PROCEEDINGS OF TIIE IRE

Absorption Techniques as a Tool for21-CM Research*

A. E. LILLEYt AND E. F. McCLAINt, SENIOR MEMBER, IRE

Summary-Hyperfine transitions between the F =0 and F= 1levels of interstellar hydrogen gas have been observed to produceabsorption lines in the spectra of radio stars. Elementary relationsare developed which describe the action of an L band radiometerconnected to an antenna, if the antenna is viewing an assembly ofgas which is illuminated by a radio star. The consequences of theabsorption line studies are reviewed. They include: high resolutionstudies of the interstellar gas, the structure and physical state ofinterstellar HI regions, minimum distances to radio stars, and ex-periments of interest in cosmology.

INTRODUCTIONE ARLY efforts in radio astronomy were concerned

primarily with radiation from the Sun, the MilkyWay, and certain of the stronger discrete radio

sources. In all cases the radiation studied was, in effect,broad-band noise or continuum radiation. In someinstances the radiation has a flat spectrum, in others,the intensity per unit bandwidth is an inverse functionof the frequency. Such radiation, while yielding valuableinformation regarding the physics of the generatingbody, nevertheless possesses limitations as far as dy-namical studies of these objects in their environmentare concerned. A spectral line was required which wouldpermit investigations in radio astronomy, based on theDoppler effect, similar to those which had been em-ployed by optical astronomers. The paper by van deHulst in 1945, which predicted the existence of lineradiation from the neutral hydrogen atom, was the firststep which led to the present major investigative pro-grams employing the 1420-mc hydrogen line.' While theexistence of the line was predicted in 1945, it remainedfor Ewen and Purcell to make the first astronomicalmeasurement of this radiation from a cosmic source.2As an indication of the significance which astronomersattached to the existence of this line, scientists in Hol-land and Australia confirmed the work of Ewen andPurcell within weeks, so that the issue of Nature whichcarried the discovery by Ewen and Purcell also con-tained the confirmations by the Dutch and Australians.Since the initial discovery in 1951, hydrogen line inves-tigations have accounted for a major part of the effortin radio astronomy.

* Original manuscript received by the IRE, October 3, 1957.t Yale University Observatory, New Haven, Conn. Formerly

with Radio Astronomy Branch, U. S. Naval Research Lab., Wash-ington, D. C.

t Radio Astronomy Branch, U. S. Naval Research Lab., Wash-ington, D. C.

I H. C. van de Hulst, Nederl. Tij. Natuurkunde, 's-Gravenhage-martinus Nyhoff, vol. 11, p. 201; 1945.

2 H. I. Ewen and E. M. Purcell, "Observation of a line in thegalactic radio spectrum," Nature, vol. 168, p. 356; September, 1951.

In their original paper, Ewen and Purcell pointed outthat while the hydrogen responsible for 1420 mc radi-ation has a characteristic or state temperature, theradiation background against which the hydrogen mightappear also would have a finite temperature and there-fore, the hydrogen might be detected either as an emis-sion line against a cold background or an absorption lineseen against a relatively hot background. As it turnedout, except for selected regions or positions in the sky,the background radiation is normally cooler than thehydrogen state temperature and the initial detectionand much of the subsequent investigation utilized theemission from ground state hydrogen. Notable amongthe studies based primarily on emission have been thework of Heeschen,3 Lilley,4 and Menon' at Harvard,who were concerned with specific regions of the MilkyWay the work of the Dutch investigators who, untilrecently, have been concerned primarily with mappingthe northern Milky Way, and the Australians who, inaddition to mapping the southern Milky Way, havemade extensive investigations of emission from theMagellanic Clouds.

Early in 1954, Hagen and McClain, at the Naval Re-search Laboratory, became interested in continuumemission from radio sources at a wavelength of 21 cmjust outside the hydrogen emission band.' A hydrogenradiometer was employed for the continuum studiesand since the radio sources constitute the necessary hotbackground, the first measurements of hydrogen inabsorption were obtained. The initial measurementsemployed the source in the direction of the galacticcenter and the radio source Taurus A as sources ofbackground radiation. Almost simultaneously, Williamsand Davies made similar studies in the direction of theradio sources Cassiopeia A and Cygnus A.7 Subse-quently, Hagen, Lilley, and McClain, made a detailedstudy of absorption in the direction of Cassiopeia Awhich revealed unexpected fine structure in the ab-

3 D. S. Heeschen, "Some features of interstellar hydrogen in thesection of the galactic center," Astrophys. J., vol. 121, pp. 569-584;May, 1955.

4 A. E. Lilley, "Association of gas and dust from 21-cm hydrogenradio observations," Astrophys. J., vol. 212, pp. 559-568; May, 1955.

5 T. K. Menon, "A 21-cm investigation of the Orion region," Doc-toral dissertation, Harvard University, Cambridge, Mass.; April, 1956.

