OBSERVATIONS OF GALACTIC RADIATION AT 4 7 Mcjs By G. R. A. ELLIS,* R. J. GREEN,* and P. A. HAMILTON* [Manuscript received June 3, 1963]

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1 OBSERVATIONS OF GALACTIC RADIATION AT 4 7 Mcjs By G. R. A. ELLIS,* R. J. GREEN,* and P. A. HAMILTON* [Manuscript received June 3, 1963] Summary This paper describes bservatins f galactic radiatin at 4 7 Mcls using an antenna with a beam f 11 by 3. A range f declinatins frm -1 t -72 and f R.A. frm 12 h t 5 h was surveyed. Brightness cnturs are given. INTRODUCTION At frequencies less than abut 2 Mcjs the prpagatin f radi waves thrugh different parts f the galaxy begins t be substantially affected by attenuatin in HI! regins. Shain (1954), fr example, shwed that at 18 3 Mcjs the ptical depth in the directin f the centre is abut 3 kpc. Since the attenuatin is given by exp( - fkds), where K ""'.4N2jT3/2j2, it might be expected that at much lwer frequencies the pacity f the interstellar medium might make it pssible t btain infrmatin abut the distributin in depth f bth the emitting and absrbing regins. Here the results f a survey at 4 7 Mcjs are described. PLAN STAYS ARE NOT SHOWN ELEVATION AA Fig. I.-Layut f 4 7 Mcls antenna. ELEVATION BB RECEIVING EQUIPMENT The antenna was sited at Hbart (lat S., lng. 147 E.). It cnsisted f an array f 272 hrizntal half-wave diples 1 wavelengths abve grund and arrayed as in Figure 1. Each diple was cnnected thrugh a quarter-wavelength * Physics Department, University f Tasmania, Hbart. Aust. J. PhY8., 1963, 16,

2 546 G. R. A. ELLIS, R. J. GREEN, AND P. A. HAMILTON transmissin line t a terminated main transmissin line which extended fr the whle length f each rw f diples. Each f the eight main transmissin lines was cnnected at its centre thrugh a matching netwrk t a caxial feeder which led t the receiving hut. Each rw f diples was thus cnnected independently t the receiver. The signals passed thrugh an eight-channel preamplifier, an eight-channel mixer and thence int six eight-channel i.f. amplifiers where they were summed with six different phase relatinships t prvide six antenna beams. Minimum recrders were used in the d.c. amplifiers and, in additin, t reduce interference frm transmitting statins, the frequency f the main scillatr was swept thrugh 12 kc/s five times per secnd. The final i.f. amplifiers had mechanical ffiters with a bandwidth f 2 7 kc/s. Figure 2 shws the calculated antenna pattern and Figure 3 the blck diagram f the receiving equipment., 1\ 2.I\J li\. -J h EAST WEST Fig. 2.-Calculated antenna pattern NORTH SOUTH The phase f the signals in each channel was measured by cnnecting a signal generatr t the eight inputs and bserving the Lissajus patterns f pairs f the utput signals at intermediate frequency n a cathde-ray tube. The phases fr individual diples were measured in the same way, except that the signal generatr was cnnected t pairs f diples via cupling lps instead f t the preamplifiers. In setting the phase differences between channels t prduce an ff-vertical antenna beam, a delay caxial cable was inserted int ne channel f a pair, the phases and gains f the pair were adjusted fr equality, and the cable remved. Successive pairs were then aligned in this way. Phase adjustments were always made by slightly altering the resnant frequency f the i.f. transfrmers in the eight-channel i.f. amplifiers. The receivers were calibrated using nise generatrs. The verall lss in the antenna system was estimated by cmparing the csmic nise level with that frm an antenna cnsisting f three full-wave diples perating at the same time and at the same frequency. Fr directins f high galactic latitude differences due t the spatial averaging f the smaller antenna were insignificant. THE OBSERVATIONS It 'was fund that, prviding the inspheric critical frequency ff2 was in the vicinity f 2 Mc/s and there was n spread-f r spradic E apparent n inspheric

3 GALACTIC RADIATION AT 4 7 MC/S 547 sundings, reprducible recrds f the sidereal prfile culd be btained. In general the bserved intensity f the galactic radiatin increased as f F 2 decreased belw 5 Mc/s and reached a maximum fr ff2,..., 3 Mc/s. It has been shwn that this FROM ANTENNA SIX 8 CHANNEL PHASING I.F. AMPUFIERS I.F. AMPLIFIERS AND D.C. AMPLIFIERS Fig. 3.-Blck diagram f the receiving equipment. 1.r--- -!:O_-!O ' ff2 (Me/s) Fig. 4.-Variatin in the received pwer at h R.A. with f F2. The slid line shws the calculated variatin (after Ellis 1963). behaviur may be expected frm theretical cnsideratins f absrptin in the F regin (Ellis 1963). Figure 4 shws the bserved variatin f the intensity with fof 2. Figure 5 shws the variatin f f F 2 with lcal time fr Hbart in June 1962.

