beam 300

Parijsky 9,400 Fan-beam 150 3,200 Fan-beam 150

Biraud, Lequex, and Leroux 1,400 Interferometer 280

Moffett and Maltby 960 Interferometer 140

Burke and Firor 405 Fan-beam 150

short extrapolation from the centimeter data in Table I to Westerhout's 1390-Mc/s observations reinforces such a conclusion. A simple inverse-square power law, fitted to the data of Table I, predicts a surface brightness at 1390-Mc/s of 75° K The Leiden map does not have adequate resolution to separate the extended source completely from the bright central source, but if the surface brightness is measured at longitudes ± 0°.6, one obtains tem­peratures of 65° K and 88° K The contribution from the central source is approximately 6° K, due to the finite resolution of the antenna, which yields an estimated surface brightness of 59° K and 80° K fo r the north and south sides, respectively. It can be concluded, therefore, that the 1390-Mc/s data support the interpretation of the extended source as a thermal emitter at centimeter wavelengths, with only negligible nonthermal contributions. The emission measure f ne2ds, assuming the usual convention of electron density in units of cubic centimeters and path length in parsecs, is approximately 4XI04•

The principal central source is much easier to discuss, since it can be more easily separated from the other components and its intensity is large. The source is too small to resolve with most instruments, and the source flux is therefore more meaningful than brightness temperature. The results of all authors are given in Table II. The pencil-beam results, at centimeter wave­lengths, are consistent with a nonthermal spectrum, with flux varying ac­cording to a power law, of exponent -0.55 ±O.l. The fan-beam observations

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288 BURKE

show that the flux does not continue to rise with decreasing frequency, but reaches a maximum between 1000 and 2000 Mc/s, and then flattens or ac­tually turns down. The departure from a power-law spectrum can be ex­plained by H II absorption, the existence of which is strongly implied by the saddle point at the center that shows clearly in Mills' map. The extended source, whose spectrum appears to fit a thermal source at centimeter wave­lengths, is a possible thermal absorber at 3.S m, but detailed examination of the data indicates that it is probably not the main absorber. The emission measure of the extended source is too small to give sufficient absorption at 400 and 960 Mc/s, unless the electron temperature in the H II region has the improbable value of 2600 K. The only possibilities seem to be: either (a) the intrinsic emission is lower, which would require some means of removing low energy electrons from the system; or (b) the principal central source also contains an H II component that absorbs the synchrotron radiation at low frequencies. The latter case requires an emission measure of about S X 105, which is not excessive.

The saddle in brightness that shows so clearly in Mills' 8S.5-Mc/s con­tours therefore remains to be explained. The angular size of the principal central source is so small (3') compared to the beamwidth of the Mills cross (50') that its obscuring effect would be negligible, no matter how large its emission measure might be. The northeast and southwest sources are like­wise too small to have an important obscuring effect, even though they, too, would be optically thick. The extended source, however, does have an appreciable angular size, comparable to the beam size of the Mills cross, and from its observed emission measure at centimeter wavelength, 4 X 104, one can predict an optical depth of approximately 2.5 at a wavelength of 3.5m• Consequently, it would severely attenuate the radiation from the far side of the Galaxy within 10 of the center even though its optical depth at higher frequencies would not be sufficient to modify the spectrum of Sgr A. The residual surface brightness, then, represents the sum of the H II region associated with the extended source, plus the contribution of the disk component between the Sun and the central region of the Galaxy.

Summary.-The principal central source is largely non thermal, although it may include a thermal component of emission measure ,,-,S X 105• The size and total energy of the non thermal source is comparable to that of the supernova remnant Cassiopeia A, and about an order of magnitude larger than the Crab nebula. The secondary central source, the northeast source, and the southwest source are most probably thermal objects, but are as­sociated with the central region rather than being chance forground objects. The extended source appears to be largely thermal, and may be responsible for the central minimum observed at 85.5 Mc/s. Because of its large physical extent, 200 pc minimum, it would be one of the largest H II regions known. Its total radio emission is about 1600 times that of the Orion nebula.