6 J. P. Hagen and E. F. McClain, "Galactic absorption of radiowaves," Astrophys. J., vol. 120, pp. 368-369; September, 1954; paperread at joint meeting of URSI, IRE, and AGU (abstr), Washington,D. C.; May, 1954.

7 D. R. W. Williams and R. D. Davies, "A method for the meas-urement of the distance of radio stars," Nature, vol. 173, pp. 1182-1183; June, 1954.

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sorption profile.8 As will be explained later, these initialinvestigations, utilizing absorption rather than emission,have resulted in the beginnings of an independent dis-tance scale to the radio sources. In addition, they haveopened new studies of astrophysical and cosmologicalinterest.A block diagram of one type of hydrogen receiver is

shown in Fig. 1. Two pass bands are so arranged thatone having a width of approximately 2 mc is used as areference against which to compare a second narrowband which is tuned across the hydrogen line. If sucha receiver is connected to a directive antenna andpointed at an isolated hydrogen cloud the hydrogenrecorder will display only an emission profile. The heightor intensity of the profile is determined by the statetemperature of the gas, the number of atoms in the lineof sight and the line broadening mechanism. The widthis determined primarily by the Doppler effect due tothermal motions and any turbulence which may bepresent in the cloud. If the cloud possesses gross motionstoward or away from the antenna, the center frequencyof the profile will of course be shifted to a higher orlower frequency respectively. In this case, with no back-ground radiation field, the comparison band does notsee a signal and no indication will be present on thecontinuum recorder.

Ist IF AM SECONDV VIOEO_ __sIF AMP CONVERTE IF AMP_

FREQUENCY LOCALCOUNTER OSCILLATOR

SERVO

_ LOCK

I~~~

5MC XTALWWV AND

MONITOR MULTIPLIER

SIGNAL COMPARISON|FILTER FILTER

LDETECTOR DETECTOR

BRIDGE CIRCUIT

DIFFERENCEAMPLIFIER

L RECORDER

Fig. 1-Block diagram of NRL hydrogen radiometer. The receiveris an L band radiometer employing a crystal mixer and doubleconversion. Signal and comparison bands provide a comparisonof the spectral intensity received by the antenna in two regionsof the spectrum. A stable local oscillator is tuned in frequency,translating the signal and comparison bands through the spec-trum. The difference in energy received in the two bands is pre-sented at the recorder.

If the same radiometer system is pointed at a discreteradio source or "radio star," the signal and comparisonbands will see equal temperatures and the continuumrecorder will display the antenna temperature of theradio star, while the hydrogen recorder will indicatezero temperature difference between signal and compari-son bands. If one now places a hydrogen cloud behindthe radio source, and provided the angular extent of the

8 J. P. Hagen, A. E. Lilley, and E. F. McClain, "Absorption of 21cm radiation by interstellar hydrogen," Astrophys. J., vol. 122, pp.361-375; November, 1955.

radio star is small compared to the antenna beam width,the signal band would receive additional radiation andwould plot the true hydrogen profile as the receiver wastuned. The continuum recorder would continue to indi-cate the antenna temperature of the radio source. Theterm antenna temperature requires some clarification.

ANTENNA TEMPERATUREIn the absorption problem we are confronted with a

gaseous medium which is a monochromatic emitter andabsorber. Imbedded in this medium are the radio starswhich are emitting a continuum of radiation. We willexamine some simple relations which govern the actionof the interstellar gas on the spectrum of the radiostars.

It is convenient in treating many problems in radioastronomy to make use of "temperatures" by use of theRayleigh-Jeans formula relating temperature and in-tensity. The radiation transfer problem can then betreated in terms of temperatures. Both the optical andsome radio investigations indicate that radio stars arenot uniformly bright in appearance but rather, exhibita filamentary appearance. The surface brightness of sucha radio star may be represented by a temperature dis-tribution T(O, 4), the effective temperature presentedby the source in an element of solid angle whose positionis specified by 0 and b. Effective temperature is usedwithout specifying whether the radiation mechanism ofthe source is thermal or nonthermal. By effective tem-perature is meant the temperature required by a blackthermal emitter (having the same geometrical configura-tion and location as the actual source) in order to pro-duce the same flux observed at the antenna.With the definition given above for the source, we

may write the antenna temperature presented by thesource as

TA = - T(G, qs)G(O, O)dw (1)

where G(O, X) is the gain function of the antenna over anisotropic radiator.