4 548 G. R. A. ELLIS, R. J. GREEN, AND P. A. HAMILTON Because f the abve restrictins the number f nights fr which usable recrds f the galactic backgrund radiatin were btained was nly 52 during the mnths June thrugh September in 1962 and The facility f six simultaneus antenna beams prved very valuable in these circumstances. Much mre limited was the number f ccasins n which discrete detail was bserved cmparable in angular size with the antenna beamwidth. Nrmally n nights when smth recrds f the backgrund radiatin were btained discrete surces were suppressed almst cmpletely by scintillatins. Only 14 f the abve 52 nights prduced relatively clear recrds f discrete surces. The suppressin f scintillating surces n the recrds resulted partly frm the minimum recrding technique, and future bservatins will make use f cathde-ray tube recrding as described by Reber and Ellis (1956). 8, , O L L MD MD LOCAL TIME (HR) Fig. 5.-Mean values fff2 fr Hbart, June Figure 6 shws sample recrds f the backgrund radiatin fr five different declinatins. Features immediately evident include the absrptin trugh near 17 h R.A. and the external galaxy NGC 5128 at declinatin _42 The recrds fr Dec. -37 and -47 in Figure 6 give a misleading impressin f the prfile f NGC 5128 since these recrds were btained n a different night frm that fr Dec. -42 and they shw cnsiderable scintillatin, which is mstly absent n the latter recrd. Figure 7 shws the brightness cnturs f the backgrund radiatin in galactic crdinates btained by using all the recrds which extended t 5 h R.A. Additinal recrds t 8 h R.A. were btained but their quality was nt adequate fr reliable brightness cnturs. The survey f this part f the sky is being repeated. Because f uncertainty abut the ttal amunt f inspheric absrptin (Ellis 1963) it is nt cnsidered that the abslute values f the intensities shwn in Figure 7 can be relied upn t an accuracy better than 5%. Hwever, the reprducibility f the recrds was such that the relative intensities in different parts f the sky culd be measured t within 5%. Fr applicatins dealing with changes in brightness and galactic absrptin with directin, the cnturs f Figure 7 are satisfactry, but deductins based n changes in abslute brightness with frequency shuld be made with cautin until the inspheric effects are investigated in mre detail.

5 GALACTIC RADIATION AT 4 7 MC/S 549 Althugh the recrds shwed n ccasins a cnsiderable amunt f reprducible fine detail this was in general t weak fr useful deductins. In Figure 6, fr example, Fig. 6.-Recrds f the galactic prfile at 4 7 Mc/s. The external galaxy NGC 5128 can be seen n the recrd fr - 42 Dec. in the absrptin trugh between 16 h R.A. and 19 h R.A. small irregularities are marked with crsses. They appear t be less in angular size than the antenna beam Z Q -3 Z -4 " :::; -6-7, hl-5 'Ihl' 3 v / v I I /, Iv"-I" \, e 6'SX 1sOK 1 \ S' '" X L / v/ 1::::= \ vn,l '[, Dilli j 1 -"" ' V.fa ' '- IY..J Jz 6 L / l \ \... :,.lterial "t::::-, BEAM =r-- = ' a 17 RIGHT ASCENSION (HR) Fig. 7.-Brightness cnturs at 4 7 Mc/s. \ ", l "-.. ))).:::--,2 l-:::c 1 l J width, but it was nt pssible t be certain whether they were caused by bright r dark bjects.

6 55 G. R. A. ELLIS, R. J. GREEN, AND P. A. HAMILTON The three strngest discrete surces recrded were Centaurus A (NGC 5128), Frnax A (NGC 1316); and Jupiter. Recrds f these are shwn in Figure 8. The recrd fr Jupiter was btained n a night when ther equipments shwed that the Jupiter radiatin was quasi-cntinuus and it therefre prvides an indicatin f the east-west pattern f the antenna. The recrd fr Frnax is typical in that in all cases this surce appeared t be f large angular width. It is uncertain t what extent the insphere was respnsible fr the apparent size f Frnax and new bservatins are being made. FORNAX A CENTAURUS A :i --,---- III > :- III - JUPITER ;r'1i1ille -:':'--2--::_:, ----2ji --:-,--:=---' :.:'-:::.2:::-::,--: :!::;,:_, RiGHT ASCENSION (HR) Fig. S.-Recrds f three discrete surces. CONCLUSIONS A detailed cmparisn between the brightness cnturs at 4 7 Mc/s and thse at higher frequencies will be made in anther paper. The verall features f Figure 7 agree with what might be expected frm bservatins at higher frequencies. The darker parts crrespnd rughly with extensive regins f inized hydrgen (Gum 1955), althugh they are brighter than calculated using existing galactic mdels (Hyle and Ellis 1963).

7 GALACTIC RADIATION AT 4 7 Mcjs 551 The bservatins shw that the distributin f brightness f the Galaxy may be measured at frequencies in the vicintiy f 5 Mcjs frm beneath the insphere. Hwever, the difficulties encuntered thrugh the inspheric scintillatins f discrete surces suggest that the reslutin f the antenna used exceeded that needed fr ptimum results. Shain 'and Higgins (1954), fr example, detected 37 galactic r extragalactic surces with an antenna 17 by 17 at 18 3 Mcjs cmpared t the tw reprted here with an antenna 3 by II. The angular scale f the backgrund radiatin is such that it wuld adequately have been recrded using an antenna with a beamwidth f abut 1 by 1. ACKNOWLEDGMENTS This investigatin is supprted financially by the Australian Radi Research Bard. REFERENCES ELLIS, G. R. A. (1963).-Aust. J. Phys. 16: 49. GUM, C. S. (1955).-Mem. R. Astr. Sc. 67: 155. HOYLE, F., and ELLIS, G. R. A. (1963).-Aust. J. Phys. 16: l. REBER, G., and ELLIS, G. R. A. (1956).-J. Gephys. Res. 61: l. SHAIN, C. A., and HIGGINS, C. S. (1954).-Aust. J. Phys. 7: 46.