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RADIO RADIATION FROM GALACTIC NUCLEAR REGION 289 SPECTROSCOPIC OBSERVATIONS

Introduction.-The occurrence of discrete line emission and absorption in the direction of the galactic center introduces velocity as an additional pa­rameter, allowing a discussion of dynamics as well as position. Since all known radio lines have extremely narrow natural line widths, and can be measured with great accuracy in the laboratory, the measured Doppler shifts give accurate values for the velocities of the gas clouds responsible for the observed emission and absorption. Two classes of lines have been observed at the present time: the well-known 21-cm hydrogen line at 1420.4057517 Mc/s; and the 18-cm OH lambda-doublet multiplet, at 1612,1665,1667, and 1720 Mc/s, discovered in 1963 by Weinreb et al. (24) and first observed in the direction of the galactic center by Bolton et al. (25) and Dieter & Ewen (26). The 21-cm observations are most complete at the present time, although the recent OH observations have raised interesting new problems that so far pre­clude the construction of a definitive model.

Hydrogen observations.-In the outer parts of the Galaxy, it has been shown clearly by the joint Leiden-Sydney study (27) that a reasonable repre­sentation of the distribution of galactic hydrogen can be constructed on the assumption of purely circular orbits about the center of the Galaxy. Later studies have shown that there are deviations from purely circular motion, but such deviations are probably small at distances greater than 4 kpc from the galactic center. (A distance of 10 kpc will be assumed for the distance of the Sun from the galactic center.)

Within 4 kpc, however, Oort & Rougoor (23) discovered that the radial motions are no longer small compared with the circular velocity. Their ob­servations, combined with those made at Washington by Burke & Tuve (28), are shown in Figure 6. Since surface brightness Tb is now measured as a func­tion of position (galactic longitude and latitude) and frequency (Doppler

shift), contour maps can only be drawn if one variable is eliminated in

Tb(l, b, vr). Figure 6 shows (Tb t, vr) , galactic latitude being held constant, and equal to -1.50 in the old galactic coordinate system, or -0.10 in the new galactic coordinate system. (New galactic coordinates (tIl, bIl) will be adopted here.) This is, therefore, a cross section of the hydrogen motions near the galactic plane; evidently it cannot be interpreted in terms of motion in circular orbits, since circular orbits give zero Doppler shift in the direction of the galactic center. The most easily interpreted feature is the "4 kpc ex­panding arm," which forms the intense ridge starting at longitude +5°, crossing the center at a velocity of -55 km/s, and continuing almost linearly to tIl = -120, where a discontinuity occurs. The feature then continues, curving to merge with the main galactic features at approximately tIl = - 230, The feature is clearly between the Sun and the galactic center, since it shows in absorption against Sagittarius A, and is probably at a distance of about 4 kpc, since it is apparently viewed tangentially at III = _230. At negative

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290 BURKE

longitudes, it is clear that this is the dominant feature. On the positive­

longitude side, there is no such dominating feature, although of the several possible arms, one seems fairly easy to follow. This arm, called the" + 135 km/s expanding arm" by Rougoor (23), forms the positive-velocity boun­dary to approximately III

= +5°; there it curves to lower velocity, crossing the center at a velocity of + 135 km/s; and finally it disappears at III = _5°, where it lies about 0.5° above the plane and hence does not show clearly in the bII

= -0.1° cross section. Since it does not show in absorption against Sagittarius A, it must lie beyond the center, and, like the 4-kpc arm, it is also expanding. The distance of the feature from the center is not known.

The discovery of such expanding features raises the difficult question of their origin. In view of the singular nature of the galactic nucleus, which con­tains at least one strong non thermal source, it is natural to suspect a relation-

+200

v

g,;-1?5 (g.�-O?l)

-200 -100 +)00

K � 5-10

11 10- 20 20 - 30

30- 40

iii >40

+200 FIG. 6. Hydrogen observations along the galactic plane in the vicinity of the

galactic center, showing the expanding features. For III> -5, contours are derived from Leiden Observatory and for III < -5, from work at the Carnegie Institution of Washington [Rougoor (23)).

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RADIO RADIATION FROM GALACTIC NUCLEAR REGION 291 ship between it and the zone of expansion. If the expanding hydrogen comes from the nucleus, it can either be a steady process or an intermittent one, but in either case a substantial quantity of hydrogen must be supplied. The 4-kpc arm represents an outward flux of approximately 0.025 solar masses per year; it extends through an arc of at least 90° about the center, and if the remaining features contribute similar outward fluxes, a total of at least 0.1 solar masses per year is flowing outwards from the center.