However, because of the generally small size of radiotelescopes, the beam widths in use are quite large com-pared to the angular sizes subtended by the radio stars.This condition enables us to make use of an averagebrightness temperature, TB and an average solid angle,Us, for each source. Then for a particular antenna, themain beam may also be assigned an effective solid angleQB and the antenna temperature may be written simplyas

TA = TB -uTA =TB1BQB

(2)

THE ABSORPTION EFFECT

If an antenna views a cloud of gas which is spectrallyemitting and absorbing microwave radiation (see Fig. 2)then the observed line intensity may be written as

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Lilley and McClain: A bsorption Techniques as a Tool for 21-CM Research

TH(v) = Ta(1 -e-l(W). (3)

In (3), TG is the temperature which is effective in estab-lishing the population of the two states responsible forthe spectral line, and r(v) is the opacity of the cloud.The opacity of the cloud may be regarded in one senseas an attenuation factor, viz.: if lo were the intensity ofsome extraneous radiation passing through the cloud,then the emergent radiation intensity would be givenby Ioe-(v).

Profiles produced by hydrogen gas in the interstellarmedium are spectrally confined to widths generally lessthan one megacycle and thus their radiation excites onlythe signal band of the radiometer.

AT'(v) = (TG- TB)(1 -e-r()). (6)

It is evident from (6) that the relative sizes of TG andTB determine whether there will result an emission line,no line, or an absorption line.Most of the radio stars are small in angular extent,

however, and (6) must be modified to take this intoaccount. We may still regard the gaseous medium asfilling the beam, but we shall assign a solid angle Q, tothe radio star.

Using the diagram shown in Fig. 4, we may againwrite down the contributions to the signal and com-parison bands

Ts = TG(1-erT(v)) + TB-e-T ()

Te= TBTB--BQQB

(7)

(8)

Fig. 2-Antenna viewing an interstellar cloud of hydrogen gas. Thisis a schematic representation of an antenna viewing a spectrallyemitting gas cloud which completely fills the antenna beam.

TX ~~~~~~~~~B-

Fig. 3-Antenna viewing an interstellar cloud illuminated by a con-tinuum source. The spectrally-emitting cloud fills the antennabeam and the cloud is illuminated from behind by a continuumsource of uniform brightness temperature TB.

The gaseous profile can be altered in shape and ampli-tude from the form given by (3) if the antenna alsoviews a continuum radiation field. Suppose that behindthe cloud there exists a uniform radiating source of sur-face brightness temperature TB and that both the cloudand the continuum source completely fill the antennabeam as shown in Fig. 3. We write down the thermalcontribution to both the signal and the comparisonbands, respectively Ts and Tc,

Ts = TO(- e-e(v)) + TBe-T(v) (4)

T= TB (5)and the difference between (4) and (5) is the quantitydisplayed by the radiometer, given by

l22l UNOBSERVABLE, EMISSION OR ABSORPTION

i OBSERVABLE IN EMISSON

E3EMISSION AND ABSORPTION PRESENT

Fig. 4-Antenna viewing an assembly of interstellar gas clouds and aradio star included within the beam. Two beams are evident: thecone subtended by the radio star, and the cone defined by thebeam of the antenna. Emission and absorption effects are indi-cated in the figure.

We distinguish between r(v), the effective opacity spreadover QB, and Ts(v), that part of r(v) which is confined toUs. We also replace TG(1 -e-(v)) with AT(v) and, be-cause this represents simply the contribution of the gasalone, we call it the expected profile. The expected pro-file is the profile which would be observed in the absenceof the radio star, and may be determined by obtainingprofiles from adjacent comparison regions or by driftcurves through the source at various fixed frequencies.The difference between (7) and (8) again yields the

observed profile,

AT'(v) = AT(v) - TA(1 - (9)

All the quantities in (9) are available through the ob-servational data with the exception of rg(v). The solution

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amolml caggm3l

0

I.,

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PROCEEDINGS OF THE IRE

for this quantity in terms of the observables is

AAT(v) - AT'(v)-rs(v) = -nn1 - TA . (10)

The absorption data thus permit the determinationof the opacity rs(v), and it is this quantity which con-tains considerable information of astronomical interest.One of the striking consequences is the angular resolu-tion involved. Because rs(v) is produced by gas confinedto the solid angle subtended by the radio star, the reso-lution is equivalent to truly enormous antennas. Ab-sorption investigations on some of the brighter radiosources present angular resolutions equivalent to an-tennas having diameters from one-tenth to approxi-mately a full mile. The greatest resolution available to21-cm research is likely to remain in absorption ratherthan emission studies.

DISTANCE DETERMINATIONS TO RADIO STARS

We shall take the absorption effects observed on theCassiopeia A radio star to illustrate the problems of dis-tance determination. At wavelengths near 21-cm,Cassiopeia A has the greatest apparent intensity of allthe radio stars. This source of radio radiation was identi-fied optically by Baade and Minkowski of MountWilson and Palomar Observatories, and a new type offilamentary nebulosity is now known by virtue of theirstudies.9

In addition to their optical studies of the physicalprocesses operating in this new type nebulosity, theyhave obtained spectra yielding radial velocities andphotographs which demonstrate changes of position ofthe emission filaments, or transverse motions. Statisti-cally combining the position change and radial velocitydata, Baade and Minkowski have concluded that thedistance to the radio source nebulosity is about 1500light years.Now let us examine the Cassiopeia A radio absorp-

tion data for distance information. All 21-cm absorptiondata show the spectrum, and hence the radial velocitydistribution, of the absorbing gas. It is the velocity dis-tribution, and in addition, the knowledge that the ab-sorbing gas must lie between the observer and the radiosource, which allows one to estimate a minimum dis-tance to the radio source by radio observations.The radio data consist of the intensity distribution of

the hydrogen radiation as a function of frequency. Theability to determine distances by means of hydrogenline profiles requires a knowledge of the large-scale dy-namics of the interstellar medium, which allows one tointerpret radial velocities and hence frequencies as meas-ures of distances. In order to interpret quantitativelythe relation between radial velocity and distance-from-the-earth, the gross behavior of the material in the plane

9 W. Baade and R. Minkowski, "Identification of the radiosources in Cassiopeia, Cygnus A, and Puppis A," Astrophys. J., vol.119, pp. 206-214; January, 1954.

of our own galaxy must be known. The phenomenon ofgalactic rotation describes the fundamental feature ofthe large-scale motions of material in the galactic plane.The interpretations of the hydrogen-line profiles, ob-

tained in the plane of our Milky Way by Dutch andAustralian workers, show that the interstellar hydrogengas is concentrated into spiral arms. These concentra-tions are revealed by separate maxima in the hydrogenspectrum at particular celestial positions.The general shape of emission profiles which are ob-

served in directions adjacent to the Cassiopeia A radiostar is characterized by two distinct emission maximaas shown in Fig. 5(a). By employing a model of galacticrotation the variation of radial velocity of the inter-stellar clouds with distance from the sun along a particu-lar line of sight may be predicted. Conversely, if theprofile produced by the radio receiver shows maximaat particular radial velocities, the maxima are inter-preted as arising from condensations of gas lying inspiral arms located at different distances along the lineof sight. On this interpretative basis, the two maximawhich characterize the profiles originating in directionsnear the Cassiopeia A radio source have been assigneddistances from the sun. The Dutch group places the mostdistant of the Cassiopeia region spiral arms at a distanceof about 9000 light years. Now the 21 cm profile ob-tained for the exact direction of the Cassiopeia A source,Fig. 5(b), shows theeffects of absorption in both maxima.8

cozwz

w

-Jw

WAVE LENGTH (VELOCITY) WAVE LFNGTH (VELOCITY)

(a) (b)Fig. 5-Behavior of 21-cm profiles in the direction of Cassiopeia

A. Profiles obtained in the general direction of Cassiopeia Ashow a shape characteristic of the curve marked A. (a) Onlyemission is present. (b) In the exact direction of Cassiopeia A,the profile shows considerable distortion in shape and exhibits ab-sorption features. The higher level of (b) is due to the antennatemperature of the Cassiopeia A source.

Moreover, the absorption profile displays the strongestabsorption effects in the frequency interval occupied bythe gas which exists in the second and most distant ofthe two major spiral arms. Immediately there is thereasonable interpretation that the radio source must lieat a distance which is at least as great as the secondspiral arm. If there were no other information existingwhich enabled the distance to the source to be evalu-ated, the minimum distance to the source of 9000 lightyears resulting from the radio data would not be dis-turbing. But the minimum radio distance is a factor of

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Lilley and McClain: Absorption Techniques as a Tool for 21-CM Research

six greater than the optically evaluated distance. Arecent absorption investigation of the Cassiopeia sourceby Muller,'0 using the Dutch 83-foot instrument, hasverified the principal absorption features and left theradio distance interpretation unchanged.The distance problem remains unresolved and, until it

is clarified, the exact distance to the Cassiopeia A sourceremains in doubt. The ability of the radio or optical datato provide valid distance information also remainsdoubtful. Admittedly, the optical determination is basedon a statistical study of limited scope imposed by thesource itself, but to stretch the optical data to accommo-date the minimum radio distance is a stretch whose mag-nitude is unacceptable to the optical investigators. Afactor of two error in the optical distance would stillleave the optical distance only one third of the way outto the minimum radio distance. Clearly, one of the dis-tance determinations has an unknown effect which ismisleading either the optical or the radio investigations.The vulnerable point of the radio data is clear. All onereally knows about the radio absorption effect is that thegas responsible for the absorption lies between the ob-server and the Cassiopeia A source. If the source is reallyas close as the optical studies place it, then the radialvelocity distribution of the absorbing gas must be givenan interpretation which is not straightforward. Twopossibilities present themselves. First, we may be deal-ing with two nearby interstellar clouds which happento have anomalous radial velocities which are just equalto the value expected from galactic rotation in the sec-ond spiral arm. Or second, the absorbing gas may belocated very close to the radio source and dynamicallyassociated with it. The random cloud argument is pos-sible, but it also is "special." Gas associated with thesource is a possibility but an adequate physical discus-sion would be complicated. The proximity of the intenseradio source would bathe the adjacent hydrogen gasclouds in microwave radiation density of a magnitudemuch greater than gas clouds normally experience intypical interstellar regions. The presence of the high-radiation density would affect the absorbing propertiesof the gas, adjusting the state temperature and essen-tially reducing the attenuating property of the gas. Butthe opacity is fixed and may be determined with reason-able accuracyfrom the radio data. Therefore, the amountof gas present on this hypothesis is, for a fixed opacity,greater than the amount needed to produce the sameopacity in more normal regions of interstellar space.