In the immediate vicinity of the galactic nucleus, one can see that an­other qualitative change in the hydrogen motion occurs. Between longitudes ±4°, there exists hydrogen with very much larger Doppler shifts, extending to velocities of ± 250 km/s. The abrupt change is most evident in the quadrant of negative longitude and negative velocity (the third quadrant), where there is no confusion with features belonging to the zone of expansion. The opposite quadrant, containing positive longitudes and velocities (the first quadrant), is confused by several features, most notably the "135 km/s expanding arm." When due allowance is made for this arm, which extends into the negative-longitude, positive-velocity quadrant (the second quad­rant), the remaining high-velocity feature is much like its counterpart in the third quadrant. Since, in the corrected profile, there is much less high-velocity hydrogen in the second and fourth quadrants than in the first and third quadrants, one may conclude that within this region, rotational motions pre­dominate over radial motions.

Rougoor & Oort have called this region "the nuclear disk, " and Rougoor has constructed a model in which, by correcting the profiles for the contribu­tions from the zone of expansion, he represents the disk to be in nearly pure rotation. The correction procedure is subject to errors, particularly in the first quadrant where the correction is large; but the reduction is probably reliable in the third quadrant for I v. I > 100 km/s. If the motion is predomi­nantly circular, a mass distribution for the central region can be derived from the apparent rotation law. Rougoor concludes that, for values of R lying between 350 and 870 pc., there exist approximately 2.1 X 106 solar masses of hydrogen, and a total mass of 7XI09M0 of all matter, including stars. The hydrogen mass within R=350 pc cannot be well determined, but from the observed rotation, a total mass of 3 X 109 M0 is indicated. The high­est-velocity hydrogen, lying at absolute velocities greater than 1 75 km/s, appears to be relatively free of confusion with gas in the zone of expansion, and a comparison of positive and negative velocity profiles indicates good symmetry about the center for these high velocities.

The resolving power of the Leiden 25-m telescope is not sufficient to show the fine details within the galactic nucleus that are known to exist from the· continuum observations. These were confined to longitudes within ± 0.6°, except for the extended source, which may reach to longitudes ± 1.0°. The­nuclear region defined by the hydrogen observations extends to longitudes of approximately ± 4°, which covers a much larger volume of space. Within. this region, it can be said that most of the hydrogen appears to be moving in.

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predominantly circular motion, with a dynamical center (after correction for the most prominent expanding arms) that lies close to the principal central source, Sagittarius A. Detailed comparisons are only possible with larger instruments, notably those with the Parkes 210-ft telescope of the CSIRO, which has a resolution of 12' at a wavelength of 21 cm.

In his first investigations, Kerr (29) discovered complexities which are shown in Figure 7, a more recent but unpublished plot of Tb (1, vr) along the galactic equator. The absorption seen against Sagittarius A is the most no­table feature, forming the unshaded band across the center of the diagram. The zero-velocity absorption, caused by local hydrogen, and the -55 km/s absorption, which is due to the 4-kpc arm, are most prominent. Absorption occurs over a much wider range of velocities, however, extending from +80 km/s to at least -175 km/s, and probably further. The 4-kpc arm would be expected to show in absorption, but neither the high negative nor the mod­erately high positive velocities would be expected for a purely rotating disk. Kerr has remarked that the +80 km/s absorption could easily be explained if the source Sagittarius A is not at the dynamical center of the Galaxy. Then the negative-velocity hydrogen would be interpreted as an outward stream­ing of gas, with a velocity comparable to the rotational velocity. If Sagit­tarius A is truly at the dynamical center, both gas outflow and inflow must be occurring.

One might expect to gain some information about the distance of the out­flowing (and possibly inflowing) hydrogen near the center by absorption observations against the other members of the central complex. Absorption can only be seen, however, against the northeast source, which shows the 4-kpc arm clearly (lII=o.r, Vr= -40 km/s in Fig. 7); but one can easily see that the effect is small, and the weaker, more extended features are lost in the complexities of the background. The fundamental difficulty is that only

-100 -80 -- ---

-60 -40 -20 o Radial velocity (km/sec)

--

+20 +40 +60 +80

FIG. 7. CSIRO 21O-ft hydrogen observations along the galactic plane within 1° of the center [Kerr (29) J.

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RADIO RADIATION FROM GALACTIC NUCLEAR REGION 293

-200 o

Radial velocity (km/s) -100 0 +100 +200

.'.- �, . ' ...... ,

, \ \ '.

FIG. 8. OH absorption-line profile against Sagittarius A, as observed with 210-ft telescope [Bolton et al. (33)].

Sagittarius A has a surface brightness that is much higher than the state temperature of the gas, and hence can show a strong absorption.

OH observations.-The first observations of the OR lines were made at the frequencies predicted by laboratory measurements, corrected for the ex­pected Doppler shift observed in the hydrogen line. The lines were found as predicted, in the spectra of both Cassiopeia A and Sagittarius A, but further observations of the galactic center by Robinson et aL (30) showed that at +40 km/s, where there is a secondary hydrogen absorption, there is a corre­sponding OR absorption that is stronger than expected by two orders of magnitude. Goldstein et aL (31), observing with the Harvard 60-ft telescope, showed not only that the +40-km/s line was easily observable against the continuum near the center, but also that there was a line of comparable intensity, at -130 km/s. Both lines are so broad that the wings of the 1665-and 1667-Mc/s lines overlap. It is interesting that the strong OR absorption lines always seem to correspond with the hydrogen lines observed at 21 cm., but the relative intensities vary widely. The state temperature of the OH is certainly very low, and all attempts to observe it in emission have failed. An upper limit to the state temperature was placed by Goldstein et aL, who measured the surface brightness in the core of the strongest component at tIl == +4°, bII =0°, and found a brightness of go ± 2° K. An upper limit of 10° K could then be placed on the state temperature.

The preliminary survey by Bolton et aL (33) of the OR absorption in the immediate vicinity of the galactic center has shown further peculiarities, best seen in the absorption profile shown in Figure 8. The strong absorptions

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at +40 km/s and -135 km/s show clearly, with the zero velocity feature, which dominates the hydrogen profiles, showing as a subsidiary absorption on the wing of the +40-km/s line. The major lines are seen to be subdivided, however, into smaller partially resolved components much narrower than the blend. These components, apparently, represent individual clouds, as can be seen in Figure 9, which shows the run of the peaks as a function of longitude and velocity. Since all the components appear to have roughly con­stant velocity over distances of the order of 1° in longitude, they most prob­ably indicate the existence of discrete clouds, and the observed changes of intensity with velocity and longitude must therefore be due to a superposi­tion of individual clouds, each keeping its integrity. Since all the clouds are seen in absorption against the central source, they must all be located be­tween the Sun and the center, and hence all clouds with positive velocities are moving inwards.

The nuclear disk model is not easily adapted to the OH observations. Since the inflowing clouds are seen over a large part of the central region, from longitude 0° to +2°, the possibility of explaining the positive velocities by an off-center Sagittarius A must be rejected. While the model of a disk, with rotational motion predominating over radial motion, can still be ac­cepted as a first approximation, it is clear that there are regions, at least, where the radial motions are large, and it is also clear that the disk must exhibit significant departures from circular symmetry .

..... .. a .:<I ..... I>-� 'S i c; :a P!

+100

0

-100

....... , - -- " .... ---� ---;..., ",

' ........ ..... ---...

.::----- -----

., .... � .. ,.... . .. ...... -• .....:..&1 ..::. __ ..... -�--

______ �e·.::.- - � ..:... .:--.. , II •

••

•••• ...... . . ... .. . ... -

IZI1111ZI Bondwldth • Boom � .

. ... . ..... . .

-200 � __ � __ � __ � __ � __ L-__ L·_·_ . . _ . . L ··_·_ . . _ . .

L .. _·_·· _ .. L·_·_··_··L ··_··�'-L __ -L-£'�-J' 80 2° -1· 0· 859·

Galactic longitude (ZU)

FIG. 9. OH clouds in vicinity of the galactic center. Heavy lines show location of peaks; closed contours are zones of heaviest absorption; and dotted contour is ap­proximate limit of observed absorption [Bolton et al. (33)].