Either random nearby clouds or gas physically verynear the source with radial motions away from thesource could produce an absorption profile which wouldmislead the radio minimum distance determined bymeans of a galactic rotation analysis. Whether the radiodata have effects such as these involved, or whether

10 C. A. Muller, "21 cm absorption effects in the spectra of twostrong radio sources," Astrophys. J., vol. 125, pp. 830-834; May,1957.

there are unknown systematic dynamical properties ofthe optical filaments are problems that have stimulatedstudies presently in process which hopefully will satis-factorily resolve the distance problem.

SAGITTARIUS A

One of the most intriguing problems today is thematter of the galactic center region. In this direction theclassical model of the galaxy would display no radialvelocity and one might expect a narrow intense emissionline. Actually, the region exhibits a complex hydrogenemission profile as first pointed out by Heeschen.3 Inaddition, the region contains an intense discrete sourceof continuum radiation at longer wavelengths which hasbeen suggested as the galactic center by McGee andBolton." Investigations of the continuum at cm wave-lengths by Haddock and McCullough, and Haddock,Mayer, and Sloanaker suggest the possibility of twosources, one behind the other.'2"3 More recently Millshas obtained additional evidence at 3.5 meters, that thecontinuum radiation may arise from two regions at dif-ferent distances and that the nearer source may be anHII or ionized hydrogen region.'4The first hydrogen absorption measurement made by

Hagen and McClain used the galactic center as a sourceof background radiation.6 This measurement employed areceiver bandwidth of 55 kc which was too wide to reveala great amount of detail subsequently found by McClainusing a bandwidth of 5 kc.'5 Fig. 6 shows the expectedand observed profiles obtained in the latter experimentusing the NRL 50-foot antenna. The extent of the ab-sorption features in velocity was compared with theangular extent of the gas at various velocities and a dis-tance scale assigned to the hydrogen profile. This treat-ment of the data indicated the probable existence of acontinuum source at a distance between 2 and 6 kilo-parsecs (kpc) with a most probable position at 3.4 kpcbut did not exclude the possibility of an additional weaksource at a greater distance. Measurements by Williamsand Davies, utilizing a somewhat different absorptiontechnique, also suggest the existence of an H II region atapproximately 3 kpc.16Dutch investigators utilizing the absorption tech-

nique in combination with their new 83-foot antennahave recently published a partial profile of the galactic

11 R. X. McGee and J. G. Bolton, "Probable observation of thegalactic nucleus at 400 mc," Nature, vol. 173, pp. 985-987; May, 1954.

12 F. T. Haddock and T. P. McCullough, "Extension of radiosource spectra to a wavelength of 3 cm," Astrophys. J., vol. 60, pp.161-162; June, 1955.

13 F. T. Haddock, C. H. Mayer, and R. M. Sloanaker, "Radioemission from the Orion nebula and other sources at 9.4 cm," A stro-phys. J., vol. 119, pp. 456-459; March, 1954.

14 B. Y. Mills, "The radio source near the galactic center," Ob-servatory, vol. 76, pp. 65-67; April, 1956.

15 E. F. McClain, "An approximate distance determination forradio source Sagittarius A," Astrophys. J., vol. 122, pp. 376-384;November, 1955.

16 R. D. Davies and D. R. W. Williams, "An alternative identifica-tion of the radio source in the direction of the galactic center," Na-ture, vol. 175, pp. 1079-1081; June, 1955.

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center region exhibiting an absorption feature in theblue wing at -50 km. This investigation by Woer-den, Rougoor, and Oort suggests the presence of a smallspiral arm near the center and requires the existence ofa source of continuum radiation at the center or be-yond.17

Fig. 6-Expected and observed profiles for Sagittarius A. In thisdiagram the ordinate is in °K and the abscissa is in kilometersper second. The upper curve is the emission profile expected inthe absence of the source while the lower curve is the observedcurve.

HI13HI r

Fig. 7-Possible configuration of gas clouds and continuum emittersin the direction of Sagittarius A. The configuration is discussed inthe text.

Based-on present evidence one is forced to consider atleast two sources of continuum radiation and threeinterleaved hydrogen regions in discussing the region ofthe galactic center. A possible configuration is shownin Fig. 7. An alternative concentric configuration hasbeen suggested by Mills." In such a complex situationwhere the continuum sources may be either thermal or

nonthermal and of different sizes, one should expect

17 H. van Woerden, W. Rougoor, and J. Oort, "Expansion d'unestructure spirale dans le noyau du systeme galactique, et position dela radiosource Sagittarius A," Compt. Rend. Acad. Sci., vol. 244, pp.1691-1695; March 25, 1957.

profiles in which certain portions are a function of an-tenna size and other portions may remain relativelyunchanged with larger antennas. A complete quanti-tative solution of the case presented in Fig. 7 requiresthe determination of at least 27 parameters. The in-herent advantage of a larger antenna results from areduction in the number of angular diameters that onemust consider. It is probable that this region will remainan interesting and fruitful area of study for some time.

EXTRAGALACTIC STUDIESFinally, in our survey of the observational possibilities

of the absorption effect, we shall consider its applicationto extragalactic objects. In the early studies which wereconfined to observing the line in emission, the first stepof the hydrogen line out of our galaxy was made by theAustralian team of Kerr, Hindman, and Robinson.'8They succeeded in detecting 21 cm emission from theMagellanic Clouds and the investigators in Australiahave profitably pursued the initial detection by study-ing the dynamics and distribution of hydrogen gas inour closest extragalactic system.The period following the detection of the Magellanic

Clouds saw several hydrogeni line groups attempting todetect other of our nearby galactic neighbors. A favoriteobject was Messier 31, the great spiral nebula in theConstellation Andromeda. But the early attempts failed,primarily because of the poor resolution of antennasavailable in 1954-1955, anid attempts were shelved untilrecently when larger antennas were available.'9 It be-came clear that in the detection of the hydrogen line re-mote galactic objects were going to be difficult. But thepossibility of finding the radio line in a remote systemwas intriguing. Ever since the discovery of the linie, aninteresting experiment was evident; a comparison of op-tical and radio red shifts on a common extragalactic.From the work of Hubble, Humason, and others, we,

have the red-shift law through which arises the conceptof the expansion of the universe. The Doppler effect in-terpretation of red-shifts of extragalactic objects showsthat galaxies in space are receiving from each other withvelocities which are proportional to their distancesapart.There have been alternative suggestions advanced to

explain the red-shift-distance relation exhibited byextragalactic nebulas which allows a red-shift-dis-tance relation in a static universe. Even though theDoppler interpretation is the straightforward one,astronomers, physicists, and cosmologists have soughtadditional tests of the Doppler interpretation. Suchtests are important because of their intrinsic interestand because the fundamental expansion property of the

18 F. J. Kerr, J. F. Hindman, and B. J. Robinson, "Observationsof the 21 cm line from the Magellanic Clouds," Aust. J. Phys., vol. 7,No. 2, pp. 297-314; 1954.

19 Heeschen and Dieter, "Extragalactic 21 cm line studies," thisissue, p. 234.

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universe rests observationally on the Doppler interpre-tation of the red-shift law.

Several tests have been proposed for examining theDoppler hypothesis. An obvious possibility which pre-sented itself with the discovery of the hydrogen line, wasthe measurement of the hydrogen line red-shift on adistant extragalactic object having a large and an opti-cally determined red-shift. The optical and radio red-shifts could then be compared over a wavelength basein the electromagnetic spectrum that covers a wave-length ratio of about 500,000 to 1. If the red-shifts areproduced by the Doppler effect, then for a common testobject, the observed shift should be the same for the op-tical and radio determinations.The early emission studies, as outlined above, were

not able to move very far into extragalactic space. Butthe discovery by Baade and Minkowski that the secondmost intense radio star was a collision of two galaxies ata distance of approximately 108 light years, and with thesimultaneous development of 21-cm absorption linestudies, an experiment suggested itself that stimulatedinterest in a red-shift experiment.

Considering the two galaxies in collision which com-prise the Cygnus A radio star, there is a chance thatperipheral hydrogen associated with the two systemsmight exist in neutral, unionized form. (See Fig. 8.)This associated gas could absorb spectrally part of thecontinuum radiation originating in the deeper regions ofthe collision producing an absorption line in the con-tinuum which originated at the source. Baade andMinkowski, using the 200-inch Hale telescope, measuredthe optical red-shift of the Cygnus A colliding systemand found that the red-shift corresponded to a reces-sional velocity of approximately 16,830 kilometers persecond.9 Since the peripheral gases in the Cygnus A sys-tem are dynamically part of the system, the radio ab-sorption line should show a Doppler shift correspondingto the recessional velocity of 16,830 km. For the 1420-mc rest frequency of the 21-cm line, a radial velocity of16,830 km would decrease the observed frequency byabout 81 mc.With these considerations in mind, the authors began