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RADIO RADIATION FROM GALACTIC NUCLEAR REGION 295 Summary.-The galactic nuclear region, as defined by the highest­

velocity hydrogen motions, extends over a larger range of longitude than the continuum sources, whose extent is harder to define because of confusion with foreground objects. The hydrogen motions are not clearly defined, because of confusion with the zone of expansion, but appear to be predominantly circular. Although hydrogen is usually observable at the position and velocity of the OH absorptions, the relative density distributions are very different. The positive and negative velocities exhibited by the individual OH clouds are clear evidence that parts of the nuclear regions have appreciable radial motions, both inwards and outwards.

The predominantly outward motions in the zone of expansion are sus­pected to arise from physical processes in the nuclear region, but it is not clear whether the process is continuous or intermittent. Various authors have suggested that the bright radio galaxies are probably short-lived objects that were caused by violent events in their nucleus. The singular character of the galactic nucleus has raised the question of whether similar events, smaller in scale, occur from time to time in all galaxies. The observed expanding arms, if of explosive origin, would have been generated by an explosion in the nu­cleus between 106 and 107 years ago, depending upon the deceleration pro­cesses that operate. The question cannot be settled, however, before we have a more complete understanding of the galactic nucleus.

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LITERA TURE CITED

1 . Jansky, K. G., Proc. I.R.E., 20, 1920 21. Biraud, F., Lequeux, J., and Leroux, E., (1932) Observatory, 80, 1 1 6 (1960)

2. Ibid., 21, 1387 (1933) 22. Burke, B. F., and Firar, J. W., Astron. 3. Ibid., 23. 1 158 (1935) J., 66. 49 (1961) 4. Reber, G., Astrophys. J., Ql, 621 (1940) 23. Rougoor, G. W., The Neutral Hydrogen S. Ibid., 1 00, 279 (1944) in the Central Region of the Galactic 6. Piddington, J. H., and Minnett, R. C., System (Doctoral thesis, Univ.

A ustralian J. Sci. Res., 4A, 459 Amsterdam, 1964) (1951) 24. Weinreb, S., Barrett, A. H., Meeks,

7. Baade, W., Evolution of Stars and M. L., and Henry, J. c., Nature, Galaxies (Payne-Gaposchkin, C . , 200, 829 (1963) Ed., Harvard Univ. Press, Cam- 25. Bolton, J. G., van Damme, K. J., bridge, Mass., 1963) Gardner, F. F., and Robinson, B. J.,

8. Seyfert, C. K., Astrophys. J., 97, 28 Nature, 201, 279 (1964) (1943) 26. Dieter, N. R., and Ewen, R. I., Nature,

9. Drake, F. D., Astron. J., 64, 329 (1959) 201, 279 (1964) 10. Bull. Astron. Inst. Neth., No. 475 (1957) 27. Oort, J. R., Kerr, F. J., and Wester-1 1 . Hagen, J. P., Lilley, A. E., and McClain, hout, G., Monthly Notices Roy. E. F., Astrophys. J., 122, 361 (1955) Astron. Soc., 1 18, 379 (1958) 1 2 . Rougoor, G. W., and Oott, J. H., Proc. 28. Burke, B. F., and Tuve, M . A., in The

Natl. A cad. Sci. U.S., 46, 1 (1960) Galaxy and the Magellanic Clouds, 13. M ills, B. Y., Ann. Rev. Astron. A stro- 1 83 (1964)

phys., 2, 185 (1964) 29. Kerr, F. J., in The Galaxy and the

14. Westfold, K. C., A strophys. J., 130, 241 Magellanic Clouds, 187 (1964)

(1959) 15. Mills, B. Y., Observatory, 76, 65 (1956) 30.

16. Westerhout, G., Bull. A stron. Inst. N eth., 14, 215 (1958)

1 7. Drake, F. D., quoted in A nnual Review 3-1. of the Director, NRAO, 1, 2 (1960)

18. Cooper, B. F. C., and Price, R. M., in The Galaxy and the Magellanic 32. Clouds, 168 (Kerr, F. S., and Rodgers, A. W., Eds., Australian Acad. Sci., Canberra, 1964)

19. Hollinger, J. P. (In preparation) 20. Parijsky, Y. N., in The Galaxy and the

Magellanic Clouds, 172 (1964)

33.

Robinson, B. J., Gardner, F. F., van Damme, K. J., and Bolton, J. G. , Nature, 202, 989 (1964)

Goldstein, S. ]., Gunderman, E. J., Penzias, A. A., and Lilley, A. E., Nature, 203. 65 (1964)

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