a study in late 1955 of various ways to search observa-tionally for the suspected absorption line. Two problemswere evident: the suspected line was likely to be bothweak and broad. Both the anticipated weakness and thebreadth of the line meant that the usual scanning tech-niques using signal and comparison bands would prob-ably be unsatisfactory and a completely different ap-proach would be required.The breadth of the absorption line is fixed and not

subject to control but the observed line intensity woulddepend on the antenna size viewing the Cygnus Asource. A very large antenna would assist the studysignificantly. The Cygnus A radio source has a declina-tion such that it passes near the zenith for the geo-graphical latitude of Washington, D. C. and so a special

possibility presented itself. A large stationary antennacould be constructed which would have the Cygnus Asource transit the antenna main beam once a day nearthe zenith. A special receiver could also be constructedwhich would record the transit of Cygnus A on a num-ber of channels distributed in frequency outside andwithin the range where the absorption line was ex-pected.

HOT COLLISIONAL ZONE

Fig. 8-Schematic representation of colliding galaxies in the CygnusA system. The collision produces a source of continuous radiationand nearby associated hydrogen can spectrally absorb part of thecontinuum radiation.

Fig. 9-Proposed 500-foot antenna for Cygnus A absorptionmeasurements. This antenna is discussed in the text.

A 500-foot parabolic antenna made of chicken wirepanels and mounted on telephone poles seemed to be areasonable approach for a special experiment and theantenna was designed (see Fig. 9).2°

During the interval when the 500-foot chicken wireantenna was being designed, it seemed reasonable tomake a serious effort with the 50-foot antenna and itsassociated comparison radiometer. Scanning techniqueswere eliminated because of the anticipated line widthand the receiver was converted to a straight radiometer,displaying only the energy received in the comparisonband.

20 W. R. Ferris, 'Design of a 500-Foot-Diameter Faceted Parab-oloidal Antenna,' U. S. Naval Research Lab. Rep., No. 4881; pp. 1-7; January 25, 1957.

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PROCEEDINGS OF THE IRE

There are many effects that can distort the output ofa receiver connected to an antenna and a broad weakline is particularly susceptible to these effects. Groundreflections and variation of the radiometer zero with fre-quency are two troublesome examples. Trouble can arisein two forms: the equipment can present a "line" whichis purely instrumental in origin, or the equipment tuningeffects can obscure the actual line originating in thegaseous assembly.

In order to avoid such instrumental difficulties asthese, a second source, Cassiopeia A, was used as astandard and a comparison was made of the relative in-tensities of Cassiopeia A and Cygnus A at various fre-quencies. The problem of detecting a weak line could beassisted by the usual method of accumulating enoughdata so that a statistical increase in sensitivity wouldresult.The search was carried out with the 50-foot antenna

and a weak absorption feature was found in the positionanticipated from the Doppler interpretation.2' Theposition of the radio absorption feature agrees very wellwith the optical value. Slight differences would be ex-pected because of the physically different locations andvelocity distributions of the gases involved in the radioand optical studies. -Since the absorption line red-shiftmeasurement on the Cygnus A system, the constancy ofA'X/X for radio and optical measurements has been con-firmed with a larger antenna. Heeschen, using thenew 60-foot Agassiz Station Radio Telescope of HarvardObservatory, has succeeded in detecting a faint 21 cmemission line from the rich Coma cluster of galaxies. Thered-shift of the Coma cluster is approximately only onethird of the red-shift of the Cygnus A system, but theradio red-shift agrees satisfactorily with the optical red-shift of the cluster.22Although the Cygnus A radio red-shift represents a

crude first attempt, the resulting demonstration of theconstancy of AX/X for optical and radio wavelengths im-proves our knowledge of the constancy by about 105.

In addition, the radio results suggest that one is likelyto find AX/X constant for a particular extragalactic ob-ject wherever a red-shift measurement is made in theelectromagnetic spectrum.The constancy of the radio and optical red-shifts is a

natural consequence of the Doppler interpretation. It isnot a categorical demonstration of the expansion of theuniverse but it is very suggestive.The successful detection of an absorption line in the

Cygnus A collision raises an interesting question con-cerning the number of similar measurements whichmight be performed with larger antennas. There aretwo additional radio sources, NGC 5128 in Centaurusand NGC 4486 in Virgo which, although not studied as

21 A. E. Lilley and E. F. McClain, "The hydrogen line redshiftof radio source Cygnus-A," Astrophys. J., vol. 123, pp. 172-175;January, 1956.

22 D. S. Heeschen, "21-cm line emission from the Coma cluster,"Astrophys. J., vol. 124, pp. 660-662; November, 1956.

yet, might be detected in absorption using the NRL50-foot antenna. While both sources are probably richin hydrogen, they exhibit antenna temperatures ap-proximately 25 per cent of the Cygnus value, and thepossibility of a successful measurement is reduced cor-respondingly. If we assume that colliding galaxies aredistributed uniformly throughout space, the number de-tected should go as the third power of the antennadiameter. We might, therefore, expect an antenna in the500- to 600-foot range to have between 1000 and 3000such sources available for study.23

ADDITIONAL ABSORPTION LINE EXPERIMENT

In our survey of the potentialities of the absorptiontechnique, two points concerning the content and physi-cal state of the interstellar medium require mentioning.First, concerning the content of the interstellar medium,the absorption technique with larger antennas presentsan opportunity for the detection of other spectral lines.As antennas increase in size, the antenna temperature ofa small source increases at a rate proportional to thesquare of the aperture. Thus the thermal amplitude of agiven absorption line increases with increasing antennasize thereby increasing the signal size with respect to theminimum detectable temperature difference of theradiometer. This results in an effective increase in sensi-tivity for line detection by virtue of significantly im-proving the chances of detecting lines which have verysmall opacities.24 Small opacities will result because ofthe small cosmic abundances of atoms and radicalswhich have expected lines.A second point concerns the direct detection of inter-

stellar magnetic fields. Bolton and Wild have proposedusing the Zeeman-splitting of the radio absorption linesto measure interstellar magnetic fields of the order 10-'to 10-6 Gauss.25 The splitting of the two of componentsof the hydrogen line (the upper level splits into threecomponents) is approximately 2.8 mc per Gauss. For aGaussian line the minimum detectable magnetic field isgiven approximately by

Hmin c 4 X 10-v AvATminTi

(11)

where T, is the maximum absorption line depth ex-pressed as a temperature, ATmin is the minimum tem-perature change detectable by the radiometer, andAv is the half-width of the absorption line.

For an antenna of the 140-foot size to be built at theNational Radio Observatory, (11) leads to Hmin inthe range between 10-' and 10-1 Gauss, provided theobservations are made on the Cassiopeia A radio star.

23 E. F. McClain, "A note on the potentialities of large radio tele-scopes," Astrophys. J., vol. 123, pp. 367-368; March, 1956.

24A. E. Lilley, "A note on the galactic microwave spectrum,"Astrophys. J., vol. 122, pp. 197-198; July, 1955.

25 J. G. Bolton and J. P. Wild, "On the possibility of measuringinterstellar magnetic fields by 21-cm Zeeman splitting," Astrophys. J.,vol. 125, pp. 296-297; January, 1957.

228 January

Lilley and McClain: Absorption Techniques as a Tool for 21-CM Research

A variety of evidences suggest that magnetic fields of theorder 10-5 and 10-6 Gauss may exist in the interstellarmedium.A second interesting possibility exists in the measure-

ment of the density of the intergalactic medium. As yetno measurement has revealed the existence of materialin average regions of intergalactic space although it isgenerally supposed that very low density hydrogenmight be present. If ground state hydrogen were presentit would be effective in absorbing continuum radiationfrom distant radio sources of the Cygnus A type.The intergalactic gas may be ionized; the time for

recombination is expressed in billions of years. How-ever, the physical state of the intergalactic medium isat present unknown and it is of considerable interest toask what density of ground state hydrogen could be de-tected in the intergalactic medium. Intergalactic gaslying between the observer and a source of the CygnusA type would absorb continuum radiation from thesource. The density detectable under this circumstanceis given approximately by

TsHA Tminp constant (12)

TA

where TA is the antenna temperature of the source usedas a background, H is Hubble's constant, ATmin is the

minimum detectable temperature of the radiometer em-ployed and Ts is the state temperature of the inter-galactic gas.26 A 140-foot radio telescope viewing theCygnus A source could detect a density of the order10-3°Ts gm cm-3. It is apparent that two major unknownswill control the fate of this experiment, the state tem-perature and fractional ionization of the intergalacticmedium. It remains nevertheless an experiment whichradio astronomers are obligated to attempt.A new approach to the absorption line problem is

being carried ahead by Bolton at the CaliforniaInstitute of Technology. He plans to apply inter-ferometer techniques to the absorption line studies ofgalactic and extragalactic radio sources. It is clear thatobjects of the Cygnus A type can be detected and usablemeasurements made at distances significantly greaterthan distances at which similar optical measurementscan be made provided means can be found for selectionof suitable sources. Therefore it may be possible to ex-tend the radio red-shift measurements to velocitieswhich are significant fractions of the velocity of light.The future of absorption line studies with larger an-tennas and improved receivers should be very interest-ing and rewarding.

26 A. E. Lilley, "Radio Astronomical Measurements of Interest toCosmology," Conference on Role of Gravitation in Physics, WrightAir Dev. Ctr. Tech. Rep., 57-216; March, 1957.

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